Effect of the Heterotrophic Dinoflagellate Oxyrrhis Marina and the Copepod Acartia Tonsa on Vertical Carbon Flux in and Around Thin Layers of the Phytoflagellate Isochrysis Galbana

Dynamics of material and energy flow through food webs differ when resources are allocated in patches in comparison to situations in which the same resources are distributed evenly throughout the water column. Thin layers of plankton are special cases of such resource patches. While previous studies have predominantly focused on the response of organisms to these layers, we investigated how 2 types of grazers in turn affect thin layers. In an experimental study with tightly controlled environmental conditions, we monitored the redistribution of particulate organic (POC), dissolved organic (DOC) and inorganic (DIC) carbon from thin layers of Isochrysis galbana. The 2 grazers (the protist Oxyrrhis marina and the copepod Acartia tonsa) had significant grazing impact on the thin layers despite the fact that their population maxima were observed outside the layers. Both grazers exported carbon from the thin layer as body burden (i.e. incorporated into cell tissue) and through release of DOC and DIC into the environment above and below the layers, albeit at different rates. The copepods released larger amounts of DIC and DOC within the thin layer, while the protist grazer exported more dissolved carbon (DOC and DIC) from the thin layers. In the copepod treatments, a net increase of DIC was observed inside the thin layer (as a result of increased respiration during feeding) and into the atmosphere above the water column due to their vertical migration between the thin layer and the water surface. Whether or not grazers made a positive contribution to DOC net release depended on the strength of grazing, with a negative effect when phytoplankton— itself releasing DOC—was depleted.