Nutrient depletion indicates high primary productivity in the Weddell Sea

Abstract

The Southern Ocean, and in particular the Weddell Sea, have long been considered areas of high biological productivity1, but recent isotopic measurements of primary productivity have not confirmed this view2,3. Because the large Zooplankton and marine mammal populations of the Southern Ocean depend ultimately on phytoplankton as the base of the food web, accurate knowledge of primary productivity is essential to our understanding of the Antarctic ecosystem. Oceanographie data collected aboard the Soviet icebreaker Mikhail Somov have allowed us to derive a new productivity estimate, based on the seasonal depletion of nitrate, phosphate and silicic acid in the surface layer. From these depletions and data on the elemental composition of Southern Ocean phytoplankton, we estimate average primary productivity in the Weddell Sea in the springtime to be 220–420 mg C m−2 day−1. Our most conservative estimate is 1.5–4 times higher than recently reported measurements of productivity in the open ocean areas of the Southern Ocean2–5. Our estimates are inherently averages over time and space, including the effects of brief, intense spring blooms of phytoplankton which may occur near the receding ice edge6–8. Studies of primary productivity based on isotope uptake experiments, particularly in the austral summer, may fail to account for the significance of such blooms.