Abstract

The semi-enclosed bays impacted by heavy anthropogenic activities have weak water exchange and purification capacities. Most of the sea bays have suffered severe eutrophication, water quality deterioration, ecosystem degradation and other problems. Although many countries and local governments have carried out corresponding environmental protection actions, the evaluation of their effectiveness still requires monitoring technology and data support for long-term water environment change. In this study, we take Yueqing Bay, the fourth largest bay in China, as a case to study the satellite-based water quality monitoring and variation analysis. We established a nutrient retrieval model for Yueqing Bay to produce a long-term series of nutrient concentration products in Yueqing Bay from 2013 to 2020, based on Landsat remote sensing images and long-term observation data, combined with support vector machine learning and water temperature and satellite spectra as input parameters, and then we analyzed its spatiotemporal variations and driving factors. In general, nutrient concentrations in the western part of the bay were higher than those in the eastern part. Levels of dissolved inorganic nitrogen (DIN) were lower in summer than in spring and winter, and reactive phosphate (PO4-P) levels were lower in summer and higher in autumn. In terms of natural factors, physical effects (e.g., seasonal variations in flow field) and biological effects (e.g., seasonal differences in the intensity of plankton photosynthesis) were the main causes of seasonal differences in nutrient concentration in Yueqing Bay. Nutrient concentration generally increased from 2013 to 2015 but decreased slightly after 2015. Over the past decade, the economy and industry of Yueqing Bay basin have developed rapidly. Wastewater resulting from anthropogenic production and consumption was transported via streams into Yueqing Bay, leading to the continuous increase in nutrient concentrations (the variation rates: aDIN>0, aPO4−P>0), which directly or indirectly caused high nutrient concentrations in some areas of the bay (e.g., Southwest Shoal at the mouth of Yueqing Bay). After 2015, the various ecological remediation policies adopted by cities around Yueqing Bay have mitigated, to some extent, the increasing nutrient concentration trends (the variation rates: aDIN<0, aPO4−P<0), but not significantly (P > 0.1). The environmental restoration of Yueqing Bay also requires continuous and long-term ecological protection and restoration work to be effective. This research can provide a reference for ecological environment monitoring and remote sensing data application for similar semi-enclosed bays, and support the sustainable development of the bay.

Highlights

  • With the rapid development of the coastal economy and continuous urbanization, over60% of the world’s population live within 200 km of a coastline [1], and more than 40% ofChina’s population are centered in coastal areas [2]

  • Nutrients are to some extent related to the physiical properties of water, such as water temperature, and optically active substances in cal properties of water, such as water temperature, and optically active substances in wawater, such as suspended solids and chlorophyll-a [63,64,65], which are associated with specter, such as suspended solids and chlorophyll-a [63,64,65], which are associated with spectra tra [66,67,68,69,70]

  • Waveband information was used as a variable input to retrieve nunutrient concentrations in the waters of Yueqing Bay

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Summary

Introduction

With the rapid development of the coastal economy and continuous urbanization, over60% of the world’s population live within 200 km of a coastline [1], and more than 40% ofChina’s population are centered in coastal areas [2]. With the rapid development of the coastal economy and continuous urbanization, over. Of sea areas worldwide, especially estuaries and bays, have been seriously disturbed by anthropogenic activities [3]. As a result of ever-increasing urbanization, continuous mariculture scale-up, and rapid development of the ocean economy, land-derived pollutants (e.g., nitrogen and phosphorus) are continuously entering into waters, resulting in water quality deterioration and eutrophication in local areas. As typical ecologically sensitive areas heavily disturbed by land–sea interactions and anthropogenic activities, semi-enclosed and enclosed bays exhibit weaker water exchange and purification capacities, and are highly vulnerable environments [6,7,8]. To monitor variations in water quality of nearshore seas and bays, it is essential to conduct dynamic monitoring of nitrogen and phosphorus elements in water and investigate their driving factors, and implement corresponding measures and policies to control and improve water quality status

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