The linkage between Upper Circumpolar Deep Water (UCDW) and phytoplankton assemblages on the west Antarctic Peninsula continental shelf

Intrusion of Upper Circumpolar Deep Water (UCDW), which was derived from th e Antarctic Circumpolar Current (ACC), onto the western Antarctic Peninsula (WAP) shelf region in January 1993 provided a reservoir of nutrient-rich, warmer water below 150 m that subsequently upwelled into the upper water co lumn. Four sites, at which topographically-induced upwelling of UCDW occurr ed, were identified in a 50 km by 400 km band along the outer WAP continent al shelf. One additional site at which wind-driven upwelling occurred was a lso identified. Diatom-dominated phytoplankton assemblages were always asso ciated with a topographically-induced upwelling site. Such phytoplankton co mmunities were not detected at any other shelf location, although diatoms w ere present everywhere in the 80,000 km(2) study area and UCDW covered abou t one-third the area below 150 m. Phytoplankton communities dominated by ta xa other than diatoms were restricted to transition waters between the UCDW and shelf waters, the southerly flowing waters out of the Gerlache Strait, and/or the summertime glacial ice melt surface waters very near shore. We suggest that in the absence of episodic intrusion and upwelling of UCDW, th e growth requirements for elevated silicate/nitrate ratios and/or other upw elled constituents (e.g. trace metals) are not sufficiently met for diatoms to achieve high abundance or community dominance. One consequence of this is that the ice-free regions of the outer WAP continental shelf will not ex perience predictable spring diatom blooms. Rather, this region will experie nce episodic diatom blooms that occur at variable intervals and during diff erent seasonal conditions, if the physical structuring events are occurring . Preferential drawdown of silicate relative to nitrate was observed at eac h of the offshore upwelling sites and resulted in a reduction in the ambien t silicate:nitrate ratio relative to the corresponding value for unmodified UCDW (1.5 versus 3.0 for UCDW). The magnitude of the nutrient drawdown in areas of topographically-induced upwelling suggested that diatom growth had been elevated in response to recent upwelling but that the resulting incre ased algal biomass was either dispersed by advective processes and/or consu med by the larger krill that were observed to be associated with each offsh ore upwelling site. Thus, diatom bloom conditions on the outer WAP shelf ma y not be recognized based on elevated biomass and/or rates of carbon fixati on. It was likely that similar physical forcing of significant phytoplankto n growth, especially diatoms, may occur but be undetected in regions where the southern boundary of the ACC nears the Antarctic continental shelf edge . Our analyses from the west Antarctic Peninsula demonstrate coupling of th e structure of the physical environment with nutrient distributions and phy toplankton assemblages and through to the higher trophic levels, such as An tarctic krill. This environment-trophic coupling may also occur in other re gions of the Antarctic, as suggested by correspondences between the distrib ution of Southern ACC boundary and regions of high concentrations of Antarc tic krill. The many mechanisms underlying this coupling remain to be determ ined, but it was clear that the ecology and biology of the components of th e marine food web of the Antarctic continental shelf cannot be studied in i solation from one another or in isolation from the physical environment.