Phytoplankton

Phytoplankton are microscopic algae that are suspended in the water column, and include diatoms and a number of non-diatom algal taxa. Assessment of rarefied alpha diversity within ecoregions indicated that phytoplankton diversity was highest in Fennoscandia and lowest in Russia and the Canadian High Arctic. Beta diversity was high in a number of ecoregions in Alaska, Russia, Fennoscandia, and southern Canada. Ecoregions in these areas showed the highest differentiation in phytoplankton assemblages and large among-lake differences in water body types (e.g., size/depth and water quality). Low and high Arctic lakes generally had higher beta diversity than sub-Arctic lakes. Turnover was the predominant component of beta diversity in all ecoregions, which is indicative of the introduction of new species across stations. This result suggests that spatially extensive monitoring of lake phytoplankton is required to provide reliable estimates of species turnover and biodiversity.

Cyanobacteria, which often include toxin-producing species, did not show long-term unidirectional trends in biovolume. However, there were similar peaks in Cyanobacteria biovolume across a number of lakes during years with high temperatures, with two-thirds of the Cyanobacteria peaks happening during one of the 10 hottest years on record. Since rising temperature and decreased ice cover potentially enhance cyanobacterial dominance (Paerl and Huisman 2008), continued monitoring of cyanobacteria in all Arctic regions may be useful in tracking associated climate and nutrient changes in Arctic water bodies. Longterm monitoring data for the full phytoplankton assemblage indicated a decrease in total biovolume in a highly productive lake in Greenland, while conversely, biovolume in a number of low productivity lakes in Finland and Sweden increased. If these trends continue into the future, phytoplankton biovolume will be expected to be more similar across these Arctic lakes.

Phytoplankton are not regularly monitored in all Arctic countries, therefore, data are patchy both in spatial and temporal coverage. The most extensive monitoring occurs in Fennoscandia and Greenland. In contrast, very little sampling occurs in the high Arctic and there is a need for increased monitoring across North America, Russia, and other northern areas of the Arctic. Future monitoring efforts for lake phytoplankton must improve consistency in sample processing methods, particularly with respect to the estimation of biovolume, and improve taxonomic resolution to the species-level where possible.