Nitrogen cycling throughout the distinct stages of oyster cultivation: From a multiple-isotope perspective

Oyster aquaculture has expanded worldwide in recent decades as a highly productive ecosystem. In these areas, oysters contribute to the removal of dissolved inorganic nitrogen (DIN) from the water column by taking up large quantities of suspended particulate organic matter (POM), thereby playing a critical role in water purification. However, a quantitative description of nitrogen (N) cycling throughout the distinct stages of oyster cultivation in such areas is lacking. This study measured δ¹⁵N values in various N pools, i.e., ammonium (δ¹⁵N-NH₄⁺), nitrate (δ¹⁵N-NO₃^-), and particulate N (δ¹⁵N-PN) in the water column, oyster adductor muscle, oyster excreta, and surface sediment to systematically assess N cycling in estuarine systems with intensive oyster aquaculture. During the oyster seeding and breeding periods, oyster-related removal of POM and DIN was weak. POM and DIN were regulated by their sources during the former period, whereas soil organic matter transported from terrestrial systems may contribute to in situ POM degradation in estuarine systems with intensive oyster aquaculture during the latter period. By contrast, during periods of high oyster filter-feeding activity, oyster-related removal of POM and DIN increased, and DIN was assimilated by phytoplankton to form POM. During periods of intensive oyster filter feeding, the removal of DIN and POM from the water column was facilitated by oysters, thus suppressing phytoplankton blooms. Our study showed that oyster growth activities were an important seasonal regulator of the seasonal dynamics of POM and DIN, thereby inhibiting phytoplankton blooms and contributing to in situ POM degradation in estuarine systems with intensive oyster aquaculture.