Impacts of large-scale aquaculture activities on the seawater carbonate system and air-sea CO2 flux in a subtropical mariculture bay, southern China

Abstract

In this study, the variations of the seawater carbonate system parameters and air-sea CO 2 flux ( F CO 2) of Shen’ao Bay, a typical subtropical aquaculture bay located in China, were investigated in spring 2016 (March to May). Parameters related to the seawater carbonate system and F CO 2 were measured monthly in 3 different aquaculture areas (fish, oyster and seaweed) and in a non-culture area near the bay mouth. The results showed that the seawater carbonate system was markedly influenced by the biological processes of the culture species. Total alkalinity was significantly lower in the oyster area compared with the fish and seaweed areas, mainly because of the calcification process of oysters. Dissolved inorganic carbon (DIC) and CO 2 partial pressure ( p CO 2 ) were highest in the fish area, followed by the oyster and non-culture areas, and lowest in the seaweed area. Oysters and fish can have indirect influences on DIC and p CO 2 by releasing nutrients, which facilitate the growth of seaweed and phytoplankton and therefore promote photosynthetic CO 2 fixation. For these reasons, Shen’ao Bay acts as a potential CO 2 sink in spring, with an average F CO 2 ranging from -1.2 to -4.8 mmol m -2 d -1 . CO 2 fixation in the seaweed area was the largest contributor to CO 2 flux, accounting for ca. 58% of the total CO 2 sink capacity of the entire bay. These results suggest that the carbonate system and F CO 2 of Shen’ao Bay were significantly affected by large-scale mariculture activities. A higher CO 2 sink capacity could be acquired by extending the culture area of seaweed.