Diatom and crustacean zooplankton communities, their seasonal variability and representation in the sediments of subarctic Lake Saanajärvi

Present and past diatom and crustacean zooplankton communities of subarctic Lake Saanajärvi in Finnish Lapland were studied with special emphasis on their representation in the sediment surface. Two years monitoring of the present state of the lake revealed it to be ultraoligotrophic with a biomass peak during autumn overturn. Chrysophytes and diatoms were the most abundant phytoplankton groups, with four species forming the main bulk of the crustacean zooplankton community. The comparison of the present plankton community with the species composition in the sediment surface showed that only some plankton groups are suitable for paleolimnological studies. The value of diatoms for paleolimnological studies was supported by the observation that all of the species recorded from various habitats in the lake were detectable in the sediment. In addition, some species (e.g. Cyclotella glomerata) that limnological sampling had not detected were found in the sediment record. Similarly, the zooplankton class Chydoridae and Daphnia hyalina species were equally or better represented in the sediment than in the water column. On the other hand, the sediment trap and core evidence of the whole crustacean zooplankton community suggested many zooplankton species have little or no significance in paleolimnological studies. For example the two dominant copepod species, Eudiaptomus graciloides and Cyclops abyssorum, were not detectable in sediment trap samples. These findings indicate that only a small fraction of the living biota are preserved in the sediments, and this should be taken into account when drawing conclusions based on the detection of just one such species group.