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

Abstract

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.

Inter- and intra-specific diurnal habitat selection of zooplankton during the spring bloom observed by Video Plankton Recorder

“…an essential part of their life history is thus to accumulate and store reserves during the phytoplankton bloom period, but also to survive until the next winter (e.g., Falk-Petersen et al 2009;Varpe 2012). In addition, they may encounter sufficient feeding opportunities at depth by foraging on detritus or marine snow (alldredge and Silver 1988; Hansen et al 1996;Möller et al 2012). an income breeding species with a shorter life cycle, such as C. finmarchicus (Conover 1988), is more dependent on current food intake.…”

“…Besides the precise depth position, an additional benefit of the VPr is the relative ease of fitting sensors that provide concurrent data on hydrography (temperature, salinity, density) and phytoplankton biomass (chlorophyll fluorescence) from the same parcel of water as imaged by the VPr, thus providing finely resolved information on zooplantkon distributions in relation to the environment. These data allow for investigation into the presence of thin layers and hot spots of predator-prey activity which can be overlooked if sampling gear does not have fine resolution (e.g., Möller et al 2012).…”

Fine-scale vertical distributions of Mnemiopsis leidyi ctenophores: predation on copepods relative to stratification and hypoxia

“…Woodson & McManus 2007); however, evidence of increased feeding in situ is rare (see Möller et al 2012).…”

“…Keister et al 2000, Schaber et al 2011 and at finer scales with acoustic, particlecounting, and camera systems (e.g. Cheriton et al 2007, Möller et al 2012, Bi et al 2013). Concentration of organisms at water column discontinuities (clines) is hypothesized to enhance production, trophic interactions, and dispersion (reviewed by McManus & Woodson 2012).…”

“…Evidence of behavioral concentra-tion of organisms and enhanced feeding at clines abounds in the laboratory (e.g. Woodson & McManus 2007); however, evidence of increased feeding in situ is rare (see Möller et al 2012).…”