The Marine Ecosystem Research Group combines researchers that conduct marine ecosystem research, from biodiversity and ecology to biogeochemistry, physics and ecosystem modelling.

This sub-unit is comprised of researchers at Tvärminne Zoological Station (focusing on marine biodiversity and the impacts of, primarily, eutrophication and climate change on marine ecosystem dynamics), and groups working on aquatic/marine biogeochemistry and genetics at the main campus in Viikki and guest researchers from Stockholm University. Research conducted by the MERG sub-unit has a strong interdisciplinary approach and addresses the structure and function of marine ecosystems, with a focus on understanding how stressors, such as eutrophication and climate related factors affect the ecosystem and its biodiversity.

Researchers in the group collaborate broadly (see publication list), but has a strategic partnership with Stockholm University (Sweden) through the Baltic Bridge, with the Institute of Marine Science, University of Auckland (New Zealand) and with the University of Southern Denmark, Department of Biology.