SPATIAL PATTERNS OF PHYTOPLANKTONIC PIGMENTS AND PRIMARY PRODUCTION IN A SEMIENCLOSED PERIANTARCTIC ECOSYSTEM - THE STRAIT OF MAGELLAN

The spatial patterns of nitrate, phytoplankton size-fractionated chlor ophyll a, photosynthetic pigments detected by HPLC and primary product ion were studied at the end of the austral summer (February-March 1991 ), in the Strait of Magellan to provide insight into the mechanisms go verning this semi-enclosed periantarctic ecosystem. Climatological pec uliarities, hydrodynamic constraints, runoff and land forcing were con sidered. The most important features identified for this area were the confinement of the microphytoplankton fraction to the external parts of the Strait and the rather uniform dimensional structure of the phyt oplankton communities (< 5 mum) within the internal sectors. In partic ular, the nanoplanktonic fraction (10(-2) mum) comprised 33%, while th e picoplanktonic one (2-0.5 mum) represented 62% of the total. The con centration of active Chl. a and plant pigments detected by HPLC, nitra te and primary production showed an uneven but similar distribution, a llowing for the characterization of different sectors within the Strai t, according to different types and intensities of forcing factors. Th e different ecological sectors identified along the Strait were associ ated with distinct hydrographic typologies: an oligotrophic, wind-mixe d coastal area, adjacent to the Pacific opening; a high runoff fjord a nd a divergence zone in the Andean sectors; a stratified inland sea in the basin-like part of the channel (Paso Ancho) and a shallow, tidall y mixed system in the Patagonian sectors. Areas of relatively low and high phytoplankton biomass and primary production alternate along the Strait, according to the hydrographic structures of the different sect ors. Phytoplankton biomass, in terms of active Chl. a (spectrofluorome tric determination) ranged between 10 and 51 mg m-2 and primary produc tion between 275 and 1170 mg C m-2 d-1. The assemblage of plant pigmen ts detected by HPLC indicated that the high levels of phytoplankton pi gments and production were most likely attributable to Chlorophytes, P rasinophytes, Prymnesiophytes and/or Gyrodinium and Gymnodinium-like D inoflagellates that were submitted to natural senescence and to superf luous feeding activity; while, when Diatoms, Cryptomonads and Prymnesi ophytes were dominant, efficient zooplankton grazing occurred and char acterized a balanced food chain. The spatial distribution of the param eters considered was related to hydrologic conditions indicating a Pac ific water influence in the entire channel.