Oyster cultivation regulating seasonal nitrate cycling in the estuarine water

Abstract

Effects of oyster cultivation on nitrate (NO₃^-) cycling exhibit discrepancies between model results and filed studies, necessitating seasonal investigations to clarify these differences. By combining geochemical analysis including water chemistry, NO₃^- dual isotopes (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-), and N-cycling functional genes (nitrifier genes and denitrifier genes), this study found that NO₃^- cycling is regulated by oyster growth activities. During the oyster's seeding periods, oyster cultivation insignificantly affected NO₃^-, and NO₃^- displayed a conservative pattern in the estuarine waters. Bayesian mixing model showed that manure and sewage were primary NO₃^- sources (>50 %), followed by atmospheric deposition (23 %). During the oysters' breeding periods, suspended particulate matter accumulation and relatively high abundance of AOA amoA gene in particle-associated communities promoted particulate matter associated nitrification. In the oysters' high filter-feeding periods, NO₃^- consumption (assimilation) occurred in surface water and NO₃^- production (nitrification) occurred in bottom water, as supported by increased nitrifier abundances. The increased nitrification was related to the consumption of NO₃^-, and thus gives rise to the remaining NH₄⁺ nitrified to NO₃^-. In the oysters' intensive filter-feeding periods, intensive nutrients demanding by oyster growth caused significant decrease in NO₃^- concentrations in estuarine waters. However, massive discharge of oyster feces into surface sediments may stimulate partial denitrification in the sediments and nitrification in the waters, leading to the decoupling of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- against ln [NO₃^-]. Overall, nitrification and partial denitrification in the estuarine ecosystems driven by oyster cultivation may be conducive to the production of nitrous oxide.

Keywords: Nitrate dual isotopes; Nitrogen; Oyster; Source; Zhanjiang Bay.