Distributions and biogeochemistries of methylamines and ammonium in the Arabian Sea

The distributions of monomethylamine (MMA), dimethylamine (DMA), trimethylamine (TMA) and ammonium (NH+4) were investigated in the Arabian Sea. The data set presented is the first to describe the distribution of MAs on an oceanic scale. Throughout the region concentrations of NH+4 were up to two orders of magnitude greater than those of the MAs. MMA (0–66 nM) was generally the most abundant MA, whilst TMA was only found at concentrations <4 nM. Low concentrations of MAs in open-ocean meso- and oligotrophic regions contrasted with the elevated levels recorded in the highly productive coastal upwelling waters of the NW Arabian Sea. In total the MAs contributed <1% dissolved organic nitrogen (DON). Depth maxima of MMA and DMA were generally associated with those of Chla, and in offshore regions, also with those of NH+4 (above the thermo-, oxy- and nitrataclines). Maxima of TMA were recorded at the base of the thermo- and oxyclines, resolved from the other analytes. Through correlation studies, a degree of diatom specific MMA production was inferred (R=0.65, p<0.001) and microzooplankton grazing found to influence significantly all aqueous MA concentrations. Enhanced correlation of MMA concentrations with mesozooplankton abundance was attributed to their ability to graze diatoms. These observations are analogous to those made of equivalent oceanographic regimes in the Mediterranean Sea (Gibb et al., 1994) and support the idea that MA concentrations in seawater are primarily regulated by the productive aspects of their biological dynamics. We postulate that the nitrogen taken up in nutrient-rich, diatom-dominated regions of the Arabian Sea will be used both biosynthetically and anabolically. This may be accompanied by introduction of MMA and DMA into the aqueous phase through enzymatic precursor degradation, nitrogen detoxification, senescence or lysis and accelerated through grazing pressures, particularly that of mesozooplankton on diatoms. In contrast, under the more oligotrophic conditions recorded in the remote Arabian Sea, those species of phytoplankton with a lower nitrogen demand are favoured, e.g., prymnesiophytes and dinoflagellates. Correspondingly lower MA concentrations are recorded in these regions.