The abundance, size spectra and bacterial colonization of exopolymer particles were investigated in Antarctic sea ice and underlying water in the Bellingshausen Sea during April 2001. In addition to exopolymer particles (EP), different abiotic (temperature, salinity, ice texture, oxygen isotopic composition, inorganic nutrient concentrations) and biotic (particulate organic carbon/nitrogen, algal pigments, abundance and biomass of bacteria and diatoms) parameters were measured from the samples. The sea ice showed different communities occurring in physically distinct layers of the ice. Algal and bacterial biomass in the ice showed strong vertical gradients and ranged between 59.4 and 5140.4 µg C l -1 and 8.8 and 119.4 µg C l -1 , respectively. EP concentrations in the sea ice were high, with EP abundance ranging between 10.2 and 260.1 × 10 6 particles l -1 and EP area between 3.4 and 92.1 cm 2 l -1 . Median EP concentrations in the ice exceeded under-ice values by 1 order of magnitude. Crude estimates of integrated sea ice EP carbon were equivalent to 14-32% of the integrated POC, and to 34-78% of the integrated diatom biomass. The estimated integrated EP carbon in sea ice exceeded the bacterial biomass by a factor of 10 to 20. The abundance of EP was inversely correlated with size of the particles. EP size spectra showed relatively flat slopes, indicating a relatively large contribution of larger particles. The bacterial colonization of individual EP in the ice and in the underice water was not significantly different. In contrast, due to the large difference of EP concentrations in the 2 habitats, the median proportion of attached bacteria was much higher in the ice (14.8% of the total bacterial number) than in the under-ice water (1.9% of the total bacterial number). The data suggest that EP are an integral component of Antarctic sea ice communities and that EP serve as important substrates for ice-associated bacteria. After the ice melts in spring, large amounts of EP are available to be released to the water, where they may significantly contribute to and alter the particle flux of the ice-covered Southern Ocean.

In the Baltic Sea ice, the spectral absorption coefficients for particulate matter (PM) were about two times higher at ultraviolet wavelengths than at photosynthetically available radiation (PAR) wavelengths. PM absorption spectra included significant absorption by mycosporine‐like amino acids (MAAs) between 320 and 345 nm. In the surface ice layer, the concentration of MAAs (1.37 µg L−1) was similar to that of chlorophyll a, resulting in a MAAs‐to‐chlorophyll a ratio as high as 0.65. Ultraviolet radiation (UVR) intensity and the ratio of UVR to PAR had a strong relationship with MAAs concentration (R2 = 0.97, n = 3) in the ice. In the surface ice layer, PM and especially MAAs dominated the absorption (absorption coefficient at 325 nm: 0.73 m−1). In the columnar ice layers, colored dissolved organic matter was the most significant absorber in the UVR (< 380 nm) (absorption coefficient at 325 nm: 1.5 m−1). Our measurements and modeling of UVR and PAR in Baltic Sea ice show that organic matter, both particulate and dissolved, influences the optical properties of sea ice and strongly modifies the UVR exposure of biological communities in and under snow‐free sea ice.

Climate-change driven increases in temperature and precipitation are leading to increased discharge of freshwater and terrestrial material to Arctic coastal ecosystems. These inputs bring sediments, nutrients and organic matter (OM) across the landocean interface with a range of implications for coastal ecosystems and biogeochemical cycling. To investigate responses to terrestrial inputs, physicochemical conditions were characterized in a river-and glacier-influenced Arctic fjord system (Isfjorden, Svalbard) from May to August in 2018 and 2019. Our observations revealed a pervasive freshwater footprint in the inner fjord arms, the geochemical properties of which varied spatially and seasonally as the melt season progressed. In June, during the spring freshet, rivers were a source of dissolved organic carbon (DOC; with concentrations up to 1410 µmol L −1). In August, permafrost and glacial-fed meltwater was a source of inorganic nutrients including NO 2 + NO 3 , with concentrations 12-fold higher in the rivers than in the fjord. While marine OM dominated in May following the spring phytoplankton bloom, terrestrial OM was present throughout Isfjorden in June and August. Results suggest that enhanced land-ocean connectivity could lead to profound changes in the biogeochemistry and ecology of Svalbard fjords. Given the anticipated warming and associated increases in precipitation, permafrost thaw and freshwater discharge, our results highlight the need for more detailed seasonal field sampling in small Arctic catchments and receiving aquatic systems.

[1] Samples of land-fast sea ice collected along the Finnish coast of the Baltic Sea, between latitudes 60.2°N and 65.7°N, in January to April 2000 were analyzed for physical, biological, and chemical parameters. Both spatial and temporal variability were investigated. Snow-ice contributed in average a third of the total ice thickness, while the snow fraction (by mass) of the ice was 20% on average. Snow-ice formation increased the nitrogen concentrations substantially, mainly in the upper parts of the ice cover. Phosphorus on the other hand was controlled by biological uptake, with distinct maxima in the bottommost parts of the ice cover. The chlorophyll-a concentrations were dependent on the physical properties of the ice to some extent. In more saline waters the chlorophyll-a concentrations in the ice were variable (1-17 mg l À1 ). However, in the less saline waters of the Bothnian Bay the concentrations were generally considerably lower (<1 mg l À1 ) than elsewhere. This is presumably caused by formation of ice of low salinity, due to the low ambient salinity in the area and the under-ice flow of river waters, and formation of ice that has no habitable space for ice algae. Atmospheric nutrients possibly enhance the magnitude of the ice algae bloom, through downward flushing of surface deposited nutrients during periods when the ice was permeable. We surmise that atmospheric supply of nutrients plays an important role in biological productivity within the Baltic Sea ice sheet and potentially also in under-ice waters.INDEX TERMS: 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 4540 Oceanography: Physical: Ice mechanics and air/sea/ice exchange processes; 4504 Oceanography: Physical: Air/ sea interactions (0312); KEYWORDS: sea ice, nutrient dynamics, atmospheric nutrient deposition, deposition, sea ice algae, Baltic Sea Citation: Granskog, M. A., H. Kaartokallio, and K. Shirasawa, Nutrient status of Baltic Sea ice: Evidence for control by snow-ice formation, ice permeability, and ice algae,