Methane is a known greenhouse gas that severely enhances climate change on earth, yet not all methane sources into atmosphere have been identified. A process that might be of importance is the production of methane by microorganisms within the anoxic guts of certain zooplankton species and their fecal pellets. This production takes place in the upper oxygenated water column and thus could have a direct impact on the methane flux between ocean and atmosphere. We hypothesize that highly productive regions like marginal seas, which have never been studied in detail in this context before, are areas of enhanced zooplankton-mediated methane production, which most probably causes the subthermocline methane anomaly that have been sporadically identified in the oxygenated water column of the Baltic Sea. In the ZooM project we will combine methane chemistry, microbiology, and zooplanktology in a multidisciplinary approach to investigate zooplankton-associated methanogenesis in detail using the Baltic Sea as a model system. We plan to investigate the following key questions: (1) Is the subthermocline methane anomaly a widespread phenomenon in the Baltic Sea, which shows a temporal and spatial variability? (2) Does zooplankton-associated methane production have the potential to support the methane anomaly in the shallow water and how are copepod species and environmental factors like food composition influencing methane production? (3) Which microbes are involved in zooplankton-associated methane production and can we detect differences in methanogenic assemblages and their activities between copepod guts and their fecal pellets?