Use of tritiated substrates in the study of heterotrophy in seawater

An improved method is described for the study of heterotrophic utilization of dissolved organic substances by marine microorganisms. The method is based on the use of ³H-labelled organic substrates of very high specific activity, rather than the conventionally used ¹⁴C-labelled substrates. Direct measurement of the rate of tracer uptake at near ambient concentration can thus be made instead of extrapolation using the Michaelis-Menten equation. The method also permits comparison between the rates of tracer uptake in sub-samples exposed to different physico-chemical conditions (temperature, light, pollutants, etc.) without the necessity of determining the ambient substrate concentration. The method was applied to the determination of D-glucose uptake by nearshore and pelagic natural microbial populations, and was found to be sensitive and convenient.     

A method to determine in situ zooplankton grazing rates on natural particle assemblages

In situ zooplankton grazing rates on natural particle assemblages were stimated by measuring zooplankton uptake of labelled autotrophic (with Na¹⁴CO₃) and heterotrophic (with [methyl-³H]-thymidine) particulate matter in 1-h incubations in clear, Plexiglas, Haney chambers. The in situ grazing rates are in the same range as those measured for zooplankton in the laboratory using standard particle counting techniques. A negative selection coefficient for ³H-labelled particles indicated a lower filtration efficiency or avoidance of these particles by zooplankton.     

Assessment of simultaneous uptake of nitrogenous nutrients (15N) and inorganic carbon (13C) by natural phytoplankton populations

The simultaneous uptake of nitrogenous nutrients and inorganic carbon was measured in shipboard incubations of natural phytoplankton populations, using tracer additions of ¹³C-bicarbonate and ¹⁵N-labelled nitrogenous substrates. From March 1991 through March 1992, three stations on the Scotian Shelf (eastern Canada) were sampled monthly at ten depths in the euphotic zone. Additions of labelled nitrogen compounds ranged between 0.5 and 98% of ambient concentrations. Most of the C/N (at/at) uptake ratios were lower than the Redfield ratio, suggesting that nitrogen was not limiting. The fixation of carbon with and without addition of nitrate, ammonium or urea was generally similar. Some samples presented significant differences in carbon uptake rate between the four treatments, but these differences were not related to nitrogen enrichment (percent or nitrogen species). Given these results, the double-labelling method appears to be a reliable tool for measuring the simultaneous uptake of carbon and nitrogen by natural phytoplankton.     

Effects of zooplankton diel vertical migration on a phytoplankton community: A scenario analysis of the underlying mechanisms

A mechanistic model was applied to study the influence of diurnal vertical migration (DVM) of planktonic crustaceans on the succession and composition of the phytoplankton community. While zooplankton was restricted to only one functional group, the phytoplankton community was divided into two functional groups which are distinguished by their maximum growth rates and vulnerability to zooplankton grazing. DVM causes a pulsed grazing regime and may also entail a corresponding reduction of the cumulative daily rates of ingestion and losses of zooplankton. To study the relative importance of these two mechanisms of DVM to phytoplankton we performed a scenario analysis consisting of 5 different scenarios. The results show that DVM has a strong influence on the phytoplankton community. Well edible algae benefit during the first 3–4 weeks of summer stratification by reduced daily grazing. The typical shift from small, well edible algae to larger, poorly or non-edible phytoplankton is distinctly delayed. Under the assumption of unchanged daily grazing, however, a pulsed grazing regime has nearly no influence on the resulting phytoplankton composition. As similar effects are also found for completely non-edible phytoplankton, indirect effects via phosphorus availability must be assumed. Thus, the scenario analysis reveals that the observed effects of DVM on phytoplankton can be explained by a combination of two mechanisms: (1) reduction of the daily zooplankton grazing, and (2) changed assimilation and remineralisation of phosphorus. Surprisingly and in contradiction to earlier reports there is almost no DVM effect on phytoplankton due to the sole action of a pulsed grazing regime.     

Dependence of phytoplankton production on rhythm and rate of elimination

This paper investigates the dependence of phytoplankton production upon rhythm and rate of zooplankton grazing and presents a mathematical model for calculating the most important parameters. Both uniform and non-uniform grazing are described mathematically. Non-uniform grazing, expressed by a sinusoidal curve, is usually found in bathyplanktonic ecosystems with migratory consumers. Phytoplankton production depends on the time of grazing; the nearer grazing occurs toward nightfall, the higher is the phytoplankton production. In order to calculate phytoplankton productivity and the amount of food consumed by the zooplankton, experimental data on generation time of phytoplankters, their mortality rates, initial and final standing stocks, and information on diurnal grazing rhythms must be available. If the distribution of grazing rates is sinusoidal and mortality rate constant, the equations presented allow the calculation of phytoplankton productivity with an error of about 6%.     

Phytoplankton carbon dynamics during a winter-spring diatom bloom in an enclosed marine ecosystem: primary production,biomass and loss rates

Phytoplankton production, standing crop, and loss processes (respiration, sedimentation, grazing by zooplankton, and excretion) were measured on a daily basis during the growth, dormancy and decline of a winter-spring diatom bloom in a large-scale (13 m³) marine mesocosm in 1987. Carbonspecific rates of production and biomass change were highly correlated whereas production and loss rates were unrelated over the experimental period when the significant changes in algal biomass characteristic of phytoplankton blooms were occurring. The observed decline in diatom growth rates was caused by nutrient limitation. Daily phytoplankton production rates calculated from the phytoplankton continuity equation were in excellent agreement with rates independently determined using standard ¹⁴C techniques. A carbon budget for the winter bloom indicated that 82.4% of the net daytime primary production was accounted for by measured loss processes, 1.3% was present as standing crop at the end of the experiment, and 16.3% was unexplained. Losses via sedimentation (44.8%) and nighttime phytoplankton respiration (24.1%) predominated, while losses due to zooplankton grazing (10.7%) and nighttime phytoplankton excretion (2.8%) were of lesser importance. A model simulating daily phytoplankton biomass was developed to demonstrate the relative importance of the individual loss processes.     

A spatial model of phytoplankton patchiness

The one-dimensional theory of critical-length scales of phytoplankton patchiness is developed to include phytoplankton growth and herbivore grazing as functions of time and space. The critical-length scale L _c for the pathch is then determined by the initial spatial distribution and concentration of the limiting nutrient and herbivores in addition to the daily averaged values of the growth and loss processes. The response of an initial phytoplankton patch to the stresses of turbulent diffusion, nutrient depletion, light periodicity, and nocturnal or continuous herbivore grazing is investigated numerically for several oceanic conditions. Nocturnal grazing, while less stressful on primary production than continous grazing, results in lower phytoplankton standing stocks. Increase in biomass of vertically migrating zooplankton results in a net loss of nutrient which might otherwise be egested, recycled, and utilized in the euphotic zone under continuous grazing conditions. The Ivlev constant is shown via sensitivity analysis to be a significant parameter ultimately influencing phytoplankton production. It is demonstrated numerically that diffusion of phytoplankton cells from areas of high concentration to low concentration prevents the local extinction of the standing stock, thereby rendering a positive herbivore grazing-threshold unnecessary for ecosystem stability.     

Marine food-web analysis: An experimental study of demersal zooplankton using isotopically labelled prey species

Short-term incubations in seawater containing H¹⁴CO₃ ^- or ³H₂O in place of the naturally predominant isotopes can yield highly radioactive preparations of living phytoplankton or zooplankton. Subsequent in situ incubation of these labelled organisms with the community from which they were taken results in the rapid transfer of radioisotope to those species which prey upon them. This technique has been employed to map a portion of a marine food web involving demersal zooplankton; experiments were conducted in summer and autumn on a coral reef and in a subtropical estuary. Similar results were obtained from these initial experiments at each study site during both seasons. Prey supplied as zooplankton (124 to 410 m nominal diameter), which consisted mainly of Oithona oculata, was fed upon by zooplankton size classes ranging from 410 to 850 m and containing amphipods, ostracods, cumaceans and polychaetes. In experiments employing labelled phytoplankton as prey a wide size spectrum was used (10 to 106 m) in order to include representative samples of most of the available planktonic autotrophs as estimated by primary production measurements. In two separate experiments, only 7 out of 63 samples evidenced grazing of phytoplankton by demersal zooplankters. In contrast, labelled diatom auxospores, employed in one experiment as they constituted the most numerically abundant species in the water column, were found to be grazed upon in nearly half the samples examined.     

Influence des conditions hivernales sur les productions phyto-et zooplanctoniques en Méditerranée Nord-Occidentale. V. Biomasse et production zooplanctonique — relations phyto-zooplancton

The cruise “Mediprod I” of the R.V. “Jean Charcot” covered an area of the Mediterranean Sea situated North of the 40th parallel and West of Corsica; during two 15-day legs, the first when Winter conditions ended, in March, the second in Spring conditions, in April, a 48-station network was surveyed as to primary and secondary production, as well as hydrological conditions. The first survey revealed a higher concentration of zooplankton in hydrologically stable areas, especially in the South-East, although zooplankton values were rather low throughout the whole area surveyed. The total zooplankton per unit surface was lower at the edges of the surveyed area, where phytoplankton was scarce. An increase in zoo-plankton biomass was observed between the two surveys, mostly in the central areas and near the surface. An important difference between both legs is in the proportion of organisms of different sizes collected by the Clarke-Bumpus sampler: the-200 μ:+200 μ organisms ratio, which is around 1 during the first leg, is much lower during the second leg. Two methods were used in estimating the biomass: the +200 μ fraction of a Clarke-Bumpus sample (Cl-B towed with a 50-μ net fitted on the sampler) was collected by sieving the sample through a 200-μ mesh nylon; standard vertical WP 2 hauls were performed (200 μ mesh). Both roughly show the same zooplankton (weight per unit surface) distribution pattern. However, higher estimations of the total biomass of the volume investigated were sometimes provided by the Clarke-Bumpus method during the first leg, probably due to the distribution of the animals according to their size-classes. A graph of chlorophyll versus zooplankton for surface waters suggests that zooplankton has a limiting effect on the development of phytoplankton in April only. Apparent growth rates of cooplankton are less than those for populations in the laboratory or enclosed environments. Values of the mean secondary production vary from 18 mg C.m^-2.day^-1 for the first leg, to 230 mg C.m^-2.day^-1 for the second leg. Estimations of net efficiency for energetic transfer between phyto- and zooplankton lie between 7 and 26%. As far as our hypotheses as regards physiological coefficients are valid, we can assume that the effect of grazing relative to primary production is greater in the border areas than in the central area, thus increasing the contrast between both areas with time. Phosphorus excretion rate by zooplankton seems to be less than that measured in the Atlantic Ocean. We suggest that in Spring zooplankton excretion is not the main phosphorus recycling process. Primary productivity measurements, apparent growth coefficients, and estimated grazing rate have been used to calculate the expected mean biomass per unit area during the second leg. A 7% loss of phyto- and zooplankton from the upper 100-m layer must be assumed to explain the observed biomass variation. Vertical mixing and sinking of surface water on an isopycnal slope are responsible for such loss, which can also affect, to an undetermined degree, the phosphorus stock introduced into the surface layer by the Mediterranean deep-water formation mechanism. We suggest that nearly half the total loss of phytoplankton from March to April is attributable to animal grazing.     

Microzooplankton grazing and selectivity of phytoplankton in coastal waters

Microzooplankton grazing activity in the Celtic Sea and Carmarthen Bay in summer 1983 and autumn 1984 was investigated by applying a dilution technique to high-performance liquid chromatographic (HPLC) analysis of photosynthetic pigments in phytoplankton present within natural microplankton communities. Specific grazing rates on phytoplankton, as measured by the utilisation of chlorophyll a, were high and varied seasonally. In surface waters during the autumn, grazing varied between 0.4 d^-1 in the bay and 1.0 d^-1 in the Celtic Sea, indicating that 30 and 65% of the algal standing stocks, respectively, were grazed daily. Grazing rates by microzooplankton within the thermocline in summer suggest that 13 to 42% of the crop was grazed each day. Microzooplankton showed selection for algae containing chlorophyll b, in spite of a predominance of chlorophyll c within the phytoplankton community. Changes in taxon-specific carotenoids indicated strong selection for peridinin, lutein and alloxanthin and selection against fucoxanthin and diadinoxanthin. This indicates a trophic preference by microzooplankton for dinoflagellates, cryptophytes, chlorophytes and prasinophytes and selection against diatoms, even when the latter group forms the largest crop within the phytoplankton. Interestingly, those algal taxa preferentially grazed also showed the highest specific growth-rates, suggesting a dynamic feed-back between microzooplankton and phytoplankton. Conversion of grazing rates on each pigment into chlorophyll a equivalents suggests firstly, that in only one experiment could all the grazed chlorophyll a be accounted for by the attrition of other chlorophylls and carotenoids, and secondly that in spite of negative selection, a greater mass of diatoms could be grazed by microzooplankton than any other algal taxon. The former may be due either to a fundamental difference in the break-down rates of chlorophyll a compared to other pigments, or to cyanobacteria forming a significant food source for microzooplankton. In either case, chlorophyll a is considered to be a good measure of grazing activity by microzooplankton.     

Impact of copepod grazing on the red-tide flagellate Chattonella antiqua

Although planktonic copepods are major suspension feeders in the sea, the impact of their grazing pressure upon red-tide flagellates has not been fully investigated. In the present study, the grazing of adult females of several copepod species is examined using three food types: viz. natural suspended particles, natural suspended particles mixed with cultured Chattonella antiqua, and cultured C. antiqua. The functional response on C. antiqua was investigated for five species of copepods (Acartia erythraea, Calanus sinicus, Centropages yamadai, Paracalanus parvus and Pseudodiaptomus marinus). Ingestion rates increased linearly with increasing cell concentrations until a maximum level was reached, beyond which the rates were constant. This cell concentration was higher for larger copepods. The weight-specific maximum ingestion rates were higher in the small species. In general, copepods tended to feed selectively on larger particles when feeding on natural particles. This tendency was strongest in a simulated red-tide environment. Thus, it can be surmised that copepods may selectively graze on C. antiqua during the outbreak of a red tide. Grazing pressure by the natural copepod community in Harima Nada, the Inland Sea of Japan, was calculated by integration of the laboratory determined feeding rates and field measurements of zooplankton biomass. The daily removal rate was 3.4 to 30.8% (mean: 12.3%) of C. antiqua biomass at 20 cells ml^-1 and decreased to 0.6–4.3% (mean: 1.8%) at 500 cells ml^-1. Therefore, the grazing pressure by the copepod community is important at the initial stage of the red tide.     

In situ feeding rates and grazing impact of zooplankton in a South African temporarily open estuary

Recent studies in temporarily open estuaries of South Africa have shown that phytoplankton biomass is at times low, when compared to the high standing stock of the grazers. In situ grazing rates of the dominant zooplankton species were estimated at the Mpenjati Estuary once during the winter closed phase, in August 1999, and once during the summer open phase, in February 2000. The study aimed at determining what proportion of the energetic demands of the dominant grazers of the estuary is met by the available phytoplankton. Results show that the gut of all species exhibited higher pigment concentrations during the night than during the day, both in winter and summer. Gut pigment contents ranged from 0.27 to 5.38 ng pigm individual^-1 in the mysid Gastrosaccus brevifissura, from 0.16 to 1.63 ng pigm individual^-1 in the copepod Pseudodiaptomus hessei, from 0.12 to 0.45 ng pigm individual^-1 in the copepod Acartia natalensis, and from 0.8 to 5.44 ng pigm individual^-1 in the caridean Palaemon sp. [where pigm is the sum of chlorophyll-a (chl-a) and phaeopigments]. During the winter closed phase, gut evacuation rates for G. brevifissura, P. hessei, and A. natalensis were 0.62, 0.42, and 0.46 h^-1, respectively. In summer, gut evacuation rates were 0.68, 0.48, and 0.46 h^-1 for G. brevifissura, P. hessei, and Palaemon sp., respectively. The rate of gut pigment destruction for G. brevifissura was 99.6% of the total ingested, one of the highest values ever recorded for any crustacean. A gut pigment destruction of 79.0% was measured for Palaemon sp., 95.7% for P. hessei, and 93.8% for A. natalensis. During winter the total grazing impact of the dominant zooplankton species ranged from 5.05 to 22.7 mg chl-a m^-2 day^-1 and accounted for 34-69% of the available chl-a in the water column. During summer, the grazing impact ranged between 0.45 and 0.65 mg chl-a m^-2 day^-1, accounting for 17-41% of the available chl-a in the water column. This shows that the dominant zooplankton species of the Mpenjati have a very high grazing impact on algal cells. At times this may exceed 100% of the available phytoplankton production, suggesting that the zooplankton community may often resort to other food sources to meet all its energetic demands.     

Determination of ammonium uptake and regeneration rates using the seawater dilution method

We have developed a method for the determination of ammonium uptake and regeneration rates applying the principle of the seawater dilution technique. The method is based on the separation of uptake and regeneration processes in the dilution series. A model is used to estimate ammonium uptake and regeneration rates simultaneously, in addition to phytoplankton growth and grazing rates. The method was applied to dilution experiments conducted during a two-year study of the upwelling region off Oregon, USA. Ammonium uptake and regeneration rates determined with our method ranged from 0.5 to 3 mol l^-1d^-1 and from 0.2 to 2.9 mol l^-1d^-1, respectively. These values agree well with those from other studies applying ¹⁵N tracer techniques in the same or similar environments. We found a close coupling between ammonium uptake and regeneration, and a strong relationship between ammonium regeneration and grazing rates. In addition, the nutritional status of the phytoplankton community could be assessed by comparing instantaneous ammonium uptake rates with the specific phytoplankton growth rates. Using the dilution technique to determine ammonium uptake and regeneration rates of the plankton community is a promising alternative to the application of tracer techniques conventionally used to determine these rates.     

Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters

Phytoplankton growth and microzooplankton grazing were measured in two productive coastal regions of the North Pacific: northern Puget Sound and the coastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09–2.69 day⁻¹) and microzooplankton grazing (range: 0.00–2.10 day⁻¹) varied seasonally, with lowest values in late fall and winter, and highest values in spring and summer. Chlorophyll concentrations also varied widely (0.19–13.65 μg l⁻¹). Large (>8 μm) phytoplankton cells consistently dominated phytoplankton communities under bloom conditions, contributing on average 65% of total chlorophyll biomass when chlorophyll exceeded 2 μg l⁻¹. Microzooplankton grazing was an important loss process affecting phytoplankton, with grazing rates equivalent to nearly two-thirds (64%) of growth rates on average. Both small and large phytoplankton cells were consumed, with the ratio of grazing to growth (g:μ) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton grazing and phytoplankton growth was tighter during phytoplankton blooms than during low biomass periods, with g:μ averaging 0.78 during blooms and 0.49 at other times. This tight coupling may be a result of the high potential growth and ingestion rates of protist grazers, some of which feed on bloom-forming diatoms and other large phytoplankton. Large ciliates and Gyrodinium-like dinoflagellates contributed substantially to microzooplankton biomass at diatom bloom stations in the Gulf of Alaska, and microzooplankton biomass overall was strongly correlated with >8 μm chlorophyll concentrations. Because grazing tended to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzooplankton was often substantial. In nearly two-thirds of the experiments (14/23), more chlorophyll was ingested by microzooplankton than was available for all other biological and physical loss processes combined. Microzooplankton were important intermediaries in the transfer of primary production to higher trophic levels in these coastal marine food webs. Received: 12 November 1999 / Accepted: 4 October 2000     

In situ grazing rates of deep-sea benthic boundary-layer zooplankton

In situ grazing rates of mixed assemblages of deep-sea benthic boundary-layer zooplankton were measured in July 1983 at a site in the North Atlantic Ocean at 2 175 m depth using a short-term radioisotope-uptake method. Zooplankton were collected with an opening-closing net system from the bottom 1 m of the ocean and incubated in situ with mixed tritiated amino acids in special cod-end chambers. Incubations were terminated at depth by the addition of MS-222. Radioisotope uptake beyond that of dead controls was shown by both the zooplankton and particulate fractions. Grazing rates in the deep-sea experiments were surprisingly high, being comparable to wintertime Narragansett Bay zooplankton grazing rates determined in a separate series of laboratory experiments. These laboratory experiments also documented nonparticle-associated uptake of dissolved amino acids by Narragansett Bay zooplankton, but the importance of this in the deep sea is unknown. The deep-sea benthic boundary layer may be a region of elevated rates and activity because of its higher particulate concentrations, and our experiments may also have measured maximum rather than average rates.     

Zooplankton abundance and grazing at Davies Reef,Great Barrier Reef,Australia

Zooplankton abundance and grazing on autotrophic and heterotrophic particulate matter were measured along a transect across Davis Reef (18°5S; 147°39E) and in the back-reef lagoon over tidal and diel cycles during austral winter (August 1984). Zooplankton entering the reef from the surrounding shelf waters decreased in abundance over the reef flat, presumably because of predation. Within the reef lagoon, maximum daytime densities of pelagic copepods occurred during high water, suggesting an external input. At night, water-column zooplankton biomass increased by a factor of 2 to 3 due to the emergence of demersal reef zooplankton. Zooplankton grazing rates on heterotrophic particulate matter (bacteria + detritus and Protozoa) compared to phytoplankton were higher on the reef flat than on the fore-reef or lagoon. Within the lagoon, zooplankton grazing rates on heterotrophic material were maximum during high water, coincident with maximum tidal concentrations of particulate organic carbon. The combined demersal and pelagic zooplankton community were often able to crop 30% of the daily primary production by >2µm phytoplankton. However, >50% of phytoplankton biomass was in cells <2µm, presumably unavailable to these zooplankton. Our particulate production and ingestion measurements, together with zooplankton carbon demand extrapolated from respiration estimates, suggest that the zooplankton community of Davies Reef derives much of its nutrition from detritus.Joint contribution from the University of Maryland, Center for Environmental and Estuarine Studies (No. 2015), and the Microbial Ecology on a Coral Reef Workshop (MECOR No. 19)     

Variability and its effect on the 24h chlorophyll budget of a small marine basin

In an intensive study (lasting 25 h) of the production, export and grazing of phytoplankton in a small marine basin, it was found that 58% of the production (11% of the total standing stock) was lost by exchange with the sea and 34% was consumed by grazing of zooplankton. The measured production of phytoplankton could be balanced, to within a few percent, against grazing, export, and a small, measured, net change in the total standing stock of the basin. Large variations were observed in concentrations of chlorophyll and zooplankton at the mouth of the basin over the 25 h period. These variations were associated with changes in the height of the tide, but were about 2^1/2 h out of phase with it. Strong negative correlations were observed between chlorophyll and transport, such that only 35% of the chlorophyll exported was exchanged via the mean flow, while 65% was exchanged via the fluctuations. The correlation was even more striking with zooplankton, for which virtually all the export was associated with the fluctuations in the transport. Time series observations in the centre of the basin revealed considerable short-term variability in both chlorophyll and zooplankton, but the variations were smaller than those observed at the mouth of the basin, and the phase lag with the tide was longer. The variability studies enable suggestions to be made about more economical design of sampling programs, but illustrate the difficulty of providing verification data for any continuous model of primary production in such a basin.Bedford Institute Contribution No. 231.Canadian Contribution to IBP No. 97.     

Excretion of ammonia by zooplankton and its potential contribution to nitrogen requirements for primary production in the Catalan Sea (NW Mediterranean)

Excretion of ammonia by mesozooplankton (>200 m zooplankton) and its potential contribution to the nitrogen requirement for phytoplankton growth has been estimated for different hydrographical situations along a transect across the Catalan Sea (Northwestern Mediterranean). The nitrogen excreted as ammonia was estimated from mesozooplankton biomass and specific excretion rates. Nitrogen requirements of phytoplankton were estimated by means of carbon fixation rates and C:N ratios of <200 m particulate organic matter. Minimum C:N ratios and maximum primary production, zooplankton biomass, phytoplankton nitrogen requirements, and nitrogen excretion of zooplankton occurred near the Catalan density front. On average, the nitrogen regenerated by the mesozooplankton accounted for 43% of the nitrogen requirements of the phytoplankton. The specific excretion rates of ammonia and the percentage of phytoplanktonnitrogen requirements supplied by excreted nitrogen were higher at coastal stations. In some coastal and frontal stations, the ammonia excreted exceeded the phytoplanktonnitrogen demand. Bacteria competing for nutrient supply and the possible uncoupling between rate processes and standing stocks of phyto- and zooplankton could explain the apparent excess of regenerated ammonia.     

Estimating carbon flux to pelagic grazers in the ice-edge zone of the eastern Bering Sea

Zooplankton ingestion of phytoplankton carbon in the iceedge zone of the Eastern Bering Sea was measured using a deck incubation approach in 1982. Using further samples collected in 1983, the plant cell carbon to cell volume ratio was estimated at 0.0604 pg m^–3 from an experimentally determined particulate carbon to seston volume relationship. The application of this conversion to the results of experimental incubations of natural plant stocks with net-caught zooplankton produced ingestion rates of 68.8 and 10.26 mg C g^–1 grazer d^–1 for copepods and euphausiids, respectively. Extrapolating these rates to in situ zooplankton biomass at the edge of the seasonal ice pack yielded carbon flux rates through the zooplankton community ranging between 6.5 and 32.8 mg C m^–2 d^–1. This consumption amounted to less than 2% of the daily phytoplankton production in the ice-edge zone.     

Numerical modelling of diel carbon production and zooplankton grazing on the Scotian shelf based on observational data

Using high resolution vertical distributions of chlorophyll and zooplankton, and field observations of photosynthetic parameters, it is shown that on the Scotian Shelf the peak in the vertical profile of primary production generally lies shallower than the chlorophyll maximum, but coincides with the peak in the vertical profile of copepods. A simple numerical model shows that the 24th carbon budget can be balanced using the best available estimates of the rate constants for phytoplankton growth, zooplankton grazing and vertical migration. This calculation is very sensitive to the size of the weight-specific ration and favors values of ～40% d^?1 for it.