The Pearl River estuary is an ecologically dynamic region located in southern China that experiences strong gradients in its biogeochemical properties. This study examined the seasonality of nutrient dynamics, identified related environmental responses, and evaluated how river discharge regulated nutrient sink and source. The field investigation showed significant differences of dissolved nutrients with seasons and three zones of the estuary regarding the estuarine characteristics. Spatially, nutrients exhibited a clear decreasing trend along the salinity gradient; temporally, their levels were obviously higher in summer than other seasons. The aquatic environment was overall eutrophic, as a result of increased fluxes of nitrogen and silicate. This estuary was thus highly sensitive to nutrient enrichment and related pollution of eutrophication. River discharge, oceanic current, and atmospheric deposition distinctly influenced the nutrient status. These factors accordingly may influence phytoplankton that are of importance in coastal ecosystems. Phytoplankton (in terms of chlorophyll) was potentially phosphate limited, which then more frequently resulted in nutrient pollution and blooms. Additionally, the nutrient sources were implied according to the cause–effect chains between nutrients, hydrology, and chlorophyll, identified by the PCA-generated quantification. Nitrogen was constrained by marine-riverine waters and their mutual increase-decline trend, and a new source was supplemented along the transport from river to sea, while a different source of terrestrial emission from coastal cities contributed to phosphate greatly.

Human society and ecosystems worldwide are increasinAagly threatened by water shortages. Despite numerous studies of climatic impacts on water availability, little is known about the influences of socioeconomic development on streamflow and water sustainability. Here, we show that the streamflow from the Yellow River to the sea has decreased by more than 80% in total over the last 60 years due to increased water consumption by agricultural, industrial and urban developments (76% of the streamflow decrease, similarly hereinafter), decreased precipitation (13%), reservoir construction (6%) and revegetation (5%). We predict that if the past trends in streamflow will continue, year-round dry-up in the lower Yellow River will commence in the late 2020s or early 2030s, unless effective countermeasures such as water diversion from the Yangtze River are taken. These results suggest that streamflow in semiarid basins is highly vulnerable to human impacts and that streamflow decline would in turn hinder further socioeconomic development and endanger river-sea ecosystems.

Lingding Bay (LDB), on the Pearl River Delta (PRD) of southern China, is a typical example of a large river mouth that is strongly affected by anthropogenic perturbations that have changed the boundary conditions of hydro- and sediment dynamic processes. An analysis of recent sedimentary patterns can shed light on the role of anthropogenic impacts on delta evolution. In this study, we collected surficial sediments from the LDB in December 2016 (dry season) and August 2017 (flood season) to analyze their grain size and organic geochemical compositions, with the aim of investigating recent depositional patterns in the bay and evaluating human impacts. The results reveal two major mud depocenters in the northeastern and southwestern parts of the bay, which are characterized by high values of grain-size end member 1 (EM1) and increased contributions of terrestrial organic carbon in the flood season. We propose that this sedimentary pattern is a manifestation of a system regime shift due to the strengthening of the fluvial function in fluvial-tide interactions and associated changes in the suspended sediment dispersal routine. We suggest that these changes are a result of recently intensified human activities, such as coastal land reclamation and sand mining. Coarsening of the surficial sediments in the LDB in the dry season and a marked increase in the terrestrial organic contribution at the mouth of the LDB indicates the redistribution of fine-grained sediments by waves and currents and increased mud export from the LDB in response to the shallowing of the bay.