No abstract

Sea ice-algae contribute significantly to Arctic primary production and play an important role in the life histories of planktonic and benthic consumers after the algae are released from the sea ice habitat. Following export from the ice, the extent to which fresh algal material is available to planktonic or benthic consumers is dependent on residence time in the water column, initially related to particle settling rate. Laboratory experiments using isolated Nitzschia frigida, a common sea ice diatom, were conducted to ascertain the effects of nutrient (N, P, and Si) and light limitation on settling characteristics of the algal material. While settling characteristics of N-and P-limited cultures were not significantly different from controls grown under light and nutrient-replete conditions, significant differences from the controls were found for light and Si limitation. Differences between treatments were evidenced by changes in the proportion of each population that had particular settling rates, rather than by changes in the range of settling rates measured within a treatment. Thus, fast (> 20 m d 21 ) and slow-sinking particles (< 2 m d 21 ) were found in all cultures, but compared to the controls, a larger percentage of fast-sinking particles were observed under Si limitation while a larger percentage of slow-sinking material was observed under light limitation. While N. frigida is just one member of the sea ice algal assemblage, its prevalence in Arctic land-fast sea ice means these results may be representative of the broader Arctic nearshore ice-algae community. As such, abiotic conditions within Arctic sea ice, such as nutrient availability and depth of overlying snow (which affects the light field in the ice), could influence the amount of algae-derived material available to different components of the underlying marine food web.

Shallow-water communities along the western Antarctic Peninsula support forests of large, mostly chemically defended macroalgae and dense assemblages of macroalgal-associated amphipods, which are thought to exist together in a community-wide mutualism. The amphipods benefit the chemically defended macrophytes by consuming epiphytic algae and in turn benefit from an associational refuge from fish predation. In the present study, we document an exception to this pattern. The amphipod Paradexamine fissicauda is able to consume Plocamium cartilagineum and Picconiella plumosa, 2 species of sympatric, chemically defended red macroalgae. In previous studies, Plocamium cartilagineum was one of the most strongly deterrent algae in the community to multiple consumers, and was found here to be unpalatable to 5 other amphipod species which utilize it as a host in nature. Paradexamine fissicauda maintained on a diet of Plocamium cartilagineum for 2 mo were much less likely to be eaten by fish than Paradexamine fissicauda maintained on a red alga which does not elaborate chemical defenses, or than a different but morphologically similar sympatric amphipod species. Halogenated secondary metabolites produced by Plocamium cartilagineum were identified from tissues of the Paradexamine fissicauda that had eaten it but not those which had eaten the undefended red alga. This indicates that P. fissicauda is sequestering the potent chemical defenses of Plocamium cartilagineum for its own use.