The Pearl River Estuary (PRE) is one of the world’s most urbanized subtropical coastal systems. It presents a typical environmental gradient suitable for studying estuarine phytoplankton communities’ dynamics and photosynthetic physiology. In September 2018, the maximum photochemical quantum yield (Fv/Fm) of phytoplankton in different salinity habitats of PRE (oceanic, estuarine, and freshwater zones) was studied, revealing a complex correlation with the environment. Fv/Fm of phytoplankton ranged from 0.16 to 0.45, with taxa in the upper Lingdingyang found to be more stressed. Community composition and structure were analyzed using 18S rRNA, accompanied by a pigment analysis utilized as a supplementary method. Nonmetric multidimensional scaling analysis indicated differences in the phytoplankton spatial distribution along the estuarine gradients. Specificity-occupancy plots identified different specialist taxa for each salinity habitat. Dinophyta and Haptophyta were the predominant taxa in oceanic areas, while Chlorophyta and Cryptophyta dominated freshwater. Bacillariophyta prevailed across all salinity gradients. Canonical correlation analysis and Mantel tests revealed that temperature, salinity, and elevated nutrient levels (i.e., NO₃⁻-N, PO₄³⁻-P, and SiO₃²⁻-Si) associated with anthropogenic activities significantly influenced the heterogeneity of community structure. The spatial distribution of phytoplankton, along with in situ photosynthetic characteristics, serves as a foundational basis to access estuarine primary productivity, as well as community function and ecosystem health.

Introduction: The Changjiang (Yangtze) River is one of the largest rivers in the world, and its estuary and offshore plume create a diversity of ecological habitats for the phytoplankton community. The phytoplankton community has to balance between light limitation in the sediment-laden inshore waters and nutrient limitation in the offshore waters. Active fluorescence measurements can provide rapid, non-intrusive estimates of photosynthetic characteristics at high spatial and temporal resolution.Methods: In the summer of 2020, a field survey of hydrodynamic characteristics, availability of nutrients, the maximum quantum efficiency of photosystem II (Fv/Fm), and rapid light curves across the Changjiang River Estuary and its adjacent sea was conducted, assessing relationships between photosynthetic physiology and biomass accumulation.Results: The photosynthetic activities significantly differed among the turbid river water, the stratified river plume water, and the oceanic East China Sea Water. The photosynthetic physiology of phytoplankton was the most active near the front of Changjiang Diluted Water, where the Fv/Fm was over 0.5.Discussion: Phytoplankton photosynthesis was alleviated from light limitation downstream of the river mouth, and benefited from phosphorus supply via tidal mixing and upwelling. The relatively suitable light and nutrients led to high photosynthetic activities, supporting increased productivity and biomass in this water. The phytoplankton in the Changjiang estuary rivermouth were under intense stress, suggested by the Fv/Fm values under 0.3. Also, the strong vertical mixing process diluted the river nutrients before the phytoplankton consumed them. Nutrients further limited the phytoplankton offshore in the East China Sea.

Variable fluorescence measurements provide automatic and rapid estimates of electron-based photosynthesis. However, electron transport rates (ETR) to phytoplankton production vary greatly due to biological physiology and environmental stress along the Changjiang River to the adjacent seawater, where the turbid riverside, nutrient-rich river plume, and seawater are divided by the sediment front and plume front. We assessed the light responses of ETR, ¹⁸O-labeled gross primary production (GPP), and O₂-based net community production (NCP) during the bloom season at the Changjiang River Estuary. The distribution of phytoplankton and potential photosynthesis efficiency were positively correlated, exhibiting offshore decreasing trends and peaking around the upwelling areas. The conversion factors of Φ^GPP (mol e^- per mol O₂ production) ranged from 0.8 to 31.8, and Φ^NCP ranged from 0.9 to 36.9, respectively. Φ_{e: O} increased from the nutrient-rich Changjiang diluted water (CDW) to the nutrient-poor seawater. Although Φ^GPP and Φ^NCP were linearly related to non-photochemical quenching (R² > 0.5) in the CDW, this relationship was decoupled in offshore seawater due to P-limitation, light stress, and shifts in the phytoplankton community. A better relationship between Φ^GPP and Φ^NCP was quantified using multiple linear analyses with the light attenuation coefficient (K_d), euphotic depth (Z_eu), salinity, nutrients, and temperature (p< 0.001). This improvement in accuracy represents a major step forward for widespread and accurate fluorometry-based GPP and NCP applications at high temporal and spatial resolutions in the future.

Cyanobacterial blooms can be stimulated by excessive phosphorus (P) input, especially when diazotrophs are the dominant species. A series of mesocosm experiments were conducted in a lake dominated by a cyanobacteria bloom to study the effects of Phoslock^®, a phosphorus adsorbent. The results showed that the addition of Phoslock^® lowered the soluble reactive phosphate (SRP) concentrations in water due to efficient adsorption and mitigated the blooms. Once settled on the sediments, Phoslock^® serves as a barrier to reduce P diffusion from sediments into the overlying waters. In short-term (1 day) incubation experiments, Phoslock^® diminished or reversed SRP effluxes from bottom sediments. At the same time, the upward movement of the oxic–anoxic interface through the sediment column slightly enhanced NH₄⁺ release and depressed N₂ release, suggesting the inhibition of nitrification and denitrification. In a long-term (28 days) experiment, Phoslock^® hindered the P release, reduced the cyanobacterial abundance, and alleviated the bloom-driven enhancements in the pH and oxygen. These results suggest that, through suppression of internal nutrient effluxes, Phoslock^® can be used as an effective control technology to reduce cyanobacteria blooms common to many freshwater systems.

Oyster culture at Rushan coastal water is one of China’s most important shellfish aquacultures. To address space limitations in aquaculture, the government promoted raft cage technology and “autumn oyster fattening”, which has significantly improved oyster quality and yield. However, environmental overcapacity and disease risks now pose critical challenges to the industry’s sustainable development. Carrying capacity evaluations (e.g. water quality, nutrient cycling, and ecosystem impacts) are essential for sustainable management. Global warming, sea level rise, and ocean acidification threaten oyster reproduction, growth, and diseases. A few developments in oyster hatcheries were planned or in operation on how to achieve key benefits, such as strengthening ecosystem adaptation by mangroves to sea level rise and erosion, addressing the climate crisis through carbon storage and reduced acidification, and enhancing the safety and prosperity of coastal communities.