Nutrient depletions in the Ross Sea and their relation with pigment stocks

The present article depicts a first attempt to relate the governing nutrien t uptake regime and phytopigment signature of the Ross Sea. Based on nutrie nt and phytopigment data obtained during two cruises in the Ross Sea, two d istinct groups were recognised. The first one was characterised by moderate nutrient (nitrate and silicic acid) depletions in combination with relativ ely high diatom and Phaeocystis abundance. The second group showed very low nutrient depletions and very poor diatom abundance. Average depth specific nitrate depletions were 8.1 and 1.1 muM and average silicic acid depletion s were 21.5 and 1.3 muM, respectively. The nutrient consumption patterns di d not match the conditions of silicic acid excess (SEA) or nitrate excess a reas (NEA), a clear trend being probably obscured by very poor seasonal mat urity of several sampling stations. The contrast between both groups is largely explained by small differences in nitrogen uptake regime of the major phytoplankters. During early season, the diatoms meet the majority of their nitrogen requirements by nitrate up take, with few exceptions where ammonium is the most important nitrogenous substrate. On average, their nitrate uptake capacity is lower than that of Phaeocystis (average specific nitrate uptake rates were 0.021 and 0.036 day (-1) for diatoms and Phaeocystis, respectively). The latter phytoplankton a lways shows predominance of nitrate uptake. Both groups are subject to inhi bition of nitrate uptake when ammonium availability increases, and it is li kely that the diatoms are more sensitive to the inhibitory effect of ammoni um. (C) 2000 Elsevier Science B.V. All rights reserved.