5. RESULTS AND DISCUSSION
5.6 Interactions between ice bacteria and the
5.6.1 Nutrients and DOM in ice and growth limitation by their availability. The main nutrient source for sea ice organisms is the initial nutrient entrapment during the freeze-up and vertical growth of the ice sheet and, in older sea ice, nutrient transport from the underlying water occurring with brine movement (Dieckmann et al. 1991, Gradinger et al. 1992, Golden et al. 1998).
Other important nutrient sources are the regeneration processes inside the ice (Cota et al. 1991). In the Gulf of Bothnia, the accumulation of ice algal biomass is dependent on the amount of nutrients, particularly phosphorus, trapped inside the ice and available during the ice algal bloom period (Haecky & Andersson 1999). The main ice algal bloom in the Gulf of Bothnia can occur in the intermediate layers of ice rather than ice layers near the ice-water interface (Haecky et al. 1998). The
highest dissolved nutrient concentrations were measured in these intermediate ice layers, which underlines the importance of initial entrapment of nutrients and subsequent recycling of this nutrient pool in the Gulf of Bothnia (II). In the Gulf of Finland, conditions may be different because nutrient supply into the ice driven by physical forcing (i.e. meltwater fl ushing or brine exchange processes) can be the determining factor in nutrient dynamics and also be important for primary productivity (Granskog et al. 2003, I). However, the observed correlations between PO4-P and bacterial secondary production (as TTI;
V), elevated PO4-P and ammonium (NH4 -N) concentrations in lower ice as well as high PO4-P concentrations observed during the heterotrophic post-bloom situation (I), indicate active nutrient remineralization by ice heterotrophic assemblages, specifi cally bacteria, in the sea-ice of the Gulf of Finland.
The fi rst attempt to study growth limi-tation of Baltic sea-ice algae and bacteria experimentally is presented in IV. The ex-periments show that light, nutrient and sub-strate limitation of ice algae and bacteria changes with progress of the ice winter and ice layer (IV). Algal growth appeared to be sequentially light- or nutrient-limited along with the winter progression, as shown in studies from Antarctica (Robinson et al.
1998), the Canadian Arctic (Gosselin et al.
1990), and the Baltic Sea (Haecky et al.
1998). The light conditions may even have an indirect effect on bacterial productivity because light increases primary production, which in turn enhances the exudation of DOM from algae, which is assumed to be the primary substrate for sea-ice bacteria.
The results (II) point to continuous nutrient limitation in the upper portion of the ice, which is consistent with results in I and V
and point to the fact that phosphorus is the main growth-limiting nutrient. In addition to regenerated phosphorus, the main phos-phorus supply available to ice organisms during the ice-covered period is in the un-derice water and it is thus probable that the upper ice layers farthest from water are the most nutrient-limited. Organism communi-ties in the lower ice are able to benefi t from this storage, since phosphorus accumulates in the lower ice in the study area (I, Gran-skog et al. 2005b).
The mechanisms that transport nutrients across the ice-water interface are not well understood, but active movement of organ-isms across the ice-water interface cannot be ruled out next to brine exchange proc-esses (IV). During the open-water period, coastal waters in the main study area are limited by nitrogen or co-limited by both nitrogen and phosphorus, and the role of nitrogen transported by river water is un-questionably crucial for the productivity of the ice organism assemblages studied (Kivi et al. 1993; V). In offshore areas, a potential source of nitrogen would be nitro-gen precipitated on the upper surface of the ice from the atmosphere and subsequently introduced into the ice e.g. by meltwater fl ushing (Granskog et al. 2003) or a propa-gating freezing front (Fritsen et al. 1994).
In the ice studied, the bacterial commu-nities were either phosphate- or substrate-limited, as was found experimentally in the Baltic Sea during the open-water period (Lignell et al. 1992, Kuparinen & Heinänen 1993). Periods of bacterial substrate limi-tation occurred when the algae were nutri-ent-limited (IV), which together with the observed signifi cant correlations between bacterial biomass and production and algal biomass and potential productivity in the explorative studies (I, V) implied tight cou-pling between the DOM derived from algae
and thus the supply of carbon for the micro-bial loop. Haecky & Andersson (1999) and Mock et al. (1997) reported a strong linkage between primary and bacterial production from Baltic sea ice in the Gulf of Bothnia and southern Baltic Sea, respectively. Sev-eral reports on Arctic sea ice indicate tight coupling between bacterial biomass and DOM (Gradinger et al. 1992, Thomas et al.
1995 and references therein).
The fi rst report of actual simultaneous measurements of bacterial production and DOM in a sea-ice system was presented (V). Linear correlations between ice DOC and bacterial parameters were not observed;
however, coupling between DON and bac-terial parameters and also PO4-P was ob-served (I, V). It is therefore suggested that the DON concentration may refl ect the readily utilizable fraction of DOM in the ice studied (see also Thomas et al. 1995, Guglielmo et al. 2000). DOC and DON were not interrelated (V; discussed in detail by Granskog et al. 2005b), which is thought to illustrate the complex nature of the ice DOM pool, originating partially from allo-chthonous (parent water, terrestrial sources) and autochthonous (ice algal production, degradation of fresh POM) sources.
5.6.2 Nitrogen transformations. Accumula-tion of temporary intermediate compounds of the nitrogen cycle was frequently observed in the sea ice of both polar areas (Oradiovskiy 1974, Clarke & Ackley 1984, Meese 1989, Garrison et al. 1990, Thomas et al. 1995). High concentrations of nitrate and organic carbon, as well as abundance of actively respiring heterotrophic organisms create suboxic microsites in the brine channels and pockets, a potential site for nitrogen reduction analogous to water-fi lled soil pore spaces (Tiedje 1988). Thomas et al. (1995) observed a high correlation
between nitrite (NO2-N) and DOC in Arctic multiyear ice cores and suggested that nitrite accumulation is associated with decomposition of organic carbon, which indicates active nitrate reduction. Antarctic sea ice hosts a high diversity of psychrophilic bacteria, and the bacterial strains closely related to denitrifying species are abundant (Gosink & Staley 1995, Bowman et al. 1997, Zumft 1997). The fi rst focused attempt to study nitrate reduction or denitrifi cation in a sea-ice environment was presented (II).
An indirect culture method (that does not measure actual denitrifi cation, but instead the denitrifi cation potential of the bacterial assemblage present in the sample) was used in the study. Rysgaard & Glud (2004) provided direct evidence of anoxia and active denitrifi cation in Arctic sea ice. Denitrifying activity in the Gulf of Bothnia was present in ice layers with high heterotrophic biomass, nutrient regeneration and accumulation of nitrite (II). These interior layers of 2–3-month-old ice were probably the sites where nitrogen transformation occurs in sea ice in the Gulf of Bothnia.
Even if the culture method used to esti-mate the activity of denitrifying organisms in this study does not provide information on the actual denitrifi cation rates, it reveals the enrichment of denitrifying organisms in the intermediate layers of thick ice and to-gether with elevated nitrite concentrations indicates active nitrate reduction and also possible denitrifi cation. Although phospho-rus probably plays a more signifi cant role in limiting primary and bacterial production in the Baltic sea ice, the removal of nitro-gen from the ice during winter may affect the amount of nitrogen released from melt-ing sea ice prior the annual phytoplankton spring bloom in the water column. Other possible nitrogen transformations mediated by ice bacteria, e.g. nitrifi cation, were not
examined in this thesis, although high cor-relations between nitrogenous nutrients in the ice data (II) suggest that they may be active in the area.
5.7 Bacteria as a trophic link in the