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.

► Antarctic krill are patchy but locally very abundant in the Southern Ocean.
► Krill biomass at South Georgia can be sufficient to deplete their algal food supply.
► But their nitrogen excretion enhanced its uptake by the remaining, un-grazed cells.
► Krill grazing and excretion thus help to structure their food assemblages.