Food WebsFood Webs

OCN 621OCN 621

Relatively few speciesRelatively few speciesYet:1) High diversity in terms of trophic

mode, e.g., herbivory, carnivory,mixotrophy, omnivory

2) Trophic level changes withdevelopmental phase (egg toadult) within a species

3) Prey selection based on size, butnot necessarily at a ratio of 1:10,especially for raptorial/directinterception consumers

4) Behaviors lead to nichepartitioning, even thoughenvironment relatively uniform,e.g., diel vertical migration

Given this background, howwould we expect food webs

to look?

Integrating Classical and Microbial LoopIntegrating Classical and Microbial LoopFood WebsFood Webs

Lalli & Parsons 1997

Comparison of food web structuresComparison of food web structures

oligotrophic

eutrophic

Lalli & Parsons 1997

BUT, diagrams such as these are misleading, as the microbial food web is:1) ubiquitous in the world’s oceans and2) likely has more steps than shown

NW Atlantic Food WebNW Atlantic Food Web

Phytoplankton (1)

Copepods (2 - 4*)

Ctenophores/Chaetognaths (3 - 5)

Small fish (4 - 6)

Squid (5 - 8)

Whales/porpoises/birds (6 - 9)

Humans (7 - 10)

Bigger fish (4 - 7)

Link, 2002

Why are ocean regionsWhy are ocean regionsdifferent?different?

Why does the North Atlantic bloom so dramatically?Why doesn’t the North Pacific?Why aren’t there ever blooms in the vast open ocean regions?

All systems have microbial organisms, as well as the largerphytoplankton and consumers, but physical processes forcethe system towards dominance of one ecosystem over another.

Extraordinarily Simplistic Answer

Dominant Pathways are determinedDominant Pathways are determinedby physical processesby physical processes

Small cells are more efficient in competing for low NSmall cells are more efficient in competing for low N(high surface area:volume)(high surface area:volume)

General size hierarchy of consumers based on energeticGeneral size hierarchy of consumers based on energeticconsiderations, i.e., for like organisms, reduced size andconsiderations, i.e., for like organisms, reduced size andbiomass of prey makes the environment more suitable forbiomass of prey makes the environment more suitable forsmaller consumerssmaller consumers

Energetic reasons why small primary consumers areEnergetic reasons why small primary consumers arefavored in favored in oligotrophic oligotrophic open ocean systems (subtropicalopen ocean systems (subtropicalgyres):gyres): reduced [reduced [phytophyto]] II declines for given declines for given FFmaxmax decreaseddecreased phyto phyto sizesize FFmaxmax declines for declines for

consumer of given sizeconsumer of given size increased Tincreased T°°CC higher higher I I is required foris required for

maintenance or to sustain a given level of growthmaintenance or to sustain a given level of growth

Diatoms are the Diatoms are the ““dynamicdynamic”” component in component inthe food webthe food web

Diatoms are responsive to high nutrient conditions andDiatoms are responsive to high nutrient conditions andcan escape can escape ““controlcontrol”” of grazers. of grazers.

In the absence of In the absence of ““external energyexternal energy”” (mixing or advection (mixing or advectionof nutrients) to stimulate diatom blooms, a of nutrients) to stimulate diatom blooms, a eutrophiceutrophicsystem will rapidly assume the characteristics of ansystem will rapidly assume the characteristics of anoligotrophic oligotrophic system -- this occurs seasonally (spring tosystem -- this occurs seasonally (spring tosummer in temperate systems) and spatially (distancesummer in temperate systems) and spatially (distancefrom upwelling source).from upwelling source).

Diatoms decrease in relative abundance from:Diatoms decrease in relative abundance from: Eutrophic Eutrophic SystemsSystems Oligotrophic Oligotrophic SystemsSystems High LatitudeHigh Latitude Low LatitudeLow Latitude Spring SeasonSpring Season Summer SeasonSummer Season Upwelling SourceUpwelling Source Distance from UpwellingDistance from Upwelling

Low Energy Stable SystemsLow Energy Stable Systems

Low nutrients(oligotrophic)

Small Phytoplankton(high surface:volume ratio)

Long food chains(small consumers at base)

Low energy Lack of nutrient re-supply

Relatively stablesystem (high recycling)

High Energy Unstable SystemsHigh Energy Unstable Systems

High nutrients(eutrophic)

Large Phytoplankton(small, too)

High energy(storm activity, eddy

action, upwelling, etc.)

Unstable (dynamic) system

(High “new” production)

Short food chain (dynamic)(superimposed on stable long food chain)

Composite Spring PictureMean Chlorophyll (µg/l) at the surface

How doesHow doesbiomassbiomasschangechangeover aover a

seasonalseasonalcycle andcycle andwhat doeswhat doesit mean?it mean?

Observations of SeasonalObservations of SeasonalCyclesCycles

Using net tows, catch diatoms, largeUsing net tows, catch diatoms, largedinoflagellates and zooplanktondinoflagellates and zooplankton

From these catches, infer food webFrom these catches, infer food webrelations and seasonal cyclesrelations and seasonal cycles

Did use Did use in situin situ chlorophyll measurements chlorophyll measurementsaround the worldaround the world’’s seas to generate mapss seas to generate maps

(note: didn(note: didn’’t have large scale, synoptic mapst have large scale, synoptic mapssuch as we have today with satellites)such as we have today with satellites)

Spring bloomsSpring blooms

North Atlantic BloomNorth Atlantic Bloom1) Phytoplankton low through the

winter:light limited, nutrients sufficientdeep winter mixing

2) Spring Bloomreduced winds, stratification nearsurfaceincreased light, nutrients sufficient

3) Summer: Low phyto biomass grazers consume thephytoplankton nutrients depleted and notrenewed

End of North Atlantic BloomEnd of North Atlantic Bloom

4) 4) Fall: Second bloomFall: Second bloomFewer grazers: non-feeding stageFewer grazers: non-feeding stageIntermittent stormsIntermittent stormsInject nutrients, but still stratifiedInject nutrients, but still stratifiedLight sufficientLight sufficient5) Early winter:5) Early winter:Storm mixingStorm mixingRe-supply of nutrients to surfaceRe-supply of nutrients to surfaceSet for next Spring BloomSet for next Spring Bloom

In places where phytoplankton cycles are strongly different (most of therest of the world’s oceans!), they are usually discussed in contrast tothe spring bloom cycle.

Main Pacific Open OceanMain Pacific Open OceanEcosystemsEcosystems

Subarctic Subarctic Gyre (HNLC)Gyre (HNLC) Subtropical Gyre (Subtropical Gyre (oligotrophicoligotrophic, location of, location of

HOTS)HOTS) Eastern Equatorial (HNLC)Eastern Equatorial (HNLC) Southern Ocean (HNLC)Southern Ocean (HNLC) Antarctic (Polar) -- permanently under ice orAntarctic (Polar) -- permanently under ice or

seasonal ice zoneseasonal ice zone Continental Shelf/Upwelling ZonesContinental Shelf/Upwelling Zones

Poles: e.g.,Poles: e.g.,ArcticArctic

Ice effect on Bloom Timinge.g., bloom relatively late inyear as ice needs to melt• Results in short growingseason (tied to light)• Zooplankton: grow slowly,have short feeding season,rest at depth over winter• May take two-three yearsto complete growth cyclee.g., Calanus glacialis & C.hyperboreus

HNLC EcosystemsHNLC Ecosystems Nitrate/Fe supply dynamics (bottom up effects)Nitrate/Fe supply dynamics (bottom up effects)

High energy systems so plenty of new fixed nitrogenHigh energy systems so plenty of new fixed nitrogen Ratio of Fe:Nitrate in deep water too lowRatio of Fe:Nitrate in deep water too low Other sources: Atmospheric inputs & coastalOther sources: Atmospheric inputs & coastal

advective advective processesprocesses Fe limitation of phytoplankton growthFe limitation of phytoplankton growth Small cells (higher S:V) selected for over Small cells (higher S:V) selected for over LgLg. cells. cells

Large scale experiments to test hypothesisLarge scale experiments to test hypothesis Martin 1990 (Martin 1990 (PaleoceanographyPaleoceanography, 5, 1-13, 5, 1-13): Fe-limitation): Fe-limitation

of phytoplankton growthof phytoplankton growth Experiments: First -- Eastern Equatorial PacificExperiments: First -- Eastern Equatorial Pacific

(followed by others in Southern Ocean, (followed by others in Southern Ocean, SubarcticSubarcticPacific)Pacific)

HNLC Region: HNLC Region: SubArctic SubArctic PacificPacificCirculation

Miller et al. 1991

ObservationsObservations

• Seasonal blooms do not occur --Canadian weathership station P• Occupied station from 1950suntil mid-1981

Characteristics ofCharacteristics ofSubarcticSubarctic

EcosystemsEcosystems

N. Pacific

N. Atlantic

• deep winter mixing in Atlantic, but a permanent halocline in the Pacific• low summer nitrate in Atlantic, butstill high in summer in Pacific

• Phytoplankton concentration low& nearly constant year rounddespite excess nutrients (nitrate) &physical conditions favoring aseasonal bloom.• Seasonal signal in phytoplanktonproduction, but not abundance• Phytoplankton dominated bytiny species, similar to tropics, notlarge forms associated with highnitrate

PlanktonPlankton

Seasonal migration ofSeasonal migration of Neocalanus Neocalanusplumchrusplumchrus -- subarctic subarctic PacificPacific

Miller et al. 1984

• Zooplankton dominated by large species, Neocalanus (4-5 mm)

• Zooplankton biomass peaks in summer:leads to historical hypothesis thatphytoplankton controlled by grazing bylarge copepods.

SUPER:SUPER: SUSUbarctic barctic PPacific acific EEcosystem cosystem RResearchesearch

• Objective: to test the “Major Grazer” Hypothesis (1980)• Would do experiments to see if grazing capacity wasconsistently greater than phytoplankton stock capacity to increase• Found that phytoplankton stock could not be controlled byavailable grazing stock• Also found that copepodite stages of Neocalanus were noteating enough phytoplankton to provide for their growth: inferredthat must also be eating microherbivores (protists): omnivores.• Also inferred that protist grazing must be important incontrolling phytoplankton biomass• Also found that if ammonia available, it would be usedpreferentially to nitrate (already in a reduced form), so ammoniaavailability suppressed nitrate uptake

Control Capabilities of Control Capabilities of NeocalanusNeocalanus

Left: Rate estimatesfrom bottle expts.Right: Experimentsin 60-L microcosms

Landry & Lehner-Fournier 1988

• Could consume cells 2-30µm, but food limited atambient concentrations• Could keep phytoplanktonin check at an abundance of 1copepod/Liter• Without them present,phytoplankton did bloom• But: not present insufficient density to controlblooms (only ~0.2/L)• Another role: to consumesmaller grazers, and therebyinfluence community sizestructure.

Role of ProtistsRole of Protists

Landry et al. 1993

Fe-limitationFe-limitationhypothesishypothesis

Martin & Fitzwater 1988

Fe limiting plant productionIn deckboard experiments,demonstrated adddition of Felead to increase inphytoplankton -- diatomsSUPER project mostly over bythe time this hypothesispresented: but still a couple ofcruises left and experimentsdesigned to test it.Data was suggestive that thiswas an important factor