Spatial patterns of phytoplanktonic pigments and primary production in a semi-enclosed periantarctic ecosystem: the Strait of Magellan

The spatial patterns of nitrate, phytoplankton size-fractionated chlorophyll a, photosynthetic pigments detected by HPLC and primary production were studied at the end of the austral summer (February–March 1991), in the Strait of Magellan to provide insight into the mechanisms governing this seemi-enclosed periantartic ecosystem. Climatological peculiarities, hydrodynamic constraints, runoff and land forcing were considered.

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 phytoplankton communities ( < 5 μm) within the internal sectors. In particular, the nanoplanktonic fraction (10 - 2 μm) comprised 33 %, while the picoplanktonic one (2 - 0.5 μm) represented 62% of the total.

The concentration of active Chl. a and plant pigments detected by HPLC, nitrate and primary production showed an uneven but similar distribution, allowing for the characterization of different sectors within the Strait, according to different types and intensities of forcing factors.

The different ecological sectors identified along the Strait were associated with distinct hydrographic typologies: an oligotrophic, wind-mixed coastal area, adjacent to the Pacific opening; a high runoff fjord and a divergence zone in the Andean sectors; a stratified inland sea in the basin-like part of the channel (Paso Ancho) and a shallow, tidally 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 sectors. Phytoplankton biomass, in terms of active Chl. a (spectrofluorometric determination) ranged between 10 and 51 mg m−2 and primary production between 275 and 1170 mg C m−2 d−1.

The assemblage of plant pigments detected by HPLC indicated that the high levels of phytoplankton pigments and production were most likely attributable to Chlorophytes, Prasinophytes, Prymnesiophytes and/or Gyrodinium and Gymnodinium-like Dinoflagellates that were submitted to natural senescence and to superfluous feeding activity; while, when Diatoms, Cryptomonads and Prymnesiophytes were dominant, efficient zooplankton grazing occurred and characterized a balanced food chain.

The spatial distribution of the parameters considered was related to hydrologic conditions indicating a Pacific water influence in the entire channel.