Zooplankton

Zooplankton are microscopic invertebrates that live suspended in the water column and provide an important food source for fish in lakes. Zooplankton include crustacean taxa and rotifers, the latter of which are often not identified in samples. Crustacean zooplankton showed the highest alpha diversity for lakes in northern Russia, Fennoscandia, and Alaska. A limited set of stations with rotifer information indicated that rotifers added a small to moderate number of taxa to regional zooplankton diversity. Assessment of the full zooplankton assemblage provided evidence of high alpha diversity in coastal regions, particularly in Fennoscandia, Russia, and Alaska. This pattern is consistent with predictions that high richness would be found in areas that were unaffected by recent glaciation (e.g., Alaska) and in coastal areas (Rautio et al. 2008, Samchyshyna et al. 2008).

Beta diversity of zooplankton (crustaceans and rotifers) varied, with some ecoregions in Alaska, Russia, and Fennoscandia indicating high assemblage differences among lakes, and other ecoregions in the high Arctic or where few lakes were sampled indicating low differences in species composition among lakes. These findings highlight the importance of monitoring zooplankton in a wide variety of lakes within an ecoregion, to ensure the full diversity in an ecoregion is captured. Diversity was generally dominated by species turnover in ecoregions where more lakes were sampled over a wider spatial extent. Consequently, widespread sampling would be necessary to accurately summarize the full diversity of species in an area and ensure differences among lakes were captured.

The most diverse groups in the zooplankton dataset were the calanoid copepods, cyclopoid copepods, cladocerans, and rotifers. Common species of rotifers and crustaceans are also common and abundant outside the Arctic. Cladocerans were numerically dominant in sub-Arctic lakes (approximately 50% of all specimens), however, this group decreased in the presence of cyclopoid copepods in the low Arctic and high Arctic. The relative abundance of calanoid copepods was similar between the sub-Arctic and low Arctic, and declined in the high Arctic zone. Ongoing climate change may provide opportunities for the spread of Eurasian species, such as Bythotrephes longimanus and Limnosida frontosa, to the North American continent and lead to potential shifts in biodiversity and food web structure.

Greenland and Norway are the primary regions with routine monitoring at established stations for zooplankton, whereas data from other regions often come from environmental impact studies (e.g., Canada) rather than long-term programs intended to evaluate natural variation or monitor for effects of climate change. The lack of data in some European countries may be due to the fact that zooplankton are not considered an “ecological quality element” according to the European Water Framework Directive and thus have lower priority in monitoring. The necessary reliance on data from academia, industry, or other non-governmental organizations means that there are few time series, and in some areas, limited sampling of the full zooplankton assemblage (e.g., areas with research focused on Crustacea or just on cladocerans or copepods). Future monitoring efforts should be based on a set of permanent monitoring sites covering all climatic regions in each country, with an aim to standardize collection methods and the habitats sampled.