ENVIRONMENTAL BIOGEOGRAPHY OF NEAR-SURFACE PHYTOPLANKTON IN THE SOUTHEAST PACIFIC-OCEAN

Biogeographic interpretation of large-scale phytoplankton distribution patterns in relation to surface hydrography is essential to understan ding pelagic food web dynamics and biogeochemical processes influencin g global climate. We examined the abundance and biomass of phytoplankt on in relation to physical and chemical parameters in the southeast Pa cific Ocean. Samples were collected along longitude 110 degrees W, bet ween 10 degrees N and 60 degrees S during late austral summer. Pattern s of taxa abundance and hydrographic variables were interpreted by pri ncipal components analysis. Five distinct phytohydrographic regions we re identified: (i) a north equatorial region of moderate productivity dominated by small flagellates, low nitrate and low-to-moderate pCO(2) ; (ii) a south equatorial region characterized by high primary product ivity dominated by diatoms, high nutrient levels, and relatively high pCO(2); (iii) a central gyre region characterized by low productivity dominated by small flagellates, low nitrate, and high pCO(2); (iv) a s ub-Antarctic region with moderate productivity dominated by coccolitho phores, moderate nitrate concentrations, and low pCO(2); and (v) an An tarctic region with high productivity dominated by diatoms, very high nitrate, and low pCO(2). Productivity and average phytoplankton cell s ize were positively correlated with nitrate concentration. Total phyto plankton abundance was negatively correlated with pCO(2), photosynthet ically active radiation, and ultraviolet-B radiation. The interaction between phytoplankton carbon assimilation, atmospheric CO2, and the in hibitory effect of ultraviolet radiation could have implications for t he global climate. These data suggest that the effects would be greate st at southern mid-latitudes (40-50 degrees S) where present phytoplan kton production and predicted future increases in UV-B are both relati vely high. Copyright (C) 1996 Elsevier Science Ltd