Periphyton is an association of autotrophs and saprophytes that is the primary source of energy and elements for many aquatic food webs. Periphyton-mat structure affects consumer access to food elements, also limiting resource transfer from basal to consumer portions of the food web. Few studies evaluate the relationship between environmental gradients and periphyton nutritional quality in wetlands, limiting our capacity to link it to population, and ultimately ecosystem, function. The aim of this work was to determine the impact of increasing water velocity on resource quality and resource-consumer relationships in the Everglades. We hypothesized that increased velocity would increase the efficiency of P update by biofilms, affecting their stoichiometry. We created a food-web fragment within enclosures established at three locations experimentally differing in water velocity (0 cm/s, 3-4 cm/s, 5-7 cm/s). The food web consisted of primary producers (periphyton mats and biofilm), a snail grazer (Planorbis rubrum), an omnivorous fish (Gambusia holbrooki), and a carnivorous fish (Enneacanthus gloriosus). The resource-consumer relationship across locations was evaluated using nutrient and fatty acid (FA) profiles, tools that relate the dietary sources of energy from basal resources to consumers.
Results/Conclusions
We found that phosphorous (P) concentrations increased in periphyton mats and in biofilm with increasing water velocity, which led to significant decreases in the C:P and N:P ratios. Differences in P enrichment did not affect the nutrient ratios in G. holbrooki held in the enclosures. In contrast, N:P and C:P ratios in E. gloriosus were significantly lower at the site with low water flow. The proportion of essential PUFAs in biofilm was also affected by variation in water flow. Essential FA in biofilm was significantly lower at mid flow velocity while being similar at the two other locations. In periphyton mats, on the other hand, essential FAs were less abundant under low and mid velocities. The essential FA found in fish followed the same pattern as the one observed in the biofilm. Our results indicate that the quality of the food source both in terms of nutrients and FA can be altered by local hydrological conditions and that such alteration can be reflected in consumers.