Contemporary mesozooplankton communities of the Beaufort Sea

Abstract

Zooplankton are critical trophic links and important modifiers of organic carbon cycles, yet are poorly characterized for much of the Arctic's Beaufort Sea, particularly in mesopelagic (> 200 m) waters. Zooplankton were sampled with 150 and 505 μm mesh nets in the upper 200 m in sections of the Beaufort Sea between Barrow Canyon and the Mackenzie River during August and September 2010-2013 to characterize the species composition, abundance, and biomass of epipelagic Beaufort Sea zooplankton communities. I observed 106 taxonomic zooplankton categories during four field seasons across both mesh sizes; copepods exhibited the highest species richness (38 species), followed by cnidarians (16 species) and amphipods (14 species). Average holozooplankton abundance ranged from 1110-3880 ind. m⁻³ in the 150-μm net and 47-215 ind. m⁻³ in the 505-μm net. Average holozooplankton biomass ranged from 23.8-76.9 mg dry-weight (DW) m⁻³ and 13.9-57.6 mg DW m⁻³ in the 150-μm and 505-μm nets, respectively. Spatial structure of zooplankton communities reflected a blending of across- and along-shelf temperature and salinity gradients that were driven by relative contributions of different water mass types. To characterize mesopelagic zooplankton communities of the Beaufort Sea, I collected stratified zooplankton samples and physical oceanographic data at stations along the Beaufort Sea slope during August 2013. I documented 93 taxonomic categories; greatest diversity was observed in the copepods (48 species), followed by the cnidarians (10 species) and amphipods (8 species). Distinct zooplankton communities were associated with the three main water masses in the study region: the Polar Mixed Layer (PML), Arctic Halocline Water (AHW), and Atlantic Water (AW). Average abundance and biomass were highest (1150 ind. m⁻³ and 27.1 mg DW m⁻³ , respectively) in the PML (0-50 m) and declined with depth, to a minimum in the 500-1000 m layer of AW (15 ind. m⁻³ and 0.6 mg DW m⁻³). Conversely, species richness increased with depth. Community structure was highly correlated with salinity and depth, both in terms of abundance (Spearman correlation (ρ) = 0.84, p < 0.01) and biomass (ρ = 0.81, p < 0.01). Zooplankton communities in the Beaufort Sea exhibit structure along three axes: alongshore, across-shore, and depth-related. Community structure along these axes reflects hydrographic gradients created by different water masses and physical factors in the study region. This work provides a contemporary benchmark for Beaufort Sea zooplankton community species composition, abundance, and biomass from which future change may be assessed.