Sinking vs. remineralisation: Active controls of zooplankton on marine snow dynamics

Klas Ove Moeller, Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Geesthacht, Germany and Morten H. Iversen, Alfred Wegener Institute Helmholtz-Center for Polar and Marine Research Bremerhaven, Polar Biological Oceanography, Bremerhaven, Germany; MARUM - University of Bremen, Bremen, Germany
Marine aggregates of biogenic origin, known as marine snow, are considered to play an important role in the oceans particle flux. These aggregates are the major vector for the transfer of carbon from the upper ocean to deeper layers and a potential food source for zooplankton. Understanding and predicting the distribution, characteristics and dynamics of marine snow in the ocean is hence a fundamental part in understanding the marine ecosystem, from food web dynamics to global biogeochemical cycles. However, our mechanistic understanding of the processes controlling the biological carbon pump is limited by a lack of observational data at appropriate scales since conventional sampling methods do not collect the vertical distribution of marine snow quantitatively.

We here showcase results from towed- and stationary high-resolution underwater imaging systems in various coastal- and open ocean marine ecosystems. We present data on zooplankton and particle small-scale distribution patterns during the transition from the convective winter regime to late spring bloom conditions and provide indirect evidence of zooplankton feeding on marine snow aggregates by images indicating a trophic interaction. Furthermore, we observed and quantified differences in sinking speeds and utilization of aggregates by copepods depending on the origin and size. Our observations suggest the heterotrophic dinoflagellate Noctiluca scintillans to be an important gatekeeper of the biological carbon pump in coastal oceans ranging from temperate to tropical regions, which was supported by negative correlations between N. scintillans abundances and marine snow concentrations, as well as high ingestion and clearance rates during on-board incubations. Our observations highlight the trophic significance of aggregates in marine ecosystems and provide new insights into marine snow dynamics to better understand and quantify the variability of the oceans biological carbon pump.