LAUSR

Abstract Spatial, seasonal, and inter-annual variation in zoobenthos was examined in Lake Winnipeg which has experienced accelerated nutrient loading and multiple environmental stressors over the last several decades leading to increased overall productivity. Qualitative zoobenthos data (1928–29) and quantitative data (1969–2013), revealed two prevailing patterns: (1) a substantial increase in density, mainly Oligochaeta (Tubificinae), Mollusca (Sphaeriidae), and Diptera (Chironomidae) in the North basin, but a less consistent pattern of increase in density overall and of Ephemeroptera (Ephemeridae) and Trichoptera (Leptoceridae) in the South basin, and (2) disappearance of Amphipoda ( Diporeia ) from the South basin and a decline in Diporeia as a dominant taxon in the North basin, although it remained prevalent in the Narrows where higher total organic carbon (TOC) concentrations have persisted. Permutational analysis of variance (PERMANOVA) revealed that over time, inter-basin variation overwhelmed the seasonal component of variation in zoobenthos abundance. The benthic environment has changed (e.g. food resources, sediment particle sizes and nutrients) over the decades, contributing to the substantial changes in zoobenthos abundance and community composition that may constitute a regime change in the lake.