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

Abstract

Abstract. The size distribution and mean spatial trends of large particles (>100 µm, in equivalent spherical diameter, ESD) and mesozooplankton were investigated across the Mackenzie Shelf (southeast Beaufort Sea, Arctic Ocean) in July-August 2009. Our main objective was to combine results from an Underwater Vision Profiler 5 (UVP5) and traditional net tows (200 µm mesh size) to characterize the structural diversity and functioning of the Arctic shelf-basin ecosystem and to assess the large-scale correspondence between the two methodological approaches. The core dataset comprised 154 UVP5 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 mean abundance 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 followed those of chlorophyll a (chl a) concentration, with maximum values between 30 and 70 m depth. Based on the size-spectra derived from the UVP5 dataset, living organisms (0.1-10 mm ESD) accounted for an increasingly large proportion of total particle abundance (from 0.1 % to >50 %) when progressing offshore and as the ESD of particles was increasing. Both the UVP5 and net tows determined that copepods dominated the zooplankton community (∼78-94 % by numbers) and that appendicularians were generally the second most abundant group (∼1-11 %). The vertical distribution patterns of copepods and appendicularians indicated a close association between primary production and the main grazers. Manual taxonomic counts and ZooScan image analyses shed further light on the size-structure and composition of the copepod community -which was dominated at ∼95 % by a guild of 10 typical taxa. The size distributions of copepods, as evaluated with the 3 methods (manual counts, ZooScan and UVP5), showed consistent patterns co-varying in the same order of magnitude over the upper size range (>1 mm ESD). Copepods <1 mm were not 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 copepods was overwhelmingly dominated (∼93-97 %) by copepods >1 mm ESD. Our results illustrate that the combination of traditional sampling methods and automated imaging techniques is a powerful approach that enabled us to conclude on the prevalence of a relatively high productivity regime and dominant herbivorous food web over the shelf when compared with the low-productive recycling system detected offshore.

Modeling plankton ecosystem functioning and nitrogen fluxes in the oligotrophic waters of the Beaufort Sea, Arctic Ocean: a focus on light-driven processes

“…Mortality is assumed to be mainly due to predation (Eiane et al, 2002) and is described by a density-dependant quadratic function. The latter implicitly represents cannibalism as well as predation by appendicularians observed during the Malina cruise (Forest et al, 2012) and limits the occurrence of oscillations generated in such non-linear systems (Edwards and Bees, 2001). The constant of mortality is set to 0.2 (mmol N m −3 ) −1 to simulate realistic mortality rates (e.g., Ohman et al, 2004).…”

“…Production rates estimated in pmol Leu L −1 h −1 were converted into carbon equivalent using a conversion factor of 1.5 kg C (mol Leu) −1 (Kirchman et al, 2009). Copepod biomasses were obtained from Underwater Video Profiler data converted into carbon units (Forest et al, 2012) and then into nitrogen using a molar C : N ratio of 8.1 (Forest et al, 2010).…”

“…Mathematical formulations and parameters related to large zooplankton (LZ) dynamics were chosen to reflect copepods as they dominate in abundance at the study station (Forest et al, 2012). Grazing (d −1 ) is described by an Ivlev function:…”

Modelling the impact of riverine DON removal by marine bacterioplankton on primary production in the Arctic Ocean

“…Bacterioplankton (BACT) are explicitly represented following the model of Fasham et al (1990). They grow on NH 4 , dDON, and on the usable fraction of RDON (see the Appendix for details).…”

“…The simulated NH 4 uptake supplements their nitrogen requirements. The growth function is formulated using the Fasham et al (1990) model. It assumes, in a balanced growth situation, where N and C assimilation occurs simultaneously and where bacterioplankton have fixed stoichiometry, that the ratio of NH 4 uptake to DONl uptake is constant (0.6; see Appendix A) to ensure that the biomass of the required C : N ratio is produced from DONl with a given C : N ratio.…”