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

Abstract

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 (d 13 C-CH 4 of 267.6&) indicated its in situ biogenic origin from CO 2 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 21 d 21 was needed to maintain the methane enrichment, and that the estimated production rate from our incubation experiments of 0.3 pmol CH 4 d 21 per adult T. longicornis (about 1 pmol L 21 d 21 ) 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 21 d 21 are needed to maintain the observed methane air-sea flux.

Controls on zooplankton methane production in the central Baltic Sea

“…Stable carbon isotopes indicated an in situ biogenic methane origin, whereas clonal sequences pointed towards methanogenic Archaea as potential producers (Schmale et al, 2018). It was further shown that zooplankton-associated methane production contributed to the subthermocline methane enrichment (Schmale et al, 2018). The authors suggested that changes in the copepod community and food web structure influenced the spatial heterogeneity of methane accumulation in the upper part of the 20 water column.…”

“…Then the phytoplankton, which remained in the bottle, was sampled. The target copepod T. longicornis has been known to migrate diurnally from the light-penetrated surface layer towards greater depths to escape predation (Hansen et al, 2006;Schmale et al, 2018). For lipid biomarker studies, a concentrated sample of zooplankton rich in T. longicornis was retrieved, avoiding 30 major co-sampling of phytoplankton.…”

“…However, recent studies in the central Baltic Sea revealed a recurring methane accumulation in oxic waters 15 immediately below the thermocline during the summer months (Jakobs et al, 2014;Schmale et al, 2018). Stable carbon isotopes indicated an in situ biogenic methane origin, whereas clonal sequences pointed towards methanogenic Archaea as potential producers (Schmale et al, 2018).…”

“…The authors suggested that changes in the copepod community and food web structure influenced the spatial heterogeneity of methane accumulation in the upper part of the 20 water column. In the same study, Schmale et al (2018) investigated natural bulk zooplankton communities obtained from the depth of the subthermocline methane enrichment, and reported an increase of methane with increasing amount of zooplankton. A mass balance established for the oxic part of the water column indicated that zooplankton-associated methane production rates alone were not sufficient to fully explain the observed methane enrichment.…”

“…A mass balance established for the oxic part of the water column indicated that zooplankton-associated methane production rates alone were not sufficient to fully explain the observed methane enrichment. However, the production rates were obtained from incubations with very dense and probably food limited zooplankton communities (1000 25 times the natural density), leading to results that are difficult to transfer into the natural environment (Schmale et al, 2018).…”