Zooplankton is crucial for lake ecosystems as it is an important component in most of the food webs of these ecosystems. However, environmental changes have caused reductions in ecosystems nutrients and stoichiometry, which directly affects phytoplankton’s development and quality. Studies were carried out in 33 different lakes throughout Sweden, divided into 4 regions (Abisko, Jämtland, Västerbotten and Värmland) to assess whether differences in environmental characteristics induced differences in phytoplankton abundance and zooplankton communities. Using data from 3 different seasons of the year, I found that zooplankton composition differed between northern and southern regions, and greater differences were found between the mountain regions than between the boreal regions. Dissolved inorganic N (DIN) and dissolved inorganic N to total P ratio (DIN:TP) concentrations were lower in northern regions than in southern regions. Phytoplankton biomass increased with dissolved organic carbon (DOC), likely through the positive effects of DOC on overall nutrient availability. DOC concentrations were higher in boreal regions than in mountain regions, as also happens with chlorophyll-a. Positive correlations between DOC and 3 different zooplankton genera were proven, whereas 3 different zooplankton taxa were correlated with DIN:TP (two negatively and one positively). Lakes with lower DIN:TP ratios had higher abundances of calanoids, which were the major contributor of the dissimilarity in zooplankton composition among the regions. Therefore, the DIN:TP ratio possibly has stronger effects than DOC on zooplankton composition in Swedish oligotrophic lakes. But further increases in DOC concentrations will likely reduce the differences in zooplankton composition caused by the declines in lake DIN:TP observed in this study.

Zooplankton is crucial for lake ecosystems as it is an important component in most of the food webs of these ecosystems. However, environmental changes have caused reductions in ecosystems nutrients and stoichiometry, which directly affects phytoplankton’s development and quality. Studies were carried out in 33 different lakes throughout Sweden, divided into 4 regions (Abisko, Jämtland, Västerbotten and Värmland) to assess whether differences in environmental characteristics induced differences in phytoplankton abundance and zooplankton communities. Using data from 3 different seasons of the year, I found that zooplankton composition differed between northern and southern regions, and greater differences were found between the mountain regions than between the boreal regions. Dissolved inorganic N (DIN) and dissolved inorganic N to total P ratio (DIN:TP) concentrations were lower in northern regions than in southern regions. Phytoplankton biomass increased with dissolved organic carbon (DOC), likely through the positive effects of DOC on overall nutrient availability. DOC concentrations were higher in boreal regions than in mountain regions, as also happens with chlorophyll-a. Positive correlations between DOC and 3 different zooplankton genera were proven, whereas 3 different zooplankton taxa were correlated with DIN:TP (two negatively and one positively). Lakes with lower DIN:TP ratios had higher abundances of calanoids, which were the major contributor of the dissimilarity in zooplankton composition among the regions. Therefore, the DIN:TP ratio possibly has stronger effects than DOC on zooplankton composition in Swedish oligotrophic lakes. But further increases in DOC concentrations will likely reduce the differences in zooplankton composition caused by the declines in lake DIN:TP observed in this study.