Structure and functioning of the sympagic community at Terra Nova Bay (Antarctica)

During the fifteenth Italian Antarctic expedition, in the framework of the PIED (Pack Ice Ecosystem Dynamics) program, we investigated structure and functioning of sympagic communities in the annual pack ice at Terra Nova Bay (74°41.72' S, 164°11.63' E). To do this, we collected at 3 interval days (from November 1 to November 30, 1999) both intact sea ice and platelet ice samples which were analysed for inorganic nutrients, autotrophic biomass and productivity, pigment spectra, extracellular enzymatic activities and bacterial carbon production, micro-algal and metazoan community structure. In addition, mesocosm experiments were carried out in order to investigate photosynthetic and photo-acclimation processes of sympagic flora associated with intact sea ice and platelet ice. Autotrophic biomass in the bottom ice increased up to two order of magnitude from November 1 to November 30 (from 4 to 400 mg chlorophyll-a m-3, respectively). Similar pattern was observed for inorganic nutrient concentrations which significantly increased (from 5 to 111 ?M NO3 and from 0.05 to 14.0 ?M PO4). The observed increase of autotrophic biomass in the bottom sea ice can not be only explained by in situ growth as we estimated, from the photosynthetic parameters, a doubling time of ca. 3 days. Pigment spectra and microscopic analyses revealed that bottom ice communities were different from those of the platelet ice. Bottom sympagic flora was mainly represented by cryobenthic species, whereas the cryopelagic population was confined to the pack-platelet ice interface. Zooplankton community in the bottom sea ice was largely dominated by copepods. In particular, the calanoiod Stephos longipes and the harpacticoid Harpacticus furcifer accounted for more than 90% of the sympagic fauna. These two species displayed different stages of their life cycle, as S. longipes was largely represented by nauplii, whereas for H. furcifer mainly exuviae were observed. Aminopeptidase activities were very high and double in platelet ice (29.8 ± 3.1 ?M h-1) than in bottom sea ice (13.0 ± 6.1 ?M h-1). Platelet ice displayed also high bacterial carbon production values (from 0.80 to 4.18 ?gC l-1 h-1), whereas in bottom sea ice were very low (0.01-0.08 ?gC l-1 h-1). This was even more evident, when autotrophic biomass in the bottom ice reached very high values, suggesting that autotrophic biomass accumulation in the bottom sea ice determines inhibiting conditions for bacterial growth, possibly due to extracellular substances released by micro-algae. Mesocosm experiments revealed that platelet ice algal community became adapted to 60% and 10% incident irradiance within a few days and, in both conditions, a bloom was observed until total disappearance of nutrients (N and P). In contrast, the bottom ice community was photo-damaged at irradiance levels commonly occurring in ice-free water column, suggesting that sympagic flora of the bottom ice might play a minor role in pelagic phytoplankton bloom.