Biogeochemistry of dissolved and particulate phosphorus speciation in the Maowei Sea, northern Beibu Gulf

Abstract

Phosphorus (P) is an important biogenic element that limits algal growth, and influences primary productivity and biogeochemical cycling of other biogenic elements in freshwater and marine ecosystems. Surface water samples were collected along the salinity gradients from the western and eastern flanks of the Maowei Sea (MWS) during July 2018 for measurements of different P species, including dissolved nitrogen and silicate. Total dissolved P (TDP) dominated the total P (TP) accounting for 64.9 ± 9.7% of TP, possibly due to relatively less suspended particulate matter (<100 mg/L). Among the dissolved P species, dissolved organic P (DOP) was more abundant (DOP/TDP = 58.1 ± 20%) than dissolved inorganic P (DIP). Considering the entire dataset, the average DIP concentration (1.53 ± 0.83 μM) was lower to medium range, while DOP was relatively higher, compared to other estuaries and coastal waters bodies worldwide. Particulate P was mainly present as particulate inorganic P (PIP), which accounted for 68.5 ± 9.0% of the total particulate P (TPP). The behavior of different P species along the salinity gradients indicated that their concentrations were dominantly influenced by riverine inputs and biogeochemical processes. Intensive mariculture activities in the MWS may have also contributed to elevated DOP compared to other coastal marine systems worldwide. Apart from similar sources of input and cycling processes, the stoichiometric relationship between DIP, dissolved inorganic N (DIN) and dissolved silicate (DSi) suggested that P is potentially limiting, while Si may co-limit primary productivity. The latitudinal distribution of different P species indicated that strong transformations existed between the particulate and dissolved P phases along the salinity gradients. The negative relationship between P partitioning coefficient [log(Kd)] and salinity, and with suspended particulate matter (log[SPM]) suggested that strong particle reactivity and probable competition between dissolved P with other anions for adsorption sites influenced the partitioning of P species in surface water. Additionally, the end-member binary model showed that ~1.24–1.55 μM of DIP may have been removed through biological uptake. In summary, this study shows that the dynamic changes in the concentrations, spatial distribution, and transformation process of different P species between dissolved and particulate phases can effectively reveal the sources of P and understand the internal relationship between the biogeochemical cycling of P, stoichiometric status, and eutrophication.