Abstract
We describe a new ocean-sea ice-biogeochemical model, apply it to the Bothnian Bay in the northern Baltic Sea for the time period 1991–2007 and provide the first long-term mesoscale estimates of modelled sea-ice primary production in the northern Baltic Sea. After comparing the available physical and biogeochemical observations within the study area and the time period investigated with the model results, we show the modelled spatial, intra- and interannual variability in sea-ice physical and biogeochemical properties and consider the main factors limiting ice algal primary production. Sea-ice permeability in the studied area was low compared with the polar oceans, which appeared to be a major reason for the generally low primary production rates. Although the sea ice was less saline in the northernmost parts of the basin, these parts were characterized by sea ice with a larger amount of habitable space, higher levels of photosynthetically active radiation and increased macronutrient availability near the coast, which favoured higher algal growth rates. Other parts of the southern central basin were mostly co-limited by less favourable light conditions (i.e., earlier ice breakups associated with fewer sunlight hours) and lower seawater macronutrient concentrations than in the coastal zones. Although a change towards milder winters (i.e., reduced ice cover, thickness and length of the ice season) was previously detected on a half-century timescale and could partly be seen here, analysis of the temporal evolution of sea-ice biogeochemical properties showed no significant trends over time, though these properties were characterized by large interannual variability.
Highlights
The Baltic Sea features seasonal sea ice structurally similar to that of the polar oceans (Vihma and Haapala 2009)
Brine salinity together with ice temperature determine the permeable fraction of sea ice, i.e., the habitable space for the biota, which is small in the Baltic Sea (Granskog et al 2006) compared with the more saline polar oceans
Kuparinen et al (2007) compiled several sea-ice datasets of biogeochemical properties measured from the Baltic Sea between 1996 and 2006 and concluded that Baltic sea ice is a highly active ecosystem that may contribute to annual nutrient and carbon (C) cycles, with complex controlling mechanisms not yet fully understood
Summary
The Baltic Sea features seasonal sea ice structurally similar to that of the polar oceans (Vihma and Haapala 2009). Undeformed sea ice in the Bothnian Bay has low Chl-a concentrations and low bacterial turnover rates, even when high concentrations of allochthonous dissolved organic matter (DOM) have been reported (Stedmon et al, 2007). This suggests that bacterial growth is controlled mostly by the algal production of the most labile fraction of DOM (Kaartokallio, 2001; Meiners et al, 2002; Kaartokallio, 2004; Kaartokallio et al, 2007; Kuparinen et al, 2007). We determined the controlling factors of seaice primary production among light, nutrients and sea-ice permeability, i.e., the factors related to the habitable space that affect the fluxes of organic and inorganic matter at the ice-water interface. For a more detailed description of the biogeochemical dynamics of the BFM-SI and for the model equations, the reader is referred to the BFM-SI manual (Tedesco and Vichi, 2010) and to Tedesco et al (2010)
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