Eutrophication and Hypoxia in Tropical Negombo Lagoon, Sri Lanka

Hsueh-Han Hsieh,Chin-Chang Hung,Ming-Hsiu Chuang,W Sanjaya Weerakkody,R R M K P Ranatunga,François L L Muller,Yung-Yen Shih,D S Wijethunga,Wei-Jen Huang

doi:10.3389/fmars.2021.678832

Abstract

Hypoxic events are becoming frequent in some estuaries and coastal waters due to over-enrichment of anthropogenic nutrients, organic matter, and/or due to restricted water circulation. The coastal lagoons and estuaries of Sri Lanka are facing high population pressure and lacking sufficient infrastructure. Coastal lagoons may receive high anthropogenic inputs of natural or untreated nitrogen and phosphorus wastes, and consequently result in hypoxic conditions while sluggish circulation occurred. In this study, we examined the spatiotemporal variability of eutrophication and hypoxia in the Negombo Lagoon, one of the most productive and sensitive coastal ecosystems in Sri Lanka. Based on seasonal measurements of dissolved oxygen, nutrients, chlorophyll-a (Chl-a), particulate and dissolved organic carbon (POC and DOC), we concluded that eutrophication and hypoxia occurred in both the dry and wet seasons. The main contributing factors were high seawater temperature and poor water circulation in the dry season and high nutrient loading combined with elevated POC and DOC inputs in the wet season.

Highlights

Sensitive coastal ecosystems such as wetlands, lagoons, and barrier-built estuaries are threatened by a combination of both marine and/or river discharge factors such as elevated seawater temperature, nutrient pollution, organic matter loading, and/or sediment inputs from turbid river waters (Hoegh-Guldberg and Smith, 1989; Hoegh-Guldberg et al, 2007; Mayfield et al, 2013; Breitburg et al, 2018; Cziesielski et al, 2019)
Nitrite (NO2), nitrate (NO3), silicate (SiO3), phosphate (PO4), total suspended matter (TSM), chlorophyll-a (Chl-a), particulate organic carbon (POC), and dissolved organic carbon (DOC) were determined according to the methods described in Hung et al (2000, 2013)
Concentrations of TSM and POC were measured following the methods of Hung et al (2013) while DOC was measured by the method of Hung et al (2016)

Summary

Introduction

Sensitive coastal ecosystems such as wetlands, lagoons, and barrier-built estuaries are threatened by a combination of both marine and/or river discharge factors such as elevated seawater temperature, nutrient pollution, organic matter loading, and/or sediment inputs from turbid river waters (Hoegh-Guldberg and Smith, 1989; Hoegh-Guldberg et al, 2007; Mayfield et al, 2013; Breitburg et al, 2018; Cziesielski et al, 2019). Most hypoxic zones in coastal habitats can be directly linked to population growth and increasing industrial and tourism activity which generates industrial and municipal wastes and results in increased terrestrial fluxes of nutrients (Diaz and Rosenberg, 1995; Steckbauer et al, 2011) This type of eutrophication is most commonly seen in estuaries, coastal and marginal sea Eutrophication, Hypoxia in Negombo Lagoon areas such as the Changjiang estuary (Wei et al, 2007), the East China Sea (Chen, 2008; Chen and Guo, 2020; Chen et al, 2020, 2021), the Shenzhen Bay (Yan et al, 2021) and the Gulf of Mexico (Rabalais et al, 2001; Bianchi et al, 2010). In most of these regions, anthropogenic eutrophication has results in hypoxia

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Results

Conclusion