Zooplankton methane production in the oxic surface waters of the Baltic Sea and its contribution to the methane efflux

Oxic CH₄ production in aquatic systems places the CH₄ source close to the water-atmosphere interface, thereby allowing a nearly direct emission into the atmosphere. Several processes that produce CH₄ in oxic waters have been recently identified and it is assumed that climate change will impact their source strength, with far reaching consequences for CH₄ flux and climate feedback. However, the origin of CH₄ from the oxic upper water column is still unknown.

Our own studies in the central Baltic Sea show a recurring CH₄ accumulation immediately below the thermocline that emerges during the summer months. This anomaly, together with stable carbon isotope signatures of CH₄, indicates an in situ biogenic origin. Isotope values and clonal sequences obtained from water column samples suggest methanogenic Archaea as potential producers of CH₄ in the oxic water column. To elucidate the particular contribution of zooplankton to sub-thermocline CH₄ production, we conducted community and food-specific grazing experiments during a summer cruise. Our experimental results suggest that CH₄ enrichment in the eastern basin of the central Baltic Sea may involve diet-consumer relationship between DMSP-rich dinoflagellates and the copepod T. longicornis. This assumption is further supported (1) by characteristic biomarkers in the neutral (storage-) lipids of T. longicornis dominated community, and (2) by sequence analyses, which show that methanogens concentrate on particles and inside T. longicornis, rather than in the free water column. However, our mass balance established for the upper water column of the central Baltic Sea indicates that zooplankton CH₄ production rates alone are not sufficient to fully explain the observed CH₄ enrichment. These calculations also verify that CH₄ is consumed below the thermocline and not transported into the upper water column, suggesting that other, yet unknown sources in the mixed layer are needed to maintain the observed CH₄ sea-air flux.