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 (delta 13C and delta 15N) 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 delta 15N 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 delta 13C and delta 15N values of fish and shrimp feed were -20.6 parts per thousand +/- 2.2 parts per thousand and 1.8 parts per thousand +/- 1.2 parts per thousand, respectively, which were different from the isotopic values of SOM in the study area. delta 13C and delta 15N values for SOM in different aquaculture areas were different from those of nearby reference stations, probably reflecting the influence of aquaculture. The delta 13C and delta 15N values in the oyster culture area (-25.9 parts per thousand and 6.0 parts per thousand, respectively) seemed to have reduced delta 13C and enriched delta 15N relative to those of the reference station (-24.6 parts per thousand and 5.8 parts per thousand, respectively). This may reflect the influence of organic matter on oyster culture. The delta 15N value of the station in the mixed culture area (7.1 parts per thousand +/- 0.4 parts per thousand) seemed to be relatively enriched in delta 15N relative to that of the reference station (6.6 parts per thousand). 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 delta 13C and delta 15N values. The sediment core at the mixed culture region (NS6) had lower TOC/TN ratios and more positive delta 13C and delta 15N 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 delta 15N value in the sediment of this area. This study provides new insights into the impact of aquaculture activities on SOM content.