Implications of Nutrient Enrichment and Related Environmental Impacts in the Pearl River Estuary, China: Characterizing the Seasonal Influence of Riverine Input

Lixia Niu,Qingshu Yang,Pieter van Gelder,Huayang Cai,Xiangxin Luo,Tao Zhang

doi:10.3390/w12113245

Abstract

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.

Highlights

Riverine nutrient input (e.g., N, P, and Si) and their effects on the aquatic environment in the river-estuary-shelf systems have received an increasing amount of attention over the past few decades [1,2]
The results showed that only phosphorus limitation predominantly appeared in the entire estuary in February and November, as a result of high DIN and DSi fluxes and low DIP fluxes; both P- and N-limitations were found in May and August
The levels of dissolved nutrients were relatively higher in the flood season than those in the dry season

Summary

Introduction

Riverine nutrient input (e.g., N, P, and Si) and their effects on the aquatic environment in the river-estuary-shelf systems have received an increasing amount of attention over the past few decades [1,2]. Any changes in riverine nutrient input and their forms can cause alterations of the associated marine community and might shape the ecological stability [1,5]. River discharge provides abundant nutrients and other biogeochemical properties in the plume along the salt water of an estuary that stimulates phytoplankton growth [6]. The effective management of these inputs on estuarine coastal waters requires extensive monitoring of nutrient transportation through the river-estuary-shelf systems and can provide a realistic estimate of inputs o the open seas [7,8]. Waters in estuaries are characterized by the interaction between fresh water and marine water [14]; correspondingly, the roles of associated indicators (e.g., salinity, sediment) on nutrients have raised the need to understand the biogeochemical responses and environmental impacts [15,16]

Methods

Results

Conclusion