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Data from a cruise in 1996 in the Southern Ocean near the island of South Georgia indicate that ammonium concentrations in near-surface waters (top 30 m of the water column) varied diurnally by ∼0.2–035 mmol m −3  day −1  (3 h mean values) in different regimes. Maximum values (up to 1.3 mmol m −3 ) occurred around local midnight and minimum values (down to 0.1 mmol m −3 ) close to noon. This cyclicity was not found in other nutrients analysed (silicate, nitrate, nitrite and phosphate). The potential for mixing ammonium-rich water from the pycnocline into the surface mixed layer (SML) at night is examined. Budget calculations for both ammonium and temperature suggest that complete mixing of the pycnocline water into the SML would increase concentration to observed levels. However, it would also give rise to changes in pycnocline ammonium concentration and temperatures which do not agree with observations. Moreover, such a model will not account for daytime drawdown, which is likely to be due to biological processes. A simple biological model combining night-time excretion of ammonium by zooplankton and daytime uptake by phytoplankton simulates the observed change, but only if (i) the daily-averaged phytoplankton growth rate was of the order of 0.5–1 day −1 , (ii) this growth was strongly dependent on ammonium for its nitrogen nutrition, (iii) zooplankton grazing was sufficient to keep phytoplankton biomass at a constant level and (iv) much of the ingested nitrogen was excreted. The implications of these conditions are explored in the context of other observations during the study, and on the basis of published data. It appears that the re-supply of inorganic nitrogen through zooplankton grazing may have been important in sustaining primary production, indicating a very tight coupling between grazers and their food supply. This conclusion is supported by comparison with results from a similar cruise in 1994. Then there was a reduction in the biomass of Antarctic krill, a major component of the zooplankton, to 14% of that observed in 1996. Diurnal change in ammonium concentration could not be detected and phytoplankton growth rate had been slower (about 0.25 d −1 ), providing support for the suggestion that biological processes were more important than physical mixing in generating ammonium cyclicity. The potential consequences for the biological cycling of carbon in high- and low-krill abundance years are discussed.

Diurnal changes in near-surface ammonium concentration—interplay between zooplankton and phytoplankton