The effects of seston food quality on planktonic food web patterns

In planktonic food webs, the conversion rate of plant material to herbivore biomass is determined by a variety of factors such as seston biochemical/elemental composition, phytoplankton cell morphology, and colony architecture. Despite the overwhelming heterogeneity characterizing the plant–animal interface, plankton population models usually misrepresent the food quality constraints imposed on zooplankton growth. In this study, we reformulate the zooplankton grazing term to include seston food quality effects on zooplankton assimilation efficiency and examine its ramifications on system stability. Using different phytoplankton parameterizations with regards to growth strategies, light requirements, sinking rates, and food quality, we examined the dynamics induced in planktonic systems under varying zooplankton mortality/fish predation, light conditions, nutrient availability, and detritus food quality levels. In general, our analysis suggests that high food quality tends to stabilize the planktonic systems, whereas unforced oscillations (limit cycles) emerge with lower seston food quality. For a given phytoplankton specification and resource availability, the amplitude of the plankton oscillations is primarily modulated from zooplankton mortality and secondarily from the nutritional quality of the alternative food source (i.e., detritus). When the phytoplankton community is parameterized as a cyanobacterium-like species, conditions of high nutrient availability combined with high zooplankton mortality led to phytoplankton biomass accumulation, whereas a diatom-like parameterization resulted in relatively low phytoplankton to zooplankton biomass ratios highlighting the notion that high phytoplankton food quality allows the zooplankton community to sustain relatively high biomass and to suppress phytoplankton biomass to low levels. During nutrient and light enrichment conditions, both phytoplankton and detritus food quality determine the extent of the limit cycle region, whereas high algal food quality increases system resilience by shifting the oscillatory region towards lower light attenuation levels. Detritus food quality seems to regulate the amplitude of the dynamic oscillations following enrichment, when algal food quality is low. These results highlight the profitability of the alternative food sources for the grazer as an important predictor for the dynamic behavior of primary producer–grazer interactions in nature.     

Incorporation of organic aggregates by marine mussels

Two marine mussels, Geukensia demissa (Dillwyn) and Mytilus edulis (L.) collected in 1990 in Old Silver Beach, Falmouth, Massachusetts, incorporated nitrogen when fed ¹⁵N-labelled organic aggregates produced from dissolved organic nitrogen released by the brown sea-weed Fucus vesiculosis. Uptake of ¹⁵N on the aggregate diet was linear over the course of 24 h, and unincorporated ¹⁵N was eliminated from the gut after 48 h. Both species of mussels incorporated approximately five times more N when they were fed organic aggregates than when they were fed either ¹⁵N-labelled dissolved organic material (DOM) or particulate detritus, both of which were also derived from the seaweed. Nitrogen uptake was greatest in controls fed the diatom Thalassiosira weissflogii; mussels fed phytoplankton incorporated seven times more nitrogen than those fed aggregates. However, aggregates could supply an estimated 7 to 14% of the N requirements for both mussels, whereas DOM or particulate detritus could only supply 1 to 3%. These data provide evidence that a food web pathway exists from primary producer to released dissolved organic nitrogen to microbial organic aggregate to metazoan consumer, and, further, that it can be more important in a detrital food web than either particulate detritus or DOM.     

Concentrations and vertical fluxes of zooplankton fecal pellets on a continental shelf

The contribution of fecal pellets to the benthos of the southeastern shelf of the USA is investigated through an analytic model which considers pellet production by different stage groups of the genus Paracalanus. Model results indicate that the concentration and vertical flux of pellets is a function of producer size and consumer size and abundance. Nauplii and adults, respectively, produce daily on the average 50 and 13% of total pellet mass, yet contribute 4 and 63%, respectively, to the daily pellet flux. Most of the pellets produced are consumed or degraded in the water column, with only 0.2% of the average daily primary production reaching the seafloor (35 m) as fecal pellets. This contributes to an impoverished benthos, such as that found on the southeastern continental shelf.     

The impact of detritivorous fishes on a mangrove estuarine system

Sundarban mangrove ecosystem in India is one of the largest detritus based ecosystem of the world and it supplies the detritus and nutrients to the adjacent Hooghly-Matla estuarine complex. In this estuary a group of fish completely detritivorous in nature, belonging to the genus Mugil spp. is present. This group of fish is expected to have important effects on the trophic dynamics of ecosystems, but exact nature of these effects is not known. In order to study the impact that detritivory by fish may have on the estuarine food chain, we developed mathematical formulations. We run two models, one with phytoplankton, zooplankton, carnivorous fish, detritus and nutrient and without this group of fish and a second one after including this fish in the system. In our model this group of fish has no major impact on primary productions of the estuarine system but has extensive role in total fish production. Coexistence of detritivorous fish and carnivorous fish occurs within reasonable parameter range. We have tested different growth rates of phytoplankton, grazing rates and predation rates of zooplankton, detritivorous fish and carnivorous fish for total system equilibrium. Carnivorous fish predation rate on detritivorous fish and detritivorous fish grazing rate on detritus are very important. Different foraging ratios are also tested in this study. Foraging preference of carnivorous fish on detritivorous fish appears significant for the system equilibrium.     

Role of herbivory in controlling phytoplankton abundance: annual pigment budget for a temperate marine fjord

An annual pigment budget was constructed for Dabob Bay, Washington (USA) by comparing the downward vertical loss of phytoplankton pigments (chlorophyll and pheopigments) to the production of chlorophyll within the euphotic zone. The vertical flux of pigments was measured with sediment traps deployed at intervals of 1 to 6 wk over a 2.5 yr period yielding 763 d of trap exposure (28 November 1978–16 June 1981). The production rate of chlorophyll was calculated from measurements of algal specific growth rates, chlorophyll (chl) crops, primary production (as carbon) and appropriate C: chl ratios. Sixty one to 77% of the annual chlorophyll production was accounted for by the vertical flux of pheopigments resulting from herbivorous zooplankton grazing (macrozooplankton). Algal sinking, represented by downward chlorophyll flux, accounted for only 5 to 6% of the annual chlorophyll production. The remaining fraction of chlorophyll production was estimated to be consumed by small herbivores (microzooplankton), whose fecal material contributes to the suspended pool of pheopigments found in the euphotic zone. The suspended pheopigments are continuously removed by photodegradation. In Dabob Bay, the major process controlling phytoplankton abundance is zooplankton grazing and it appears that the ultimate fate of most phytoplankton is to be consumed by herbivores.     

Origins and scales of hypoxia on the Louisiana shelf: Importance of seasonal plankton dynamics and river nutrients and discharge

Management plans for the Mississippi River Basin call for reductions in nutrient concentrations up to 40% or more to reduce hypoxia in the Gulf of Mexico (GOM), while at the same time the government is considering new farm subsidies to promote development of biofuels from corn. Thus there are possibilities of both increasing and decreasing river nutrients depending on national priorities. River flow rates which also influence the extent of hypoxia on the shelf may be altered by global climate change. We have therefore developed a series of simulations to forecast ecosystem response to alterations in nutrient loading and river flow. We simulate ecosystem response and hypoxia events using a linked model consisting of multiple phytoplankton groups competing for nitrogen, phosphorus and light, zooplankton grazing that is influenced by prey edibility and stoichiometry, sub-pycnocline water-column metabolism that is influenced by sinking fecal pellets and algal cells, and multi-element sediment diagenesis. This model formulation depicts four areas of increasing salinity moving westward away from the Mississippi River point of discharge, where the surface mixed layer, four bottom layers and underlying sediments are represented in each area. The model supports the contention that a 40% decrease in river nutrient will substantially reduce the duration and areal extent of hypoxia on the shelf. But it also suggests that in low and middle salinity areas the hypoxia response is saturated with respect to nutrients, and that in high salinity regions small increases in nutrient and river flow will have disproportionally large effects on GOM hypoxia. The model simulations also suggest that river discharge is a stronger factor influencing hypoxia than river nutrients in the Mississippi River plume. Finally, the model simulations suggest that primary production in the low salinity regions is light limited while primary production in the higher salinity zones is phosphate limited during the May to October period when hypoxia is prevalent in the Mississippi River plume.     

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)     

Modeling the seasonal autochthonous sources of dissolved organic carbon and nitrogen in the upper Chesapeake Bay

In this paper we investigate the seasonal autochthonous sources of dissolved organic carbon (DOC) and nitrogen (DON) in the euphotic zone at a station in the upper Chesapeake Bay using a new mass-based ecosystem model. Important features of the model are: (1) carbon and nitrogen are incorporated by means of a set of fixed and varying C:N ratios; (2) dissolved organic matter (DOM) is separated into labile, semi-labile, and refractory pools for both C and N; (3) the production and consumption of DOM is treated in detail; and (4) seasonal observations of light, temperature, nutrients, and surface layer circulation are used to physically force the model. The model reasonably reproduces the mean observed seasonal concentrations of nutrients, DOM, plankton biomass, and chlorophyll a. The results suggest that estuarine DOM production is intricately tied to the biomass concentration, ratio, and productivity of phytoplankton, zooplankton, viruses, and bacteria. During peak spring productivity phytoplankton exudation and zooplankton sloppy feeding are the most important autochthonous sources of DOM. In the summer when productivity peaks again, autochthonous sources of DOM are more diverse and, in addition to phytoplankton exudation, important ones include viral lysis and the decay of detritus. The potential importance of viral decay as a source of bioavailable DOM from within the bulk DOM pool is also discussed. The results also highlight the importance of some poorly constrained processes and parameters. Some potential improvements and remedies are suggested. Sensitivity studies on selected parameters are also reported and discussed.     

Climate change effects on phytoplankton depend on cell size and food web structure

We investigated the effects of warming on a natural phytoplankton community from the Baltic Sea, based on six mesocosm experiments conducted 2005–2009. We focused on differences in the dynamics of three phytoplankton size groups which are grazed to a variable extent by different zooplankton groups. While small-sized algae were mostly grazer-controlled, light and nutrient availability largely determined the growth of medium- and large-sized algae. Thus, the latter groups dominated at increased light levels. Warming increased mesozooplankton grazing on medium-sized algae, reducing their biomass. The biomass of small-sized algae was not affected by temperature, probably due to an interplay between indirect effects spreading through the food web. Thus, under the higher temperature and lower light levels anticipated for the next decades in the southern Baltic Sea, a higher share of smaller phytoplankton is expected. We conclude that considering the size structure of the phytoplankton community strongly improves the reliability of projections of climate change effects.     

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.     

Lake of flies, or lake of fish? A trophic model of Lake Malawi

Ecosystem-focused models have, for the first time, become available for the combined demersal and pelagic components of a large tropical lake ecosystem, Lake Malawi. These provide the opportunity to explore continuing controversies over the production efficiencies and ecological functioning of large tropical lakes. In Lake Malawi these models can provide important insight to the effect of fishing on fish composition, and the potential competition that the lakefly Chaoborus edulis may have with fisheries production. A mass-balanced trophic model developed for the demersal fish community of the southern and western areas of Lake Malawi was integrated with an existing trophic model developed for the open-water pelagic. Input parameters for the demersal model were obtained from a survey of fish distributions, fish food consumption studies, and from additional published quantitative and qualitative information on the various biotic components of the community. The model was constructed using the Ecopath approach and software. The graphically presented demersal food web spanned four trophic levels and was based primarily on consumption of detritus, zooplankton and sedimented diatoms. Zooplankton was imported into the system at trophic levels three and four through fish predation on carnivorous and herbivorous copepods and Chaoborus larvae. It is proposed that the primary consumption of copepods was by fish migrating into the pelagic zone. Chaoborus larvae in the demersal were probably consumed near the lakebed as they conducted a daily migration from the pelagic to seek refuge in the sediments. This evidence for strong benthic-pelagic coupling provided the opportunity for linking the demersal model to the existing model for the pelagic community so producing the first model for the complete ecosystem. Energy fluxes through the resulting combined model demonstrated that the primary import of biomass to the demersal system was detritus of pelagic origin (72.1%) and pelagic zooplankton (10.6%). Only 15.8% of the biomass consumed within the demersal system was of demersal origin. Lakefly production is efficiently utilised by the lake fish community, and any attempt to improve fishery production through introduction of a non-native plantivorous fish species would have a negative impact on the stability and productivity of the lake ecosystem.     

Kelp as a trophic resource for marine suspension feeders: a review of isotope-based evidence

Kelp forests are enormously productive, and they and adjacent habitats support large populations of suspension feeders. What do these suspension feeders eat? Intuitively, we might expect that kelp primary production is a key form of trophic support for these animals. Indeed, a large and growing number of studies using carbon stable isotope data, typically collected over short time periods, have asserted that detritus from kelps is an important, and in some cases the main, food source for coastal benthic suspension feeders. This view has been incorporated into several textbooks and review papers covering kelp forest ecosystems, and loss of trophic support for benthic suspension feeders is now often invoked as an ecosystem consequence of top-down or other impacts on kelp forests. More direct evidence, however, suggests that these animals mainly eat phytoplankton and, in some cases, bacteria or zooplankton. Because isotope values of pure coastal phytoplankton, uncontaminated with detritus, are difficult to obtain, present studies have largely relied on single measurements from offshore environments or from the literature, which typically reflects offshore values. We review the evidence showing that phytoplankton isotope values can, and are expected to, vary widely in coastal waters and that inshore phytoplankton may often be enriched in ¹³C compared to offshore phytoplankton. This unaccounted-for variation may have systematically biased the results of such trophic studies toward finding large contributions of kelp detritus to suspension-feeder diets. We review some key stable isotope studies and put forth evidence for alternative explanations of the isotope patterns presented. Finally, we make recommendations for future isotope studies and describe several approaches for progress in this area. New techniques, particularly flow cytometry and compound-specific stable isotope analysis, provide ways to shed light on this interesting and important ecological issue.     

Study of the nutrient and plankton dynamics in Lake Tanganyika using a reduced-gravity model

An eco-hydrodynamic (ECOH) model is proposed for Lake Tanganyika to study the plankton productivity. The hydrodynamic sub-model solves the non-linear, reduced-gravity equations in which wind is the dominant forcing. The ecological sub-model for the epilimnion comprises nutrients, primary production, phytoplankton biomass and zooplankton biomass. In the absence of significant terrestrial input of nutrients, the nutrient loss is compensated for by seasonal, wind-driven, turbulent entrainment of nutrient-rich hypolimnion water into the epilimnion, which gives rise to high plankton productivity twice in the year, during the transition between two seasons. Model simulations predict well the seasonal contrasts of the measured physical and ecological parameters. Numerical tests indicate that the half saturation constant for grazing by zooplankton and the fish predation rate on zooplankton affect the zooplankton biomass measurably more than that of phytoplankton biomass. This work has implications for the application of this model to predict the climatological biological productivity of Lake Tanganyika.     

Are closure terms appropriate or necessary descriptors of zooplankton loss in nutrient–phytoplankton–zooplankton type models?

Our current knowledge of plankton ecology ascribes a large proportion of zooplankton losses to zooplankton cannibalism and carnivory, rather than via the activity of higher trophic levels beyond the plankton. However, planktonic ecosystem models, such as the widely used nutrient–phytoplankton–zooplankton (NPZ) type models, typically represent all zooplankton losses by mathematically (rather than biologically) justified closure functions. Even where it is assumed that these closure functions include zooplanktonic cannibalism and carnivory, these processes are not explicitly implemented within the grazing function of the zooplankton. Here it is argued that this representation of zooplankton losses through “closure” terms within planktonic food web models is neither appropriate nor necessary. The general consequences of implementing a simple function incorporating zooplankton cannibalism and carnivory (intra-guild predation) within a planktonic food web model are compared against models implementing different types of traditional closure functions. While the modelled biomass outputs may appear similar, the fate of annual primary production and f-ratios vary widely. There appears no justification for the continued use of traditional closure term to depict zooplankton loss processes on biological or modelling arguments. To do so can seriously misrepresent the fate of primary production and thence trophic dynamics.     

Zooplankton feeding ecology: contents of fecal pellets of the copepods Temora turbinata and T. stylifera from continental shelf and slope waters near the mouth of the Mississippi River

In-situ feeding habits of the copepods Temora turbinata and T. stylifera were investigated by scanning electron microscope examination of fecal pellets, the contents of which reflected copepod gut contents upon capture. Pellet contents were compared with assemblages of available phytoplankton in the water column at the times of zooplankton sampling. Samples were collected in continental shelf and slope waters of the Gulf of Mexico near the mouth of the Mississippi River. Both species ingested a wide size range and taxonomic array of phytoplankters, and to a lesser extent, other crustaceans. Fecal pellets contained primarily the remains of the phytoplankters that were most abundant in the water at times of collection. There was considerable overlap in the food items ingested by adult females of both copepod species, or two stages of T. turbinata copepodites. Thus, T. turbinata and T. stylifera are omnivores, but primarily opportunistic herbivores.     

Using stable isotopes to reveal shifts in prey consumption by generalist predators.

The effectiveness of generalist predators in biological control may be diminished if increased availability of alternative prey causes individual predators to decrease their consumption of crop pests. Farming practices that enhance densities of microbidetritivores in the detrital food web can lead to increased densities of generalist predators that feed on pest species. The ability to predict the net biocontrol impact of increased predator densities depends upon knowing the extent to which individual predators may shift to detrital prey and feed less on crop pests when prey of the detritus-based food web are more abundant. We addressed this question by comparing ratios of stable isotopes of carbon (delta13C) and nitrogen (delta15N) in generalist ground predators and two types of prey (crop pests and microbidetritivores) in replicated 8 x 8 m cucurbit gardens subjected to one of two treatments: a detrital subsidy or no addition of detritus (control). Small sheet-web spiders (Linyphiidae) and small wolf spiders (Lycosidae) had delta13C values similar to those of Collembola in both the detrital and control treatments, indicating that small spiders belong primarily to the detrital food web. In control plots the larger generalist predators had delta13C values similar to those of the major insect pests, consistent with their known effectiveness as biocontrol agents. Adding detritus may have caused delta13C of one species of large wolf spider to shift toward that of the microbi-detritivores, although evidence is equivocal. In contrast, another large wolf spider displayed no shift in delta13C in the detrital treatment. Thus, stable isotopes revealed which generalist predators will likely continue to feed on pest species in the presence of greater densities of alternative prey.     

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.     

Carbon-based balanced trophic structure and flows in the offshore Lake Ontario food web before (1987-1991) and after (2001-2005) invasion-induced ecosystem change

Replicate mass-balanced solutions to Ecopath models describing carbon-based trophic structures and flows were developed for the Lake Ontario offshore food web before and after invasion-induced disruption. The food webs link two pathways of energy and matter flow: the grazing chain (phytoplankton-zooplankton-fish) and the microbial loop (bacteria-protozoans) and include 19 species-groups and three detrital groups. Mass-balance was achieved by using constrained optimization techniques to randomly vary initial estimates of biomass and diet composition. After the invasion, production declined for all trophic levels and species-groups except Chinook salmon. The trophic level (TL) increased for smelt, adult sculpin, adult alewife and Chinook salmon. Changes to ecotrophic efficiencies indicate a reduction in phytoplankton grazing, increased predation pressure on Mysis, adult smelt and alewife and decreased predation pressure on protozoans. Specific resource to consumer TTE changed; increasing for protozoans (8.0-11.5%), Mysis (0.6-1.0%), and Chinook salmon (1.0-2.3%) and other salmonines (0.4-0.5%) and decreasing for zooplankton (20.2-15.1%), prey-fish (9.7-8.8%), and benthos (1.7-0.6%). Direct trophic influences of recent invasive species were low. The synchrony of the decline in PP and species-group production indicates strong bottom-up influence. Mass balance required an increase of two to threefold in lower trophic level biomass and production, confirming a previously observed paradoxical deficit in lower trophic level production. Analysis of food web changes suggest hypotheses that may apply to other similar large pelagic systems including, (1) as pelagic primary productivity declines, overgrazing of zooplankton results in an increase in protozoan production and a loss of trophic transfer efficiency, (2) habitat and food web changes increased Mysis predation on Diporeia and contributed to their recent decline, and (3) production of Chinook salmon, the primary piscivore, was uncoupled from pelagic production processes. This study demonstrates the value of food web models to better understand the impact of invasive species and to develop novel hypotheses concerning trophic influences.     

Fish production and the marine ecosystems of the Scotian Shelf,eastern Canada

Fishery production and food webs have been studied on the Scotian Shelf and upper continental slope along a transect running 270 km SE of Halifax, Canada. The area (TCNAF Division 4W) supports a fishery of roughly 0.15x10⁶ metric tons. Overall primary production of the shelf waters is 102 g C m^-2 year^-1 and of the slope wacers about 128 g C m^-2 year^-1. Demersal fish production (average 4.1 kcal m^-2 year^-1) is highest over the shelf and declines in an offshore direction, while pelagic fish production (average 16 kcal m^-2 year^-1) is highest over the slope and declines in an inshore direction. Hypothetical food webs of these two intergrading ecosystems have been constructed, based on data for primary production, fish catches, and the biomasses of zooplankton and macrobenthos. These lead us to suggest that there are basic differences in food chains and efficiencies between the two ecosystems that account for their differences in production. Although primary production is 17% higher on an average on the Nova Scotian transect than in the North Sea, the apparent zooplankton and macrobenthos production is 31% lower and macrobenthos production may also be lower. Overall fish catch from the Scotian Shelf and slope is about 47% lower per unit area than the catch in the North Sea, despite the fact that the demersal catches are identical. This is accounted for by a much lower overall pelagic catch from the Nova Scotian area, centered in a region that is small compared to the total area. Fish production in different regions cannot be predicted merely on the basis of differences in level of primary production, but must take into account differences in the structure of the ecosystems.