Marine snow, zooplankton and thin layers: indications of a trophic link from small-scale sampling with the Video Plankton Recorder

Marine aggregates of biogenic origin, known as marine snow, are considered to play a major role in the ocean's particle flux and may represent a concentrated food source for zooplankton. However, observing the marine snow-zooplankton interaction in the field is difficult since conventional net sampling does not collect marine snow quantitatively and cannot resolve so-called thin layers in which this interaction occurs. Hence, field evidence for the importance of the marine snow-zooplankton link is scarce. Here we employed a Video Plankton Recorder (VPR)to quantify small-scale (metres) vertical distribution patterns of fragile marine snow aggregates and zooplankton in the Baltic Sea during late spring 2002. By using this non-invasive optical sampling technique we recorded a peak in copepod abundance (ca. 18 ind. l(-1)) associated with a pronounced thin layer (50 to 55 m) of marine snow (maximum abundance of 28 particles l(-1)), a feature rarely resolved. We provide indirect evidence of copepods feeding on marine snow by computing a spatial overlap index that indicated a strong positively correlated distribution pattern within the thin layer. Furthermore we recorded images of copepods attached to aggregates and demonstrating feeding behaviour, which also suggests a trophic interaction. Our observations highlight the potential significance of marine snow in marine ecosystems and its potential as a food resource for various trophic levels, from bacteria up to fish.