The contribution of zooplankton to methane supersaturation in the oxygenated upper waters of the central Baltic Sea

We report on methane enrichments that were observed during summer in the upper water column of the Gotland Basin, central Baltic Sea. In the eastern part of the basin, methane concentrations just below the thermocline varied between 15 nM and 77 nM, in contrast to the western part where no methane enrichments could be detected. Stable carbon isotope ratios of methane (C-13-CH4 of -67.6 parts per thousand) indicated its in situ biogenic origin from CO2 reduction, which was supported by clonal sequences that clustered with Methanomicrobiaceae, a family of methanogenic Archaea. Incubation experiments with a Temora longicornis dominated seston fraction obtained from the relevant depth showed a positive correlation between seston concentration and methane production rates. Our results, in combination with previous literature outcomes, suggest that the methane enrichment in the eastern basin might be sustained by a diet-consumer relationship between the dinoflagellate Dinophysis norvegica and the copepod T. longicornis. However, our mass balance indicates that a local methane production of 110 pmol L-1 d(-1) was needed to maintain the methane enrichment, and that the estimated production rate from our incubation experiments of 0.3 pmol CH4 d(-1) per adult T. longicornis (about 1 pmol L-1 d(-1)) was too low to maintain the methane enrichment by zooplankton associated methane production only. These calculations also showed that methane was consumed below the thermocline and not transported into the upper-ocean, suggesting that other sources in the mixed layer in the range of 95 pmol L-1 d(-1) are needed to maintain the observed methane air-sea flux.