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

Abstract

The present article depicts a first attempt to relate the governing nutrient uptake regime and phytopigment signature of the Ross Sea. Based on nutrient and phytopigment data obtained during two cruises in the Ross Sea, two distinct groups Ž . were recognised. The first one was characterised by moderate nutrient nitrate and silicic acid depletions in combination with relatively 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 mM and average silicic acid depletions were 21.5 and 1.3 mM, respectively. The nutrient consumption patterns did not match the conditions of silicic acid excess Ž . Ž . SEA or nitrate excess areas NEA , a clear trend being probably obscured by very poor seasonal maturity 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 uptake, with few exceptions where ammonium is the most important nitrogenous substrate. On average, their nitrate uptake capacity is lower Ž y1 than that of Phaeocystis average specific nitrate uptake rates were 0.021 and 0.036 day for diatoms and Phaeocystis, . respectively . The latter phytoplankton always shows predominance of nitrate uptake. Both groups are subject to inhibition of nitrate uptake when ammonium availability increases, and it is likely that the diatoms are more sensitive to the inhibitory effect of ammonium. q

A coupled ocean‐ecosystem model of the Ross Sea: 2. Iron regulation of phytoplankton taxonomic variability and primary production

“…In addition, the Ross Sea region of the Southern Ocean is an excellent candidate for modeling because of its relative trophic simplicity. Estimates of the f ratio in the Ross Sea indicate that most of the primary production is supported by NO 3 rather than by a regenerated N source such as NH 4 [ Arrigo et al , 1999; Goeyens et al , 2000; Cochlan and Bronk , 2001; Cochlan et al , 2002], so that NH 4 pools need not be explicitly included in the model. In addition, evidence suggests that heterotrophic processes such as bacterial production and zooplankton grazing are greatly reduced in the Ross Sea compared with other marine ecosystems [ Biggs , 1982; Dunbar et al , 1998; Carlson et al , 1999; Ducklow et al , 1999, 2000; Goffart et al , 2000; Caron et al , 2000; Dennett et al , 2001], allowing simple formulations of these processes to be employed.…”

Living in a Phaeocystis colony: a way to be a successful algal species

“…In the case of P. antarctica both phosphorus and nitrogen in the ambient water remain high throughout the growth season in the Southern Ocean (Goeyens et al, 2000) and should therefore be considered to be non-limiting for growth of colonies. In contrast, P. globosa blooms in the coastal zones of the North Sea after the spring bloom of diatoms.…”