Seasonal and spatial variation in the sources and fluxes of long chain diols and mid-chain hydroxy methyl alkanoates in the Arabian Sea

Long chain 1,14-diols and 12-hydroxy methyl alkanoates may be useful indicators for high-nutrient conditions as their inferred sources, diatoms belonging to the genus Proboscia, are often abundant in upwelling regions. In order to test this hypothesis, the lipids of three different Proboscia species in culture were determined and the fluxes of different long chain diol isomers and mid-chain hydroxy methyl alkanoates in the Arabian Sea were studied. The culture studies showed that long chain 1,14-diols and 12-hydroxy methyl alkanoates are indeed major lipids in Proboscia indica, Proboscia inermis and Proboscia alata. Time-series sediment trap data from the Arabian Sea showed increased fluxes of long chain 1,14-diols and 12-hydroxy methyl alkanoates in periods of upwelling and low fluxes for the rest of the year. High fluxes were found primarily at stations in the upwelling area close to the coast, whereas at a station located 590 km off the coast of Oman fluxes were substantially lower. These results show that long chain 1,14-diols and 12-hydroxy methyl alkanoates can be used as proxies for upwelling conditions. Flux patterns of 1,15-diols did not, however, resemble those of 1,14-diols: 1,15-diols reached maximum flux earlier than 1,14-diols, annual flux of 1,15-diols was much lower than 1,14-diols and upwelling did not seem to affect 1,15-diol flux values. This agrees with the idea that the Eustigmatophytes rather than Proboscia spp. should be considered as the major source of 1,15-diols.