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Subarctic lakes are sentinels of climate change, showing responses in their physical, chemical, and biological properties. However, climate-induced changes in invertebrate diversity and their underlying mechanisms are not fully understood. We explored the relationship between past climate change and taxonomic composition of subfossil cladocerans in a subarctic lake during the last ca. 5700 years. The Cladocera community shifted from specialist to generalist species at ca. 3500 cal years BP, corresponding to the long-term cooling period between the Holocene Thermal Maximum and the Late Holocene. Taxonomic diversity declined driven by the collapse of the keystone herbivorous Daphnia longispina group, pelagic and littoral predators, and phytophilous benthic species, therefore resulting in a simplification of the food web and a reduction of trophic levels. Furthermore, the shift in cladoceran composition was associated with the decline of aquatic primary producers and the development of birch forest, suggesting a potential causal link between dissolved organic carbon dynamics and cladoceran community composition. This study provides empirical evidence of the response of cladocerans to climatic fluctuations and their underlying mechanisms through catchment-mediated processes and direct temperature-induced changes. [ABSTRACT FROM AUTHOR]

We combined paleolimnological reconstructions and space-for-time substitutions to unravel chironomid biodiversity responses to climate change in subarctic mountains across temporal and spatial scales. Using sediment records, we found that long-term temporal changes in chironomid taxonomic diversity were mainly induced by the temperature tolerance/optimum of species, while little changes in functional diversity were found due to the replacement of similar functional-type taxa within the community. Overall, paleolimnological reconstructions suggested the selection of larger chironomid species by long-term climate cooling and little changes in trophic guilds. Space-for-time substitutions showed, however that low-elevation lakes with forested have more sediment-feeding taxa and larger larvae than high-elevation lakes, thus, suggesting the selection of large chironomid morphotypes with a sediment-feeding mode under warmer climate. Space-for-time substitutions and paleolimnological reconstructions, therefore, gave contrasting results for the link between climate and functional diversity of chironomid communities, likely because space-for-time substitutions failed to match the extent of both spatial and temporal climatic gradients. We suggest that future studies must address biodiversity issues across both temporal and spatial scales as an improved understanding of biodiversity responses to climate change may help us to understand how biodiversity will be affected by ongoing and future change. [ABSTRACT FROM AUTHOR]

Blooms of the nontoxic raphidophyte Gonyostomum semen have shown a recent increase in frequency and distribution in the Fennoscandian region. Due to large cell size and several grazer-avoidance strategies, G. semen is hypothesized to be inedible for most zooplankton species and therefore may constitute a bottleneck for the transfer of energy and nutrients in pelagic food webs. Repression of other phytoplankton through increased competition and induced mortality could further exacerbate this effect. In a field study of four lakes with recurring blooms of G. semen and four lakes without blooms, we found significant differences in community structure between the two lake groups during the bloom period. Bloom-lakes had lower biovolumes of small chrysophytes and chlorophytes and zooplankton assemblages were predominated by small, potentially bacterivorous cladocerans, suggesting a limited availability of edible phytoplankton and an increased importance of microbial pathways during G. semen blooms. Low biovolumes of large cladocerans in bloom-lakes may be due to interference of G. semen with filter feeding. Moreover, high abundances of the phantom midge Chaoborus flavicans in bloom-lakes suggest that the flow of energy and nutrients is directed more towards this invertebrate predator than fish. This could have negative impacts on fish populations, especially if bloom periods are prolonged. [ABSTRACT FROM AUTHOR]

Phototrophic primary production is a fundamental ecosystem process, and it is ultimately constrained by access to limiting nutrients. Whereas most research on nutrient limitation of lacustrine phytoplankton has focused on phosphorus (P) and nitrogen (N) limitation, there is growing evidence that iron (Fe) limitation may be more common than previously acknowledged. Here we show that P was the nutrient that stimulated phytoplankton primary production most strongly in seven out of nine bioassay experiments with natural lake water from oligotrophic clearwater lakes. However, Fe put constraints on phytoplankton production in eight lakes. In one of these lakes, Fe was the nutrient that stimulated primary production most, and concurrent P and Fe limitation was observed in seven lakes. The effect of Fe addition increased with decreasing lake water concentrations of total phosphorus and dissolved organic matter. Possible mechanisms are low import rates and low bioavailability of Fe in the absence of organic chelators. The experimental results were used to predict the relative strength of Fe, N, and P limitation in 659 oligotrophic clearwater lakes (with total phosphorus ≤ 0.2 μM P and total organic carbon < 6 mg C l−1) from a national lake survey. Fe was predicted to have a positive effect in 88% of these lakes, and to be the nutrient with the strongest effect in 30% of the lakes. In conclusion, Fe, along with P and N, is an important factor constraining primary production in oligotrophic clearwater lakes, which is a common lake-type throughout the northern biomes. [ABSTRACT FROM AUTHOR]

The effect of temperature and viscosity on the ingestion rate of Daphnia galeata and Chydorus sphaericus was investigated in short-term feeding experiments with labelled Scenedesmus quadricauda. Viscosity was manipulated independently from temperature by addition of inert polymers in two experiments. In the first experiment, the ingestion rate of D. galeata was found to be 50% lower at 10°C than at 25°C due to the combined effects of increased viscosity and lower temperature. Viscosity alone accounted for 61 % of the decline in ingestion rate. In the second experiment, the ingestion rate of a different clone of D. galeata was 46-53% lower at 10°C than at 20°C and changes in viscosity alone accounted for most (>92%) of that decline. The ingestion rate of (sphaericus was 80% lower at 10°C than at 20°C, and viscosity accounted for 65% of that decline. Thus, viscosity plays an important role in determining the ingestion rate of crustacean zooplankton. Adaptations to life in waters that undergo considerable variation in temperature and viscosity may therefore influence energy budgets and may eventually be of importance for zooplankton species succession, distribution and evolution. [ABSTRACT FROM AUTHOR]

Reconstructions of past food web dynamics are necessary for better understanding long-term impacts of climate change on subarctic lakes. We studied elemental and stable isotopic composition of sedimentary organic matter, photosynthetic pigments and carbon stable isotopic composition of Daphnia (Cladocera; Crustacea) resting eggs (δ13CClado) in a sediment record from a small subarctic lake. We examined how regional climate and landscape changes over the last 5800 years affected the relative importance of allochthonous and autochthonous carbon transfer to zooplankton. Overall, δ13CClado values were well in line with the range of theoretical values of aquatic primary producers, confirming that zooplankton consumers in subarctic lakes, even in the long-term perspective, are mainly fuelled by autochthonous primary production. Results also revealed greater incorporations of benthic algae into zooplankton biomass in periods that had a warmer and drier climate and clearer water, whereas a colder and wetter climate and lower water transparency induced higher contributions of planktonic algae to Daphnia biomass. This study thus emphasizes long-term influence of terrestrial-aquatic linkages and in-lake processes on the functioning of subarctic lake food webs. [ABSTRACT FROM AUTHOR]