Climate change and permafrost thaw are unlocking the vast storage of organic carbon held in northern frozen soils. Here, we evaluated the effects of thawing ice‐rich permafrost on dissolved organic matter (DOM) in freshwaters by optical analysis of 253 ponds across the circumpolar North. For a subset of waters in subarctic Quebec, we also quantified the contribution of terrestrial sources to the DOM pool by stable isotopes. The optical measurements showed a higher proportion of terrestrial carbon and a lower algal contribution to DOM in waters affected by thawing permafrost. DOM composition was largely dominated (mean of 93%) by terrestrial substances at sites influenced by thawing permafrost, while the terrestrial influence was much less in waterbodies located on bedrock (36%) or with tundra soils unaffected by thermokarst processes (42%) in the catchment. Our results demonstrate a strong terrestrial imprint on freshwater ecosystems in degrading ice‐rich permafrost catchments, and the likely shift toward increasing dominance of land‐derived organic carbon in waters with ongoing permafrost thaw.

Abstract. Over Antarctica, aerosol particles originate almost entirely from marine areas, with minor contribution from long-range transported dust or anthropogenic material. The Antarctic continent itself, unlike all other continental areas, has been thought to be practically free of aerosol sources. Here we present evidence of local aerosol production associated with melt-water ponds in continental Antarctica. We show that in air masses passing such ponds, new aerosol particles are efficiently formed and these particles grow up to sizes where they may act as cloud condensation nuclei (CCN). The precursor vapours responsible for aerosol formation and growth originate very likely from highly abundant cyanobacteria Nostoc commune (Vaucher) communities of local ponds. This is the first time freshwater vegetation has been identified as an aerosol precursor source. The influence of the new source on clouds and climate may increase in future Antarctica, and possibly elsewhere undergoing accelerating summer melting of semi-permanent snow cover.

We addressed the extent to which autochthonous and allochthonous carbon sources contribute to zooplankton diet throughout the year in oligotrophic subarctic Lake Saanajä rvi. Optical measures of dissolved organic matter (DOM) indicated high water discharge and associated terrestrial DOM during winter and low inputs in summer. Bulk particulate organic matter (POM) showed d 13 C values consistent with allochthonous inputs of DOM. The most positive POM d 13 C values (227%) occurred during winter, when heterotrophic bacteria and nanoflagellates peaked in abundance; the isotopically lighter autotrophic phytoplankton shifted the POM d 13 C to 229% in summer. The d 13 C values of zooplankton were up to 4% more negative than those of POM, most likely because detritus and other nonliving material with higher d 13 C values comprised 45-90% of POM. The d 13 C of the cladoceran Daphnia umbra mirrored the trend of POM d 13 C, indicating a dietary shift from heavy dependence on autochthonous carbon in summer to allochthonous or heterotrophic food in winter. The d 13 C of the most abundant zooplankton in Lake Saanajä rvi, Eudiaptomus graciloides and Cyclops abyssorum, remained low and constant in winter, suggestive of little or no food intake during the 8 months of winter. There are significant differences in the zooplankton utilization of nonphytoplankton carbon in different seasons and among species. Although autochthonous carbon seemed to be mainly responsible for the growth of zooplankton, the ability to utilize allochthonous carbon-fueled microorganisms explains the survival of Daphnia through winter in northern lakes.

Colony-forming cyanobacteria of the genus Aphanizomenon form massive blooms in the brackish water of the Baltic Sea during the warmest summer months. There have been recent suggestions claiming that the Baltic Sea Aphanizomenon species may be different from Aphanizomenon flos-aquae found in lakes. In this study, we examined variability in the morphology and 16S-23S rRNA internal transcribed spacer (ITS) sequences of A. flos-aquae populations along a salinity gradient from a string of lakes to a fjord-like extension of the Baltic Sea to the open Baltic Sea. Morphological differences among the populations were negligible. We found that the Baltic Sea was dominated (25 out of 27 sequences) by one ITS1-S (shorter band of ITS 1 [ITS1]) genotype, which also was found in the lakes. The lake populations of A. flos-aquae tended to be genetically more diverse than the Baltic Sea populations. Since the lake ITS1-S genotypes of A. flos-aquae are continuously introduced to the Baltic Sea via inflowing waters, it seems that only one ITS1 genotype is able to persist in the Baltic Sea populations. The results suggest that one of the ITS1-S genotypes found in the lakes is better adapted to the conditions of the Baltic Sea and that natural selection removes most of the lake genotypes from the Baltic Sea A. flos-aquae populations.