Benthic carbon is inefficiently transferred in the food webs of two eutrophic shallow lakes

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Contribution of benthic microalgae to the temporal variation in phytoplankton assemblages in a macrotidal system

Suspended marine benthic microalgae in the water column reflect the close relationship between the benthic and pelagic components of coastal ecosystems. In this study, a 12‐year phytoplankton time‐series was used to investigate the contribution of benthic microalgae to the pelagic system at a site along the French‐Atlantic coast. Furthermore, all taxa identified were allocated into different growth forms in order to study their seasonal patterns. The highest contribution of benthic microalgae was observed during the winter period, reaching up to 60% of the carbon biomass in the water column. The haptobenthic growth form showed the highest contribution in terms of biomass, dominant in the fall‐winter period when the turbidity and the river flow were high. The epipelic growth form did not follow any seasonal pattern. The epiphytic diatom Licmophora was most commonly found during summer. As benthic microalgae were found in the water column throughout the year, the temporal variation detected in the structure of pelagic assemblages in a macrotidal ecosystem was partly derived from the differentiated contribution of several benthic growth forms.     

Spatial and Temporal Variability in the Ecosystem Metabolism of a High-elevation Lake: Integrating Benthic and Pelagic Habitats

We characterized spatial and temporal variability in net ecosystem production (NEP), community respiration (CR), and gross primary production (GPP) over an ice-free season in an oligotrophic high-elevation lake using high-frequency measurements of dissolved oxygen. We combined the use of free-water and incubation chamber measurements to compare pelagic and benthic habitats and estimate their relative contributions to whole-lake metabolism. Despite a brief period of predominant heterotrophy after snowmelt, both free-water and incubation chamber measurements confirmed autotrophy of the epilimnion in all habitats throughout the ice-free season. In contrast, benthic incubation chambers showed the benthos to be consistently heterotrophic. Although temperature was the strongest seasonal driver of benthic metabolism, bacterioplankton density and indexes of organic matter quality explained the most variability in pelagic metabolism. Driven largely by benthic metabolism, free-water measurements of GPP and CR were twice as high in littoral than pelagic habitats. However, rates of water column primary production overlying the littoral benthos were high enough to overcome net benthic heterotrophy, and seasonal mean NEP in littoral habitats remained positive and not significantly different from pelagic habitats. Benthic rates averaged about 25% of whole-lake metabolism. Pelagic metabolism measurements were affected by littoral rates about half the time, with the degree of isolation between the two a function of advection and water column stability. These results emphasize the importance of characterizing spatial and temporal variability in metabolism within the context of physical dynamics and challenge the notion that benthic metabolism will necessarily be larger than pelagic metabolism in oligotrophic lakes.     

Light and nutrients regulate energy transfer through benthic and pelagic food chains

The amount of energy flowing to top trophic levels depends on primary production and the efficiency at which it is converted to production at each trophic level. In aquatic systems, algal production is often limited by light and nutrients, and the nutritional quality of algae depends on the relative balance of these two resources. In this study, we used a mesocosm experiment to examine how light and nutrient variation affected food chain efficiency (FCE, defined as the proportion of primary production converted to top trophic level production), using a food web with benthic and pelagic food chains. We also related variation in benthic and pelagic efficiencies to the nutritional quality of primary producers, i.e. carbon:nitrogen:phosphorus stoichiometry. As predicted, pelagic and benthic FCEs were highest under low light/high nutrient conditions, the treatment with the best algal food quality, i.e. the lowest C:nutrient ratios. Pelagic FCE and pelagic herbivore efficiency (HE_P) were more responsive than benthic FCE to variation in light and nutrients. Furthermore, pelagic FCE and HE_P were highly correlated with algal C:P, suggesting ‘carryover effects’ of algal food quality on carnivores (larval fish) via effects on herbivore (zooplankton) quality. Benthic (tadpole) production was primarily explained by primary production rate, suggesting food quantity rather than quality drives their production. However, benthic FCE was also highest at low light/high nutrients and was significantly correlated with food quality. The stronger effect of food quality in mediating pelagic compared to benthic efficiencies, is consistent with differences in the stoichiometric mismatches between algae and consumers. Pelagic FCE and HE_P were more likely to be P‐limited, whereas benthic FCE was more likely N‐limited. This study is the first to examine both pelagic and benthic FCE within the same system, and highlights the importance of differential consumer needs in determining how food quality affects energy transfer efficiency.     

The food web in the lower part of the Seine estuary: a synthesis of existing knowledge

The Seine estuary illustrates the alterations to estuaries due to human activities heavy releases of pollutants of various origins and significant morphological changes beginning in the middle of the 19th century. The intertidal mudflat surface has been seriously reduced (< 30km²) since the channels of the Seine River came under management. While the role of the Seine estuary in the dynamics of the eastern English Channel ecosystem is recognized as important, the biological characteristics of the estuary remained relatively unknown until the 1990s. Biological diversity was progressively impoverished from the polyhaline zone to the oligohaline zone. In spite of a heavily contaminated environment, the macrobenthic and planktonic fauna of the Seine estuary remains similar to those of other northeastern Atlantic estuaries. The fauna exhibit clear contrasts between areas with very high abundance and others with very low abundance. The pelagic fauna, especially the copepod Eurytemora affinisand the shrimp Palaemon longirostris, are more abundant in the Seine estuary than in other estuaries. Diversified and abundant, Abra alba-Pectinaria koreni and Macoma balthica benthic communities occur, respectively, in the outer and inner parts of the estuary. In subtidal flats, benthic fauna is especially poor in terms of specific richness, abundance and biomass. Paradoxically, considering the high abundance of prey, fish are particularly scarce. Two food webs have been identified. In the oligohaline zone, where turbidity is maximum, the food web is exclusively planktonic, due to dredging that prevented benthic fauna from settling. In the polyhaline zone, fish populations that feed particularly on benthic fauna benefit from low turbidity and high oxygen concentrations. So, in spite of heavy organic and metallic contamination and human activities, the Seine estuary remains a highly productive ecosystem, which provides a nursery for marine fish and feeding grounds for migratory birds. A global management plan appears to be necessary in order to guarantee that the Seine estuary continues to function as it currently does.     

Nitrogen cycling in coastal marine ecosystems

It is generally considered that nitrogen availability is one of the major factors regulating primary production in temperate coastal marine environments. Coastal regions often receive large anthropogenic inputs of nitrogen that cause eutrophication. The impact of these nitrogen additions has a profound effect in estuaries and coastal lagoons where water exchange is limited. Such increased nutrient loading promotes the growth of phytoplankton and fast growing pelagic macroalgae while rooted plants (sea-grasses) and benthic are suppressed due to reduced light availability. This shift from benthic to pelagic primary production introduces large diurnal variations in oxygen concentrations in the water column. In addition oxygen consumption in the surface sediments increases due to the deposition of readily degradable biomass. In this review the physico-chemical and biological factors regulating nitrogen cycling in coastal marine ecosystems are considered in relation to developing effective management programmes to rehabilitate seagrass communities in lagoons currently dominated by pelagic macroalgae and/or cyanobacteria.     

Pelagic and benthic net production of dissolved inorganic carbon in an unproductive subarctic lake

1. Both the pelagic and benthic net dissolved inorganic carbon (DIC) productions were measured in situ on four occasions from June to September 2004, in the unproductive Lake Diktar-Erik in subarctic Sweden. The stable isotopic signal ( δ ¹³C) of respired organic material was estimated from hypolimnion water data and data from a laboratory incubation using epilimnion water.
2. Both pelagic and benthic habitats were net heterotrophic during the study period, with a total net DIC production of 416 mg C m⁻² day⁻¹, of which the pelagic habitat contributed approximately 85%. The net DIC production decreased with depth both in the pelagic water and in the sediments, and most of the net DIC production occurred in the upper water column.
3. Temporal variations in both pelagic and benthic DIC production were small, although we observed a significant decrease in pelagic net DIC production after the autumn turnover. Water temperature was the single most important factor explaining temporal and vertical variations in pelagic DIC production. No single factor explained more than 10% of the benthic net DIC production, which probably was regulated by several interacting factors.
4. Pelagic DIC production, and thus most of the whole-lake net production of DIC, was mainly due to the respiration of allochthonous organic carbon. Stable isotope data inferred that nearly 100% of accumulated DIC in the hypolimnion water had an allochthonous carbon source. Similarly, in the laboratory incubation using epilimnion water, c. 85% of accumulated DIC was indicated to have an allochthonous organic carbon source.     

Nutrient dynamics and ecosystem metabolism in the Bay of Blanes (NW Mediterranean)

The dynamics of the nutrient pools and their stoichiometry as well as their control by ecosystem metabolism (benthic and planktonic) and benthic–pelagic exchanges (sedimentation rates and sediment waterfluxes) were examined in the Mediterranean littoral (Blanes Bay, NE Spain). Dissolved organic nitrogen comprised about half of the nitrogen present in the water column and the carbon pool was dominated by the inorganic pool (95% of the carbon present in the water column). The dissolved and particulate organic pools were deficient in P relative to C and N, indicating a rapid recycling of P from organic matter. The pelagic compartment was heterotrophic, supported by significant allochthonous inputs of land material, which also contributed greatly to the sedimentary inputs (37% of total sedimenting carbon). In contrast, the benthic compartment was autotrophic, with the excess net benthic community production balancing the deficit in pelagic community production, leading to metabolic equilibrium at the station studied. Sedimentary inputs of nitrogen, phosphorus and silicon exceeded the benthic release, indicating that the benthic compartment acted as a sink for nutrients, consistent with its autotrophic nature. Carbon inputs to the benthic compartment also exceeded requirements, due to the allochthonous subsidies to the system, so that the benthic compartment stored or exported organic carbon. An erratum to this article can be found at .     

Bottom‐up and top‐down effects of browning and warming on shallow lake food webs

Productivity and trophic structure of aquatic ecosystems result from a complex interplay of bottom‐up and top‐down forces that operate across benthic and pelagic food web compartments. Projected global changes urge the question how this interplay will be affected by browning (increasing input of terrestrial dissolved organic matter), nutrient enrichment and warming. We explored this with a process‐based model of a shallow lake food web consisting of benthic and pelagic components (abiotic resources, primary producers, grazers, carnivores), and compared model expectations with the results of a browning and warming experiment in nutrient‐poor ponds harboring a boreal lake community. Under low nutrient conditions, the model makes three major predictions. (a) Browning reduces light and increases nutrient supply; this decreases benthic and increases pelagic production, gradually shifting productivity from the benthic to the pelagic habitat. (b) Because of active habitat choice, fish exert top‐down control on grazers and benefit primary producers primarily in the more productive of the two habitats. (c) Warming relaxes top‐down control of grazers by fish and decreases primary producer biomass, but effects of warming are generally small compared to effects of browning and nutrient supply. Experimental results were consistent with most model predictions for browning: light penetration, benthic algal production, and zoobenthos biomass decreased, and pelagic nutrients and pelagic algal production increased with browning. Also consistent with expectations, warming had negative effects on benthic and pelagic algal biomass and weak effects on algal production and zoobenthos and zooplankton biomass. Inconsistent with expectations, browning had no effect on zooplankton and warming effects on fish depended on browning. The model is applicable also to nutrient‐rich systems, and we propose that it is a useful tool for the exploration of the consequences of different climate change scenarios for productivity and food web dynamics in shallow lakes, the worldwide most common lake type.     

Benthos as the basis for arctic lake food webs

Plankton have traditionally been viewed as the basis for limnetic food webs, with zooplankton acting as a gateway for energy passing between phytoplanktonic primary producers and fish. Often, benthic production has been considered to be important primarily in shallow systems or as a subsidy to planktonic food web pathways. Stable isotope food web analyses of two arctic lakes (NE14 and I minus) in the Toolik Lake region of Alaska indicate that benthos are the primary source of carbon for adults of all species of benthic and pelagic fish present. We found no effect of turbidity, which may suppress benthic algae by shading, on food web structure. Even though Secchi transparency varied from 10.2 m in NE14 to 0.55–2.6 m in I minus, food webs in both lakes were based upon benthos, had four trophic levels, and culminated with omnivorous lake trout. We suggest that the importance of benthos in the food webs of these lakes is due to their extreme oligotrophy, resulting in planktonic resources that are insufficient for the support of planktivorous consumers.     

Cross-habitat effects shape the ecosystem consequences of co-invasion by a pelagic and a benthic consumer

Invasive species can have major impacts on ecosystems, yet little work has addressed the combined effects of multiple invaders that exploit different habitats. Two common invaders in aquatic systems are pelagic fishes and crayfishes. Pelagic-oriented fish effects are typically strong on the pelagic food web, whereas crayfish effects are strong on the benthic food web. Thus, co-invasion may generate strong ecological responses in both habitats. We tested the effects of co-invasion on experimental pond ecosystems using two widespread invasive species, one pelagic (western mosquitofish) and one benthic (red swamp crayfish). As expected, mosquitofish had strong effects on the pelagic food web, reducing the abundance of Daphnia and causing a strong trophic cascade (increase in phytoplankton). Crayfish had strong effects on the benthic food web, reducing the abundance of benthic filamentous algae. Yet, we also found evidence for important cross-habitat effects. Mosquitofish treatments reduced the biomass of benthic filamentous algae, and crayfish treatments increased Daphnia and phytoplankton abundance. Combined effects of mosquitofish and crayfish were primarily positively or negatively additive, and completely offsetting for some responses, including gross primary production (GPP). Though co-invasion did not affect GPP, it strongly shifted primary production from the benthos into the water column. Effects on snail abundance revealed an interaction; snail abundance decreased only in the presence of both invaders. These results suggest that cross-habitat effects of co-invaders may lead to a variety of ecological outcomes; some of which may be unpredictable based on an understanding of each invader alone.     

Modelling marine ecosystem response to climate change and trawling in the North Sea

The marine ecosystem response to climate change and demersal trawling was investigated using the coupled hydrodynamic-biogeochemical water column model GOTM-ERSEM-BFM for three contrasting sites in the North Sea. Climate change forcing was derived from the HadRM3-PPE-UK regional climate model for the UK for the period 1950–2100 using historical emissions and a medium emissions scenario (SRESA1B). Effects of demersal trawling were implemented as an additional mortality on benthic fauna, and changes in the benthic–pelagic nutrient and carbon fluxes. The main impacts of climate change were (i) a temperature-driven increase in pelagic metabolic rates and nutrient cycling, (ii) an increase in primary production fuelled by recycled nutrients, (iii) a decrease in benthic biomass due to increased benthic metabolic rates and decreased food supply as a result of the increased pelagic cycling, and (iv) a decrease in near-bed oxygen concentrations. The main impacts of trawling were (i) reduced benthic biomass due to the increased mortality, and (ii) the increased benthic–pelagic nutrient fluxes, with these effects counteracting each other, and relatively small changes in other variables. One important consequence was a large decrease in the de-nitrification flux predicted at the two summer-stratified sites because less benthic nitrate was available. The effects of trawling scaled linearly with fishing effort, with greatest sensitivity to fishing in summer compared to fishing in winter. The impacts of climate change and trawling were additive, suggesting little or no non-linear interactions between these disturbances.     

Impact of a low salinity year on the metabolism of a hypersaline coastal lagoon (Brazil)

Araruama Lagoon is a large, continuously hypersaline coastal lagoon (mean salinity 52 g l^-1). However, during certain years the evaporation-precipitation balance is modified and results in lower salinity conditions, e.g. 1989–1990 when the mean salinity measured 41 g l^-1. These atypical conditions are responsible for a different behavior of the system, normally characterized by the presence of extensive microbial algal mats, low pelagic primary production compared to benthic primary production, and low turbidity. However, during this study (1989–1990), the presence of microbial algal mats was restricted to the shallow eastern embayment of the lagoon representing c.a. 8% of the total area, the mean concentration of suspended matter was relatively high (21.2 mg l^-1) when compared with a typical year, and primary production measured was 447 mg C m^-2 d^-1, higher than benthic primary production during hypersaline years. These conditions therefore influence the trophic state of the system, during low salinity conditions, the lagoon was meso-eutrophic.     

Carbon flows in the Westerschelde estuary (The Netherlands) evaluated by means of an ecosystem model (MOSES)

The autotrophic production and heterotrophic consumption of organic matter in the Westerschelde, a highly turbid and eutrophic estuary in the Southwest Netherlands is examined by means of a dynamic simulation model. The model describes the ecologically relevant processes in thirteen spatial compartments and adequately fits most observed data.Three autotrophic processes are included in the model. Net pelagic photosynthetic production is relatively low (average 41 gC m^–2 yr^–1) and three spatial compartments near the turbidity maximum zone are respiratory sinks of phytoplankton biomass. According to the model, net phytobenthic primary production is more important than pelagic primary production in the upstream half of the Westerschelde. On the scale of the entire estuary, benthic primary production amounts to about 60% of pelagic primary production. Water-column nitrification, which is very important in the nitrogen cycle, is most pronounced near the turbidity zone where it accounts for the major autotrophic fixation of carbon (up to 27 g C m^–2 yr^–1). Viewed on the scale of the total estuary, however, the process is not very important.Less than 20% of total organic carbon input to the estuary is primary produced, the remainder is imported from waste discharges and from the river.The degree of heterotrophy of the Westerschelde estuary proved to be one of the highest yet reported. On average 380 g carbon per square metre is net lost per year (range 200–1200 gC m^–2 yr^–1). The yearly community respiration (bacterial mineralization, respiration of higher trophic levels and sedimentation) is 4 to 35 times (estuarine mean of 6) higher than the net production. This degree of heterotrophy is highest near the turbidity maximum and generally decreases from the freshwater to the seaward boundary. About 75% of all carbon losses can be ascribed to pelagic heterotrophic processes; the sediment is only locally important.Mineralisation rates are highest in the turbidity region, but as only a fraction of total carbon resides here, less than 20% of all organic carbon is lost in this part of the estuary. This result is in contradiction with a previous budget of the estuary, based on data of the early seventies, where more than 80% of all carbon was estimated to be lost in the turbidity zone. Part of this discrepancy is probably caused by changes that have occurred in the estuary since that time.Due to the high heterotrophic activity, nearly all imported and in situ produced carbon is lost in the estuary itself and the Westerschelde is an insignificant source of organic matter to the coastal zone.The model estuary acts as a trap for reactive organic matter, both from the land, from the sea or in situ produced. Internal cycling, mainly in the water column, results in the removal of most of the carbon while the more refractory part is exported to the sea.     

Coupled energy pathways and the resilience of size-structured food webs

Size-based food-web models, which focus on body size rather than species identity, capture the generalist and transient feeding interactions in most marine ecosystems and are well-supported by data. Here, we develop a size-based model that incorporates dynamic interactions between marine benthic (detritus-based) and pelagic (primary producer based) pathways to investigate how the coupling of these pathways affects food web stability and resilience. All model configurations produced stable steady-state size spectra. Resilience was measured by the return speed obtained from local stability analysis. Return times following large perturbations away from steady-state were also measured. Resilience varied nonlinearly with both predator and detrital coupling, and high resilience came from predators (1) feeding entirely in the slow benthic zone or (2) feeding across the two energy pathways, with most food coming from the fast pelagic pathway. When most of the energy flowed through the pelagic pathway, resilience was positively related to turnover rate. When most of the energy flowed through the benthic pathway, resilience was negatively related to turnover rate. Analysis of the effects of large perturbations revealed that resilience for pelagic ecosystems depended on the nature of the perturbation and the degree of benthic–pelagic coupling. Areas with very little or no benthic–pelagic coupling (e.g. deep seas or highly stratified water columns) may return more quickly following pulses of detrital fallout or primary production but could be much less resilient to the effects of human-induced mortality (harvesting).     

Viruses in subarctic lakes and their impact on benthic and pelagic bacteria

Virus–bacterium interactions were investigated in the pelagic and benthic habitats in a set of lakes along an altitudinal gradient in the subarctic northern Sweden. Viral and bacterial abundances showed a significant variation between the lakes, with the highest benthic microbial abundances recorded in a high-altitude lake [993 m above sea level (a.s.l.)], whereas the highest pelagic microbial abundances were found in a low-altitude lake (270 m a.s.l.). In the pelagic habitat, there was also a distinct difference in microbial abundances between the summer–autumn and the winter sampling occasion. A positive relationship was noted between viruses and bacteria in both the pelagic and the benthic habitats. Visibly virus-infected bacterial cells were uncommon in the pelagic habitat and undetectable in the benthos. Both lytic and lysogenic pelagic viral production rates were undetectable or low; thus, a possible explanation for the relative high viral abundances found in the water column could be an allochthonous input of viruses or release of sediment-derived viruses. Overall, our results provide novel information about the relevance of viruses in the subarctic region and indicate that viruses play only a minor role in the nutrient and carbon cycling in the microbial communities of subarctic lakes.     

Benthic–pelagic coupling in the population dynamics of the harmful cyanobacterium Microcystis

1. In eutrophic lakes, large amounts of the cyanobacterium Microcystis may overwinter in the sediment and re‐inoculate the water column in spring. 2. We monitored changes in pelagic and benthic populations of Microcystis in Lake Volkerak, The Netherlands. In addition, sedimentation rates and the rate of recruitment from the sediment were measured using traps. These data were used to model the coupling between the benthic and pelagic populations and to calculate the contribution of overwintering benthic and pelagic populations to the magnitude of the pelagic summer bloom. 3. Changes in the benthic Microcystis population showed a time lag of 3–14 weeks compared with the pelagic population. This time lag increased with lake depth. The largest amount of benthic Microcystis was found in the deepest parts of the lake. These observations suggest horizontal transport of sedimented Microcystis from shallow to deep parts of the lake. 4. Recruitment from and sedimentation to the sediment occurred throughout the year, with highest recruitment and sedimentation rates during summer. Model simulations indicate that the absence of benthic recruitment would reduce the summer bloom by 50%. 5. In spring, the total pelagic population was three to six times smaller than the total benthic population. Yet, model simulations predict that the absence of this small overwintering pelagic population would reduce the summer bloom by more than 64%. 6. Reduction of the overwintering pelagic populations, for instance by flushing, may be a useful management strategy to suppress or at least delay summer blooms of Microcystis.     

Macrophyte development in unimpacted lowland rivers in Poland

Three ecologically and morphologically distinct forms of Arctic charr (Salvelinus alpinus L.) have been identified in Loch Rannoch, Scotland, whose evolutionary status and origins are incompletely understood. A study was made of restriction fragment length polymorphism (RFLPs) detected variation in the D-loop, ND1 and cytochrome b regions of the mitochondrial genome, encompassing >3500 bp. Eight RFLP haplotypes were identified that clustered into three distinct clans based on restriction differences and into four clans based on sequence differences. Significant differences in RFLP frequencies were found among all morph groups. The pelagic morph was highly divergent from the two benthic forms, with the benthic forms having variants from only one genetic clan while the pelagic was dominated by a single variant from another clan. The relative divergence observed among benthic and pelagic forms is ~10 fold greater when nucleotide divergence among the haplotypes, as well as haplotype frequency differences, is taken into account. Sequence divergence between haplotypes in the two main clans is of a similar order to that between haplotypes in these clans and a charr from North America. In contrast, divergence among the two benthic morphs relates entirely to differences in haplotype frequencies. The study confirms the genetic distinctiveness of the pelagic and benthic forms as well as of the two benthic forms. It strongly supports previous evidence that the genetic divergence between the pelagic and benthic populations is allopatric in origin. Additionally, the results strongly suggest that the two benthic populations have undergone peripatric divergence through the sequential colonisation of the two basins by one lineage, followed by their spatial separation and reproductive isolation.     

Fish Reliance on Littoral–Benthic Resources and the Distribution of Primary Production in Lakes

Pelagic, littoral, and terrestrial resources can all play a role in supporting consumers in lakes. The role of benthic algal-derived food web pathways in lakes is perhaps the least understood because limnologists have historically focused on pelagic (open-water) production and processes. We compiled carbon stable isotope data from 546 fish populations (75 lakes), and used a two end-member mixing model to calculate littoral–benthic reliance for each fish species in each lake. Fish littoral–benthic reliance values were averaged by lake to assess overall fish species benthic reliance for each lake. Lake-specific mean littoral reliance (BR_L; fish species not weighted according to production or biomass) averaged 57% and was independent of lake morphological and limnological attributes. For these same lakes, water column nutrients, light, and morphometry data were used to estimate whole-lake benthic algal and phytoplankton primary production. On average, benthic algae comprised 36% of whole-lake primary production (BP_f = 0.36). BP_f and BR_L were weakly correlated: BR_L tends to be high even in large/deep lakes in which benthic algae is a minor contributor to whole-lake primary production. The high littoral–benthic contribution to individual fish species appears to reflect the high concentration of fish species diversity in the littoral zone. Our work cannot be extrapolated to whole-lake fish production. However, the result is consistent with other work indicating that most fish species inhabit the littoral zone, whereas relatively few exclusively inhabit the pelagic. Our results suggest that it takes less primary production to support a single fish species in the littoral zone than is required to support a species in the pelagic.     

The mutual relationship between the monitoring and modelling of estuarine ecosystems

A general simulation model developed for the Ems estuary (a part of the Wadden Sea), to describe the main carbon flow through the foodweb, was applied for monitoring purposes. In this model, the estuary is divided into five compartments, in each of which a pelagic, an epibenthic and a benthic submodel operates. A transport model regulates the distribution of dissolved and particulate substances over the area. Two parameters that partly regulate the carbon flow are the concentrations of phosphate and of the particles (turbidity) in the water. From the sensitivity analyses for phosphate and turbidity it can be concluded that the relative effects of changes in turbidity and changes in the influx of phosphate from the rivers are large in the upper reaches and small in the lowest reaches of the estuary. This is due to the increasing influence of the coastal waters in the lower reaches. These results indicate that the reaches, which represent the beginning and the end of a gradient in the estuary, are also very suitable for monitoring the quality status of this estuary in terms of production and standing stock of groups of organisms. The need to use monitored data from the boundaries (sea and rivers) of those ecosystems as input in simulation models is discussed. Moreover, the possibilities of using simulation models to generate new ideas on the functioning of estuarine ecosystems under changing environmental conditions and to help administrators to decide on specific future management strategies are also discussed. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988).