Effects of food‐web structure on periphyton stoichiometry in eutrophic lakes: a mesocosm study

Abstract

1. Aquatic herbivores typically have much higher concentrations of nutrients (e.g. N and P) in their tissues than there is in the food they eat. These stoichiometric differences can cause herbivores to be limited by the elemental quality of their food, which could affect, in turn, the structure of consumer communities and even alter key ecosystem processes. 2. In streams and in the littoral zone of shallow lakes, periphyton is an important food resource for benthic animals. Studying the elemental composition of periphyton may help us to understand food-web structure, and any reciprocal effect of this structure on periphyton stoichiometry. 3. To understand how alterations in the food-web structure affect the elemental composition of periphyton in a eutrophic lake, we carried out a long-term experiment (14 months) in large-scale mesocosms (40 m 3 ), in which we manipulated food-web structure, and which were dominated either by planktivorous fish (Rutilus rutilus) or herbivorous invertebrates (without fish). Periphyton was sampled monthly at three depths (0.5, 1.5 and 2.5 m) to determine its biomass and elemental composition (C ⁄ N ⁄ P ratio). Food-web structure, physical and chemical parameters were monitored throughout the experiment. 4. Fish had indirect positive effect on periphyton biomass, leading to twofold higher levels than in herbivore-dominated mesocosms. This result was probably due to control of benthic consumers by fish, suggesting a strong top-down control on periphyton by their consumers in fishless enclosures. 5. The elemental ratios C ⁄ P and C ⁄ N were lower in deep water in both treatments, mainly mediated by light availability, in accordance with the light ⁄ nutrient ratio hypothesis. These ratios were also lower in fishless treatments, probably due to increases in inorganic nutrient availability and grazing pressure in herbivore-dominated systems. During winter, periphyton elemental composition was similar in both treatments, and was unrelated to inorganic nutrient availability. 6. These results indicate that any alteration of food-web structure in lakes, such as in biomanipulation experiments, is likely to modify both the biomass and elemental quality of periphyton. Resultant effects on the consumers of periphyton and macrophytes could play a key role in the success of biomanipulations and should be taken into account in further studies.

Abundance and Diversity of the Phototrophic Microbial Mat Communities of Sulphur Mountain Banff Springs and Their Significance to the Endangered Snail, <i>Physella johnsoni</i>

“…Chl a was the main photosynthetic pigment (absorption peak maxima at 661 -664 nm) throughout all seasons for Sites 3,6,8,11,12 and 1 Temperature taken with an alcohol thermometer to the nearest half degree, 2 Relative percent development: the seasonally observed expanse of the bacterial mat expressed as a percentage of the maximum development at that site as witnessed in August 2003. 3 Maximum absorption peaks of sonicated whole-community samples expressed in nm.…”

“…Conversely, opposite population trends may be expected if grazer consumption of Chl-and Bchl-containing organisms improved the competitive capabilities of other bacteria. Grazer impact on periphyton biomass and composition has been well-documented [6] [62] [63]. If such an impact occurred at Banff, it may be due to increased access to detrital or water column-based nutrients and snail mucous trails as carbon sources and fecal matter as phosphorous reservoirs [64], benefitting anoxygenic phototrophs, which are primarily heterotrophic.…”

“…The importance of phototrophic microbes as primary producers of fixed carbon in aquatic environments has been demonstrated [6] [7], as dense metabolically diverse mats can attain production rates comparable to those of tropical rain forests [8]. Excreted photosynthates, extracellular polymeric substances and cell lysates [9]- [11] are then available to support anabolism of heterotrophs.…”

Des réseaux trophiques au fonctionnement des écosystèmes lacustres : vers une intégration de l’hétérogénéité et de la complexité

“…Les contraintes stoechiométriques sont susceptibles de modifier la nature des interactions entre compartiments biologiques, comme l'a démontré le passage d'une compétition nette à un mutualisme net entre les producteurs primaires et les bactéries, le long d'un gradient d'apport en phosphore modifiant l'équilibre des éléments nutritifs disponibles (DANGER et al, 2007b). L'intégration de la théorie stoechiométrique à l'échelle de systèmes plus complexes intégrant par exemple des producteurs primaires, des décomposeurs et des consommateurs est en cours sur le plan théorique (CHERIF et LOREAU, 2007), alors que des travaux expérimentaux ont déjà été réalisés à l'échelle de systèmes écologiques complexes (DANGER et al, 2008;ELSER et al, 2000b). Les travaux relatifs à la théorie stoechiométrique réalisés à ce jour ont démontré l'intérêt de ces approches dans la compréhension de nombreux processus écologiques.…”

“…La plupart des travaux ayant eu trait à des tests d'hypothèses dans ce domaine ont donc été conduits dans des environnements aquatiques pélagiques (DANGER et al, 2007a(DANGER et al, , 2007bELSER et al, 1988;STERNER, 1990). Ces études tendent désormais à se généraliser à d'autres milieux, tels que les systèmes littoraux et benthiques (CROSS et al, 2005;DANGER et al, 2008;FROST et ELSER, 2002), et progressivement les écosystèmes terrestres (ELSER et al, 2000a). Il est probable que la stoechiométrie écologique jouera un rôle déterminant dans les prochaines années pour mieux comprendre les convergences et les contrastes entre les systèmes terrestres et les systèmes aquatiques.…”