One of the shortcomings of the analysis of subfossil Cladocera (water flee) remains is that preservation of remains is selective. Of Daphnia spp. which are very common in zooplankton assemblages of lakes, usually only postabdominal claws and ephippia are found. In the present paper I describe Daphnia shell margins and some tail spines from the Holocene sediments of a lake in southern Finland where the margins were much more abundant than the postabdominal claws, indicating that postabdominal claws may be underrepresented. Daphnia claws, shell margins and tail spines were found also in surface samples of 17 Finnish lakes and thus the abundance of tail spines could be compared with that of postabdominal claws. The results showed that in most cases the tail spines are more abundant than postabdominal claws and may give a closer estimate of the true abundance of Daphnia. However, in some lakes claws were clearly more frequent than tail spines. Apparently, there are differences in preservation of different types of Daphnia remains between lakes, possibly connected with water chemistry. Overall, the results indicate that probably Daphnia remains are always underrepresented in lake sediments.

The present work is a part of the development of a method which uses the relative proportions of asexually and sexually reproducing chydorid females to reconstruct the length of the open-water season. Surface sediments (5 cm) of Lake Aitajärvi, northern Finnish Lapland, were examined for modern and recent proportions of chydorid cladoceran ephippia in subarctic climate near the pine limit. The total chydorid ephippium proportions (TCE) were steadily 9.5-9.7 % in the Aitajärvi sediment but declined to 8.4 % in the uppermost sample. The result was compared with the surface sediment TCE from four lakes in southern Finland where it varied between 3-6%. It was also compared with the TCE from two lakes in northernmost Finnish Lapland above the treeline in very severe climate, where it was 26-30 %. These very high values suggest that there might be a threshold in climate conditions between Aitajärvi and the two northernmost lakes that alters the reproduction of chydorids towards an even more important role of sexual reproduction.

Alona protzi is a rare species of Cladocera, occurring in lake littoral throughout Europe. However, little is known about this animal, and so far it has not been included in provisional lists of species found in Finland. In this short report we present our findings of Alona protzi, both recent and subfossil material, as well as one previous, unpublished finding site of the species in Finland. We found three subfossil shells of this species in the bottom sediments of two lakes. In a third lake we found intact animals, an ephippial female and a male, while sampling stony bottom of lake littoral.

Subfossil remains of a new species of Cladocera (water fleas) of the family Chydoridae in Finland, Alona werestschagini Sinev, were found in the sediments of four lakes above the treeline in northernmost Finnish Lapland. The remains were found in surface sediments of three lakes and in early Holocene sediments of one lake where the species was a pioneer which soon disappeared. The remains of A. werestschagini, except the male postabdomen, closely resemble Alona guttata. In Eurasia A. werestschagini has a wide but patchy distribution in cold climates, suggesting that it is a postglacial relict adapted to cold climate and oligotrophic lakes. Recently it has been found also in Norway and Kola Peninsula. The early Holocene finds indicate that the species spread to northernmost Finland after the retreat of the Scandinavian Ice Sheet. Since the species has been found in lakes in very severe conditions it may be used as a palaeolimnological indicator in sediment studies.

In this study we compared chydorid cladoceran (Chydoridae) taxa and assemblages from sediments of 6 Polish and 6 Finnish lakes and investigated if the difference in climate of these two countries can be detected in the cladoceran data. The data were analysed in terms of I) average relative proportions of chydorid taxa during the history of each lake and by 2) redundancy analysis (RDA) to explain the present effect of environmental variables (altitude, area, maximum depth, mean annual temperature, mean summer temperature and length of the growing season) on species abundances. The redundancy analysis (RDA) enabled us to distinguish groups of taxa I) with a high thermal preference 2) associated with small, cold-water lakes and 3) associated with shallow lakes. There are clear differences in the dominant chydorid taxa and in the relative proportions of many other chydorid taxa between the two countries since the end of the last glaciation. Although these differences first of all appear to reflect the climatic difference, the influence of many other environmental factors, controlling the living conditions of particular chydorids have been raised and considered. Further studies with larger data are needed before the role of climate can be reliably separated from other elements of environment.

Analyses of subfossil cladocerans (Crustacea: Cladocera) and chironomids (Diptera: Chironomidae) were applied to examine water-level changes in a small and oligotrophic lake in southern Finland over the past 2000 years. Major changes in the invertebrate communities occurred ca. 400 AD onwards when the littoral cladoceran Alonella nana started to replace the planktonic Eubosmina as the dominant species and chironomids Psectrocladius sordidellus group and Zalutschia zalutschicola increased. These changes were most likely due to a decreasing water level and an enlarging proportion of the littoral area, providing suitable vegetative habitats, e.g. aquatic bryophytes (mosses), for these taxa. The lowering water level reached its minimum just before the Medieval Warm Period, ca. 800-1000 AD, after which the lake level rose again and remained high until modern times. A prominent change in the chironomid assemblages occurred during the 20th century when Ablabesmyia monilis and Chironomus anthracinus type increased, presumably due to changes in water chemistry, caused by anthropogenic load of pollutants.

This paper gives a description of the head shield of Alona protzi, a rare species of Cladocera (water fleas) whose separated head shield has not yet been described in detail. Subfossil head shields of A. protzi were found in sediment cores taken from lakes in Denmark, Sweden, Finland, Estonia and Poland. Despite the rarity of the species this suggests a wide distribution of A. protzi in northern Europe. The ecology of A. protzi is poorly known. The environmental spectrum of the finding sites was wide and ranged from relatively nutrient poor clear water lakes to eutrophic turbid water lakes, indicating that A. protzi is not narrowly restricted. Most of the lakes were, however, meso-eutrophic with neutral to high pH, and with a relatively low abundance of submerged macrophytes. However, we cannot exclude the possibility that A. protzi mainly lives in groundwater and is only occasionally transported into lakes.