Nutrient budgets and biogeochemical dynamics in the South China Sea
Figure 1. Sampling sites and investigated estuaries in the northern Beibu Gulf. Black dots represent stations where surface water samples were collected, while green dots represent the stations where surface, overlying and pore water samples were collected. The dotted boxes represent the investigated estuaries analyzed with the LOICZ box model.
This study examined the nutrient budgets and biogeochemical dynamics in the coastal regions of northern Beibu Gulf, in the South China Sea. Field investigations were conducted in August and December 2017. A total of 264 surface water samples were collected at 88 stations, with three separate samples at each site. Similarly, a total of 66 overlying (bottom) water samples were collected at 22 selected stations. Surface and overlying water samples were immediately filtered through a 47 mm mixed cellulose ester membrane. Filtered samples were then stored in polyethylene bottles containing saturated HgCl2 and kept frozen at 20 ◦C until nutrient analysis. At the selected 22 stations, surface sediment samples were also collected and divided into two replicates, one for the measurement of chemical parameters and the other for pore water extraction. Porewater was extracted from sediments by centrifugation (3500 rpm, 40 min) and preserved similarly to the surface water samples. Main results are discussed below. The full article is available at https://doi.org/10.1016/j.marenvres.2024.106447
Figure 2. Nutrient concentrations in rivers of northern Beibu Gulf (n =6)
Figure 3. Spatial and temporal distribution of nutrients in the northern Beibu Gulf
Nutrient concentrations varied spatially and seasonally among the different bays. High nutrient levels were found in the regions with high riverine inputs and intensive mariculture. Using a three end-member mixing model, nutrient biogeochemistry within the ecosystem was estimated separately from complex physical mixing effects. Nutrient consumption dominated in most bays in summer, whereas nutrient regeneration dominated in winter, likely due to phytoplankton decomposition, vertical mixing and desorption.
Figure 4. Relationship between nutrient and salinity in the northern Beibu Gulf
We observed a significant seasonal difference in river discharge and, consequently, a significant seasonal difference in the fluxes of nutrients, freshwater, and sediments to the gulf. There was a significant negative correlation between DIN/Si(OH)4 and salinity in both summer and winter, while the relationship between DIP and salinity was not obvious. In summer, the fitting curve of DIN and salinity was lower than the theoretical dilution line, indicating that in addition to the simple mixing dilution of DIN, other DIN consumption processes occurred, such as absorption and assimilation of phytoplankton, denitrification, anammox, etc. In winter, the fitting curve of DIN and salinity basically coincided with the theoretical dilution line, indicating that DIN exhibited a conservative dilution behavior in winter. The possible reason was that the growth and reproduction ability of phytoplankton in winter was weakened due to low temperature, thus reducing the absorption and assimilation of DIN.
Figure 5. Nutrient budgets in the bays of northern Beibu Gulf
Nutrient budgets were constructed using the Land-Ocean Interaction Coastal Zone (LOICZ) model. Results of the modeling indicates that Beibu Gulf behaves as a sink of dissolved inorganic nitrogen, phosphorus and silicon. River-borne nutrient inputs were the dominant nutrient source, while residual flows and water exchange flows transported nutrient off the estuaries.
The full article, published in the journal Marine Environmental Research, is available at https://doi.org/10.1016/j.marenvres.2024.106447