Phytoplankton ecology of subarctic lakes in Finnish Lapland

The climate is warming and it is most noticeable in the arctic and subarctic areas, where the warming trend is expected to be the greatest. Arctic and subarctic freshwater ecosystems, which are a very characteristic feature of the northern landscape, are especially sensitive to climate change. They could be used as early warning systems, but more information about the ecosystem functioning and responses are needed for proper interpretation of the observations. Phytoplankton species and assemblages could be especially suitable for climate-related studies, since they have short generation times and react rapidly to changes in the environment. In addition, phytoplankton provides a good tool for lake classifications, since different species have different requirements and tolerance ranges for various environmental factors. The use of biological indicators is particularly useful in arctic and subarctic areas, where many of the chemical factors commonly fall under the detection limit and therefore do not provide much information about the environment. This work brings new information about species distribution and dynamics of subarctic freshwater phytoplankton in relation to environmental factors. The phytoplankton of lakes in Finnish Lapland and other European high-altitude or high-latitude areas were compared. Most lakes were oligotrophic and dominated by flagellated species belonging to chrysophytes, cryptophytes and dinoflagellates. In Finnish Lapland cryptophytes were of less importance, whereas desmids had high species richness in many of the lakes. On a Pan-European scale, geographical and catchment-related factors explained most of the differences in the species distributions between different districts, whereas lake water chemistry (especially conductivity, SiO2 and pH) was most important regionally. Seasonal and interannual variation of phytoplankton was studied in the subarctic Lake Saanajärvi. Characteristic phytoplankton species in this oligotrophic, dimictic lake belonged mainly to chrysophytes and diatoms. The maximum phytoplankton biomass in Lake Saanajärvi occurs during the autumn, while spring biomass is very low. During years with heavy snow cover the lake suffers from a pH drop caused by melt waters, but the effects of this acid pulse are restricted to surface layers and last for a relatively short period. In addition to some chemical parameters (mainly Ca and nutrients), the length of the mixing cycle and physical factors such as the lake water temperature and the thermal stability of the water column had a major impact on phytoplankton dynamics. During a year with long and strong thermal stability, the phytoplankton community developed towards an equilibrium state, with heavy dominance of only a few taxa for a longer period of time. During a year with higher windiness and less thermal stability, the species composition was more diverse and species with different functional strategies were able to occur simultaneously. The results of this work indicate that although arctic and subarctic lakes in general share many common features concerning their catchment and water chemistry, big differences in biological features can be found even in a relatively small area. It is most likely, that lakes with very different algal flora do not respond in a similar way to differences in the environmental factors, and more information about specific arctic and subarctic lake types is needed. The results also show considerable year to year differences in phytoplankton species distribution and dynamics, and these changes are most probably linked to climatic factors.