Seasonal Changes in Pack Ice Communities
Sea ice is a dynamic environment with constantly changing physical properties such as temperature, salinity and brine volume. In the Arctic, winter minimum ice surface temperatures can reach -22oC with salinities of 223 and brine volumes of less than 1% (Schunemann and Werner, 2005). During spring ice temperatures increase due to rising air temperatures, this leads to a decrease in salinity and an increase in brine volume (Werner, 2005). In summer the temperatures in the upper part of the ice pack can get as high as 1 oC, leading to the formation of meltwater pools with very low salinities (down to 0) and low brine volume (<5%) (Schunemann and Werner 2005). As autumn approaches cooling of the ice begins and with it a corresponding increase in salinity and decrease in brine volume (Werner, 2005). Many metazoans that live within the sea ice have adapted their life history strategies to cope with these changes in environment, which have stopped mass colonization of the upper and middle parts of the ice pack (where the extremes in temperature, salinity and brine volumes are found). The lower parts of the ice are heavily influenced by the water column and so do not experience the same strong seasonal changes that are related to the atmosphere (Werner, 2006). In winter the brine volume in the lower part of the ice is usually well above 5% (Werner, 2005).
Algal Biomass ranges in concentration from detection limit in the winter to 67 ug l-1 in the summer. This is in contrast to the integrated abundances of sea ice metazoans which does not vary much between summer (0.6-34.1 103 ind. m-2) and winter (3.7-24.8 103 ind. m-2) (Schunemann and Werner, 2005). Although the abundance is relatively similar throughout the year, the composition of the ice metazoan community differs greatly (Fig.7). The lowest diversity of metazoans occurs in winter when copepod nauplii dominate the community (93%), all other taxa were only found as single specimen examples. This distribution of taxa becomes more even during the spring and summer months when the community is made up of turbellarians (3%), rotifers (33%), nematodes (16%), copepod nauplii (5%) and juvenile and adult copepods (43%) (Schunemann and Werner, 2005).
This difference in community structure must be related to the difference in abundance of algal biomass (a major source of food for the metazoans) as this is the only property of the lower ice pack (where the greatest concentration of metazoans are found) that is significantly different between summer and winter. The nauplii of many copepod species have non feeding stages, during this time they could survive a food limited period, it is thought that also receive energy from an internal lipid reserve. Little is known about other organisms overwintering in the ice, it is hypothesized that low numbers survive and rebuild the population in the spring when food sources are more readily available (Schunemann and Werner, 2005). This hypothesis is supported by observations such as all developmental stages of copepods and juvenile forms of turbellarians and nematodes, and the dominance of copepod nauplii in the spring ice pack. Evidence of resting stages of metazoans in the ice or the presence of ice metazoans in the under ice pelagic zone have not been found and so these methods of overwintering are probably not undertaken (Werner, 2005).