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

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 NH₄ regeneration. NH₄ and NO₃ 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 NO₃ (vNO₃) was 0.0026 h⁻¹ (range 0.0013-0.0065 h⁻¹) and for NH₄ (vNH₄) was 0.0097 h⁻¹ (0.0014-0.0376 h⁻¹). vNH₄ was related positively with NH₄ availability, which ranged from 0.1 to 1.5 mmol m⁻³ within the upper mixed layer. Ambient NH₄ concentrations and vNH₄ 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 NH₄ concentrations and uptake rates, vNO₃ was more uniform across the sampling sites. Under these conditions, increasing NH₄ concentration appeared to represent an additional N resource. However, high vNH₄ 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, vNH₄ 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 NH₄ excretion supported enhanced uptake rates by the remaining, ungrazed cells.