| Popis: |
Sea ice algae play a critical role as major primary producers in the Southern Ocean ecosystem. Each autumn, algae are incorporated into newly forming sea ice in vast numbers, but the mechanisms by which algal cells are incorporated into young sea ice remain unclear. Early experiments with ice-forming tanks suggested that frazilharvesting is the most significant process leading to algal accumulation. In the present study, both field and laboratory experiments are used to investigate factors related to algal inclusion in sea ice. Field experiments were used to evaluate the natural extent of algal cell incorporation, while laboratory experiments tested specific processes. Field samples were collected in areas of new ice formation off eastern Antarctica during the spring of 1992 and the autumns of 1993 and 1996. One to three orders of magnitude higher algal biomass was found in ice and interstitial water samples than in the underlying water column. Diatoms contributed approximately two thirds of the biomass with small unidentified flagellates contributing the remainder. Field experiments indicate some diatom species were selectively incorporated into ice which forms in different conditions (ie that which forms in rough versus calm conditions). There was evidence of size specific incorporation of algae and microspheres into the natural and laboratory-formed sea ice. This size selectivity discriminated against large microspheres and large algal cells. Experiments using a laboratory-based ice tank and a similar tank at Casey Station Antarctica, revealed that laboratory-based ice tanks are an effective way of simulating both sea ice production and the incorporation of phytoplankton into the sea ice. Experiments were conducted to simulate water being pushed through the ice matrix (similar to a wave field agitation system). Results of ice tank experiments indicate that there are several physical processes responsible for algal enrichment in sea ice, these are, 'pumping' of water through the newly forming ice, water circulation patterns and scavenging of phytoplankton by frazil ice crystals. The ability of phytoplankton to remain suspended in the upper water column is an important biological factor which influences algal incorporation into newly forming sea ice. Laboratory-based experiments using temperate water diatom species revealed a level of incorporation into sea ice similar to that of Antarctic species. Experiments with glass microspheres indicate that microspheres < 53 µ.m in diameter were incorporated in to sea ice at levels above the microsphere numbers in the underlying water. Microspheres with a greater diameter rapidly settled out of the system. Despite the spatial and temporal constraints of laboratory-based tanks, Antarctic phytoplankton species were incorporated into slurry within the ice at concentrations as great as 7 times the levels in ice and water samples. These results imply algae are actively incorporated into very young sea ice. Incorporation microspheres into sea ice at a similar level to Antarctic taxa indicates that in an agitated system the incorporation results from a physical process. The enrichment of specific species in calm conditions indicates there are important biological processes involved in the algal incorporation. Results of this study indicate that the 'pumping' of water through an ice matrix, water circulation patterns and algal cell harvesting, are important physical mechanisms responsible for algal enrichment in sea ice. Preferential incorporation of some species occurs in calm conditions as a result of a combination of biological and physical processes. |
