Ecosystem dynamics in Tokyo Bay with a focus on high trophic levels using Ecopath with Ecosim

Abstract

The present study aimed to investigate ecosystem dynamics in Tokyo Bay, a semi-enclosed bay surrounded by the Tokyo metropolitan area in Japan, which is one of the largest and most populous industrialized areas in the world. The bay has been subject to eutrophication since the 1960s, and excessive increase in phytoplankton biomass has led to hypoxia. Ecopath with Ecosim was used to simulate the phytoplankton dynamics, and it could closely reproduce the observed relative biomass. To evaluate the impacts of phytoplankton dynamics, hypoxia, and fishing on the dynamics, with a focus on high trophic levels (up to fish), 3 scenarios, “Yearly constant phytoplankton biomass,” “No hypoxia,” and “No fishing,” were tested. Comparisons with the “Control” scenario without any modifications suggested that (1) the dynamics was controlled by phytoplankton (bottom-up control), (2) hypoxia did not have a serious effect on the past dynamics, and (3) stopping fishing would not contribute to recover of the biomass of exploited fish. Predictions for future dynamics under the scenarios “DO deteriorated” and “DO unchanged” suggest that if DO deteriorates strongly enough to decrease the survival of most benthos, the ecosystem might undergo a non-negligible transformation through extinction or biomass decrease of some benthos.