Role of arrowworm Sagitta elegans (Chaetognatha) in plankton communities of the western Bering Sea

Abstract

Arrowworm Saggita elegans is one of the important and abundant species in zooplankton communities of the western Bering Sea. Interannual variability of their biomass is considered on the data of long-term surveys conducted in 1986–2018, the species production is determined for the Anadyr-Navarin region and deep-water basin in 2008–2018, and the total production of zooplankton communities is calculated for 2008, 2015, and 2018. In the modern period, the biomass of S. elegans has significantly decreased compared to 1990–2000s, with exception of the shallow areas where it is relatively stable. The last decade included two periods with different oceanographic regime: the cold years (2007–2013) and the warm ones (2014–2018), but S. elegans did not demonstrate rapid response to such change of abiotic factors. However, the water circulation patterns in the upper layer were important for them and determined their transport and accumulation. For example, the high abundance and production of arrowworms in the deep-water area in 2008 and in the Anadyr-Navarin region in 2011 were conditioned by their transport and accumulation in these areas. S. elegans are predators, preying mainly upon copepods, so their impact on structure and production of zooplankton community is determined by significant grazing estimated at 50–100 % of Copepoda production or even more. The total production of zooplankton communities was higher in «warm» 2015 and 2018 than in «cold» 2008. The arrowworm production decreased in these warm years but production of non-predatory species (basically small- and medium-sized copepods) increased sharply because of their rapid response to temperature changes realized in high growth rate under higher temperature. Thus, the role of S. elegans in formation of the community production becomes lower in conditions favorable for high growth rate of non-predatory species.