Spatial and Seasonal Variations of Sedimentary Organic Matter in a Subtropical Bay: Implication for Human Interventions

Abstract

Elemental (total organic carbon (TOC) and total nitrogen (TN)) and stable carbon and nitrogen isotope compositions (δ13C and δ15N, respectively) in the surface sediment of Zhanjiang Bay (ZJB) in spring and summer were measured to study the spatial and seasonal changes of organic matter (OM) and assess the human-induced and environment-induced changes in the area. The OM in the surface sediment of ZJB was a mixture of terrestrial and marine sources, and was dominated by marine OM (54.9% ± 15.2%). Compared to the central ZJB, the channel and coastal ZJB areas had higher δ13C and δ15N values, higher TOC and TN concentrations, and lower TOC/TN ratios, indicating higher primary productivity and higher percentages of marine OM in the latter two subregions. Mariculture activities, sewage inputs, and dredging were responsible for these phenomena. Clear seasonal variations in OM were observed in ZJB. The average proportions of terrestrial OM in summer increased by 10.2% in the ZJB channel and 26.0% in the coastal ZJB area compared with those in spring. Heavy rainfall brought a large amount of terrestrial OM into the channel and coastal ZJB areas, leading to the increase of the terrestrial OM fraction in these two subregions in summer. In summary, anthropogenic influences had a significant influence on the spatial and seasonal variations of sedimentary OM in ZJB.

Spatiotemporal Distribution Characteristics of Nutrients in the Drowned Tidal Inlet under the Influence of Tides: A Case Study of Zhanjiang Bay, China

“…However, the water environment is poor due to the small area, narrow topography, small flow velocity of part of the tidal basin, and because the coastal area of this part is the location of a densely populated area that includes the industrial zone and port of Zhanjiang City, whereby the main terrestrial pollutants are also discharged into the sea area here [ 38 , 39 ]. The tidal inlets connecting oceans to estuaries profoundly influence the ecology of estuarine water bodies [ 4 ].…”

“…Over recent decades, rapid economic development and urbanization have significantly impacted the ZJB environment. Eutrophication and harmful algal blooms have occurred frequently [ 38 , 39 ]. A scientific understanding of the response mechanism of the water quality environment of ZJB to special geomorphological units is the prerequisite and basis for solving the water pollution problem.…”

Origin of the Particulate Organic Matter in a Monsoon-Controlled Bay in Southern China

“…C 4 plants are not common in the area surrounding Zhanjiang Bay since the main cultivated botany are C 3 plants (such as rice) and the subtropical forest is mainly a natural ecosystem. Thus, the contribution of the C 4 plants to the POM in Zhanjiang Bay can be ignored [26]. According to the isotopic ranges of potential sources, the ranges of the δ 15 N and δ 13 C values in summer and winter indicate that terrestrial organic matter and marine organic matter may be the main POM sources in the study area, respectively (Figure 12).…”

“…In winter, the dominant contributions of POM were from marine organic matter (53%), C 3 plants (20%), soil organic matter (17%), freshwater algae (5%), and sewage (5%) (Figure 13d), indicating that the source was mainly dominated by phytoplankton biomass. The cause for the large credibility intervals obtained for each source component in the Bayesian analysis might be that the spatial difference of land use in Zhanjiang bay is obvious due to the different natural conditions and economic level [25][26][27]55]. However, we believe that this estimation can at least help us understand the source of nitrogen in Zhanjiang Bay to a certain extent.…”

“…Therefore, determining the sources of the POM in the bay is of great importance to controlling regional water eutrophication. Although several studies have examined the stable carbon and nitrogen isotope compositions in Zhanjiang Bay, many of these studies were confined to the surface sediments [26], and no systematic studies have been conducted to elucidate the sources and distribution of the POM in Zhanjiang Bay. Since the bay is controlled by the monsoon seasons and experiences concentrated and frequent human activities, the seasonal changes in the POM sources and their biogeochemical processes are not well constrained.…”