The relationship between species diversity and ecosystem functioning is a central topic in ecology today. Classical approaches to studying ecosystem responses to nutrient enrichment have considered linear food chains. To what extent ecosystem structure, that is, the network of species interactions, affects such responses is currently unknown. This severely limits our ability to predict which species or functional groups will benefit or suffer from nutrient enrichment and to understand the underlying mechanisms. Here our approach takes ecosystem complexity into account by considering functional diversity at each trophic level. We conducted a mesocosm experiment to test the effects of nutrient enrichment in a lake ecosystem. We developed a model of intermediate complexity, which separates trophic levels into functional groups according to size and diet. This model successfully predicted the experimental results, whereas linear food-chain models did not. Our model shows the importance of functional diversity and indirect interactions in the response of ecosystems to perturbations, and indicates that new approaches are needed for the management of freshwater ecosystems subject to eutrophication.

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To assess the effects of physical dimension and planktivorous fish on phytoplankton standing crop, we repeated an experiment at different scales in plastic enclosures during summer 1995 in Lake Créteil, France. Enclosures were scaled for a constant surface (1.5 × 1.5 m) as depth was increased from 2.5 to 4.5 m. Even-link (zooplankton and phytoplankton) and odd-link (planktivorous fish, zooplankton and phytoplankton) food webs were established in both shallow and deep enclosures. Fish densities in the deep enclosures were scaled to allow comparisons with shallow ones for both in individuals m or individuals m. We explicitly designed this experiment to examine the scale-dependent behavior of the top-down mechanism of algal biomass control in lakes, and in particular to test the hypothesis of stronger cascading effects of fish on lower trophic levels at reduced depth. Both fish and enclosure size had highly significant effects on phytoplankton biomass over the duration of the experiment. No depth × fish interaction effects were observed. The presence of planktivorous fish enhanced phytoplankton biomass in both shallow and deep enclosures, although the reduction in depth generally produced a stronger effect. The mean concentration of chlorophyll a in the deep odd-link systems (ca 5 mg m) was lower than in the shallow even-link systems (ca 17 mg m). Statistical interpretation did not change when data were expressed as phytoplankton biomass per unit of surface area. Light limitation and zooplankton grazing are the most probable mechanisms explaining our results in these nutrient-enriched systems. Moreover, we found that the strength of the cascading effect of fish on plankton was not a function of depth. We believe that further studies on scaling effects should be conducted in order to improve our understanding of ecological patterns and to extrapolate results from micro/mesocosms to natural ecosystems.

Empirical models were established, which link the biomasses or productions of the successive planktonic trophic levels in lakes of different depth and trophic status. Published data on 56 lakes were analysed, using both least-square and reduced-major-axis regressions. Reduced-major-axes regression technique was used in order to take into account variability due to both bottom-up and top-down forces. In shallow lakes, the energy transfer efficiency between phytoplankton and zooplankton decreases with increasing trophy. An inverse tendency appears in deep lakes. However, when considering only the oligo-and mesotrophic temperate lakes, the mean primary and secondary productivities per unit of area are about the same in shallow and in deep sites. On the other hand, shallow water bodies are characterized by a smaller mean size of the crustacean species, a lower mean biomass of zooplankton and a higher P zoo /B zoo ratio than deep lakes, probably as a response to the prédation by fish. While the consumers seem to have a strong influence on the size structure and biomass of plankton, the energy flows between the trophic levels could be largely controlled by bottom-up forces. Biomasse et production du plancton dans les lacs profonds et peu profonds : existe-t-il des règles générales ?Mots-clés : plancton, biomasse, production, efficience, interactions trophiques, lacs, profondeur.Des modèles empiriques ont été établis afin d'analyser les relations entre les biomasses et les productions de niveaux trophiques successifs au sein de communautés planctoniques lacustres. Des données de la littérature, concernant 56 de lacs de profondeurs et de statuts trophiques variés, ont été collectées et analysées à l'aide de modèles de régression linéaires établis selon les techniques des moindres carrés et de l'axe majeur réduit. Cette dernière technique a été utilisée afin de tenir compte simultané-ment des forces de contrôle ascendantes et descendantes au sein des réseaux trophiques. Dans les lacs peu profonds, l'efficacité des transferts énergétiques entre le phytoplancton et le zooplancton diminue avec le statut trophique des lacs. Une tendance inverse caractérise les lacs profonds. Cependant, les transferts énergétiques semblent s'effectuer avec une efficacité similaire lorsqu'on ne prend pas en compte les lacs ultra-oligotrophes. Par ailleurs, les lacs peu profonds sont caractérisés par des microcrustacés planctoniques de plus petite taille, une plus faible biomasse zooplanctonique et un rapport P^o/B^ inférieur par rapport à ce que l'on observe dans les lacs profonds, probablement du fait d'une plus forte pression de prédation par les poissons. Alors que les prédateurs semblent jouer un rôle important dans le contrôle de la structure en taille et de la biomasse du plancton, les flux d'énergie entre niveaux trophiques successifs semblent essentiellement contrôlés par les ressources.