Spatiotemporal variability, partitioning and mixing behavior of dissolved and particulate phosphorus in a typical...

Phosphorus (P) is the stoichiometrically limiting nutrient in marine ecosystem. In order to examine how terrestrial input regulates P dynamics, dissolved inorganic P (DIP) and dissolved organic P (DOP), particulate inorganic P (PIP) and particulate organic P (POP) were determined in a typical subtropical river-estuary-bay continuum, northern Beibu Gulf during wet season (strong terrestrial signal) and dry season (weak terrestrial signal). DIP was the main species in total dissolved P (TDP), comprising 88 ± 5 % and 78 ± 5 % during wet and dry seasons, respectively. Historical data (2011-2017) indicated that DIP concentrations in the Qinjiang River Estuary (QRE) were increased in recent years, mainly due to stronger terrestrial input. The distribution coefficient (logK_d) of P decreased with increasing suspended particulate matter (SPM) during wet season, suggested that P is highly particle reactive. Hydrological processes played a significant role in increasing DIP concentrations during estuarine mixing, and there were intensive removal/addition and transformation between P species, especially in wet season. The variations of stable carbon and nitrogen isotopes in SPM indicated that the SPM sources from terrigenous in wet season to more autochthonous in dry season. The reduction in DIP concentration was 1.22 μmol/L and 1.16 μmol/L in the wet and dry seasons, respectively, which is likely attributed to higher DIP bioavailability for phytoplankton uptake in the wet season (as evidenced by elevated chlorophyll a concentrations) compared to the dry season. Overall, this study highlighted the substantial impact of riverine input, hydrological and biological processes on P dynamics in coastal environments.