Seasonal Trophodynamics of the Upper Epipelagic Nekton Community in the Western Bering Sea

Abstract

Complex ecosystem analysis which includes total abundance estimation of its members as well as their quantitative trophic relationships gives an insight to ecosystem structure, its dynamic and steadiness and allows to compare large marine ecosystems. In this paper we aim to reveal and quantify the seasonal dynamics of main trophic web components and to estimate nekton (fish and squids) food capacity throughout the summer-autumn in upper epipelagic layer of deep basins in the Western Bering Sea. We used nekton and plankton abundance data as well as nekton diet data obtained in June-October throughout 2003–2015. Log-transformed abundance were averaged by 10-day periods considering the probability of occurrence of given species. We have found that the total biomass of nekton and the species ratio showed severe changes within summer-autumn. Total biomass reached the maximum in August showing two- or three-time increase from early-summer and followed by the similar decrease in autumn. The total food consumption changed in the same way. However, the grazing pressure on prey was very low as the consumer/prey biomass ratio approached zero. In addition, we have recognized diet divergence both between species and within size group of one specie. We conclude that the spatio-temporal separation of mass nekton species and differences in their diet lead to alleviation of trophic competition and grazing pressure.