Effects of hypoxia on lake food webs

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Hypoxia (low oxygen concentration) in bottom waters, which is caused by increased inputs of nutrients and organic matter, as well as potentially by climate change, is now being recognized as a global threat to the ecological integrity of coastal areas and lakes. Hypoxia in lakes can lead to losses of biodiversity and productivity through the whole food web. Identified gaps in our knowledge concerning this ecological forcing factor include a need to improve its monitoring and quantification of its extent, duration, and food-web impacts. Using recent developments in food web theory and a biogeochemical approach based on stable isotopes of sulphur, nitrogen, carbon, and oxygen, I propose to address those gaps in the present research program. I will be testing the hypothesis that a key feature of the effect of hypoxia on food web structure is the weakening of the linkage between consumers and detritus. Sulphur stable isotopes will play a central role in this project as we have shown in previous work that it can be used to quantify the trophic linkage between fish and sedimentary detritus. Hypoxia-driven perturbations in the quantity and seasonality of food sources, as well as direct exposure to low oxygen concentration conditions, are also likely to affect fish growth and reproduction. These effects, together with the sulphur isotope signature of fish, may constitute powerful indicators of this syndrome. Limited laboratory studies suggest that hypoxia may also play an important role as an endocrine disruptor in fish. Using a multi-lake approach along a gradient of hypoxia, I will examine, with collaborators, the relationship between trophic positioning in the food web and variation in seasonal growth (RNA:DNA ratios) and reproductive physiology (e.g. vitellogenin, testosterone) in brook char, an economically important and sensitive salmonid species. This proposed research program will therefore contribute to improve our ability to predict the simplification of ecosystems brought by hypoxia, a perturbation that itself may be a prelude to extinction of species.