Transformation of dissolved and particulate materials on continental shelves influenced by large...

Continental Shelf Research 24 (2004) 833–858

Transformation of dissolved and particulate materials oncontinental shelves influenced by large rivers: plume processes

M. Dagga,*, R. Bennerb, S. Lohrenzc, D. Lawrencea

aLouisiana Universities Marine Consortium, 8124 Highway 56, Chauvin, LA 70344, USAbDepartment of Biological Sciences, Marine Science Program, University of South Carolina, Columbia, SC 29208, USA

cDepartment of Marine Science, University of Southern Mississippi, Stennis Space Center, MS 39529, USA

Abstract

The world’s ten largest rivers transport approximately 40% of the fresh water and particulate materials entering theocean. The impact of large rivers is important on a regional/continental scale (e.g. the Mississippi drains B40% of theconterminous US and carries approximately 65% of all the suspended solids and dissolved solutes that enter the oceanfrom the US) and on a global scale (e.g. the Amazon River annually supplies approximately 20% of all the freshwaterthat enters the ocean; e.g. approximately 85% of all sedimenting organic carbon in the ocean accumulates in coastalmargin regions).River plume processes are affected by a suite of complex factors that are not fully understood. It is clear however,

that the composition, concentration and delivery of terrestrial materials by large rivers cannot be understood by simplyscaling up the magnitudes and impacts of dominant processes in smaller rivers. Because of high rates of particulate andwater discharge, the estuarine processes associated with major rivers usually take place on the adjacent continental shelfinstead of in a physically confined estuary. This influences the magnitude and selectivity of processes that transform,retain or export terrestrial materials.Buoyancy is a key mediating factor in transformation processes in the coastal margin. In this paper we review and

synthesize current understanding of the transformation processes of dissolved and particulate organic and inorganicmaterials associated with large river (buoyant) plumes. Chemical and biological activities are greatly enhanced by thechanged physical and optical environment within buoyant plumes. Time and space scales over which thesetransformation processes occur vary greatly, depending on factors such as scales of discharge, suspended sedimentloads, light and temperature. An adequate understanding of transformation processes in these highly dynamic,buoyancy-driven systems is lacking. In this paper, we review the biogeochemical processes that occur in large riverplumes.r 2004 Elsevier Ltd. All rights reserved.

1. Introduction

Large rivers are the primary interface betweenterrestrial and ocean environments. Approxi-

mately 40% of the fresh water entering the oceanis transported by the ten largest rivers. Riverimpacts are important on regional, continentaland global scales but are especially significant tothe continental shelf regions that receive the riverinputs. For example, the Mississippi Riverdrains B40% of the conterminous US, carries

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*Corresponding author. Fax: +1-985-851-2874.E-mail address: [email protected] (M. Dagg).

0278-4343/$ - see front matter r 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.csr.2004.02.003

approximately 65% of all the suspended solids anddissolved solutes that enter the ocean from the US,and effectively injects these materials onto thecontinental shelf as a point source in the northernGulf of Mexico. It is primarily due to large riversthat approximately 40% of the global marineburial of organic matter occurs in deltaic environ-ments (Hedges and Kiel, 1995). The discharge ofsuspended sediment, particulate organic carbon(POC) and dissolved organic carbon (DOC) by therivers with the largest freshwater inputs aresummarized in Table 1.

Schettini et al. (1998) recognized the existence ofa spectrum of conditions at the river-ocean inter-face, and categorized buoyant plumes into ‘river-ine’ and ‘estuarine’ types, based on the amount of

mixing that occurs before a plume enters the sea.In riverine plumes, freshwater discharge dominatesover tidal and other effects, resulting in fresh waterbeing directly injected over shelf waters, e.g.,Mississippi River, Amazon River. In estuarineplumes, much of the mixing takes place within anenclosed basin before being released to shelfwaters. In this case, the water released into theadjacent sea already has a significant contributionof salt water, e.g., St. Lawrence River, FraserRiver. The temporal pattern of freshwater input ishighly variable among the rivers in Table 1, suchthat their location on the riverine-estuarine spec-trum (Schettini et al., 1998) varies within anannual cycle. In addition to large amounts offreshwater input, continental shelves affected by

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Table 1Discharge data for the world’s largest rivers

River (Country) Water discharge(109m3 yr!1)

Sediment discharge(106 t yr!1)

Drainage basin(106 km2)

POC(106 t yr!1)

DOC(106 t yr!1)

Amazon (Brazil) 6300 1150 6.15 13.0 19.1Zaire (Zaire) 1250 43 3.82 2.8 10.2Orinoco (Venezuela) 1200 150 0.99 2.0 4.5Ganges-Brahmaputra(Bangladesh)

970 1050 1.48 nd nd

Yangtze (China) 900 480 1.94 4.4 11.8Yenisey (Russia) 630 5 2.58 0.17 4.9Mississippi (USA) 530 210 3.27 0.8 3.5Lena (Russia) 510 11 2.49 0.46 3.4Mekong (Vietnam) 470 160 0.79 nd ndParana/Uruguay(Brazil)

470 100 2.83 1.3 5.9

St. Lawrence (Canada) 450 3 1.03 0.31 1.6Irrawaddy (Burma) 430 260 0.43 nd ndOb (Russia) 400 16 2.99 nd 3.7Amur (Russia) 325 52 1.86 nd ndMackenzie (Canada) 310 100 1.81 1.8 1.3Xi Jiang (China) 300 80 0.44 nd ndSalween (Burma) 300 100 0.28 nd ndColumbia (USA) 250 8 0.67 nd 0.5Indus (Pakistan) 240 50 0.97 nd 0.8Magdalena(Columbia)

240 220 0.24 nd nd

Zambezi(Mozambique)

220 20 1.20 nd nd

Danube (Romania) 210 40 0.81 nd ndYukon (USA) 195 60 0.84 nd ndNiger (Africa) 190 40 1.21 0.66 0.5Purani/Fly (NewGuinea)

150 110 0.09 nd nd

Data from Milliman and Meade (1983), and Meade (1996).

M. Dagg et al. / Continental Shelf Research 24 (2004) 833–858834

large rivers receive large inputs of allochthonousmaterials, both dissolved and particulate, organicand inorganic (Table 1). Although each of theserivers (Table 1) serves as a major source ofbuoyancy and dissolved and particulate materialsto the associated coastal ocean margin, the form ofinput differs. This in turn influences the continen-tal shelf processes that transform, retain or exportterrestrial materials.

The purpose of this paper is to review thetransformation processes that occur in continentalshelf regions that are strongly affected by largerivers and to suggest some important questions forfuture study. A companion paper (McKee et al.,2004) addresses processes that occur near and onthe bottom, beneath the plumes of large rivers.

1.1. Riverine inputs

There is an extensive literature on river inputs tothe ocean (e.g., Berner and Berner, 1996; Bianchiet al., 1997; Ittekkot and Laane, 1991; Ludwig andProbst, 1996; Mayer et al., 1998; Meade, 1996) andit is not our intent to review this topic here. Thereare however, several important points that warrantemphasis.

Large river systems deliver substantial amountsof inorganic nutrients, including nitrogen (N),phosphorus (P) and silica (Si) to coastal environ-ments. Increasingly, these inputs are being en-hanced by anthropogenic sources. N, P and Si aregenerally believed to be the primary limitingmacro-nutrients for phytoplankton in coastalwaters. Although iron has been implicated as a

limiting nutrient in some systems, it is generallynot limiting in river-impacted shelf environments.The input of high concentrations of biologicallyimportant elements, along with the complexphysical structure of buoyant freshwater plumes,leads to strong gradients in concentrations of andtransformations among biogeochemical constitu-ents in plume environments.

The magnitude of riverine inputs of dissolvedinorganic nutrients to shelf waters can be esti-mated from the product of discharge and nutrientconcentration. In Table 2, we compare threetropical (Amazon, Zaire, Orinoco) and threetemperate (Changjiang, Mississippi, Huanghe)rivers. Mean discharge for these rivers spans awide range (Fig. 1). Temperate large-river systemsthat are associated with higher population devel-opment and agricultural activity (Mississippi,Changjiang) have higher nutrient concentrationsthan those of the tropical river systems (Table 2).This results in high dissolved inorganic nitrogen(DIN) flux for these rivers despite lower discharge,relative to the tropical rivers (Fig. 1).

Natural sources of dissolved inorganic nutrientsin rivers, largely mineral weathering and decom-position of terrestrial plants, have been augmentedin many rivers by anthropogenic inputs includingfertilizer, leguminous crop fixation of N, urbanrun-off, and industrial and sewage discharges(Howarth et al., 1996; Jickells, 1998; Turner andRabalais, 1999). These inputs and the subsequenttransformations within the drainage basins haveled to increases in the magnitude as well as changesin the elemental ratios of nutrient inputs in many

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Table 2Discharge, dissolved inorganic nutrient concentrations, elemental ratios and average annual primary production of major river systems

River Discharge(10m3 s!1)

DIN (mM) DIP (mM) SiOH4 (mM) N:P (mol/mol) Si:N (mol/mol) Annual PP(g Cm!2 yr!1)

TropicalAmazon 188 12 0.6 132 20.0 11 730Zaire 49 7.2 0.8 165 9.0 22.9 83Orinoco 42 6.6 0.2 86 33.0 13 137

TemperateChangjiang 27 40 0.62 125 64.5 3.13 274Mississippi 15.4 101 3.1 108 32.6 1.07 584Huanghe 1.5 134 0.99 135.4 135

M. Dagg et al. / Continental Shelf Research 24 (2004) 833–858 835

river systems. For example in the MississippiRiver, N inputs related to the increased use offertilizers and increased levels of atmosphericdeposition have contributed to higher dissolvedinorganic N:P ratios than are optimal for plantgrowth (N:P atomic ratio of 16:1) (Turner andRabalais, 1991; Just!ıc et al., 1994, 1995; Rabalaiset al., 1996). P increases can be attributed largelyto direct discharge and urban sources (Jickells,1998; Turner and Rabalais, 1999). Inputs ofdissolved Si are primarily derived from mineralweathering, and hence have not shown the type ofincrease seen for N and P related to humanactivities. In fact, inputs of dissolved Si havedecreased in some cases.

Large rivers also contribute significant amountsof dissolved organic materials to the coastal ocean.DOC typically comprises 60% or more of the totalorganic carbon load in major world rivers (seeTable 1; Degens et al., 1991). Globally, riversdischarge B0.25 Pg of DOC annually to thecoastal ocean (Hedges et al., 1997). The large loadof dissolved organic matter (DOM) in rivers alsoincludes substantial quantities of organic N andphosphorus, which often exceed concentrations ofdissolved inorganic forms of these nutrients(Meybeck, 1982). Dissolved organic nitrogen(DON) can represent a substantial fraction of thetotal N in river water. For example, an average of34% for DON was computed for the Mississippi

River during 1988–1994, and in the AmazonRiver, DIN and DON pools have been found tobe similar in concentration (16.8 and 14 mmol l!1,respectively) (DeMaster and Aller, 2001). It islikely that regeneration of DON supplements theN available for uptake by phytoplankton. How-ever, the extent to which this organic source isutilized is uncertain. A portion of the riverineDON pool is likely to be refractory. The propor-tion of total dissolved N associated with DON hasbeen shown to increase with increasing salinity(L !opez-Veneroni and Cifuentes, 1994), apparentlythe result of both preferential depletion ofinorganic N pools and in situ production ofDON. Given the predominant contributionof DOM to the total organic matter dischargedby large rivers and the rapid settling of particulatematter from river plumes, it is reasonable topresume that much of the cycling of C, N and Pin river plumes involves DOM. However, the fatesof riverine DOM in ocean margins and its impacton marine food webs are poorly understoodaspects of the global cycling of these elements(Hedges et al., 1997).

Large rivers also discharge significant amountsof particulate materials. Particulate organic matterconcentrations in many large rivers, such as theMississippi River, have been shown to be wellcorrelated with total suspended matter concentra-tions, which decrease sharply within a few km ofthe river mouth (Trefry et al., 1994). Thus, a largefraction of the river-borne particulate organicmatter is initially deposited near the mouth. Therelease of nutrients from fluvial particles throughdesorption or remineralization (Edmond et al.,1981; Mayer et al., 1998) may subsequentlycontribute to the available inorganic N in riverplumes. Particle interactions with dissolved inor-ganic P have also been argued to be important inregulating the availability of P for biologicaluptake (Zhiliang et al., 1988; DeMaster and Pope,1996; Jickells, 1998). For example, in the AmazonRiver and mixing zone, release of dissolved P fromthe particulate phase may be comparable to theinitial inorganic P flux from the river (DeMasterand Aller, 2001).

Fluxes of dissolved and particulate materialsfrom large rivers into the coastal margin are

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Fig. 1. River discharge and DIN flux for six major rivers. Notethat DIN flux is high for the temperate rivers (Changjiang,Mississippi, and Huanghe) despite lower discharge.


typically large because of the combination of highdischarge and high component concentrations.Large rivers serve effectively as point sources forthese high fluxes of dissolved and particulatematerials because, in most cases, receiving watershave much lower concentrations. There are largevariations among rivers.

1.2. Transformations (within the plumes)

Plume dynamics and associated biogeochemicalprocesses vary on several time scales. On inter-annual scales, variations in total river dischargeand meteorology modify the spatial and temporalextent of plumes. Seasonally, river discharge, solarradiation and wind forcing are important andthere is considerable variability in the phasing ofthese signals. For example, particulate discharge atthe mouth of a river is affected by the timing andmagnitude of water discharge. There is a non-linear relationship between flow and particulatedelivery resulting in short intense pulses ofterrestrial particulate inputs into the estuarineenvironment. Dissolved inputs are also pulsed,although not for the same reasons. Inputs and thesubsequent transformations are not linearly re-lated to the magnitude of fresh water discharge.On event scales, weather patterns contributesignificantly to variations in direction and strengthof plume flow and mixing. For example, in thenorthern Gulf of Mexico, these events recur every3–10 days between October and April. Dependingon location, tides can have a significant impact onshort-term discharge. Even in the Gulf of Mexicowhere tides are diurnal and small (amplitudesrarely exceed 30 cm), associated currents mayaffect the plumes (Wright and Coleman, 1971)and there is some evidence that plume edges varyon a tidal time scale. Also, in the highly stratifiedwaters near deltas, internal tides may becomeimportant and increase the currents. Thus, trans-formations within river plumes on the shelf are anexceptionally complex set of processes that occuron small temporal and spatial scales, compared totraditional oceanographic scales.

Some processes however, appear consistentamong systems. Because of the formation of abuoyant plume after discharge to the shelf

environment, there is decreased turbulence whichreduces the ability of plume waters to transportsuspended lithogenic materials. Large quantities ofterrigenous particles are rapidly lost from plumewaters leading to increased light penetration.Mixing with ocean waters dilutes riverine coloredDOM (CDOM), also enhancing light penetration.These changes in the physical and optical environ-ment have a major impact on chemical andbiological processes within buoyant plume waters.For example:

* Aggregation, flocculation and desorption pro-cesses associated with large changes in ionicstrength and composition during the mixing ofriver and marine waters lead to rapid exchangesbetween particulate and dissolved phases in thelow salinity or near-field portion of the plume;

* Increased light penetration stimulates phyto-plankton utilization of dissolved inorganicnutrients resulting in enhanced primary produc-tion that cascades through ‘‘classical’’ and‘‘microbial’’ food webs and processes;

* Photochemical and microbial transformationsof riverine DOM release organically boundnutrients that further stimulate phytoplanktonproduction, food webs and the cycling ofbiologically important elements.

All these factors affect the transformation andtransport (cross shelf and vertical) of river bornematerials, resulting in highly variable rates andproportions of various processes.

1.2.1. Aggregation, flocculation, adsorption–desorption

As the river plume mixes with the receivingocean waters, flocculation and aggregation ofdissolved and colloidal materials into largerparticles takes place. This is largely a physico-chemical process (Sharp et al., 1983; Stumm, 1990)and takes place at low salinities in the near-fieldplume. There is chemical sorption of inorganic Pand several organic constituents onto particlesduring this early mixing phase of plume-oceaninteractions (Fox, 1984; Sholkovitz, 1976; Sharpet al., 1983). Conversely for phosphate at slightlyhigher salinities there is a significant desorption,

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such that more than half of the phosphate reach-ing the open ocean from the Amazon River isreleased from the particulate phase during river/ocean mixing. Thus, suspended sediments are animportant source of P in these systems (Fox, 1984;DeMaster and Pope, 1996). On the other hand,‘‘phosphate concentrations in the Para estuaryincrease between 0 and 4 psu (probably as a resultof desorprtion).’’ There is also desorption of otherriverine constituents, especially metals, from par-ticle surfaces to the soluble phase (Li et al., 1984)in this low salinity region.

Aggregation enhances the sinking of bulkparticulate matter in the near-field region (Ittekkotand Laane, 1991). Aggregates of particulate matterthat settle rapidly, up to 5m/h (Gibbs, 1986),quickly settle from the plume and require only afew hours to reach the bottom sediments. Aggre-gation is important for removal of colloidalmaterials. DOC, DON and DOP can havesignificant colloidal fractions that are subject toremoval via aggregation (Honeyman and Santschi,1992). In contrast, dissolved silicate, nitrate andammonium do not appear to contain significantcolloidal components and are unaffected byaggregation (Hollibaugh et al., 1991). Aggregationand settling decrease the suspended sedimentconcentrations in the Mississippi River plume by>90% within 5–10 km of the river mouth (Trefryet al., 1992).

Aggregates are processed at different rates thantheir non-aggregated constituents and thereforewill be removed at different salinities and locationswithin the plumes (Sharp et al., 1983; Fox andWofsy, 1983). This selective removal is an im-portant means of separating pathways of differentriverine constituents in a down-plume direction.

1.2.2. Biological uptake of dissolved inorganicmaterials

Enhancement of biological production by land-derived nutrients in regions impacted by largerivers is evidenced by relationships between DINinputs and primary production on an areal basis(Fig. 2). Nixon et al. (1996) reported a relationshipbetween integrated primary production and inor-ganic N inputs per unit area in other systems. Useof this approach for large rivers is complicated

because the appropriate shelf area can be unclear.Nevertheless, Lohrenz et al. (1997) showed thatareal N requirements for primary production(assuming Redfield C:N of 106:16 by atoms) inthe Mississippi River delta region were generallycomparable to river inputs, supporting the viewthat inputs and biological uptake were closelycoupled in this system. Non-conservative decreasesin nutrient–salinity relationships along river/oceanmixing gradients have been attributed to biologicaluptake in various large river plumes including theAmazon (Edmond et al., 1981; DeMaster andPope, 1996), Mississippi (Lohrenz et al., 1999),Changjiang (Edmond et al., 1985; Tian et al.,1993) and Zaire (Van Bennekom et al., 1978). Inaddition to the observations of surface nutrientconcentrations below the expected conservativemixing relationships, there have been numerousobservations where surface nutrient concentra-tions were higher than expected for conservativemixing. This pattern has been observed in theMississippi region (Lohrenz et al., 1990, 1999;Dortch and Whitledge, 1992), in the Amazonplume (Edmond et al., 1981; DeMaster and Pope,1996), in the Zaire River plume (Van Bennekomet al., 1978), in the Changjiang (Edmond et al.,1985; Zhang, 1996), and in the Huanghe (Zhang,

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0

100

200

300

400

500

600

700

800

0 20 40 60Annual DIN Flux (109 mol N y -1)

Ann

ual P

P (g

C m

-2 y

-1)

A

M

CO

Z

H

80

Fig. 2. Relationship between primary production in shelfwaters impacted by major rivers and riverine DIN flux. Lettersdesignate the different rivers, including Amazon (A), Chang-jiang (C), Huanghe (H), Mississippi (M), Orinoco (O) andZaire (Z).


1996). Such observations have been attributed toinputs from other sources such as desorption fromparticles, remineralization of organic matter, orentrainment from below the plume.

Estuarine circulation can entrain large volumesof water from offshore. For example, DeMasterand Aller (2001) state the shoreword flow ofsubsurface water onto the Amazon shelf is 10times greater than riverine discharge. However,concentrations of nutrients in the river are muchgreater than in the entrained deep water so thenutrient contribution of entrainment is much lessthan 10 times. On the Amazon River shelf, theriver contribution was dominant and was esti-mated to be 83–91% for silicate, 62–76% fornitrate and 48–65% for phosphate (DeMaster andPope, 1996). For the shelf of the northern Gulf ofMexico between approximately 89.5"W and97"W, 54–67% of the N is contributed by theMississippi-Atchafalaya River system (L !opez-Ve-neroni and Cifuentes, 1994). The nutracline isgenerally deeper than the shelf break in thenorthern Gulf of Mexico and entrained waterfrom offshore will not contain high concentrationsof nitrate or silicate. Oceanic nutrients may beimportant to continental shelves in general (Wol-last, 1991) but their significance in river dominatedmargins appears greatly diminished.

A complicating factor in understanding non-conservative changes in nutrients and the relation-ship to biological uptake is the difficulty inquantifying mixing rates of plumes (e.g., Shiller,1993). Hitchcock et al. (1997) tracked Lagrangiansurface drifters in the Mississippi River plume andfound that mixing relationships between thenutrients in surface and subsurface waters withinthe rapidly flowing plume core of the MississippiRiver were conservative. This finding was atodds with numerous observations of non-conser-vative patterns in nutrient–salinity relationshipsin this system. Lohrenz et al. (1999) noted thatassumptions of simple two-end member mixingmodels used to infer uptake-related losses couldbe violated in the complex advective regimes ofriver plumes. They demonstrated that subsurfacewaters beneath the plume as well as longerresidence time waters at the plume edge couldbe depleted in nutrients relative to waters of

similar salinity within the rapidly advecting coreregion of the plume. Thus, waters similar insalinity may come from different subenvironmentsand have different physical, chemical and biologi-cal histories.

Optimal ratios of nutrients for phytoplanktongrowth are considered to be the Redfield ratiovalues corresponding to an N:P ratio of 16:1 and aSi:N ratio of approximately 1. Differences amongriver systems in N:P and Si:N ratios (Table 2) canbe attributed largely to differences in populationdensity and land use practices within the drainagebasin. Higher N:P ratios are observed for thosesystems with more heavily populated watersheds(e.g., Mississippi, Changjiang) as compared to lessdeveloped drainage basins (Amazon, Orinoco,Congo). In addition to the impact of N inputsassociated with fertilizer and sewage on N:P ratios,factors which influence denitrification or storage insoils or plants may lead to different loss rates ofinorganic nutrients. Denitrification and storagelosses of N can be expected to increase withincreasing residence time of waters within thedrainage basin (Howarth et al., 1996; Turner andRabalais, 1999; Alexander et al., 2000). This wouldaccount for the positive correlation between riverdischarge and N:P ratios observed for the Mis-sissippi River (Lohrenz et al., 1999). Shorterresidence times would be expected during highdischarge. Seasonal variations in N:P ratios of theAmazon river were also observed (DeMaster andPope, 1996), although over a much smaller range(12–24) than that for the Mississippi River (13–72based on USGS data for Belle Chase, LA). Forboth systems, highest values were observed inspring, corresponding to seasonal periods of high-er discharge.

Damming of rivers may also alter elementalratios of inorganic nutrients. Introduction of damsreduces particulate loads in river systems and,subsequently, reduces levels of dissolved phos-phorus. Uptake by freshwater diatoms in up-stream reservoirs can also reduce fluxes of silicate(Jickells, 1998; Turner and Rabalais, 1999). Forthe rivers considered here, highest Si:N ratios wereassociated with the tropical rivers (Table 2).However, this was due largely to lower DIN asopposed to higher Si concentrations.

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Despite the relatively high input of nutrientsassociated with large rivers, nutrient limitation ofprimary production has been argued in severalcases. The controlling nutrient appears to differboth temporally and spatially and among thedifferent rivers. Depletion of P was observedduring July in the Changjiang plume (Xiurenet al., 1988). Depletion of nitrate in the Amazonplume led investigators there to conclude thatphytoplankton were predominantly N limited inthe higher salinity reaches of that plume environ-ment (DeMaster and Pope, 1996). Depletion ofsilicate has also been observed in the AmazonRiver plume (DeMaster et al., 1996). In the ZaireRiver plume, biological uptake appears to besmaller than that of the Amazon or MississippiRiver plume ecosystems (Van Bennekom et al.,1978). This may be due to rapid mixing and theresulting low phytoplankton biomass. Neverthe-less, there was evidence in some cases for depletionof inorganic N that is consistent with N limitation.For the Mississippi River, nutrient limitation hasbeen inferred based on nutrient ratios andconcentration thresholds (Dortch and Whitledge,1992; Lohrenz et al., 1999). Results were consistentwith potential limitation by P during periods whenDIN:PO4 ratios of river nutrients were relativelyhigh. Potential for N limitation was generallyrestricted to higher salinities and during periods ofrelatively low DIN:PO4 in river waters (periods oflow discharge). Si limitation was also indicated forsome periods, especially during spring whendiatom abundance is relatively high.

Conclusive demonstration of nutrient limitationrequires knowledge of uptake rates for individualnutrients (e.g., Fisher et al., 1992) and composi-tional ratios of phytoplankton biomass (e.g.,Pennock and Sharp, 1994). Such data are limitedfor large river systems. Direct estimates of nutrientuptake rates are rare. Evidence for P limitation inthe Mississippi River plume waters comes fromreported high rates of P turnover times duringJuly–August 1990 and September 1991 (Ammer-man, 1992). Nelson and Dortch (1996) reportedstrong Si limitation in spring and no Si limitationin summer based on 30Si uptake kinetics. Directobservations of nitrate uptake support argumentsthat non-conservative decreases in nutrients in the

Mississippi River plume were due to biologicaluptake. Bode and Dortch (1996) observed thatnitrate in surface waters of the Mississippi Riverplume was greatly reduced at salinities between 5and 25, where the largest variance in uptake rateswas observed, and was coincident with peaks insurface chlorophyll. Bode and Dortch (1996)noted that despite the depletion of nitrate in theMississippi River plume, N limitation was a rareevent during their study because of relatively highammonium concentrations (>1 mmolNH4

+1!1)and regeneration rates.

Nutrient enrichment bioassays have been usedin various systems. Results of these efforts havesupported evidence of P limitation in the Huanghe(Turner et al., 1990) and Changjiang (Minghuiet al., 1990) river plumes. Bioassays in theMississippi River plume (Smith and Hitchcock,1994; Chen, 1994) provided evidence of both P andSi limitation during March and May and Nlimitation during low flow conditions in the latesummer.

In summary, there is evidence to support Plimitation in plumes of temperate rivers withhigher N:P ratios, while plumes of tropical systemsappear to be primarily limited by N, especially athigher salinities. Seasonal and spatial variations innutrient limitation can also occur, as observed inthe Mississippi Rive plume. The shift from P to Nlimitation along the mixing gradient of theMississippi probably reflects differential proces-sing of N and P in the plume. Losses of N due todenitrification can be substantial in coastal envir-onments (Christensen, 1994; Galloway et al., 1996;Seitzinger and Giblin, 1996). In addition, it hasbeen suggested the variations in dissolved inor-ganic P may be buffered in estuaries by particleinteractions (Zhiliang et al., 1988; Jickells, 1998;DeMaster and Aller, 2001). Clearly, numerousfactors affect the transformation of dissolvedinorganic nutrients in plumes of large rivers.

1.2.3. Phytoplankton productionConditions for phytoplankton growth in buoy-

ant discharge plumes of large rivers are typicallyvery good because of high available nutrientconcentrations and high light associated with thebuoyant plume. Phytoplankton production in

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some parts of these plumes is generally high(Fig. 3). Spatial patterns of phytoplankton pro-duction in a ‘‘down-plume’’ direction generallyshow a maximum at intermediate salinities wheregrowth rate of both large and small celled speciesis high, although large celled species, primarilydiatoms, typically dominate in the mid-salinityregions of maximum phytoplanktpon biomass(e.g., Bode and Dortch, 1996). Controlling factorsinclude light, nutrients, and loss terms such asgrazing and sinking. This mid-salinity maximum inphytoplankton biomass and productivity is attri-butable to declining turbidity in the presence ofhigh nutrient levels and has been reported forvarious large rivers including the Amazon (De-Master et al., 1986; Smith and DeMaster, 1996),Mississippi (Lohrenz et al., 1990, 1999), Huanghe(Turner et al., 1990) and Changjiang (Xiuren et al.,1988; Tian et al., 1993). This pattern has also beenobserved in many estuarine ecosystems (Kempet al., 1982; Cloern et al., 1983; Filardo andDunstan, 1985; Pennock and Sharp, 1986; Fisheret al., 1988). The location of this maximum alongthe salinity gradient differs between rivers andwithin rivers for different discharge conditions andseasons. This appears to be due to differences insalinities at which suspended matter concentra-tions decline to a level (o10mg l!1) to allowsufficient light to enhance phytoplankton growth(Edmond et al., 1985; Xiuren et al., 1988; Turneret al., 1990). A comparison of the Amazon, Zaireand Changjiang Rivers (Xiuren et al., 1988)revealed that the biomass maximum in theAmazon occurred at a much lower salinity thanthat of the Zaire or Changjiang (Fig. 3). This wasattributed to a shoal at the mouth of the AmazonRiver that resulted in rapid sedimentation ofsuspended matter and thus to an improved lightenvironment. DeMaster et al. (1996) referred tothe region of high biomass and productivity in theAmazon River plume as the ‘‘optimal growthzone.’’ In a comparison among various riversystems, Turner et al. (1990) observed that theregion of high chlorophyll, high primary produc-tion, and large phytoplankton cells spanned abroader salinity range in the Huanghe River plumethan other river plumes. They pointed out thatvarious factors in addition to light attenuation,

including surface mixed layer depths and mixingrates, were important factors influencing phyto-plankton distributions and primary production. Inthe Mississippi River, Lohrenz et al. (1999) notedthat the salinity corresponding to maxima inbiomass and productivity differed between differ-ent discharge levels. During higher dischargeconditions in March, the maxima occurred athigher salinities than during summer. The loca-tions of biomass maxima generally correspondedto regions where light availability in the surfacemixed layer increased due to decreased turbidity.Presumably, dilution or breakdown of coloreddissolved organic material may also be a factor indifferences observed, but there is less informationavailable about this term. As mixing with receivingwater continues in the ‘‘down-plume’’ direction,nutrients become limiting and phytoplanktongrowth declines. At these higher salinities, loss

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Fig. 3. Relationships between phytoplankton production andsalinity within the plumes of several large rivers: (A) Ning et al.(1988 and references therein); and (B) Lohrenz et al. (1999) forthe Mississippi River.


terms (grazing and sinking) become relativelymore important and phytoplankton biomassdeclines because of dilution and losses (Dagg andBreed, 2003; Liu and Dagg, 2003). Processescontrolling phytoplankton distributions, commu-nity composition, and rate processes requireadditional study.

1.2.4. Zooplankton and higher trophic levelprocesses

Several biotic and abiotic mechanisms lead tospecial zooplankton communities in large riverplumes. These mechanisms includes: (1) growthwithin the plume due to enhanced food concentra-tions (trophic effect); (2) physical accumulation inthe zone of frontal convergence; and (3) activemigration into the plume layer. Processes (2) and(3) mainly apply to mesozooplankton, organisms>200 mm. Alternatively, transformations asso-ciated with plumes may sometimes be destructivebecause of mortality resulting from salinity andtemperature stresses, and because the forces thataggregate zooplankton may also aggregate larvalfish and other predators. Understanding theseprocesses is essential to determining how uppertrophic levels may be affected by large rivers.

An example of physical accumulation by en-trainment/transport is seen in the Fraser Riverplume. Here, Mackas and Louttit (1988) found thedensest aggregations of the large copepod Neoca-lanus occur when and where the low salinity lensexpands outward from the river mouth. Theysuggest that as the plume advances it accumulatesanimals from underlying water. When the plumeboundaries are stationary or recede back towardthe mouth, zooplankton densities at the frontalmargins were reduced. St. John et al. (1992)further corroborated the physical mechanism ofplume entrainment in the Fraser River plume.They showed that copepods and amphipods weresignificantly more abundant in the brackishestuarine plume (surface salinities 10–15) com-pared to the area covered by freshwater of theFraser River plume (0–10) and the region of theStrait of Georgia (25–30) unaffected by theoutflow of the Fraser River.

In some large rivers, estuarine zooplankton canbe transported in plumes onto the shelf. For

example, Calef and Grice (1967) found thataverage zooplankton displacement volume on theshelf near the Amazon River was almost threetimes higher during the wet season than during thedry season. Furthermore, outflow from the Ama-zon River created a large low-salinity lens whereestuarine zooplankton species (Evadne tergestina,a cladoceran, and Lucifer faxoni, a decapod)dominated the surface waters some 370 km off-shore. It was also noted that the species composi-tion became more diverse during the wet season ofMay–June. Thus, the Amazon River appears to betransporting a large amount of zooplanktonbiomass to shelf waters, particularly duringperiods of high discharge. Large variationsin the zooplankton biomass between the floodand dry seasons have been observed in the YangtzeRiver. Yaqu et al. (1995a) found that meanbiomass during the flood season was as high as962mgm3, compared to only 68mgm3 in the dryperiod.

Growth of mesozooplankton within plumewaters also contributes to the high biomass inthese environments. For example, in the Yangtze(Changjiang) River there were trends in speciescomposition in relation to salinity (or distancefrom outflow) that could only result from acombination of in situ growth and transport froma river source. The retention of a zooplanktoncommunity within a salinity range in a rapidlymoving plume requires that gains from reproduc-tion/recruitment exceed losses from advective andmixing processes. In addition, zooplankton specieswere vertically distributed in waters influenced bythe Yangtze plume during the wet season, whilepatterns of vertical distribution were not clearduring the dry season (Zhaoli et al., 1995). Thus, itappears that the Yangtze River has a significantrole transporting zooplankton biomass seaward,has a strong stimulatory role in populationgrowth, and has a significant influence on thevertical composition of animals found in thevicinity of the plume. Distinct zooplankton com-munities also exist within and below the Mis-sissippi River plume (Al-Yamani, 1988; Ortneret al., 1989; Dagg and Ortner, 1995).

Concentrations of immature copepods (nauplii)are elevated in the discharge plume of the

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Mississippi River where abundance is 2–10 timesgreater than in nearby shelf waters (Dagg, 1988;Dagg and Whitledge, 1991), indicating high eggproduction rates and rapid population growthassociated with at least part of the plume environ-ment. In the plume, winter and spring concentra-tions of copepod developmental stages, especiallythe early naupliar stages, were typically between 50and 100 nauplii l!1, and summer concentrationswere much higher, sometimes greater than 1000 l!1

(Dagg et al., 1987; Dagg and Whitledge, 1991;Dagg and Govoni, 1996).

In the Mississippi River plume region, Grimesand Finucane (1991) found zooplankton volumeswere greater in frontal waters compared to plumeor shelf waters (1.7–5 times greater, respectively).Surface chlorophyll concentration was 20 timesgreater in frontal waters than in the core of theplume or in shelf waters. In the Rhone Riverplume, peak abundance of zooplankton occurredat the plume edge in the nighttime (Gaudy et al.,1996), although this pattern was not observed inthe daytime (Gaudy et al., 1990).

Enhanced population levels within plumes canalso occur from active migration. In the RhoneRiver plume, zooplankton aggregation in relationto freshwater outflow appears to be the result ofactive migration. In this system, tidal effects areminimal and the extent of the plume is dependentprimarily on wind mixing and freshwater inflows(Forget et al., 1990). Pagano et al. (1993) studiedthe vertical distribution of mesozooplakton over a24-h period at a fixed station in the Rhone Riverplume. Abundant taxa exhibited a nocturnalmigration into chlorophyll-rich, low salinity plumewaters. In addition, gut fluorescence of copepodswas highest at night in the plume waters. Thus,phytoplankton in the Rhone River plume providesan enhanced food environment, which supportedenhanced zooplankton feeding. These data formesozooplankton are summarized in Table 3.

One of the most significant transforming roles oflarge zooplankton within plume environments is asconsumers of particulate materials including phy-toplankton, bacteria and lithogenic particles. Thebulk of zooplankton grazing in river-dominatedmargins is done by three groups: the protozoa, thegelatinous zooplankton, and the copepods.

The copepod community can consume signifi-cant portions of the phytoplankton stock andproductivity in river plume regions. In the north-ern Gulf of Mexico, depending on the time of year,between 14% and 62% of the daily algal produc-tion can be consumed by the copepod community(Dagg, 1995). Dagg (1995) measured ingestionrates of the copepod community within the buoy-ant water plume of the Mississippi River and in afar-field location away from the immediate impactof riverine enrichment. Within the plume, gut-pigment levels and ingestion rates of copepodswere elevated relative to the shelf station. Litho-genic particles are also consumed by copepods(Turner, 1984) as are particles larger than 20 mm,including protozoans. Grazing experiments byParsons et al. (1969) carried out in the FraserRiver plume showed selective grazing of copepodson larger phytoplankton species. They observedincreased copepod standing stock in the vicinity ofthe plume, and suggest this results from thepresence of high concentrations of the rightsize and shape food organisms. In the RhoneRiver plume, Gaudy et al. (1990, 1996) documen-ted enhanced grazing rates of copepods in thebuoyant river plume, especially at the salinityinterface, compared to the more dense water layerbeneath it.

Protozoan populations respond quickly whenphytoplankton food is abundant. Protozoans areimportant grazers in plume regions where phyto-plankton growth rates are high. In the northernGulf of Mexico, there is close coupling ofheterotrophic and autotrophic abundances, parti-cularly in the o20 mm size fraction (Bode andDortch, 1996; Liu and Dagg, 2003), and proto-zoans consume large amounts of phytoplankton.For example, Fahnenstiel et al. (1995) reportedmicrozooplankton grazing rates on cells o20 mmaveraged 82% of algal growth rates, which arevery high, in the summer. In contrast, microzoo-plankton grazing rates on cells >20 mm were notsignificantly different from zero (Fahnenstiel et al.,1995), indicating that chain forming diatoms arenot subject to the same degree of protozoangrazing mortality as smaller cells in the plumeregion. Slightly farther to the west and away fromthe plume region, phytoplankton growth rates

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were nutrient limited but still quite high and rates>1.0 d!1 were measured on several occasions(Strom and Strom, 1996). Here, microzooplanktonalso consumed significant portions of phytoplank-ton; the grazing:growth ratio was typically be-tween 0.3 and 0.9, and microzooplankton biomasswas correlated with chlorophyll concentration. Incontrast to Fahnenstiel et al. (1995), Strom andStrom (1996) observed significant consumption oflarge diatoms by microzooplankton (heterotrophicdinoflagellates) at some stations.

Gelatinous zooplankton, primarily larvaceans,salps and doliolids, often form large swarms incoastal regions. For example, high concentrationsof the larvacean, Oikopleura dioica, are commonlyfound in the vicinity of the Mississippi Riverplume (Dagg, 1995; Dagg et al., 1996), and thisgrazer is an important component of the grazercommunity. During May 1992 O. dioica popula-tions filtered a mean of 20%day!1 of the upper

5m at stations within the Mississippi River plume(Dagg et al., 1996).

Terrigenous POM (detritus) may be consumeddirectly by micro- or mesozooplankton in plumeregions, converting non-living particulate matterinto organisms. This bypasses the solubilization-bacterial uptake-bacterivory route and thus in-creases retention of organic matter in the foodweb.

Feeding conditions in the highly productiveplume regions appear favorable for ichthyoplank-ton because of the high concentrations of plank-tonic prey. However, efforts to measure clearresponses in the feeding, growth and recruitmentof larval fish have had mixed success (Grimes andKingsford, 1996) in part because the study regionsare so physically dynamic but also because theplume regions offer good feeding opportunities forpredators on larval fish (Grimes and Kingsford,1996). Nevertheless, ichthyoplankton abundance is

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Table 3Influence of buoyant water plumes on secondary producers

River/Bay Buoyant water plume zooplankton vs. surrounding shelf water zooplankton References

Abundance/biomass

Grazing rates withinplume

Distinct speciescomposition?

Mississippi +1, 3, 4, 5, 7 +6, 7 +1, 2, 4, 7 1. Marum (1979), 2. Al-Yamani(1988), 3. Dagg (1988), 4. Ortneret al. (1989), 5. Grimes andFinucane (1991), 6. Dagg (1995), 7.Dagg and Ortner (1995)

Amazon + ND + Calef and Grice (1967)Yangtze +1, 2, 3 ND +1, 2, 3 1. Yaqu et al. (1995a, b), 2. Yaqu

et al. (1995a, b), 3. Xu et al. (1995)Rhone !1+2 (at night), 3 +1, 2, 3 !1 1. Gaudy et al. (1990), 2. Pagano

et al. (1993), 3. Gaudy et al. (1996)Fraser +1, 2, 3 ND ND 1. Parsons et al. (1969), 2. Mackas

and Louttit (1988), 3. St. John et al.(1992)

St.Lawrence

+ ND + Cote et al. (1986)

Columbia + ND + Cross and Small (1967)Itajai-acu + ND + Schettini et al. (1998)Fly + ND ND Robertson et al. (1993)Po ND ND + Specchi and Fonda-Umani (1983)Rhine,Meuse,Schedlt

+ ND ND Fransz (1986)

+, Variable was elevated in plume relative to surrounding waters; !, no difference; ND, no data.


significantly higher in regions at the boundarybetween plume and oceanic water than in adjacentshelf waters (Govoni et al., 1989; Grimes andFinucane, 1991).

Regions associated with large rivers are oftensites of high fisheries production (Chesney et al.,1998) suggesting stimulation of upper trophiclevels may be common to these types of systems.The northern Gulf of Mexico is a region of veryhigh fisheries production; approximately 66% ofthe total fishery landings for the US Gulf coast arein Louisiana. In general, high concentrations of‘‘new’’ N are conducive to high trophic transferfrom phytoplankton to fish (Runge, 1988; Houdeand Rutherford, 1993). Because many large riversserve as a source for ‘‘new’’ N, river plumes appearto be supportive of the ‘‘classical’’ food web andhigh fisheries production.

1.2.5. Recycling and bacterial processesAs river-borne nutrients become depleted, the

relative importance of other nutrient sourcesincreases, reducing the direct coupling betweenriver inputs and primary production. The impor-tance of biological regeneration of nutrients hasbeen noted for various large river plume systems.However, there are few quantitative assessments ofthe importance of regenerated sources to overallnutrient requirements. In the northern Gulf ofMexico, a comparison of the estimated N uptakeby phytoplankton with the supply of NO3

!+NO2!

N from the Mississippi River (Lohrenz et al., 1997)revealed that fluvial inorganic N flux was equiva-lent to the photosynthetic N requirements(mean=106%, std. dev.=58%, N=12, low lightconditions in March 1991 excluded). This situationwas in contrast to conditions reported for theAmazon shelf, where regeneration was estimatedto sustain >50% of N requirements for primaryproduction (DeMaster and Pope, 1996). Thedifferences in N utilization between the Mississippiand Amazon ecosystems could be due to the muchhigher concentrations of nitrate in the Mississippias compared to the Amazon River. Estimates ofnew production in the Changjiang (Yangtze)plume (Chen et al., 1999) indicated that nitrateuptake was a relatively small fraction of total Nrequirements for primary production, suggesting

the potential importance of regenerated sources.However, the input of non-nitrate sources of Nfrom the river was not known, and so it is unclearto what extent primary production was supportedby regenerated sources in this system.

Despite the relatively high input of riverine N inthe Mississippi River plume, there does appear tobe substantial nutrient regeneration in this system.This view is supported by observations of surfacenutrient properties (Dortch and Whitledge, 1992),close coupling between primary producers andheterotrophic compartments (Gardner et al., 1994;Dagg, 1995; Fahnenstiel et al., 1995), and highbenthic nutrient fluxes (Twilley et al., 1999; Morseand Rowe, 1999). Gardner et al. (1994) found highrates of respiration and remineralization at inter-mediate salinities in the plume during July–August1990, and noted that activities were lower insamples with microzooplankton-sized particles(>1–3 mm) removed. Bode and Dortch (1996)reported seasonal variations in the proportion ofphytoplankton N uptake supplied by regeneratedsources, with highest proportions in summer.Gardner et al. (1997) reported light dependentammonium regeneration, indicating tight couplingbetween phytoplankton production of DON andmicrobial regeneration of inorganic N. Pakulskiet al. (1995, 2000) observed intense nitrification atintermediate salinities in Mississippi plume waters.These various studies provide a glimpse of the typeand magnitude of nutrient transformations thatoccur in plumes of large rivers.

It is also interesting that NH4 concentrationswithin the Mississippi plume are always low, inspite of the high regeneration rates (especially atintermediate salinities). Mixing lines of NO3 vs.salinity showed non-conservative behavior, withdepressed NO3 at low to intermediate salinities.Incubation experiments indicated a net regenera-tion of NO3 at salinities between 18 and 27 inplume waters. Nitrifying bacteria oxidize NH4 asan energy source, and they play important roles inthe consumption of dissolved oxygen and produc-tion of NO3 at intermediate salinities (Pakulskiet al., 1995). Heterotrophic bacteria and micro-zooplankton are the major regenerators of theNH4 at intermediate salinities in the Mississippiplume.

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Benthic fluxes of nutrients provide anotherpotential source of regenerated nutrients for riverplume environments. Information about the con-tribution of benthic fluxes to total nutrient inputsis limited. DeMaster and Pope (1996) examinedthis problem for the Amazon River plume usingflux core measurements and determined thatdiffusive fluxes across the sediment/water interfacewere a minor component of the nutrient budgetsfor the shelf. Rather, they argued that the majorityof nutrient regeneration occurred in shelf bottomwaters. Benthic sources appear to be moreimportant in the Mississippi River delta region.Twilley et al. (1999) determined average DINfluxes from the sediments in the region of3.0mmolm!2 d!1. Morse and Rowe (1999) simi-larly determined a range of inorganic N flux of2.6–4.2mmolm!2 d!1. These values represent ap-proximately 12–20% of riverine DIN flux on anareal basis for shelf waters surrounding theMississippi birdfoot delta. As Morse and Rowe(1999) pointed out, such extrapolated estimateshave high uncertainty because of the spatialheterogeneity in benthic fluxes. Furthermore, thephysical transport of bottom regenerated nutrientsto surface (plume) waters needs to be considered.

Much of the organic matter in large rivers issoil-derived, diagenetically altered and relativelyresistant to microbial degradation (Meybeck,1982; Ittekkot, 1988; Hedges et al., 1994; Benneret al., 1995). In the Mississippi River, DOM isresistant to microbial decomposition and supportslow rates of respiration and microbial growth(Chin-Leo and Benner, 1992; Gardner et al., 1994).Heterotrophic processes are carbon limited in theAmazon River system as well (Benner et al., 1995)suggesting this may be a general phenomenon inlarge rivers. The highly degraded nature of riverineorganic matter is also evident from detailedanalyses of its chemical composition (Hedgeset al., 1986, 1994; Ertel et al., 1986).

Despite the widely recognized resistance ofriverine DOM to biodegradation, there is noevidence indicating riverine DOM accumulates inthe ocean (Hedges et al., 1997; Opsahl and Benner,1997). Recent evidence indicates that photochemi-cal processes play an important role in the removalof riverine DOM in ocean margins (see Section

1.2.6), but microbial degradation is likely respon-sible for the oxidation of considerable amounts ofriverine DOM during transport in plumes. Basedon community respiration rates and the organiccarbon load in the Amazon River, biologicaldegradation could remineralize most of the or-ganic matter in 40–50 days (Hedges et al., 1994;Benner et al., 1995). This presumes that the entireload of organic matter in the river is similarlyreactive, which is unlikely. However, this simpleanalysis indicates that a significant fraction(10–30%) of riverine DOM is biologically reactiveon time scales (days–weeks) relevant to plumeprocesses.

There is strong evidence for a DOC source atintermediate salinities in the Mississippi Riverplume, particularly during the spring and summer(Fig. 4). High phytoplankton biomass and produc-tion occur in this region and elevated DOCconcentrations are probably from direct releaseby phytoplankton or from grazing-mediated pro-cesses. High concentrations of combined dissolvedcarbohydrates provide clear molecular evidencefor the production of DOM at mid-salinities in theplume (Benner and Opsahl, 2001). Carbohydrateconcentrations are much higher at mid-salinities inthe plume than in the river or adjacent marinewaters.

Community respiration rates provide an inte-grative measurement of heterotrophic metabolismin river plumes, and they can provide an indicationof the sources of organic matter supportingheterotrophic activity. Respiration rates are spa-tially and temporally variable in river plumes,indicating the heterogeneous nature of organicmatter sources and biological processes in theseenvironments. In the Mississippi plume, respira-tion rates range from 0.3 to 0.8 mMh!1 in winter,0.6–3.7 mMh!1 in spring and 0.9–3.2 mMh!1 insummer (Chin-Leo and Benner, 1992; Pakulskiet al., 1995, 2000). Respiration rates are highestduring the warmest periods when plankton bio-mass and production are highest in plume waters.Respiration rates are also highest in mid-salinityplume waters where plankton production andconcentrations of DOM and dissolved carbohy-drates are highest (Benner and Opsahl, 2001).These patterns indicate the importance of

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plankton-derived organic matter in Mississippiplume food webs. Considerably lower communityrespiration rates (0.5–1.7 mMh!1) were measuredin the Fly River plume during summer (Robertsonet al., 1993). The Fly River has lower nutrientconcentrations than the Mississippi River, and Flyplume waters support lower rates of primaryproduction than Mississippi plume waters (Loh-renz et al., 1990; Robertson et al., 1993). The lowerrates of community respiration in the Fly plumepresumably result from lower rates of planktonproduction and activity. Robertson et al. (1993)estimated that community respiration across theFly plume exceeded primary production by10–100 fold, indicating that most respiration inthe plume was supported by riverine-derivedorganic matter.

As the predominant consumers of DOM,heterotrophic bacteria often account for a majorfraction of community respiration in aquaticenvironments. However, we are aware of only asingle study that attempted to directly measure thebacterial contribution to heterotrophic oxygenconsumption in the plume of a large river.Gardner et al. (1994) compared rates of respirationin filtered (o1 mm bacterial size fraction insummer; 1:5 dilution of o3 mm filtrate witho0.2 mm filtrate in winter) and unfiltered watersamples from various salinities in the Mississippiplume. Experiments were conducted with water ofvarying salinities during summer and winter.During the summer, an average of 40% ofrespiration was measured in the bacterial sizefraction, and during the winter bacteria were

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50

100

150

200

250

300

350

400

0 5 10 15 20 25 30 35

DO

C (µ

M)

July 1990

50

100

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250

300

350

400

0 5 10 15 20 25 30 35

February 1991

50

100

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0 5 10 15 20 25 30 35

DO

C (µ

M)

Salinity

May 1992

50

100

150

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250

300

350

400

0 5 10 15 20 25 30 35

Salinity

July 1993

Fig. 4. Concentrations of DOC in surface waters collected from the Mississippi River and plume. The solid line represents conservativemixing between the river and marine end members. Elevated concentrations at mid-salinities are consistent with production of DOCfrom primary production and grazing processes in plume waters (from Benner and Opsahl, 2001).


responsible for an average of 71% of respiration.These results indicate bacterial utilization of DOMaccounts for a large fraction of communityrespiration throughout plume waters.

Rates of bacterial production in river plumes aretypically higher than in river waters and adjacentmarine waters (Table 4). Enhanced rates ofbacterial production have been measured inplumes from several rivers, including the Fly,Hudson, Rhone and Mississippi rivers (Albright,1983; Ducklow and Kirchman, 1983; Kirchmanet al., 1989; Chin-Leo and Benner, 1992; Robert-son et al., 1993; Amon and Benner, 1998). Thesestudies observed strong spatial relationships be-tween plume areas with elevated bacterial abun-dance and rates of production and areas withelevated phytoplankton biomass and rates ofprimary production. An exception to this generalpattern was observed in the plume of the LenaRiver, which discharges into the Laptev Seaand the ice-covered Arctic Ocean (Saliot et al.,1996). Overall, these findings suggest a strong

coupling between autotrophic and heterotrophicmetabolism in plumes of tropical and temperaterivers.

The utilization of riverine (terrestrial) DOC alsocontributes significantly to heterotrophic bacterialproduction in river plumes. Chin-Leo and Benner(1992) determined that phytoplankton derivedsubstances supported only 18% of bacterialproduction in the Mississippi River plume duringthe winter, suggesting that riverine DOM supportsmost bacterial production at this time. During thesummer, when rates of bacterial and phytoplank-ton production are considerably higher, plankton-derived organic matter supported 68% of bacterialproduction.

During the summer, bacterial production and Nregeneration are tightly coupled in the MississippiRiver plume (Cotner and Gardner, 1993; Dortchand Whitledge, 1992). Maximum regenerationrates occur in the surface waters and at inter-mediate salinities. Cotner and Gardner (1993)developed a conceptual model for the Mississippi

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Table 4Bacterial abundance and production in surface waters of several rivers, river plumes and adjacent coastal waters

System Bacterial abundance (109 cells l!1) Bacterial production (mg C l!1 d!1) Season References

RiversMississippi 0.4–2.6 4.6–11.8 sum, win, spr 1, 2Fly nd 2.6–8.5 sum 3Lena 0.5–0.8 3.9–7.1 sum 4Rhone nd nd win 5Hudson nd nd spr 6

PlumesMississippi 0.8–10.5 5.0–192 sum, win, spr 1, 2Fly nd 4.9–128 sum 3Lena 0.2–0.9 0.1–2.9 sum 4Rhone 0.7–1.4 1.5–7.2 win 5Hudson 1.2–1.4 6.8–10.2 spr 6

Coastal waterGulf of Mexico 0.2–0.5 0.7–6.7 sum, win, spr 1, 2Gulf of Papua nd 13.4–31.2 sum 3Laptev Sea nd nd sum 4Mediterranean Sea 0.4–1.0 0.7–2.6 win 5New York Bight 1.1–1.2 4.7–6.2 spr 6

References: 1. Chin-Leo and Benner (1992); 2. Amon and Benner (1998); 3. Robertson et al. (1993); 4. Saliot et al. (1996); 5. Kirchmanet al. (1989); 6. Ducklow and Kirchman (1983).Rivers=salinityo2; Plumes=salinity between 8 and 32; and Coastal water=salinity>33.


River plume consistent with other models of plumeprocesses:

* in the river and at low salinities, NH4 regenera-tion rates by bacteria are low because of poorsubstrate conditions, DON is primarily terres-trial in origin because phytoplankton produc-tion is low, and riverine DON is refractory;

* at intermediate salinities, NH4 regenerationrates by bacteria are high because high phyto-plankton production and biomass providesplentiful labile material; and

* in high salinity waters, phytoplankton produc-tion is greatly reduced by nutrient limitationand therefore bacterial regeneration of NH4 isalso reduced.

An important observation is that bacteria are notalways the major remineralizers in this plume system.Chin-Leo and Benner (1992) observed that approx.50% of the respiration in the plume was by organisms>1mm (i.e., not bacteria). Similarly, Cotner andGardner (1993) and Gardner et al. (1994) observedthat a large fraction of the NH4 regeneration was byorganisms >1mm. This is consistent with the highmicrozooplankton and mesozooplankton grazingrates reported in plume waters.

1.2.6. Photochemical transformations of organicmatter in large river plumes

There is growing evidence indicating a majorrole of photochemical reactions in the transforma-tions and remineralization of riverine-derivedorganic matter in ocean margins (Kieber et al.,1990; Miller and Zepp, 1995; Vodacek et al., 1997;Benner and Opsahl, 2001). The ultraviolet portion(290–400 nm) of the solar spectrum is responsiblefor most photochemical transformations of or-ganic matter (Zepp, 1988), and these wavelengthsonly penetrate the upper few meters of surfacewaters in coastal regions. The extent of physicalmixing between river and marine waters prior todischarge onto continental shelves will largelydetermine the density of these waters and therebydetermine their exposure to ultraviolet radiation.Large rivers typically discharge directly ontocontinental shelves and form buoyant and shallowplumes that promote the exposure of riverine-derived DOM to solar radiation. Thus, photo-

chemical processes in these systems are likely to beimportant for biogeochemical cycling and theproductivity of ocean margins. Photochemicaltransformations of riverine DOM can enhanceprimary production through the regeneration ofammonium and phosphate (Bushaw et al., 1996;Cotner and Heath, 1990), and the photodegrada-tion of dissolved humic substances can stimulatemicrobial production (Miller and Moran, 1997). Itappears ocean margins receive a previously un-recognized ‘‘energy subsidy’’ (Odum, 1971) be-cause sunlight enhances decomposition andregeneration processes as well as provides energyfor photosynthesis.

The mechanisms of photochemical transforma-tions of natural organic matter are complex andinvolve numerous free radicals and reactive oxy-gen species (Mopper and Zhou, 1990; Vaughanand Blough, 1998; Sandvik et al., 2000). Photo-oxidation of terrigenous organic matter can lead todirect production of CO2 and CO as well as a widevariety of low-molecular-weight photoproductsthat are rapidly consumed by microorganisms(Miller and Zepp, 1995; Moran and Zepp, 1997).The susceptibility of organic matter to photo-oxidation is largely dependent upon its chemicalcomposition, and aromatic structures are particu-larly photoreactive and abundant in riverine-derived organic matter (Hedges et al., 1992;Opsahl and Benner, 1998). These same photo-reactive aromatic structures, such as those inlignins and tannins, are also relatively resistant tomicrobial degradation (Benner et al., 1985;Opsahl and Benner, 1998). The microbial andphotochemical degradation of dissolved lignin inMississippi River water was examined by Opsahland Benner (1998). Approximately 80% of dis-solved lignin was photooxidized during 28 days ofexposure to natural sunlight, whereas only B15%was degraded by microorganisms in dark incuba-tions. This experiment also demonstrated thatphotooxidation produces process-specific signa-tures in the remaining dissolved lignin that can betraced in river plumes and the coastal ocean(Opsahl and Benner, 1998; Benner and Opsahl,2001). The close association of lignins withpolysaccharides and other biopolymers of planttissues decreases the bioavailability of these

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components. Thus, the combination of photoche-mical and microbial decomposition processescould be a very efficient mechanism for the cyclingof riverine organic matter in ocean margins (Millerand Moran, 1997).

1.2.7. Vertical transport from plumes: sinkingVertical flux of plume materials, both river

derived and those produced in situ, is significant.Some estimates indicate that shelf regions aroundthe deltas of major rivers account for about 40%of the global ocean carbon burial (Hedges andKiel, 1995). Large particles that remain in suspen-sion while in the river, quickly sink once a plume isformed on the shelf (Trefry et al., 1994) andconcentrations of suspended lithogenic materialstypically decrease rapidly in plumes of large rivers.Biological activity can enhance the sinking oflithogenic materials which often comprise asignificant component of the gut and fecal pelletcontents of copepods and larvaceans in theMississippi River plume (Turner, 1984; Nelsenand Trefry, 1986). Such particles have no detri-mental effect on copepod energetics (White andDagg, 1989). It is not clear if protozoans (micro-zooplankton) ingest lithogenic particles but thereis no reason to expect otherwise. Fecal pelletscontaining lithogenic particles sink rapidly. Pelletsfrom larvaceans collected from the MississippiRiver plume contained high concentrations ofsediment particles in the 1–2 mm size range (Dagget al., 1996) and had a specific gravity of3.65 g cm!3. Larvaceans are often sufficientlyabundant to filter approximately 50% of theplume water in 24 h. This biological aggregationprocess can be an important mechanism for thevertical transport of small terrigenous particles.

The benthic environments of continental shelvesare highly productive, indicating high inputs oforganic matter. In the northern Gulf of Mexico,bottom waters of the inner shelf routinely becomehypoxic, indicative of large organic matter input(Rabalais et al., 1996). Although magnitudes ofthis flux are large, it is unclear what proportion ofplume organic productivity is represented. Redaljeet al. (1994) simultaneously measured phytoplank-ton production and vertical particulate flux at thebase of the mixed layer during 4 times of the year

in the Mississippi plume and nearby shelf waters.Flux was not closely related to production over theannual cycle. In both plume and shelf regionsduring summer, the period of highest phytoplank-ton productionm!2, vertical export was small,equivalent to only 3–9% of primary production,indicating most production was recycled within thewater column at this time. In contrast, during latewinter when production was lowest, verticalexport was large, equivalent to 64–266% ofprimary production. POC flux in the plume regionranged between 0.29 g Cm!2 d!1 in the summer to1.80 g Cm!2 d!1 in the spring, with intermediatevalues of 0.95 and 0.69 gCm!2 d!1 in the winterand fall, respectively. Riverborne suspended POMdid not make a significant contribution to theirtrap collected materials; sediment trap dC13 valuesreflected characteristics of marine production(Eadie et al., 1994). PON fluxes showed approxi-mately the same pattern as POC.

The amount of POM deposited on the bottom inregions near large rivers is a function of fluxm!2

and of the area over which the flux occurs. Thislatter component varies with river discharge. Fluxto the bottom is equivalent to a significant fractionof water column phytoplankton production, andresultant near-bottom and benthic processes areimportant components of the carbon and N flowsin large river systems (see companion paper byMcKee et al., 2004).

With the development of reliable satellite oceancolor analysis in case 2 waters (nearshore waterswith high productivity and high turbidity) comesthe possibility that new production can beestimated remotely via surface chlorophyll, butthis requires that the relationship between primaryproduction and export out of surface waters isknown on at least a regional scale. In the openocean, the relative rates of primary production andexport vary greatly as a function of food webdynamics and planktonic community structure(Michaels and Silver, 1988; Peinert et al., 1989;Legendre and Rassoulzadegan, 1996; Boyd andNewton, 1999) with export from the photic zonegenerally less than 10% of primary production,but at times as great as 50%. Quantification of thisrelationship is lacking in river-dominated marginenvironments. Estimates based on organic carbon

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remineralization rates and particle residence timespredict that well over half of the organic carbonproduced in surface waters of shallow coastalenvironments may reach the seabed in the form ofunremineralized particulate carbon (Wollast,1991). Benitez-Nelson et al. (2000) report a widerange of production: export ratios in the Gulf ofMaine (2–56%) using 234Th:238U disequilibria.Rivkin et al. (1996) found that neither food webstructure nor the magnitude of new productionwere good indicators of export of biogenic carbonfrom surface waters of the Gulf of St. Lawrence. Itis therefore likely that export in river-dominatedenvironments is a large (but variable) fraction ofproduction. A better understanding of the con-trolling processes is needed.

2. Conclusion and new directions

Large rivers serve as major sources of buoyancyfor coastal margins. The physical controls on thetransport, flow patterns, mixing, and strength ofgradients in these buoyant plumes are highlycomplex and not fully understood. Within theseplumes, there is a suite of biogeochemical trans-formation processes, also not fully understood,that affect the dissolved and particulate, organicand inorganic materials over short (and highlyvariable) time and space scales. These surfaceplumes are highly dynamic systems that areimportant for transporting and transformingterrestrial materials in coastal margins. Regionalimpacts are enhanced because many of the world’slarge rivers discharge between 30"N and 30"S andthus discharge into parts of the world ocean thatare permanently stratified and oligotrophic.

Preparation of this review has led us tonumerous questions for future work in theseimportant regions of the world’s marine environ-ment. Here we suggest a few of these.

Surface trapping of river water that occurs afterdischarge to the shelf results in a decrease inturbulence that in turn reduces the ability of plumewaters to retain suspended lithogenic material.Beginning at this point, terrigenous particlesrapidly settle from plume waters, leading toincreased light penetration. Mixing with ocean

waters dilutes riverine colored dissolved organicmatter (CDOM), further enhancing light penetra-tion. These changes in the physical and opticalenvironment have major impacts on chemical andbiological processes within the buoyant plume,including: (a) rapid drawdown of nutrients insupport of increased phytoplankton growth andproduction which in turn stimulates the ‘‘classical’’and ‘‘microbial’’ food webs; and (b) an increase inphotochemical and microbial transformations ofrefractory riverine DOM which in turn releaseorganically bound nutrients and further stimulatephytoplankton production, food webs and thecycling of biologically important elements. Timeand space scales over which these responses to thechanging optical environment occur vary greatly,depending on scales of discharge, suspendedsediment loads, mixing and turbulence. How dooptical characteristics of plume water control thetiming and magnitude of the biological and photo-chemical responses to river inputs?

In ‘classical’ food webs, productivity is domi-nated by larger eukaryotic phytoplankton (e.g.,diatoms) that support a mesozooplankton grazingpopulation. Such communities have relatively highexport to production ratios. In microbial foodwebs, primary production is dominated by smallphytoplankton, grazing is dominated by microzoo-plankton and tightly coupled to phytoplanktonproductivity, and export is small (Legendre andRassoulzadegan, 1996; Legendre and Michaud,1998). Outside the regions affected by many largerivers, water is typically stratified and oligotrophic,dominated by the microbial web. In contrast,plumes are dominated by the ‘classical’ web. Asthe near-field is transported to the far-field,biological processes, continuously affected by mix-ing and dilution, change such that the structure ofthe planktonic community evolves from a classicalweb to microbial web. What are the relationshipsbetween mixing and community structure? How doesthe rate of mixing of buoyant plume water withambient coastal water determine the structure andcomposition of the planktonic community?

In the mid-salinity zone of plumes of large riverswhere stratification, the light environment and thehigh concentrations of river nutrients result in anideal environment for phytoplankton production,

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autotrophic processes often dominate heterotro-phy. This will be maximized under conditions ofhigh nutrient supply and broad spreading of theplume (low wind). Net autotrophy should alsolead to high export production because hetero-trophic remineralization will be uncoupled fromprimary production. What controls the balancebetween autotrophy and heterotrophy within theplume and how do these factors vary from the near-field to the far-field plume regions?

By sinking, intact phytoplankton cells, fecalpellets, mucous, lithogenic particles and othermaterials form a link between the water columnand the benthos. Export appears to be a largerfraction of production on the shelf than in theopen ocean but clear seasonality or other relation-ships between phytoplankton production, zoo-plankton community ingestion, and flux are notapparent. However, flux is equivalent to asignificant portion of productivity and mesozoo-plankton fecal pellets may contribute significantlyto flux. Theoretical modeling indicates informa-tion is needed about factors causing the fraction ofzooplankton fecal material sinking from the photiczone to vary (Aksnes and Wassmann, 1993;Sarnelle, 1999). Two processes are primarilyresponsible for the retention of fecal pelletmaterials in the upper water column: (a) ingestionor destruction by other zooplankton (e.g., Gonza-lez and Smetacek, 1995) and (b) bacterial degrada-tion (e.g., Hansen et al., 1996). These processes arealso applicable to other forms of organic matter inthe plume. What controls the flux of plumematerials, especially organic production that occurswithin the plume?

Addressing these and other important questionswill require major investment in monitoring,modeling and process studies in at least severalof the world’s river-dominated shelves but thesignificance of coastal regions dominated by largerivers argues strongly that these investments bemade.

Acknowledgements

This work was supported by the Office of NavalResearch under award no. N000140010829, and

by the Louisiana Universities Marine Consortium.The assistance of Jean Rabalais and Lillie Wicheris greatly appreciated.

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