Spatiotemporal variability, partitioning and mixing behavior of dissolved
and particulate phosphorus in a typical subtropical river-estuary-bay continuum
- Introduction
Phosphorus (P) is an essential and crucial macronutrient for the living organisms in the ocean (DeDuve, 1991; Wilson et al., 2023; Lan et al., 2024). P chemical speciation in seawater mainly include dissolved inorganic P (DIP), dissolved organic P (DOP), particulate inorganic P (PIP), and particulate organic P (POP). Terrestrial input is the main P source in coastal ecosystems (Shen et al., 2020; Gurung et al., 2020),
accounting for 75–94 % (Benitez-Nelson, 2000). Refractory P from riverine sources is primarily removed from the oceanic P cycle through burial in coastal sediments (Goll et al., 2023), while reactive P is mainly taken up by algae and then lost from the water column by burial as particulate organic matter (Wang et al., 2024). In addition, other possible processes for the removal of P species include adsorption and precipitation/flocculation of particulate P, as implied by the quick decrease in suspended particulate matter concentration with increasing
- Corresponding author at: Jiangsu Key Laboratory of Marine Biotechnology, Jiangsu Ocean University, Lianyungang, 222005, China. E-mail address: binyang@jou.edu.cn (B. Yang).
https://doi.org/10.1016/j.marpolbul.2025.118854
Received 10 August 2025; Received in revised form 24 September 2025; Accepted 14 October 2025
Available online 27 October 2025
0025-326X/© 2025 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.
L. Yang et al. Marine Pollution Bulletin 222 (2026) 118854
salinity (Lin et al., 2012; Zhang et al., 2021). The uptake, reminerali- zation, and physical and biological exchanges among these various pools are the essential processes of the marine P cycle (Testa et al., 2022; Mu et al., 2023; Li et al., 2024). P chemical speciation in dissolved and particulate phases can vary significantly along the salinity gradient since several P biogeochemical processes (i.e., P removal, migration, trans- formation, and regeneration) occurs in the estuarine areas during mix- ing processes (Lin et al., 2013; Huang et al., 2025). Thus, understanding the spatiotemporal variability, partitioning, and mixing behavior of dissolved and particulate P species in estuarine systems are important for knowing P biogeochemical cycle from river-estuary-bay continuum as well as to manage the eutrophic estuaries.
P availability can limit the growth of phytoplankton (Harke et al., 2013), regulate the structure of phytoplankton community (Prentice et al., 2015), and stimulate or constrain harmful algal bloom biomass (Schindler et al., 2016). Given the importance of P for marine phyto- plankton, numerous methods are used to assess P limitation status for phytoplankton communities (Deng et al., 2022; Shen and Wang, 2025). These include direct measurements of P concentrations in water column and its molar ratios to other nutrients (Maberly et al., 2020), and nutrient stoichiometry of biomass (Sterner and Elser, 2002). For instance, the molar ratios of dissolved organic carbon (DOC): DOP, dissolved inorganic nitrogen (DIN): DIP, and dissolved silicate (DSi): DIP were used to study the responses of different phytoplankton species to P limitation, as the N/P ratio has been always suggested as an indicator of either N or P limitation (Maberly et al., 2020).
While rivers are the main sources of P for the coastal waters (Turner et al., 2003; Hu et al., 2021), therefore, estuaries are the key land to ocean exchange zones where physicochemical and biogeochemical processes could change the chemical speciation and fluxes of P dis- charged from rivers to sea (van der Zee et al., 2007; Liu et al., 2024). Qinzhou Bay (QZB) is a typical subtropical P-limitation bay influenced mostly by the discharge of the Qinjiang River (Lu et al., 2022), which will be the direct connecting water system of the first canal (Pinglu Canal) that links the river to the sea since the founding of the People’s Republic of China. Nevertheless, the dynamics of dissolved and
particulate P and their behavior during estuarine mixing remain poorly understood. Moreover, the spatiotemporal variations of stable carbon and nitrogen isotopes (δ13C and δ15N) within the aquatic ecosystems of the Qinjiang River Estuary (QRE) have received little attention. It has been reported that DIP increased by 150 % in the QZB from the rapid urbanization during 2006–2017, and eutrophication of the QZB was mainly attributed to the riverine nutrient inputs (Lu et al., 2022). Thus, the combination of field investigation and laboratory experiments can provide a new insight into understanding the dynamics of P species and its mixing behavior in estuarine environments.
Our major objectives were to (1) quantify the abundance of inorganic and organic P species between dissolved and particulate phases along a transect from the lower Qinjiang River to the QZB during wet and dry seasons when P typically limits productivity; (2) examine the spatio- temporal variability of different P species during estuarine mixing and P- limitation changes between wet and dry seasons; and (3) identify the mixing behavior of P species and the relative importance of different processes in P biogeochemical cycling from the estuarine environments, through both field investigation and laboratory mixing experiments. This study will contribute to a better understanding of P biogeochemical cycling in a typical subtropical river-estuary-bay continuum under the significant anthropogenic influences.
- Materials and methods
2.1. Study area
The QRE, located on the south coast of Guangxi province, is a shallow estuary connecting to northern Beibu Gulf, southwest China (Fig. 1). It receives freshwater from the Qinjiang River (length 179 km, drainage area 2457 km2), and the annual freshwater input is 20.3 × 108 m3 y− 1, and along with the other river also co-deliver ~8.6 × 104 t y − 1 of suspended sediment load into the QZB (Chen et al., 2018; Xu et al., 2021). In addition, more than 80 % of freshwater draining into the QRE is discharged from May to September, while the remaining 20 % is discharged between October and Apirl (Yang et al., 2019). The QZB, isalso the bay-drowned valley and the largest oyster mariculture bay, and is a semi-enclosed bay consisting of the inner bay (Maowei Sea, about 135 km2) and outer bay (Qinzhou Bay, about 200 km2) (Gu et al., 2018; Xie et al., 2024a). The average depth range of the QZB is approximately 5–7 m, with homogeneously mixed freshwater, depending on the sea- sonal river discharge. These conditions make the QZB behave like a typical subtropical estuary, especially during the wet season with sur- face salinity being generally <20 (Xu et al., 2021). Accordingly, the seasons of the study area are roughly divided into wet season (May to September), and dry season (October to April) due to the characteristics of precipitation (Lu et al., 2022). This study area represents a significant economic developing region, which has been gradually impacted by anthropogenic activities (Xie et al., 2024b). The baggy geomorphology of QZB together with the narrow outlet channel linking it with the Qinjiang River somewhat limits rapid water exchange between these two water bodies and Beibu Gulf (Zhang et al., 2019).
