Drastic and synchronous changes in megabenthic community structure concurrent with environmental variations in a eutrophic coastal bay

Abstract

This study demonstrated long-term changes in the stock size and species composition of the megabenthic community, including both vertebrates and invertebrates, in Tokyo Bay, Japan, a semi-enclosed coastal area in which the ecosystem has been affected by both natural and anthropogenic factors. We conducted fisheries-independent trawl surveys over a total of 26 yr (1977–1995 and 2003–2009) to collect megabenthos from the entire bay. We also obtained environmental data for the bay from published records to investigate relationships between biota and environment. Abundance- and biomass-based densities of the megabenthos increased until the late 1980s, followed by an abrupt decline, which we attribute to changes in the densities of small to mid-sized dominant species. In the 2000s, although the abundance-based density remained low, biomass-based density increased substantially due to an increase in the numbers of large fishes. Cluster analysis divided the survey period into six groups showing characteristic species compositions and densities. Stepwise multivariate discriminant analysis (SMDA) revealed that changes in water temperature, dissolved oxygen concentration, precipitation, and copepod density were accompanied by simultaneous changes in the megabenthic community. The SMDA cross-validation test indicated that the percentage of incorrect classifications of the model ranged from 14.3% to 66.7%, implying that factors not incorporated in the model, such as fisheries pressure, might affect the changes in the megabenthic community structure. Our results suggest that major shifts have occurred in the megabenthic community structure in Tokyo Bay, and that both natural and anthropogenic factors can trigger these shifts in the system.