Demographic patterns of quagga mussel invasion into Lake Michigan profundal zone

Although the invasion dynamics and establishment of quagga mussels (Dreissena rostriformis bugensis) into the nearshore areas of north-temperate lakes are well studied, their continued expansion into deep profundal zones is poorly understood, having implications for offshore benthic and pelagic communities. To assess the expansion of quagga mussels into the Lake Michigan profundal, we used U.S. EPA's Great Lakes National Program Office long-term benthic monitoring data (2007–2022) to document changes in density, biomass, average length, and size-frequency distributions within and across three distinct depth zones (> 30–50 m; > 50–90 m; and > 90 m). We found significant differences in mussel density, biomass, and average length across depth zones. While the average dreissenid density at the > 50–90 m and > 90 m depth zones declined throughout the study period, biomass remained unchanged, and the average length of mussels at > 50–90 m depth zone increased, conflicting trends were found at the station level. We found the greatest variation of station size-frequency in the shallowest zone, while deeper zones followed similar demographic patterns. Our results indicate that the management of quagga mussels requires a combined approach that includes demographic data and considers within station variation.