Ammonium accumulation during a silicate-limited diatom bloom indicates the potential for ammonia emission events

Ammonium (NH4+) concentration was measured at 15 stations in the NE Atlantic during the declining phase of the spring diatom bloom as part of the FISHES 2001 cruise. The NE Atlantic temperate spring bloom is one of the largest seasonal events in the oceanic cycle of primary productivity and represents the conversion of large amounts of oxidized nitrogen to organic (reduced) nitrogen by planktonic photosynthesis, much of which is recycled by heterotrophs as reduced N (DON and ammonium). We might expect recycled ammonium to be rapidly re-used by photosynthesizers. However, during the FISHES cruise the decline of the bloom was accompanied by a substantial buildup of dissolved ammonium (from 100 nM to > 500 nM near the sea surface) caused by a temporary imbalance in production and consumption processes. Calculations of air–sea ammonia (NH3) flux under assumed ‘normal’ ‘peak bloom’ and ‘post-diatom-bloom’ conditions are presented and these demonstrate that over a wide range of temperatures, post-bloom accumulation of ammonium will lead to changes in the magnitude and direction of air–sea ammonia flux. If our results are generally applicable then the seasonal succession of plankton communities may be accompanied by periods of anomalously high ammonium concentration and hence be disproportionately important for air–sea ammonia flux. Once in the atmosphere, ammonia neutralizes acidity and in doing so plays a potentially significant role in climate forcing via aerosol formation.