Effects of global brightening on primary production and hypoxia in Ise Bay, Japan

Abstract

In many parts of the world, annual mean surface solar radiation (SSR) has undergone significant decadal changes; however, its effect on the coastal water environment has not been investigated. This study investigates the effects of changes in the SSR on hypoxia and the primary production of phytoplankton in a eutrophic bay in Japan (Ise Bay), where the annual SSR increased by 13.3% from 1980 to 2010. We numerically simulated the hydrodynamics and ecosystem of 2010 using a three-dimensional model (case O). We used this model to simulate the case where SSR was reduced by 10% (case A) and estimated the effect of an increase in SSR from the difference between case O and case A. With the 10% increase in SSR, the primary production in the bay increased by only 2.8%. This limited increase was the result of the negative effects by the nonlinearity of the light limitation function (including the photoinhibition) and the limitation in PO4–P availability and a significant positive effect by the increased water temperature. Similarly, the overall volume of hypoxic water increased, and in August, it increased by 5.8%. This is because water temperature and biomass such as phytoplankton increased with the increase in SSR; consequently, all oxygen consumption terms such as biological respiration also increased. These results imply that recent global brightening has the potential to amplify the primary production and hypoxia in a eutrophic bay.