Stability of midge assemblages in productive shallow lakes exposed to point and diffuse nutrient inputs

Human activities in the headwaters of the Carleton River Watershed (southwest Nova Scotia, Canada) are suspected to have led to nutrient enrichment of freshwaters, resulting in downstream effects. However, the presence of multiple nutrient sources in the headwaters, including mink fur farming and land-based aquaculture, have made it difficult to distinguish the dominant stressor(s). We used sedimentary chironomid and chaoborid remains and visible reflectance spectroscopy of sedimentary chlorophylla (VRS chl-a) to assess the timing and nature of limnological changes in two shallow eutrophic-hypereutrophic lakes that have been directly monitored for water quality since 2008. The catchment of eutrophic Hourglass Lake contains one mink farm and an aquaculture operation, and the outlet of Hourglass Lake flows into hypereutrophic Placides Lake through a watershed with several mink fur farms. Midge assemblages at Hourglass Lake showed a compositional shift (ANOSIM: 0.65, P < 0.001) coincident with the start of aquaculture in similar to 1990. However, the Chaoborus:chironomid (chaob:chir) ratio and midgeinferred volume-weighted hypolimnetic oxygen (VWHO) did not reflect decreased concentrations of dissolved oxygen. Midge assemblages at hypereutrophic Placides Lake showed no significant compositional shift and the chaob:chir ratio and inferred of dissolved oxygen were stable over the similar to 80-year record. It is likely that hypolimnetic oxygen concentrations have not decreased markedly in these productive lakes. Trends in VRS chl-a were also relatively stable at Placides Lake despite decades of nutrient inputs from the upstream watershed. High water colouration of these eutrophic-hypereutrophic shallow lakes may have increased their resilience to nutrient inputs from the catchment. Moreover, our study confirms that midge assemblage composition does not respond directly to phosphorus inputs in these shallow, stratified lakes. We highlight the stability of midge assemblages to decades of nutrient inputs from a land-based aquaculture operation and mink fur farms.

Automated summary

Human activities in the headwaters of the Carleton River Watershed may have led to nutrient enrichment in the freshwater, but the impact on oxygen concentrations in lakes is not significant. Midge assemblage compositions in these lakes are stable and do not directly respond to nutrient inputs.