The stability of cladoceran communities in sub-arctic NW Finnish Lapland lakes

It is difficult to plan restoration projects or study the amount of disturbance in aquatic ecosystems if background conditions are not known. Zooplankton, especially cladocerans (water fleas), has proven highly useful as a reliable indicator of environmental change. Cladocerans preserve well in sediments and thus allow for the analysis of historical communities. To assess the stability of cladoceran communities in lakes with low human impact, we compared pre-industrial and modern cladoceran assemblages (top-bottom analysis) in 32 sub-Arctic lakes in NW Finnish Lapland. We used a dataset of measured environmental variables to determine their explanatory power on cladoceran assemblages. While cladoceran assemblages at the community level have remained relatively stable between the pre-industrial and modern samples, a clear change at the genus level was observed with a significant proportional increase in Bosmina (Eubosmina) spp. (Wilcoxon signed-rank test z = 2.75 p = 0.006). The amount of organic matter in the sediment [measured as loss on ignition (LOI)] explained the largest proportion of the variation in the cladoceran community. Since LOI is strongly correlated to climatic factors, the increased abundance of B. (Eubosmina) spp. may ultimately be related to climate warming. As the top-bottom approach is comprised of two temporal snapshots, it cannot provide the exact time of community change. This shortcoming is of special importance for restoration and management planning.