Methane in situ production occurs frequently in the oxygenated upper ocean. A principal pathway by which methane can be formed is methylotrophic methanogenesis, while an important methylated substrate is DMSP (dimethylsulfoniopropionate) produced by marine phytoplankton. Here we report on an in situ methane production/consumption cycle during a summer phytoplankton bloom and a potential link to DMSP concentration in Storfjorden (Svalbard Archipelago) - a polar shelf region. The study is based on measurements of δ13CCH4 values, concentrations of methane, chlorophyll-a, particulate and dissolved DMSP, as well as water temperature and salinity along four transects in August 2005. Freshwater input creates a stable surface layer in Storfjorden during summer, below which a denser subsurface layer is found. A methane surplus (between 5 and 55 nM) in relation to the atmospheric equilibrium concentration (about 3.5 nM) is detected in the water column and the carbon isotopic signatures of dissolved methane (- 52 to - 24‰ PDB) deviate from those of atmospheric methane (- 47‰ PDB). The methane plumes observed in the surface and subsurface water differed from each other, suggesting that they are generated independently. The subsurface water in summertime contained methane that was released from sediments during winter, and oxidized over time, leaving the residual methane 13C-enriched. The surface water, on the other hand, contained recently produced, 13C-depleted methane. We propose that methane in situ production occurs during the summer phytoplankton bloom. The concentration of methane increases up to a certain threshold value, above which methane consumption begins. A methane production-removal cycle is established, which is reflected in the varying methane concentrations and δ13CCH4 values. DMSP and methane are inversely correlated suggesting that DMSP could be a potential substrate for the methylotrophic methanogenesis. © 2007 Elsevier B.V. All rights reserved.