Abstract
In this study, the variations of the seawater carbonate system parameters and air-sea CO2 flux (FCO2) 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 FCO2 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 CO2 partial pressure (pCO2) 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 pCO2by releasing nutrients, which facilitate the growth of seaweed and phytoplankton and therefore promote photosynthetic CO2 fixation. For these reasons, Shen’ao Bay acts as a potential CO2 sink in spring, with an average FCO2 ranging from -1.2 to -4.8 mmol m-2 d-1. CO2 fixation in the seaweed area was the largest contributor to CO2 flux, accounting for ca. 58% of the total CO2 sink capacity of the entire bay. These results suggest that the carbonate system and FCO2 of Shen’ao Bay were significantly affected by large-scale mariculture activities. A higher CO2 sink capacity could be acquired by extending the culture area of seaweed.
Highlights
Mariculture is developing rapidly in coastal waters and is one of the fastest-growing food-producing sectors in the world
Aquacult Environ Interact 13: 199–210, 2021 respiration, resulting in lower total alkalinity (TA), HCO3− and CO32− due to calcium carbonate (CaCO3) precipitation (Li et al 2021). Another view is that TA and the buffering capacity of carbonate chemistry in aquaculture waters are reduced by the rapid calcification processes of fast-growing calcifying organisms (Mos et al 2015); lower TA decreases the relative calcite saturation state for a given partial pressure of CO2 (Mos et al 2015), causing the release of CO2 into the atmosphere (Fine et al 2017)
The uptake of nitrate (NO3−) and NH4+ by seaweed modifies TA (Brewer & Goldman 1976, Goldman & Brewer 1980). These findings suggest that the interactions between mariculture and the seawater carbonate system are very complex and that the analysis of their comprehensive effects should be based on species- and sitespecific investigations
Summary
Mariculture is developing rapidly in coastal waters and is one of the fastest-growing food-producing sectors in the world. Aquacult Environ Interact 13: 199–210, 2021 respiration, resulting in lower total alkalinity (TA), HCO3− and CO32− due to calcium carbonate (CaCO3) precipitation (Li et al 2021) Another view is that TA and the buffering capacity of carbonate chemistry in aquaculture waters are reduced by the rapid calcification processes of fast-growing calcifying organisms (Mos et al 2015); lower TA decreases the relative calcite saturation state for a given partial pressure of CO2 ( pCO2) (Mos et al 2015), causing the release of CO2 into the atmosphere (Fine et al 2017). PCO2 is sensitive to TA, pH, salinity and temperature conditions; any mariculture activity that alters these parameters could affect seawater pCO2 and air−sea CO2 exchange In addition to these direct influences, the release of ammonium (NH4+) increases TA, while the nitrification of NH4+ and remineralization of organic particles (e.g. animal feces and feed residue) decrease TA (Schlesinger 1997, Wolf-Gladrow et al 2007). The carbonate system characteristics of surface seawater in different cultivation areas were investigated, and direct influences of physiological processes and the indirect influences of hydrographic factors were examined in order to assess the impact of large-scale aquaculture activities on the seawater carbonate system and air− sea FCO2, as well as to further understand the interactions between aquaculture and the marine environment at a bay scale
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