Nutrient Cycles in Antarctic Marine Ecosystems
Studies of mineral cycling in the marine environment can be very informative in regard to understanding food web dynamics and general functioning of the ecosystem. Although emphasis is placed on the essential macro-and micro-elements, other mineral elements are also valuable tracers for various processes involving chemical transformations. Most chemical elements show a unique distribution pattern in the water column which reflects their biological and chemical reactivities and the rates at which they are resolubilized. Antarctic waters can be visualized as a giant chemostat, with nutrient rich water upwelling at the Divergence at high latitudes and the water ultimately downwelling at the Polar Front or the Sub-Tropical Convergence. In comparison with other major up-welling areas of the world, one would expect to find almost complete stripping of plant nutrients with time in the upper 50–100 m due to growth of phytoplankton. This condition of nutrient depletion would also be suggested by abundant data on rates of primary production in all sections of the Southern Ocean. Nitrate and phosphate, however, show only slight decreases in concentration in sections from close to the continent to close to the Sub-Tropical Convergence. This apparent contradiction has been explained by recent data which show that N is re-cycled 6–7 times in the euphotic zone before it settles out to deeper water as particulate organic N. These findings have focused attention on the nature of the food web which results in such very high rates of nutrient re-cycling in surface waters. Microbial populations seem to be the dominant organisms responsible for these transformations. With the exception of studies dealing with the distribution of silicon and related elements in the water column and sediments, little attention has been paid to questions of mineral cycling in Antarctic waters. Our understanding of the interaction between biological-physical-geochemical processes in the Antarctic is thus meagre in comparison with temperate and tropical waters.
KeywordsSouthern Ocean Phytoplankton Biomass Nutrient Cycle Euphotic Zone Tropical Water
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