Distribution and sources of sedimentary organic matter in different aquaculture areas of northeastern Zhanjiang Bay using stable carbon and nitrogen isotopes

Zhanjiang Bay is a major aquaculture area in China with many types of mariculture products (such as oysters, fish, and shrimp). The culture area and shrimp output in Zhanjiang Bay are ranked first in China. We investigated the total organic carbon (TOC), total nitrogen (TN), TOC/TN ratio, and stable isotopes (δ¹³C and δ¹⁵N) of the fish and shrimp feed, fish and shrimp feces, and sedimentary organic matter (SOM) in and around different aquaculture areas of northeastern Zhanjiang Bay to study the impact of aquaculture activities on SOM. The average TOC contents of fish and shrimp feed were 39.20% ± 0.91% and 39.29% ± 0.21%, respectively. The average TOC content in the surface sediments of the oyster culture area, the mixed (fish and shrimp) culture area, and the cage fish farm area were 0.66%, 0.88% ± 0.10%, and 0.58% ± 0.19%, respectively, which may indicate that mixed culture had a greater impact on SOM. The relatively high TOC and TN contents and relatively low TOC/TN ratios, and δ¹⁵N values in the upper layer of the core sediment in the mixed culture area could also support the significant influence of mixed culture. The average δ¹³C and δ¹⁵N values of fish and shrimp feed were −20.6‰ ± 2.2‰ and 1.8‰ ± 1.2‰, respectively, which were different from the isotopic values of SOM in the study area. δ¹³C and δ¹⁵N values for SOM in different aquaculture areas were different from those of nearby reference stations, probably reflecting the influence of aquaculture. The δ¹³C and δ¹⁵N values in the oyster culture area (−25.9‰ and 6.0‰, respectively) seemed to have reduced δ¹³C and enriched δ¹⁵N relative to those of the reference station (−24.6‰ and 5.8‰, respectively). This may reflect the influence of organic matter on oyster culture. The δ¹⁵N value of the station in the mixed culture area (7.1‰ ± 0.4‰) seemed to be relatively enriched in δ¹⁵N relative to that of the reference station (6.6‰). Sedimentation and the subsequent degradation of organic matter from mixed cultures may have contributed to this phenomenon. The surface sediment at the cage fish farm area seemed to be affected by fish feces and primary production based on the indication of δ¹³C and δ¹⁵N values. The sediment core at the mixed culture region (NS6) had lower TOC/TN ratios and more positive δ¹³C and δ¹⁵N values than the sediment core at the oyster culture area, suggesting a higher proportionate contribution of marine organic matter in the mixed culture area. In summary, oyster culture, mixed culture, and cage fish culture in northeastern Zhanjiang Bay had a certain degree of impact on SOM, and mixed culture had more significant influences on SOM based on the high TOC contents and the significant vertical variations of TOC/TN ratio and δ¹⁵N value in the sediment of this area. This study provides new insights into the impact of aquaculture activities on SOM content.