Seasonal community and food web dynamics of planktonic and benthic organisms in temperate lakes, with emphasis on winter

Abstract

Lakes of temperate regions are experiencing shortening of the winter period and reductions in the ice-cover duration. Despite these changes, winter ecological and biological processes are still not well understood due to the relatively small number of studies occurring during the ice-on period. However, recent studies showed that the winter season can play an important role for lower trophic level organisms, like zooplankton and benthic communities, in terms of their reproduction, succession, and food availability. With the ongoing changes in winter conditions, lower trophic level organisms could experience changes in abundances and nutritional qualities, which will consequently affect higher trophic level consumers like fish. Further, such disturbances could affect whole food web energy transfer and trophic level interactions of the lake ecosystem. Therefore, lake studies including winter could help to better understand the complete picture of intra-annual lake ecosystem processes. To better understand seasonal changes in lower trophic level organisms' population, community, and trophic dynamics during winter, I conducted research across lakes of different size classes and trophic states with a focus on the wintertime. The objectives of my research were: a) to assess seasonal variations in seston, zooplankton and benthic organisms abundances and environmental drivers affecting them; b) to determine seasonal changes in food sources and trophic positions of planktonic and benthic organisms using carbon and nitrogen stable isotopes; and c) to evaluate nutritional status of the lower-level organisms using fatty acid analysis.Here, I present the results of two studies distributed among four chapters. In the first study, I compared summer and winter environmental parameters and zooplankton communities across 13 lakes in Minnesota and Wisconsin. In the second study, I described the full-year seasonal changes in abundance and nutritional quality of seston, zooplankton, and benthos of Lake Superior, with emphasis on winter. In Chapter 1, I investigated parameters associated with changes in crustacean zooplankton densities, community composition, and their food sources between winter and summer across lakes of varying trophic status. I found that eutrophic lakes had higher zooplankton densities under the ice than dystrophic and oligotrophic lakes. Zooplankton communities were more similar across lakes during winter than during the open water period. Carbon and nitrogen stable isotopes suggested that zooplankton have higher lipid content in winter. In Chapter 2, I conducted a comprehensive full-year study of crustacean zooplankton and the effect of environmental parameters shaping zooplankton communities throughout the year in five nearshore regions of Lake Superior. My results suggested that zooplankton are still active during winter and vary greatly in densities between seasons, with greater variability at shallow than deep stations. I also found that water temperature and food availability were key drivers of total zooplankton abundance and the densities of the main taxonomic groupsthroughout the year. In Chapter 3, I provide the results of a survey study of coarse-level benthic organism seasonal changes across five locations of Lake Superior. This study showed relatively stable benthos abundances across all sampled locations, suggesting a continuous availability of littoral food sources for benthivorous fish throughout the year, including winter. I also found that the benthos diversity index was the highest in fall, while the species richness was similar across all seasons. In Chapter 4, I used stable isotope and fatty acid analysis to assess the origin of energy sources and nutritional values throughout the year in planktonic and benthic organisms. This information was intended to better assess and understand food availability for higher trophic level organisms. My results indicate low zooplankton abundances during winter but higher lipid content of total and essential fatty acids (EFA). Conversely, selected benthic organisms had higher total lipid content in summer than in winter. However, I found that concentrations of EFA (e.g., DHA, EPA, -ALA) in benthic organisms were low despite the high benthos densities throughout the year. This study suggests that different elements of the lower-level food web may vary in their importance as energy and nutrition sources for higher-level organisms like fish throughout the year. Together these four chapters provide important information on the seasonal dynamics of lower trophic level organisms across small and large lake ecosystems. These results help to address the winter ecology knowledge gap in seasonally frozen lakes and contribute to a better understanding of the effect of changing environmental conditions on lower trophic level organisms across a broad range of lake ecosystems. Additionally, the results of these studies provide new insights into the seasonality of freshwater organisms of the lower food web and their importance for higher-level consumers like fish. The detailed information provided in this research on temporal and spatial distribution along with nutritional conditions of seston, zooplankton, and benthic organisms can have important implications for fish productivity and distribution research and management. This study helps to identify future research efforts to charachterize the effect of climate change on food web dynamics in northern temperate lakes.