The origin of dissolved organic matter (DOM) within sea ice in coastal waters of the Baltic Sea was investigated using parallel factor (PARAFAC) analysis of DOM fluorescence. Sea ice DOM had distinctly different fluorescence characteristics than that of the underlying humic-rich waters and was dominated by protein-like fluorescence signals. PARAFAC analysis identified five fluorescent components, all of which were present in both sea ice and water. Three humic components were negatively correlated to salinity and concluded to be terrestrially derived material. Baltic Sea ice DOM was found to be a mixture of humic material from the underlying water column incorporated during ice formation and autochthonous material produced by organisms within the ice. Dissolved organic carbon (DOC) and nitrogen (DON) concentrations were correlated to the humic fluorescence, indicating that the majority of the organic carbon and nitrogen in Baltic Sea ice is bound in terrestrial humic material trapped within the ice. This has implications for our understanding of sea ice carbon cycling in regions influenced by riverine input (e.g., Baltic and Arctic coastal waters), as the susceptibility of DOM to degradation and remineralization is largely determined by its source.

Abstract. The microbial degradation of dissolved organic carbon and nitrogen (DOC, DON) was studied in three Finnish boreal estuaries with contrasting land use patterns (Kiiminkijoki -natural forest and peatland; Kyrönjoki -agricultural; Karjaanjoki -mixed/urban). Bioassays of 12-18 d long durations were used in 3 seasons at in situ temperatures. Besides the bulk parameters, a suite of dissolved organic matter (DOM) quality parameters were also investigated, including colored DOM (CDOM), fluorescent DOM and the molecular weight of DOM. Bioavailable DOC and DON pools varied significantly between the estuaries, from 7.9 to 10.6 % and from 5.5 to 21.9 %, respectively. DOM originating from the catchment dominated by natural forests and peatlands (Kiiminkijoki) had the lowest DOC and DON degradation rates, as well as the lowest proportions of biodegradable DOC and DON. A greater proportion of agricultural land in the catchment increased the bioavailability of DON, but not the bioavailability of DOC (Kyrönjoki). Additionally, DOM quality varied significantly between the estuaries, and DOM originating from the agricultural Kyrönjoki catchment sustained higher DOC and DON degradation rates and higher bacterial growth efficiency (BGE) compared to those of the natural forest and peat dominated Kiiminkijoki catchment. The quality of DOM, indicated by differences in CDOM, fluorescent DOM and molecular weight, varied between estuaries with differing land use and was concluded to be major driver of BGE of these systems and thereafter to the microbial CO 2 fluxes from the estuaries. The differences in BGE resulted in a 5-fold difference in the calculated daily bacterial CO 2 emissions between the study's estuaries due to bacterial activity, ranging from 40 kg C d −1 in the Karjaanjoki estuary to 200 kg C d −1 in the Kyrönjoki estuary. Lower DOC : DON ratios, smaller molecular weight and higher CDOM absorption spectral slope values of DOM resulted in higher proportion of the initial DOC and DON being transferred to microbial growth and therefore to the pelagic food web. The pristine, peatland and forest-dominated Kiiminkijoki catchment had the lowest BGE, and therefore proportionally highest CO 2 fluxes.

The flocculation of dissolved organic matter (DOM) was studied along transects through three boreal estuaries. Besides the bulk concentration parameters, a suite of DOM quality parameters were investigated, including colored DOM (CDOM), fluorescent DOM, and the molecular weight of DOM as well as associated dissolved iron concentrations. We observed significant deviations from conservative mixing at low salinities (<2) in the estuarine samples of dissolved organic carbon (DOC), UV absorption (a (CDOM254) ), and humic-like fluorescence. The maximum deviation from conservative mixing for DOC concentration was À16%, at salinities between 1 and 2. An associated laboratory experiment was conducted where an artificial salinity gradient between 0 and 6 was created. The experiment confirmed the findings from the estuarine transects, since part of the DOC and dissolved iron pools were transformed to particulate fraction (>0.2 μm) and thereby removing them from the dissolved phase. We also measured flocculation of CDOM, especially in the UV region of the absorption spectrum. Protein-like fluorescence of DOM decreased, while humic-like fluorescence increased because of salt-induced flocculation. Additionally, there was a decrease in molecular weight of DOM. Consequently, the quantity and quality of the remaining DOM pool was significantly changed after influenced to flocculation. Based on these results, we constructed a mechanistic, two-component flocculation model. Our findings underline the importance of the coastal filter, where riverine organic matter is flocculated and exported to the sediments.

The Arctic sea-ice-scape is rapidly transforming. Increasing light penetration will initiate earlier seasonal primary production. This earlier growing season may be accompanied by an increase in ice algae and phytoplankton biomass, augmenting the emission of dimethylsulfide and capture of carbon dioxide. Secondary production may also increase on the shelves, although the loss of sea ice exacerbates the demise of sea-ice fauna, endemic fish and megafauna. Sea-ice loss may also deliver more methane to the atmosphere, but warmer ice may release fewer halogens, resulting in fewer ozone depletion events. The net changes in carbon drawdown are still highly uncertain. Despite large uncertainties in these assessments, we expect disruptive changes that warrant intensified long-term observations and modelling efforts.

No abstract

Extracellular polymeric substances (EPS) are known to help microorganisms to survive under extreme conditions in sea ice. High concentrations of EPS are reported in sea ice from both poles; however, production and dynamics of EPS during sea ice formation have been little studied to date. This investigation followed the production and partitioning of existing and newly formed dissolved organic matter (DOM) including dissolved carbohydrates (dCHO), dissolved uronic acids (dUA) and dissolved EPS (dEPS), along with bacterial abundances during early stages of ice formation. Sea ice was formed from North Sea water with (A) ambient DOM (NSW) and (B) with additional algal-derived DOM (ADOM) in a 6d experiment in replicated mesocosms. In ADOM seawater, total bacterial numbers (TBN) increased throughout the experiment, whereas bacterial growth occurred for 5d only in the NSW seawater. TBN progressively decreased within developing sea ice but with a 2-fold greater decline in NSW compared to ADOM ice. There were signiWcant increases in the concentrations of dCHO in ice. Percentage contribution of dEPS was highest (63%) in the colder, uppermost parts in ADOM ice suggesting the development of a coldadapted community, producing dEPS possibly for cryoprotection and/or protection from high salinity brines. We conclude that in the early stages of ice formation, allochthonous organic matter was incorporated from parent seawater into sea ice and that once ice formation had established, there were signiWcant changes in the concentrations and composition of dissolved organic carbon pool, resulting mainly from the production of autochthonous DOM by the bacteria.

No abstract

[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,