Size distribution of particles and zooplankton across the shelf-basin system in southeast Beaufort Sea: combined results from an Underwater Vision Profiler and vertical net tows

摘要

The size distribution and mean spatial trends of large particles (>100 μm, in equivalent spherical diameter, ESD) and mesozooplankton wereinvestigated across the Mackenzie Shelf (southeast Beaufort Sea, ArcticOcean) in July–August 2009. Our main objective was to combine results froman Underwater Vision Profiler 5 (UVP5) and traditional net tows (200 μmmesh size) to characterize the structural diversity and functioning of theArctic shelf-basin ecosystem and to assess the large-scale correspondencebetween the two methodological approaches. The core dataset comprised 154UVP5 profiles and 29 net tows conducted in the shelf (<100 m isobath),slope (100–1000 m) and basin (>1000 m) regions of the study area. The meanabundance of total particles and zooplankton in the upper water column(<75 m depth) declined exponentially with increasing distance from shore.Vertical and latitudinal patterns in total particle concentration followedthose of chlorophyll a (chl a) concentration, with maximum values between 30and 70 m depth. Based on the size-spectra derived from the UVP5 dataset,living organisms (0.1–10 mm ESD) accounted for an increasingly largeproportion of total particle abundance (from 0.1 % to >50 %) whenprogressing offshore and as the ESD of particles was increasing. Both theUVP5 and net tows determined that copepods dominated the zooplanktoncommunity (~78–94 % by numbers) and that appendicularians weregenerally the second most abundant group (~1–11 %). The verticaldistribution patterns of copepods and appendicularians indicated a closeassociation between primary production and the main grazers. Manualtaxonomic counts and ZooScan image analyses shed further light on thesize-structure and composition of the copepod community – which wasdominated at ~95 % by a guild of 10 typical taxa. The sizedistributions of copepods, as evaluated with the 3 methods (manual counts,ZooScan and UVP5), showed consistent patterns co-varying in the same orderof magnitude over the upper size range (>1 mm ESD). Copepods <1 mm werenot well quantified by the UVP5, which estimated that only ~13–25 %of the assemblage was composed of copepods <1 mm ESD compared with ~77–89 % from the net tow estimates. However, the biovolume of copepodswas overwhelmingly dominated (~93–97 %) by copepods >1 mm ESD. Ourresults illustrate that the combination of traditional sampling methods andautomated imaging techniques is a powerful approach that enabled us toconclude on the prevalence of a relatively high productivity regime anddominant herbivorous food web over the shelf when compared with thelow-productive recycling system detected offshore.