Phytoplankton, sediment and

optical observations in Netherlands

coastal water in spring

Karen Wild-Allen a,*, Andrew Lane b, Paul Tett a

aSchool of Life Sciences, Napier University, 10 Colinton Road, Edinburgh, EH10 5DT, UKbProudman Oceanographic Laboratory, Bidston Observatory, Birkenhead, CH43 7RA, UK

Received 26 July 2001; accepted 7 February 2002

Abstract

Factors controlling the dynamics of suspended particulate matter (SPM), its influence on sea-leaving radiance and in-water

optical properties, and the consequences of optical variation for phytoplankton growth, were studied at the ‘Processes of

Vertical Exchange in Shelf Seas’ (PROVESS) project’s southern North Sea site during April 1999. The optical properties of

Netherlands coastal water were not unexpectedly found to be primarily determined by suspended sediment (Case 2) and were

classified as Jerlov type 7 ‘relatively turbid coastal water’. During the study period, vertical mixing periodically resuspended

optically active particles from the bed fluff layer throughout the water column and into the near-surface layer. These particles

influenced sea surface radiance reflectance, and the red/green ratio of radiance reflectance, both of which can be observed by

remote sensing. Linear relationships between sea surface radiance reflectance and SPM concentration were primarily

determined by the inorganic fraction, as organic SPM varied little in concentration throughout the cruise period. The inorganic

fraction was an important scatterer of light at all wavelengths, whereas the organic fraction displayed a greater tendency for light

absorption at shorter wavelengths. Although the euphotic layer (depth of 1% surface irradiance) was only 8–10 m deep, vertical

mixing ensured that phytoplankton throughout the water column (f 18 m) had access to PAR in excess of the estimated

compensation illumination. Growth rates of microplankton (which includes pelagic microheterotrophs as well as

phytoplankters) were calculated using an algorithm from the PROWQM model. These ranged from 0.1 to 0.3 d� 1, and

implied loss rates of 3–25% which were mostly attributed to mesozooplankton grazing. Estimated oxygen production,

however, was in near equilibrium with oxygen demand observed in dark bottles, and implied a significant oxygen demand due

to detrital respiration and nitrification. This was estimated as 3–6 mmol O2 m� 3 d� 1. In an order of magnitude timescale

analysis, vertical mixing was found to be the single most important factor controlling the dynamics of SPM under mixed or

stratified conditions. For a mixed water column microplankton aggregation and fluff layer resuspension also had the potential to

redistribute material in the water column several times per day, whilst under stratified conditions horizontal exchange and

inorganic particle sinking were more important. Resuspended material in a stratified water column remained below the

pycnocline and had little impact on the near-surface layer optics. Other factors varied in importance with the level of

1385-1101/02/$ - see front matter Crown Copyright D 2002 Published by Elsevier Science B.V. All rights reserved.

PII: S1385 -1101 (02 )00121 -1

* Corresponding author. Present address: Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen, AB24 2TZ, UK.

E-mail address: k.wild-allen@abdn.ac.uk (K. Wild-Allen).

www.elsevier.com/locate/seares

Journal of Sea Research 47 (2002) 303–315

stratification, which was recognised as a significant factor in determining the dynamics of SPM in this region of freshwater

influence (ROFI). Crown Copyright D 2002 Published by Elsevier Science B.V. All rights reserved.

Keywords: North Sea; Case 2 water; Sea surface reflectance; SPM; Phytoplankton; Vertical mixing

1. Introduction

Optical conditions in the Southern Bight of the

North Sea are complex, because there are substantial

and varying concentrations of the three main types of

optically active constituents in addition to water.

Inflow from rivers contributes yellow substance. Sedi-

ments are resuspended by tidal currents and wind-

induced wave action. Phytoplankton biomass, sup-

ported by anthropogenic nutrients, is often large. The

suspended particulate matter (SPM), in particular, has

a major impact on sea-leaving radiance and hence on

remotely sensed images of this region (Holligan et al.,

1989; Van Raaphorst et al., 1998). However, all

optically active constituents influence the light climate

for phytoplankton growth, and may ameliorate eutro-

phication in parts of the southern North Sea by

restricting light available for photosynthesis (Tett et

al., 1993; Tett and Walne, 1995).

The programme ‘Processes of Vertical Exchange in

Shelf Seas’ (PROVESS) is an EU MAST project with

the objective of integrating observations and model-

ling of processes and consequences of vertical ex-

changes in shelf seas. The work described in this paper

was carried out at the PROVESS south site (Howarth

et al., 2002), near the Netherlands coast, during a 7-d

period in April 1999. The aim of the work was to

increase understanding of the functioning of the south-

ern North Sea ecosystem in relation to interactions

between biological and physical processes controlling

the generation and vertical and temporal distribution of

optically active SPM and the consequences of such

distributions for phytoplankton dynamics.

Mixing due to vertical eddy turbulence, sinking and

resuspension from the sea bed are the main physical

factors controlling the distribution and abundance of

optically active particles. The impact of such processes

on the pelagic biota depends on their importance rela-

tive to rates of organic production, which are in turn at

least partially dependent on the light climate and hence

the mixing, sinking and resuspension of optically ac-

tive particulates. In addition, such processes influence

sea-leaving radiance and hence potentially provide a

remotely sensed signal of some effects of vertical

exchange.

2. Instrumentation and methods

2.1. CTD and moorings

CTD, moored instrument and meteorological data

were obtained from the PROVESS database at the

British Oceanographic Data Centre (BODC). The

profiling CTD (conductivity-temperature-depth) in-

strument array was equipped with a Seabird 911

CTD plus an additional SeaTech transmissometer

and Chelsea Instruments Aquatracka Mk.3 fluorom-

eter. Water-sampled chlorophyll concentrations were

used to calibrate the CTD fluorometer (Wild-Allen,

1999). Moored instrumentation located at Site A

(Howarth et al., 2002) included an Aanderaa current

meter, NAS nutrient analyser (nitrogen and silicate),

SeaTech transmissometer and Chelsea Instruments

Aquatracka Mk.3 fluorometer. These instruments

were calibrated with samples taken throughout the

cruise period. Wind speed and direction were recorded

at the Meetpost Noordwijk platform (52.27jN,

4.29jE).

2.2. Water samples and analysis

Samples were taken from 12-dm3 bottles assembled

around the CTDwhich were closed at depths of interest

(identified from the thermal and fluorescence signals),

during the up-cast. Typical sample depths were in mid-

water and near the surface, coincident with optical and

turbulence measurements. Further samples were occa-

sionally pumped from a depth of f 3 m or taken from

the surface with a bucket.

Water samples (1 dm3) were filtered through pre-

prepared Whatman glass fibre GF/F filters for SPM

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315304

analysis following the method of Strickland and

Parsons (1972). Filters (including replicate samples)

were air dried and frozen, with desiccant, for transport

to a shore-based laboratory where the analysis was

completed for total, inorganic and organic sediment

load. GF/F filters were selected as they were suitable

for the determination of organic SPM by combustion

(e.g., Moffat, 1995). Care was taken to minimise filter

fibre loss during handling which could compromise

accuracy, although with respect to the high concen-

trations of SPM sampled, any such losses were likely

to be trivial.

Further samples were filtered through Micropore

membrane (5 Am pore size) and/or Whatman glass

fibre (nominal pore size 0.7 Am) filters and analysed

for chlorophyll and pheopigment concentration. Fil-

ters were freeze dried in darkness and transferred to a

shore-based laboratory for extraction (in 90% analar

acetone) and spectrophotometric analysis (Lorenzen,

1967).

2.3. Optical observations

A Biospherical Profiling Reflectance Radiometer

(PRR600) was used to measure vertical profiles of

upwelling and downwelling light in the water column.

The instrument simultaneously recorded in-water

downwelling irradiance, upwelling radiance and (with

a separate sensor positioned on the deck of the ship)

surface irradiance. Data were collected in 20 nm

wavebands at 412, 443, 490, 510, 555 and 665 nm

corresponding to bands 1–6 of the SeaWiFs (Sea-

viewing-Wide-Field-of-view-Sensor) colour sensor on

the SeaStar satellite launched in 1997. In addition,

upwelling radiance at 683 nm was recorded, and a

cosine collector measured diffuse photosynthetically

active radiation (PAR) between 400–700 nm.

Parameters of interest were the spectra of radiance

reflectance, R(0-, k), and diffuse vertical attenuation

for downwelling light, kd(0-, k), just below the sea

surface, where k is the wavelength. These were

calculated directly from the vertical profiles of

upwelling radiance, Lu(z, k) and downwelling irradi-

ance, Ed(z, k) using Eqs. (1) and (2). Here, z is the

depth below the sea surface. The ‘least squares fit’

gradients and intercepts of the ratios Lu(z, k) / Ed(z, k)were determined and used to calculate the sea surface

radiance reflectance. Similarly, the diffuse vertical

attenuation was calculated from the gradient and

intercept of � d ln Ed(z, k) / dz.

Rð0�; kÞ ¼ Luð0�; kÞEdð0�; kÞ

cLuðz; kÞEdðz; kÞ

����z¼0

ð1Þ

kdð0�; kÞ ¼�dln Edðz; kÞ

dz

����z¼0

ð2Þ

These ‘apparent’ optical properties are related to the

‘inherent’ optical properties of absorption and scatter-

ing. (The inherent optical properties do not change

with variations in light conditions.) The absorption

coefficient, a(k), and scattering coefficient, b(k), werefound from the following empirical relationships:

Rð0�; kÞ ¼ abbðkÞaðkÞ þ bbðkÞ

ð3Þ

where ac 0.32 (Gordon et al., 1975).

kdð0�; kÞ ¼ ½aðkÞ2 þ GaðkÞbðkÞ�12 ð4Þ

where G = 0.256, for normal incidence. The ratio of

backscattering coefficient to scattering coefficient

(backscattering ratio b = bb/b) is dependent on the

particle size distribution and refractive index of the

scattering particles (Twardowski et al., 2001, have

developed a model to relate these parameters). The

value of b = 0.019 used here is applicable in coastal

and inland waters (Kirk, 1994). Other values of back-

scattering ratio have been used, e.g., 0.0125 for

coastal ocean (Sydor and Arnone, 1997), 0.044 for

clear ocean (Mobley, 1994). Concentrations of sus-

pended sediment from filtered water samples, and

other constituents in the water, are related to these

apparent and inherent optical properties.

3. Site description

Plankton, sediment and optical measurements

reported in this paper were made from RV ‘Pelagia’

at 52.3jN, 4.3jE between 2 and 9 April 1999. Addi-

tional measurements of current, attenuation, nutrients

and chlorophyll fluorescence were obtained from

moorings (located at site A) in the immediate vicinity

(Howarth et al., 2002). The study area was approx-

imately 20 m deep with a mud-sand bed. Light winds

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315 305

and a slight sea state were encountered during the first

part of the cruise, increasing to moderate winds for the

second part of the cruise (Fig. 1a). The sky was

mostly overcast with occasional fog apart from two

days of clearer weather (2 and 7 April). The cruise

commenced during a spring tide (see time series Fig. 1

in Grenz et al., 2002), and moderately strong currents

were sustained through the following week by the

prevailing wind (Fig. 1b). Currents were predomi-

nantly rectilinear and aligned parallel to the coast,

although a small cross-shore component advected

reduced salinity Rhine plume water into the site area

at semi-diurnal frequency (Fig. 1c) and gave rise to a

semi-diurnal cycle of mixing and stratification.

Detailed descriptions of the water column physics

and turbulence structures are given in Gemmrich

and van Haren (2002) and Fisher et al. (2002). Optical

conditions were ‘Case 2’ (Morel and Prieur, 1977)

with high concentrations of suspended organic and

inorganic particulate material.

4. Results

4.1. Spm abundance and composition

SPM, inferred from beam attenuation, gradually

declined throughout the cruise period (Fig. 1d) except

on 5 and 6 April when exceptionally high SPM values

were observed concurrent with the period of strongest

Fig. 1. Time series of (a) wind speed, (b) current speed (spring tides on day 93), (c) salinity*, (d) beam attenuation at 11 m depth, and (e)

chlorophyll concentration (derived from fluorescence) at 10 m depth, recorded by moored instrument arrays. (Day 92 is 2 April 1999.)

* Realistic short-term variation but absolute values are too high (Moncoiffe, pers. comm.).

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315306

wind. At this time increased wind and turbulence

resulted in the breakdown of stratification (Fig. 2a)

and suggested a redistribution of bed material through-

out the water column. A similar pattern was seen in the

bottle sample SPM analysis although the highest con-

centrations were observed a day later (Fig. 3a), with

greatest enhancement in the inorganic fraction. In

contrast, the organic fraction showed little temporal

variation remaining approximately constant at around

3–4 mg dm � 3. The temporal mis-match between

elevated beam attenuation and bottle-sampled SPM

is likely to result from local patchiness and variations

in particle size. Beam attenuation is most influenced

by scatter, which depends strongly on the small size

fraction; conversely, mass is most influenced by large

particles. A detailed description of the particle size

distribution is given in McCandliss et al. (2002).

Observations of the phytoplankton pigment chlor-

ophyll-a were used as a proxy for phytoplankton bio-

mass. Chlorophyll concentrations, inferred from the

moored fluorometer (Fig. 1e), ranged from 6–18 mg

m � 3 and illustrated semi-diurnal variations in phase

with the variation in salinity (except when the water

column was well-mixed on 5 and 6 April). Fresher

water, influenced by the Rhine plume, typically con-

tained 4–8 mg m� 3 less chlorophyll than the higher

salinity water. As the cruise progressed there was a

gradual decline in day-mean chlorophyll concentration,

which was also shown in the bottle chlorophyll samples

(Fig. 3b). Pheopigment concentrations, however,

remained steady at about 1 mg m� 3 throughout the

cruise period, suggesting that the decline in chlorophyll

concentration was unlikely to result from mesozoo-

plankton grazing, which typically releases pheopig-

ment (Strom, 1993). Size fractionated pigment

analysis showed that large particles ( > 5 Am) accounted

for approximately 75%of the chlorophyll-a at all depths

throughout the cruise period (Fig. 3c). The proportion of

larger particles generally increased slightly with depth.

Field microscope analysis (Table 1) showed an abun-

dance of diatoms, with chain-forming varieties being

commonplace, particularly Chaetoceros, Guinardia

and Rhizosolenia. The most frequently seen dinoflagel-

lates were armoured Peridinium which were often

mobile.

Fig. 3. Time series of near-surface (0–8 m) (a) organic and

inorganic SPM concentration, (b) chlorophyll and pheopigment

concentration and (c) size fractionation of chlorophyll, from

analysis of water samples. (Day 92 is 2 April 1999).

Fig. 2. Time series of vertical (a) density structure and (b)

chlorophyll concentration (derived from fluorescence) recorded by

a profiling CTD system on RV ‘Pelagia’.

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315 307

The CTD provided observations of density and

estimates of chlorophyll concentration (derived from

fluorescence) with a high vertical resolution (Fig. 2a

and b). For most of the cruise the water column was

stratified with a layer of fresher, warmer water at the

surface. Chlorophyll concentration generally increased

with depth and periodic patches of high concentration

in deep water during the spring tide (2–4 April)

suggested bed resuspension. Resuspended chloro-

phyll-rich material remained below the pycnocline

and did not contribute to near-surface layer SPM,

which remained relatively constant. Tidal resuspension

typically exhibits quarter-diurnal frequency; however,

in our observations, periods of increased near-bed

concentration corresponded with strong northward

flow when near-bed velocities exceeded 1 m s�1.

Southward currents were generally weaker (0.5 m

s�1) and may have been insufficient to resuspend bed

material. In addition there may have been small-scale

patchiness in the bottom substrate with perhaps a

source region (with the potential for resuspension in

a northward flow) to the south of the site and a deposi-

tion region (with little potential for resuspension in a

southward flow) to the north. A more detailed descrip-

tion of these resuspension events is given in McCand-

liss et al. (2002). During the later days of the cruise,

weaker wind and tidal velocities prevailed and there

was no further evidence of resuspension.

4.2. Sea-leaving radiance

Suspended sediment particles have an absorption

spectrum that decreases exponentially with increasing

wavelength in the range 400–570 nm (Bowers et al.,

1996); however, they tend to scatter light more than

absorb it, and thus have a high reflectance particularly

between 500–570 nm (Robinson, 1983). Suspended

sediments are therefore important agents for scattering

compared with other constituents in seawater.

The relationship between suspended sediment and

reflectance at a particular wavelength is linearly

dependent on the sediment concentration at concen-

trations of less than 10–20 mg dm� 3 (Van Raaphorst

et al. 1998), although Curran and Novo (1988) found

a linear relationship held up to concentrations of 30–

40 mg dm� 3. At higher concentrations, the reflec-

tance saturates (at about 1.5–2%), and shifts spec-

trally towards longer wavelengths (becomes more

red). The shift in colour means that sediment concen-

trations can be related to ratios of reflectance at

various wavelengths (Kratzer et al., 2000). At low

concentration, suspended sediment concentrations can

be estimated from absolute radiance values, whilst for

higher concentrations, reflectance ratios should be

used (Clark et al., 1980; Sugihara and Kishino,

1988; Kratzer et al., 2000).

Sea surface radiance reflectances were least in the

412 nm (violet) channel and greatest at 555 nm

(green/yellow wavelengths) with highest values

occurring on 6–7 April (Fig. 4b) coinciding with

the increase in SPM concentrations. Reflectance at

555 nm showed the largest response to variations in

SPM concentration, while values at 412 nm only

changed slightly. By comparing the radiance reflec-

tance ratio at 665:555 nm (red/green ratio), a shift in

spectral reflectance towards longer wavelength light

with increasing SPM is confirmed (Figs. 3a and 4c).

A good linear fit was found between total SPM and

both absolute reflectance, at 665 nm, and the red/green

radiance reflectance ratio [R(0 � , 665) / R(0� , 555)]

(Fig. 5a and b), although R2 values indicated that the

former relationship was more robust (with R2 values of

0.82 and 0.73, respectively). Similarly the relationship

Table 1

Plankton groups identified with the field microscope

Diatoms Dinoflagellates Other

Biddulphia sinensis Gymnodinium sp. Strombidium sp.

Ceratium sp. Peridinium sp. Tintinid

Chaetoceros sp. Phaeocystis Copepods

Fragilaria sp. Scripsiella sp. Detritus

Guinardia flaccida

Guinardia sp.

Guinardia sp.

(empty shells)

Leptocylindricus danicus

Leptocylindricus sp.

Nitzschia clostarium

Pennate diatom –

naviculoid type

Rhizosolenia alata

Rhizosolenia setigera

Rhizosolenia shrubsolei

Rhizosolenia sp.

Rhizosolenia stolterfothi

Skeletonema costatum

Thalassionema

Nitzschiodies

Thalassiosira sp.

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315308

between inorganic SPM and radiance reflectance was

good at 665 and 555 nm (Table 2). In the remaining

wavebands, poorer correlations were found between

radiance reflectance and total and inorganic SPM. The

poor correlation between organic SPM and radiance

reflectance at all wavebands was primarily due to the

lack of variation in organic SPM concentration

throughout the cruise period, but also partly due to

the tendency of organic matter to absorb light, partic-

ularly at shorter wavelengths. Overall, the relationship

between SPM concentration and reflectance was pri-

marily determined by the inorganic sediment fraction.

The relationship between in situ radiance reflec-

tance and SPM concentration evaluated from our

observations was compared with similar data from

the North Sea (Fig. 6) presented by Van Raaphorst et

al. (1998). These authors computed radiance reflec-

tance from Advanced Very High Resolution Radio-

meter (AVHRR) satellite observations (at 580–680

nm) and compared these with in situ SPM concen-

trations for September 1990 and January 1991. While

the range of reflectance and SPM concentrations

presented by Van Raaphorst et al. (1998) was much

greater than those encountered in April 1999, the

linear relationships proposed at concentrations of less

than 10 mg dm�3 were comparable. The slightly

steeper gradient of our relationship may indicate an

overestimation of reflectance by AVHRR observa-

tions, at low sediment concentrations.

Fig. 4. Time series of near-surface (a) diffuse attenuation coefficient

and (b) radiance reflectance ratio (%), at six wavelengths and (c)

ratio of reflectances at 665 nm and 555 nm. (Day 92 is 2 April 1999.)

Fig. 5. Relationship between total, inorganic and organic SPM and

(a) radiance reflectance at 665 nm (%) and (b) ratio of reflectances

at 665 nm to 555 nm.

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315 309

The linear relationship between reflectance colour

ratio and SPM is interesting, as this was expected to be

valid only at higher SPM concentrations. It indicates

that there was a spectral shift in reflectance with

increasing sediment concentration (Fig. 4c) even at

relatively low SPM concentrations, and shows that

colour ratios can be a sensitive indicator of SPM

concentration. Kratzer et al. (2000) successfully used

colour ratios in Menai Strait waters, of similar SPM

concentration.

4.3. In-water optics

Diffuse vertical attenuation was greatest in the 412

nm (violet) channel with highest values occurring on

6–7 April (Fig. 4a) when SPM concentrations were

largest. The spectra indicate rapid attenuation of short

(violet/blue) and long (red) wavelength light, with

greatest penetration of light at green/yellow wave-

lengths (555 nm).

To evaluate the water clarity, the Jerlov (1976)

classification of ‘optical water type’, was determined

from the vertical attenuation coefficient Kd(0� ,k) justbelow the sea surface. Mean and standard deviation of

Kd were calculated from 33 PRR600 profiles and

compared with Jerlov’s standard spectra (Fig. 7).

Our observations most closely match Jerlov type 7

water (a classification within the parameters of Case 2

waters) which is described as ‘relatively turbid coastal

water’. Maximum transmittance occurs at f 550 nm

giving the water a distinctive green/yellow colour.

Small deviations from the Jerlov type 7 spectra are

due to slightly lower proportions of yellow substance

and/or higher proportions of sediment in the water

(less absorption or more scattering of blue light with

more absorption or less scattering of red light).

The relationships between absorption and scattering

coefficients and SPM concentration were explored

with a multiple linear regression analysis (Table 3).

This resulted in some negative values for specific

absorption and scattering coefficients (of the order

of � 0.01 m2 mg� 1); where this occurred, the analysis

Fig. 6. Relationship between SPM concentration measured in situ

and reflectance at 665 nm derived from in situ PRR600 profiles

obtained during 2–8 April 1999. Superimposed is the relationship

between SPM concentration measured in situ and reflectance at

580–680 nm derived from NOAA/AVHRR data in 1990–91 near to

the Belgian–Dutch coast (from Van Raaphorst et al., 1998).

Fig. 7. Means and standard deviations of vertical attenuation

coefficient Kd(0� ,k) just below the sea surface, calculated from 33

in situ profiles using PRR600 colour sensor. The solid lines are the

spectra for each Jerlov classification of ‘optical water type’ (Jerlov,

1976): oceanic types I, II and III correspond to Class 1, and coastal

types 1–9 correspond to Class 2. The present results may be

assigned to type 7, that is, relatively turbid coastal waters.

Table 2

Relationship between SPM concentrations and surface radiance

reflectances at six wavebands k measured by the PRR600 colour

sensor. Tabulated values of regression coefficient R2. Values close

to 1.0 signify high correlation; conversely, values close to zero

indicate little or no correlation

Wavelength, k (nm)

412 443 490 510 555 665

Total SPM 0.582 0.521 0.530 0.589 0.703 0.819

Inorganic SPM 0.616 0.509 0.514 0.591 0.730 0.814

Organic SPM 0.099 0.056 0.053 0.077 0.163 0.300

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315310

was repeated omitting that term on the assumption that

the true value was zero. Although the regressed

variables were not strictly independent, as organic

matter typically correlates well with fine inorganic

particles, the analysis still offers some valuable

insights. The background absorption and scattering

coefficients a0 and b0 exceeded the aw and bw values

for clear seawater at all wavelengths suggesting the

presence of ultra fine particles (i.e., smaller than the

filter pore size of 0.7 Am) and/or dissolved substances

such as yellow substance. Additional scattering may

also result from microscale turbulence (Mobley, 1994).

The variation in specific absorption coefficient a *

with wavelength appears to be independent of inor-

ganic SPM, but neither is it strongly linked to the

organic fraction, since the R2 values are low. Values

derived for a *org are much larger than those found by

Kratzer et al. (2000) for (methanol soluble) pigmented

organic matter, suggesting that in our samples organic

detritus (coloured with methanol insoluble pigments)

was an important agent for absorption. At 555 nm,

where absorption by chlorophyll should be minimal,

our a *org value of 0.08 m

2 mg� 1 is of the same order as

the PROWQMmodel value for organic detritus of 0.05

m2 mg � 1 (Lee et al., 2002). The specific scattering

coefficient, in contrast, was strongly correlated to SPM

concentration and was more dependent on the inor-

ganic (which had greater dynamic range) than the

organic fraction. Values calculated for b *inorg are com-

parable to those calculated for mineral suspended

sediment in the Menai Strait (b *MSS from Harker,

1997), indicating that the southern North Sea sediment

we encountered had a similar capacity to scatter light.

4.4. Photosynthetically active radiation

Evaluation of euphotic layer PAR is critical in the

assessment of phytoplankton dynamics as it is essen-

tial for growth (discussed in Section 5.1.). In-water

PAR depends on the diffuse attenuation coefficient and

incident irradiance, which varies with season, latitude

and atmospheric conditions. During the cruise, varia-

tion in the diffuse attenuation coefficient for PAR was

similar to the Kd values described in Fig. 4a, with high

attenuation corresponding to periods of increased

SPM. Euphotic layer depth (depth of 1% surface

irradiance) varied between 6 and 10 m depending on

the turbidity (Fig. 8b). Observed sea surface PAR

irradiance approached theoretical estimates for clear

sky values (Bowers and Brubaker, 2002) on 2 and 7

April. At other times, cloud and fog reduced the

incident irradiance by as much as 80% (Fig. 8a).

While the depth of the euphotic layer remained

fairly constant throughout the cruise period, the

amount of photons illuminating the water column

changed on a daily basis with incident insolation.

Vertical mixing generated by turbulence associated

with wind mixing and strong tidal currents, allowed

phytoplankton throughout the water column (depth of

about 18 m) to be periodically exposed to PAR in the

euphotic layer. To evaluate the level of PAR exposure

of uniformly vertically mixed algae over a 24-h

period, the layer mean PAR was calculated as

I24hr

¼ I024hr

m1ð1� eð�KdhÞÞKdh

ð5Þ

where I24hr

0 is the 24-h mean PAR incident on the sea

surface, m1 accounts for reflection losses at the air-sea

interface, Kd is the day and depth mean PAR attenu-

ation coefficient, and h is the water depth. Layer-mean

PAR varied considerably between days with clear sky

conditions providing more than three times the illu-

Table 3

Relation between organic and inorganic SPM concentrations, and

coefficients of absorption a and scattering b at six wavebands kmeasured by the PRR600 colour sensor. Absorption aw and

scattering bw coefficients (in m� 1), for clear ocean water are shown

for comparison (from Smith and Baker, 1981). Tabulated values of

coefficients in the regression equations a(k) = a0(k) + a*org(k) (or-

ganic SPM) + a *inorg(k) (inorganic SPM), b(k) = b0(k) + b

*org(k)

(organic SPM) + b *inorg(k) (inorganic SPM), are a0, b0 (in m� 1),

a * and b * (in m2 g� 1) and the regression coefficient R2

Wavelength, k (nm)

412 443 490 510 555 665

Absorption

aw 0.015 0.013 0.017 0.035 0.067 0.417

a0 0.33 0.24 0.13 0.12 0.13 0.54

aorg* 0.20 0.17 0.14 0.11 0.08 0.07

ainorg* 0.00 - - - 0.00 -

R2 0.545 0.464 0.412 0.405 0.350 0.259

Scattering

bw 0.007 0.005 0.003 0.003 0.002 0.001

b0 0.45 0.48 0.46 0.42 0.31 0.03

borg* - - 0.03 0.05 0.09 0.11

binorg* 0.22 0.23 0.25 0.25 0.25 0.22

R2 0.820 0.800 0.812 0.821 0.820 0.848

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315 311

mination of cloudy skies. Values were compared with

modelled clear sky conditions to assess whether there

was sufficient light to sustain phytoplankton during

cloudy weather. In all cases, layer-mean PAR

exceeded 1% clear sky surface PAR, indicating that

a vertically mixed water column would have been

fully euphotic throughout the cruise period.

5. Discussion

5.1. Estimates of phytoplankton growth

To investigate how fluctuations in layer-mean PAR

might affect phytoplankton growth, a simple algorithm

was used to calculate potential growth rates. Algal

growth is either controlled and limited by the supply

of critical nutrients (e.g., nitrogen, silicate, phosphate),

or by ambient light conditions. Nitrogen and silicate

concentrations recorded at a mooring throughout the

cruise period indicate both nutrients were in ample

supply (I. Ezzi, J. Boyd, pers. comm., 2001) and

unlikely to limit algal growth. An algorithm for light

controlled growth from the PROVESS 2MPPD model

(Tett, 1990; Lee et al., 2002) was therefore used to

assess the potential growth rate:

l ¼ a I24hr

ChlQC � r ð6Þ

where a is the photosynthetic efficiency, ChlQC is the

chlorophyll to carbon ratio and r is respiration calcu-

lated as

r ¼ r0 þ rll ð7Þ

where r0 is the basal respiration rate and rl is the growth

rate related respiration. Appropriate values for a dia-

tom-dominated mixed population of microplankton in

the southern North Sea were taken as a = 0.07 mmol C

(mg Chl) � 1 d� 1 (AEm � 2s� 1)� 1 (Tett and Walne

1995); ChlQC= 0.35 mg Chl (mmol C)� 1, r0 = 0.035

d� 1 and rl= 0.78 (Lee et al., 2002).

Layer-mean net growth rate varied from 0.3 d� 1

on clear sky days to less than 0.1 d� 1 during cloudy

weather (Fig. 8c). Daily fluctuations in observed

depth-integrated chlorophyll did not reflect the bio-

mass increases predicted by the calculated growth

rates. In general, changes in biomass were less than

predicted indicating that the phytoplankton was grow-

ing at a substantially lower rate and/or suffering

additional losses. These losses varied from 3 to 25%

of the total chlorophyll biomass and would have been

partly due to zooplankton grazing. Tett and Walne

(1995) estimate mesozooplankton grazing at a coastal

station in the southern North Sea for March–April as

0.20–0.05 d� 1 which could account for much of our

lost material. Further losses may have resulted from

sedimentation processes at the bed, cell destruction

during periods of intense turbulence, and/or advection

of water with reduced biomass content through the

study site.

These loss rates can be used to estimate the 24-h

mean microplankton compensation irradiance at which

net production is zero. The ‘classical’ compensation

Fig. 8. Time series of (a) modelled (Bowers and Brubaker, 2002)

and observed incident sea surface PAR, (b) euphotic layer depth and

(c) estimated growth, respiration and net growth rate from Eqs. 6

and 7. (Day 92 is 2 April 1999.).

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315312

irradiance is defined as the balance between produc-

tion and respiration; however, for a mixed population

of microplankton additional losses due to microbial

loop processes, mesozooplankton grazing, advection

and dispersion are important. The microplankton com-

pensation irradiance, which includes a term (L, in d�1)

for external losses, was calculated from Tett et al.

(2002) as

Ic ¼Lð1þ rlÞ þ r0

a ChlQCð8Þ

using the same photosynthetic and respiration param-

eters as above. Values ranged from 3–19 AE m� 2 s� 1

and were exceeded by 24-h mean mixed layer PAR on

all but the dullest day (6 April, when both parameters

were approximately equal). On most days then, micro-

plankton throughout the water column had access to

sufficient PAR for production to exceed respiration and

external losses.

Our growth rate estimates can be compared with

observations of water column oxygen respiration made

at the same location later in April 1999 by Grenz et al.

(2002). Assuming an oxygen to biomass ratio of 1.0

mmol O2 (mmol C)� 1 our estimates of algal growth

produce 9–30 mmol O2 m� 3 d� 1. Grenz et al. (2002)

measured dark bottle respiration rates (including nitri-

fication, algal and detrital respiration) of 11–30 mmol

O2m� 3d� 1,which suggests thatwater columnoxygen

concentrations were in near equilibrium. By subtract-

ing algal respiration (Eq. 7) from the observations of

Grenz et al. (2002) dark oxygen demand (due to dark

algal respiration, detrital respiration and nitrification)

was estimated as 3–6 mmol O2 m� 3 d� 1.

These growth and respiration estimates assume a

uniform well-mixed layer of phytoplankton. However,

observations from the CTD fluorometer suggest this

was seldom true. The general vertical distribution of

chlorophyll observed with the CTD suggests that a

large fraction remained below the euphotic layer (f 8

m) for long periods (although it is possible that

chlorophyll was being elevated to the surface and

sinking to the bed within the distribution seen). If the

chlorophyll biomass at depth was not periodically

elevated to the euphotic layer, its growth rate would

have been significantly lower (even negative if respi-

ration exceeded growth) than that predicted for a well-

mixed layer. In addition estimates of loss rate from

layer mean growth rate could have been further over-

estimated from the biased distribution of chlorophyll

with depth.

Table 4

Timescales (d�1) of processes controlling the abundance of SPM (the depth co-ordinate, z, increases from the seabed upwards; sinking velocities

are –ve)

Process Evaluated term Mixed biomass Mixed inorganic SPM Stratified biomass Stratified inorganic SPM

Biology (growth) hli 0.02–0.16 0 0.02–0.16 0

Particle sinking � hwY ih

0.03–0.25 0.1–0.4 0.03–0.06 0.1–0.4

Vertical mixingh8Kzih2

2–200 2–200 0.3–3.2 0.3–3.2

Maximum fluff resuspension2Y ½f �hY ih 2.0 2.0 0 0

Horizontal exchange KxDðlnY Þ

Dx

� �2

0.9 0.7–1.0 0.9 0.7–1.0

Aggregation kaashcsY i 0.56–5.58 0 0.03–0.31 0

Microplankton relative growth rate l was calculated from Eq. 6 using parameter values quoted in the text. Particle sinking rates were taken from

values used in PROWQM, which were comparable to those measured in settling tube experiments during the cruise (McCandliss et al., 2002).

Vertical mixing was calculated from Bowden (1983) with vertical eddy diffusion coefficients supplied as 102–104 m2 d�1 for the fully-mixed

condition, and 1–10 m2 d�1 in the pycnocline under stratified conditions (Fisher et al., 2002). Resuspension depended on the bed shear stress

and was limited by the total amount of material in the fluff layer so that the maximum relative rate of resuspension was 2 d�1. Horizontal

exchange was calculated from spatial gradients after Prandle (pers. comm., 2001). Aggregation was evaluated as the product of particle

‘stickiness’ as (the probability that an encounter between two particles will lead to aggregation), small-scale shear cs taken from the physical

model (d�1) and the ‘collision kernel’ ka (m3 mmol C�1) (Lee et al., 2002).

K. Wild-Allen et al. / Journal of Sea Research 47 (2002) 303–315 313

5.2. Timescales of processes controlling SPM and in-

water optics

We have extracted and simplified representative

terms from the PROWQM 1-D model (Lee et al.,

2002) for comparison, to gain an order of magnitude

impression of the relative importance of different

processes for the dynamics (evolution of distribution

and abundance) of microplankton and (fine) inorganic

SPM. An additional term for horizontal exchange,

parameterised as a mixing process, is also included, to

give an indication of the coherence of a particular

water body at the site. Terms for growth, particle

sinking, vertical mixing, fluff layer resuspension and

aggregation are reduced to relative rates of change

(d� 1) and values are shown for two scenarios: a fully

mixed water column (of depth 20 m), and a stratified

water column (euphotic surface mixed layer 8 m;

pycnocline thickness 5 m) (Table 4).

For a mixed water column, vertical mixing was the

most important process controlling the dynamics of

SPM, followed by microplankton aggregation and

fluff layer resuspension. All of these processes had

the potential to change the distribution and abundance

of SPM throughout the water-column several times

per day. As horizontal exchange occurred over a

timescale of 1–2 d, there was time for a distinctive

SPM distribution to develop in a coherent patch of

water. Particle sinking and microplankton growth

played comparatively minor roles in the net dynamics

of the SPM with turnover times of 2–50 d.

In the case of a stratified water column, vertical

mixing was still the dominant process, although sig-

nificantly weaker than for the mixed water column.

This was followed by horizontal exchange and sink-

ing of inorganic SPM which operated over timescales

of 1–2 days. Fluff layer resuspension was unimpor-

tant as material remained below the pycnocline at all

times and therefore had little impact on the euphotic

layer. Microplankton sinking also played a very minor

role due to assumed vertical gradients in nutrient.

The observed water column oscillated between

periods of stratification and mixing depending on

weather conditions and the tidal cycle (Fisher et al.,

2002). Whilst vertical exchange was the most signifi-

cant process controlling the dynamics of SPM and

hence in-water optics at all times, other processes

varied in importance with the presence or absence of

stratification. Stratification itself must therefore be

considered as a key factor in understanding SPM

distribution and abundance and associated in-water

optical properties in this region of freshwater influ-

ence (ROFI).

Acknowledgements

This work was partly funded by the EU MAST III

PROVESS project MAS3-CT97-0159 and supported

by the UK MAFF ‘Improved Assessment of Eutro-

phication Effects in Coastal Waters’ project AE1024.

We are grateful for all assistance received from

PROVESS colleagues; for the support of the captain

and crew of the RV Pelagia; for use of equipment and

laboratory space of the Marine Optics Group at the

University of Wales, Bangor; and to Gwen Moncoiffe

at BODC for the supply of banked data. The authors

would also like to thank the referees for their

constructive criticism.

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