Spatial Variability in Recent Phototroph Production in a Large, Fluvial Lake Driven by Local Land‐Use Changes (St. Lawrence River, Canada)

Increases in lake production and the abundance of cyanobacteria have been reported in many temperate freshwater systems, with their rise typically attributed to elevated nutrient influx or climate change. On the St. Lawrence River (SLR), recent evidence suggests that cyanobacterial abundances have increased since ca. 2000, although the precise mechanisms causing this increase are unclear. This study aims to evaluate the importance of local nutrient inputs and climate as factors contributing to changes in lake production in Lake St. Francis, a large fluvial lake on the SLR. Eight sediment cores from similar water depths were collected and contrasted from five depositional basins with varying nutrient influxes, ranging from intensive agriculture to minimal land‐use disturbance. Cores were radiometrically dated and analysed for sedimentary geochemistry and photosynthetic pigment concentrations to quantify changes in algal and cyanobacterial production and composition. Broad‐scale changes in δ 13 C and carbonate content reflected known periods of hydrological change on the SLR over the past ~85 years. Most algal and cyanobacterial pigments from cores taken from nearshore basins showed significantly increasing trends suggesting the importance of local land‐use effects, whereas cores from the mid‐channel basin showed generally non‐significant trends that were variable or decreasing, perhaps reflecting recovery of upstream Lake Ontario from prior eutrophication. Similarly, increases in δ 15 N were two‐fold greater in the nearshore regions, consistent with enhanced nutrient inputs from local tributaries. Differences in biogeochemical trends across sediment cores were more influenced by local nutrient effects than climate‐change impacts, particularly in regions proximal to agricultural catchments. This study confirms the importance of local land‐use practices to the degradation of water quality in a large, complex fluvial ecosystem and highlights the role of paleolimnology in evaluating historically understudied lotic systems.