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 the 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 column. Four sites, at which topographically-induced upwelling of UCDW occurred, were identified in a 50 km by 400 km band along the outer WAP continental shelf. One additional site at which wind-driven upwelling occurred was also identified. Diatom-dominated phytoplankton assemblages were always associated with a topographically-induced upwelling site. Such phytoplankton communities were not detected at any other shelf location, although diatoms were present everywhere in the 80,000 km 2 study area and UCDW covered about one-third the area below 150 m. Phytoplankton communities dominated by taxa 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, the growth requirements for elevated silicate/nitrate ratios and/or other upwelled 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 experience predictable spring diatom blooms. Rather, this region will experience episodic diatom blooms that occur at variable intervals and during different seasonal conditions, if the physical structuring events are occurring. Preferential drawdown of silicate relative to nitrate was observed at each of the offshore upwelling sites and resulted in a reduction in the ambient 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 increased algal biomass was either dispersed by advective processes and/or consumed by the larger krill that were observed to be associated with each offshore upwelling site. Thus, diatom bloom conditions on the outer WAP shelf may not be recognized based on elevated biomass and/or rates of carbon fixation. It was likely that similar physical forcing of significant phytoplankton 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 the structure of the physical environment with nutrient distributions and phytoplankton assemblages and through to the higher trophic levels, such as Antarctic krill. This environment-trophic coupling may also occur in other regions of the Antarctic, as suggested by correspondences between the distribution of Southern ACC boundary and regions of high concentrations of Antarctic krill. The many mechanisms underlying this coupling remain to be determined, but it was clear that the ecology and biology of the components of the marine food web of the Antarctic continental shelf cannot be studied in isolation from one another or in isolation from the physical environment.