Opening the black box of winter: Full-year dynamics of crustacean zooplankton along a nearshore depth gradient in a large lake

Past studies of zooplankton seasonality in large temperate lakes have often neglected the winter period. Winter conditions are rapidly changing (e.g., reduced ice cover extent and duration, altered thermal and mixing regimes) in northern lakes, making it important to fill the existing winter knowledge gap. In this study, we sampled five stations in Lake Superior across a nearshore depth gradient through the full year to assess the phenology of crustacean zooplankton communities and the effect of environmental drivers on them. Across stations, zooplankton densities were the lowest in winter (0.9 +/- 0.6 Ind. L-1) and highest in summer (14.2 +/- 15.1 Ind. L-1). Zooplankton abundances and community composition were less seasonally variable at deeper stations compared to shallower and more terrestrially affected regions. Cladocerans were the dominant taxonomic group during the summer across all stations, while cyclopoid and calanoid copepods were more important during the fall, winter, and spring. Among feeding groups, herbivores were most abundant in summer while omnivores and carnivores dominated in winter. We found that water temperature and food availability were the main drivers of total zooplankton densities through the year and during the cold seasons, but the effect of these factors varied among the main taxonomic groups. Our study demonstrates seasonal and spatial variation in crustacean zooplankton and environmental parameters, with the highest fluctuation at shallower stations. This study offers new information on seasonal crustacean zooplankton dynamics and contributes to understanding the effects of climate change on large lake ecosystems.

Automated summary

This study examines the seasonal variation of crustacean zooplankton communities and the impact of environmental factors in Lake Superior. Zooplankton density was lowest in winter and highest in summer. Variation in zooplankton abundances and community composition was smaller at deeper stations compared to shallower and more terrestrially affected regions. Water temperature and food availability were identified as the main drivers of total zooplankton densities throughout the year and during the cold seasons.