Close coupling between ammonium uptake by phytoplankton and excretion by Antarctic krill, Euphausia superba

Abstract

In this study we examined the hypothesis that, under conditions of replete macronutrients and iron in the Southern Ocean, phytoplankton abundance and specific N uptake rates are influenced strongly by the processes of grazing and NH 4 regeneration. NH 4 and NO 3 uptake rates by marine phytoplankton were measured to the northeast and northwest of the island of South Georgia during January–February 1998. Mean specific uptake rate for NO 3 ( vNO 3) was 0.0026 h −1 (range 0.0013–0.0065 h −1) and for NH 4 ( vNH 4) was 0.0097 h −1 (0.0014–0.0376 h −1). vNH 4 was related positively with NH 4 availability, which ranged from 0.1 to 1.5 mmol m −3 within the upper mixed layer. Ambient NH 4 concentrations and vNH 4 were both positively related to local krill biomass values, computed from mean values along acoustic transect segments within 2 km of the uptake measurement stations. These biomass values ranged from ∼1 g krill fresh mass m −2 in the northwest to >4 kg krill wet mass m −2 in the northeast. In contrast to the variability found with NH 4 concentrations and uptake rates, vNO 3 was more uniform across the sampling sites. Under these conditions, increasing NH 4 concentration appeared to represent an additional N resource. However, high vNH 4 tended to be found for stations with lower phytoplankton standing stocks, across a total range of 0.24–20 mg chlorophyll a m −3. These patterns suggest a coupling between phytoplankton biomass, vNH 4 and krill in this region of variable but high krill biomass. Locally high concentrations of krill in parts of the study area appeared to have two opposing effects. On the one hand they could graze down phytoplankton stocks, but on the other hand, their NH 4 excretion supported enhanced uptake rates by the remaining, ungrazed cells.