Impact of Variation in Nutrient Concentration on Ecosystem, Biomass, and Production in a Semi-enclosed Embayment: Osaka Bay

Abstract

In Osaka Bay, where total nitrogen (TN) and dissolved inorganic nitrogen (DIN) have decreased significantly, we investigated changes in biomass and seasonal fluctuation patterns at each trophic level of the ecosystem. The water temperature, which can affect the structure of an ecosystem, did not increase during the survey period of 1990 to 2019. The decrease in DIN caused the decrease in phytoplankton abundance represented by chlorophyll-a and the primary production. This resulted in a decline in the population of zooplanktons, such as ciliates and copepods, and a decrease in larval fish abundance. The ecosystem in the bay was a bottom-up system in which the biomass of each trophic level responded almost linearly to the amount of primary production. The decrease in TN caused the expansion of the DIN depletion period from only summer in the 1990s to spring–summer in the 2010s. The biomass of fish larvae, especially that of inner-bay species, greatly decreased, whereas anchovy, a wide-area migratory fish, did not decrease, resulting in the dominance of anchovy over other fish species.