Phytoplankton Nitrogen Nutrition in the Western Indian Ocean: Ecophysiological Adaptations of Neritic and Oceanic Assemblages to Ammonium Supply

Abstract

The nitrogen nutrition of the phytoplankton community in the neritic and oceanic waters of the western Indian Ocean was investigated during the south-east monsoon (June–July) and intermonsoon periods (November–December). The region is very oligotrophic, characterized by very low nutrient concentrations (surface NO3<0·5μM), low phytoplankton biomass (PON ∼0·85μmoll−1) and predominance of regenerated production (maximum f -ratio <0·47). Ammonium was the major nitrogen substrate during the two seasons, supplying 53–99% of the phytoplankton’s nitrogen requirement. Nevertheless, both the uptake of nitrate and its relative contribution to total nitrogen removal (f -ratio) were significantly higher during the intermonsoon period than during the south-east monsoon period. While nutrient concentrations and nitrate uptake rates varied little, ammonium uptake and regeneration rates as well as f -ratio values showed significant spatial variability (i.e. between neritic and oceanic regions), which reflected the difference in the plankton assemblage and its ecophysiology. The oceanic assemblage exhibited higher ammonium uptake capacity, tuned to the activity of an efficient regenerating community that supplied about 68% of the daily nitrogen requirement of the phytoplankton. Analysis of ammonium uptake in relation to seasonal changes in ammonium availability showed that the neritic and oceanic assemblages had different uptake responses. While the ammonium uptake rates of the neritic assemblage varied according to the ambient ammonium availability, the oceanic assemblage maintained a relatively high specific ammonium uptake rate throughout the two seasons despite large variations in ammonium availability. Maintaining a relatively high ammonium uptake rate in the oceanic stations is interpreted as a physiological adaptation to ammonium supply.