The thesis investigates some of the roles marine copepods play in the carbon turnover in the ocean. Copepods are key organisms in the transfer of energy from primary production to fish. They also at times contribute substantially to the fuelling of the microbial community through their production of dissolved organic carbon (DOC). During a simulated bloom of diatoms the importance of two of the mechanisms by which copepods produce DOC was investigated in relation to the copepod grazing activity. Sloppy feeding, i.e. loss of material due to inefficient feeding, was found to be 49% of what the copepod cleared. Although, freshly expelled pellets lost more than 20% of their carbon content as DOC during the first hour, this only corresponded to 6% of the carbon cleared. Thus, the contribution to the DOC production due to leakage from faecal pellets was of minor importance compared to that by sloppy feeding in this situation. The DOC production by sloppy feeding can, however, be expected to depend on the relative size of the copepod prey. Few studies have measured the DOC production in relation to the relative size of the prey and it has not, until now, been possible to predict when DOC production will be of quantitative importance. A model that allows this indirectly was established from the relationship between the copepod/prey size ratio and the apparent growth efficiency of the copepod. It suggests that for copepod/prey size ratios below ~55 the DOC production by sloppy feeding should be considered. In the Arctic the large copepods Calanus spp. are often some of the most importance pelagic grazers of primary production. Calanus spp. are well adapted to the harsh Arctic environment, storing large amount of lipids to survive the winter during hibernation. However, coexisting with Calanus spp. are a community of small copepods that are less specialised for the Arctic environment. Investigations of the zooplankton community structure and its influence on the carbon turnover was carried out just after the spring bloom in Disko Bay of western Greenland and in the Greenland Sea east of Greenland during summer. In Disko Bay Calanus spp. constituted 88% of the copepod biomass. Calanus spp. were effectively grazing the phytoplankton, and the removal of phytoplankton due to grazing was at least five times higher than that by sedimentation. In addition to phytoplankton sedimentation, equal amounts sedimented as faecal pellets and other detritus. The major part of what Calanus ingested (85%) were respired or retained in the euphotic zone as DOC or biomass. In the Greenland Sea, on other hand, the small copepods were found to be important in terms of both biomass and grazing impact. In this environment with low chlorophyll concentrations the copepods were highly dependent on heterotrophic feeding. Together with the protozooplankton the small copepods in the Greenland Sea were crucial in the recycling and respiration of primary production during summer, resulting in the retainment of carbon fixed by photosynthesis in the surface waters.