Hypolimnetic anoxia in oligotrophic lakes and its effects on nutrient supply and phytoplankton communities

Low concentrations of dissolved oxygen (DO) in hypolimnetic waters have been reported to increase nutrient supplies available to phytoplankton communities in meso- and eutrophic lakes. We examined the connections between hypolimnetic DO, nutrient concentrations and phytoplankton communities in 26 oligotrophic lakes in the Kawartha Highland region of south-central Ontario, Canada. Lakes were divided into 2 categories based on their mid-summer hypolimnetic DO concentration: oxic, >2.0 mg/L and anoxic, <2.0 mg/L. Despite differing in DO concentration, nutrient concentrations in surface or deep waters did not strongly differ among lake categories. Furthermore, we found little evidence that low hypolimnetic DO was linked to differences in phytoplankton communities either deep in the water column or in surface waters of these lakes. Our results show that low hypolimnetic DO, by itself, is insufficient to alter nutrient concentrations in hypolimnions and affect algal communities in oligotrophic lake ecosystems.