Distributions and biogeochemistries of methylamines and ammonium in the Arabian Sea

The distributions of monomethylamine (MMA), dimethylamine (DMA), trimethyla mine (TMA) and ammonium (NH4+) were investigated in the Arabian Sea. The da ta set presented is the first to describe the distribution of MAs on an oce anic scale. Throughout the region concentrations of NH4+ were up to two ord ers 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 cont rasted with the elevated levels recorded in the highly productive coastal u pwelling waters of the NW Arabian Sea. In total the MAs contributed < 1% di ssolved organic nitrogen (DON). Depth maxims of MMA and DMA were generally associated with those of Chla, and in offshore regions, also with those of NH4+ (above the thermo-, oxy- and nitrataclines). Maxima of TMA were record ed at the base of the thermo- and oxyclines, resolved from the other analyt es. Through correlation studies, a degree of diatom specific MMA production was inferred (R = 0.65, p < 0.001) and microzooplankton grazing found to i nfluence significantly all aqueous MA concentrations. Enhanced correlation of MMA concentrations with mesozooplankton abundance was attributed to thei r 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 postu late that the nitrogen taken up in nutrient-rich, diatom-dominated regions of the Arabian Sea will be used both biosynthetically and anabolically. Thi s may be accompanied by introduction of MMA and DMA into the aqueous phase through enzymatic precursor degradation, nitrogen detoxification, senescenc e or lysis and accelerated through grazing pressures, particularly that of mesozooplankton on diatoms. In contrast, under the more oligotrophic condit ions recorded in the remote Arabian Sea, those species of phytoplankton wit h a lower nitrogen demand are favoured, e.g., prymnesiophytes and dinoflage llates. Correspondingly lower MA concentrations are recorded in these regio ns. (C) 1999 Elsevier Science Ltd. All rights reserved.