Phytoplankton nitrogen nutrition in the western Indian Ocean: Ecophysiological adaptations of neritic and oceanic assemblages to ammonium supply

The nitrogen nutrition of the phytoplankton community in the neritic and oc eanic 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 concentrati ons (surface NO3 <0.5 mu M), low phytoplankton biomass (PON similar to 0.85 mu mol l(-1)) and predominance of regenerated production (maximum f-ratio <0.47). Ammonium was the major nitrogen substrate during the two seasons, s upplying 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 an d nitrate uptake rates varied little, ammonium uptake and regeneration rate s as well as f-ratio values showed significant spatial variability (i.e. be tween neritic and oceanic regions), which reflected the difference in the p lankton assemblage and its ecophysiology. The oceanic assemblage exhibited higher ammonium uptake capacity, tuned to the activity of an efficient rege nerating community that supplied about 68% of the daily nitrogen requiremen t of the phytoplankton. Analysis of ammonium uptake in relation to seasonal changes in ammonium availability showed that the neritic and oceanic assem blages had different uptake responses. While the ammonium uptake rates of t he neritic assemblage varied according to the ambient ammonium availability , the oceanic assemblage maintained a relatively high specific ammonium upt ake rate throughout the two seasons despite large variations in ammonium av ailability. Maintaining a relatively high ammonium uptake rate in the ocean ic stations is interpreted as a physiological adaptation to ammonium supply . (C) 1999 Academic Press.