Structures of coexisting marine snow and zooplankton in coastal waters of Svalbard (European Arctic)

How plankton and particles are arranged spatially and the configurations of their co-occurrence shape the rates of organic matter production, utilization, and export within marine systems. The aim of this study was to examine whether the composition of marine snow (particles and aggregates >500 mm) and its coexistence with zooplankton change with depth layer, level of zooplankton dominance, chlorophyll fluorescence, and turbidity across the coastal-offshore gradients of hydrographically different Arctic fjords. The distribution and concentrations of zooplankton and marine snow were assessed in situ using an underwater vision profiler (UVP) in Svalbard waters during summer 2019. UVP counts of marine snow drastically outnumbered zooplankton at glacial stations, whereas zooplankton dominated offshore and in upper water layers, even in coastal waters. The most common compositional structure was dominance by an elongated morphotype of marine snow, often co-occurring with small dark (opaque) particles below 40 m depth, implying that these were the typical forms exported directly from surface layers. The other widespread type of structuring was dominance of UVP counts by copepods. They often coexisted with a flake morphotype of marine snow associated with high chlorophyll fluorescence. Structuring dominated by dark morphotypes was observed mainly near glaciers and in deep fjord basins. The highest amount of marine snow, represented by a high degree of dark morphotype, was observed in Hornsund, the most Arctic-type fjord. A Phaeocystisassociated agglomerated morphotype of marine snow occurred scarcely and only in more Atlantic-influenced fjords. A bimodal distribution pattern, with one abundance peak at the surface and another in deeper layers (>80 m) was observed offshore and in Kongsfjorden. This study emphasizes the high potential of UVPs for tracking links between plankton and detritus directly in their natural environment, and that variation in their co-occurrence may provide a proxy for the state of a pelagic ecosystem.