Climate Impacts on Remote Subarctic Lakes in Finnish Lapland: Limnological and Palaeolimnological Assessment with a Particular Focus on Diatoms and Lake Saanajärvi

Limnological characteristics and recent environmental history of five remote subarctic lakes in NW Finnish Lapland were investigated using both limnological and palaeolimnological methods with particular attention to diatoms. The studied sites were dilute, oligotrophic, circumneutral clear-water lakes with low productivity. Two different thermal structures were recognised; dimictic lakes with brief spring overturn, a few weeks of stratification, and a relatively long autumn overturn, and isothermal lakes with only one mixing period during the entire open-water season. A single phytoplankton production maximum occurred in the autumn overturn at the main study site, Lake Saanajarvi. The inter-annual variability within the dominant algae groups (chrysophytes and diatoms) was common. Typical benthic habitat preferences were found among diatom communities in Lake Saanajarvi. Epilithic diatoms, such as Achnanthes, Brachysira and Denticula species, were common in the upper littoral zone while epipelic Fragilaria and Navicula species predominated the deeper littoral zone. Extremely slow but constant sedimentation rates were characteristic for the studied lakes indicating low allochthonous and authochtonous inputs. Palaeolimnological studies demonstrated a concurrent diatom floristic shift that occurred in all the studied lakes about 100 years ago. The change in diatom assemblages was from benthos to plankton in dimictic lakes, while in isothermal lakes the shift took place among the tychoplanktonic and benthic forms. Multi-proxy analyses from Lake Saanajarvi showed that the change in diatom assemblages was accompanied by synchronous changes in other biological indicators, such as cladocerans, chrysophyte cyst assemblages, and fossil pigments, indicating an overall ecosystem response. The diatom record was compared to the 200-year long monthly air temperature record, specifically reconstructed for the study region using an European-wide instrumental data and all available proxy data. A significant relationship was found between the reconstructed air temperatures and the diatom species shift in each lake, with spring temperatures explaining most of the variation in the diatom species data. A change in climate represents the only process that is regionally synchronous and can potentially account for the pronounced diatom floristic shift. No other process – such as acidification or catchment disturbance – can explain satisfactorily the diatom assemblage change from benthos to plankton that occurred within the circumneutral taxa. It is suggested that climate affected the diatom flora mainly via the increased spring temperatures, which in turn lengthened the open-water season, and strengthened the thermal structures in dimictic lakes. The changes in the duration of the overturns and summer stratification may have favoured the growth of small Cyclotella species through the increased nutrient availability from the hypolimnion during the autumn mixing. The study strongly reinforces the suitability of diatoms for biomonitoring both present and past environmental changes in subarctic aquatic ecosystems.