Algae from Benthic Samples

Algae are key primary producers in Arctic freshwaters, and benthic samples include diatoms and a number of classes of other algal groups. This assessment focused on diatoms, as this is a major group in Arctic freshwaters and data availability was high. Lake diatom stations were the most evenly distributed across the circumpolar region of all the FECs, although coverage was patchy in Russia and lacking in the High Arctic of Greenland or Svalbard. The highest alpha diversity for lake sediments was found at low- to mid-level latitudes and in coastal ecoregions, including coastal Alaska, the Arctic archipelago and southern coast of Hudson’s Bay in Canada, Iceland, and Norway. Beta diversity indicated that there was generally moderate to high dissimilarity in community structure among lake stations. Lake beta diversity was dominated by the turnover component in all ecoregions indicating that there was a high degree of species replacement across stations. The highest alpha diversity of river diatoms was in coastal Alaska and western Canada, and high diversity was also evident in Fennoscandian ecoregions. The lowest alpha diversity was found in eastern and southern Canadian ecoregions, which had on average half as many diatom taxa as in the most diverse ecoregions. Beta diversity within an ecoregion was highly variable for river diatoms, but turnover was the predominant component of beta diversity for river diatoms.

Samples with the highest diatom richness for both lakes and rivers were generally between 60-75°N latitude. However, the decline in richness outside this latitudinal range was small, and partly due to the fact that fewer samples were collected at the highest latitudes (above 75°N), particularly in rivers. Diversity was lower in the high Arctic than in the sub- or low Arctic, particularly for lakes, and analysis identified groups of taxa in both lakes and rivers that were characteristic of high latitude samples. Diatom taxa that were dominant across the circumpolar region are generally also common to other regions of the world. This is consistent with the observation that although temperature may affect diatom diversity, the distribution of species is also driven by local geology and water chemistry conditions. Many of the taxa found across the Arctic are typical of waters with low nutrient levels and neutral pH, although indicators for nutrient-rich conditions were also found. Assessment of paleolimnological data indicated that temporal change in diatom assemblage composition was lowest in the eastern Canadian Arctic, which has historically been subjected to less warming than other areas of the Arctic. Shifts in dominant taxa over time were indicative of strong community changes, likely due to changes in the thermal stratification regimes of lakes since circa 1800. 

Lake diatoms are so far not generally included as part of routine monitoring programs, and thus assessment must rely on academic data. Although time series for these data are largely absent, the advantage of diatom samples in lakes is that long-term changes can be inferred from diatoms stored in sediment cores. However, the collection of cores should be expanded to a broader spatial area across the Arctic to facilitate broad-scale assessment of long-term trends for the circumpolar region. River samples were more sparse than lake samples, and were lacking from Russia, Iceland, Greenland, Svalbard, and central and western Canada. Although river algae monitoring is done routinely in some Arctic countries (e.g., Norway, Sweden, Finland), it is limited elsewhere in the circumpolar region. Furthermore, even in countries where monitoring occurs, the samples may not always be comparable if they focus on soft algae (nondiatoms, e.g., in Norway) or do not follow comparable sample processing procedures. Thus, there is a clear need to increase the spatial scope of river diatom monitoring in order to better capture biodiversity of this important group across the circumpolar region.