Prezelin¹, Barbara, B., E.E. Hofmann², J. M. Klinck² and Mark Moline³

¹Marine Science Institute, University of Santa Barbara, Santa Barbara, CA, Tel: 805-893-2879, Fax: 805-893-4319, E-mail: prezelin@lifesci.ucsb.edu, ²Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, VA and ³Biological Sciences Department, California Polytechnic State University

Recognizing shelf break forcing of phytoplankton community structure in continental shelf waters of the western Antarctic Peninsula

Subsurface intrusions of nutrient-rich Upper Circumpolar Deep Water (UCDW) which flow onto the west Antarctic Peninsula (WAP) continental shelf via shelf break forcing by the Antarctic Circumpolar Current, and the subsequent upwelling of this water, set up a physical and chemical structure that promotes diatom-dominated communities and localized increases in rates of primary productivity. The intrusions of UCDW explain almost all of the variation in the patterns of phytoplankton community distributions observed for WAP shelf waters during four sequential cruises. Diatom-dominated communities were never observed outside the nearby influences of recently upwelled UCDW. This previously unrecognized mechanism differs from the more traditional control of biological activity and community composition through mixed layer dynamics. The episodic nature of this mechanism also explains why diatom-dominated communities were evident in large loosely-linked patches across much of the WAP in summer 1993 when UCDW covered much of the shelf, but were completely absent in summer 1994 when this water mass was absent. In the intervals between UCDW upwelling events, phytoflagellates became increasingly dominant as the surface waters 'aged' with progressive nutrient depletion. A pattern of progressive changes in phytoplankton community composition over the mesoscale and on time scales of weeks to months was related to changes in the balance of available inorganic nutrients. Our analyses shows that the phytoplankton community structure on the WAP shelf is determined by episodic and non-seasonal physical forcing, similar to other coastal regions where subsurface intrusions of oceanic waters are observed.