Eutrophication and Hypoxia in Tropical Negombo Lagoon, Sri Lanka

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.

Monitoring of fecal indicator bacteria in two Salvadoran estuaries

“…Apparently, Barra de Santiago presented important inputs from confluent rivers and the Pacific Ocean, and this influenced local dissolved oxygen concentrations. Same trend was also observed by Hsieh et al (2021) in a tropical coastal lagoon at Sri Lanka. The dissolved oxygen concentrations observed at both aquatic ecosystems were significantly different (Kruskall-Wallis, H = 9.411, p = .0022 (1, N= 50) α = .05), allowing to consider that each place has different hydrodynamics.…”

“…Highest value (8.1) was observed once at Barra Salada at sampling station close to the ocean in September 2021 which, may be related to strong ocean influence (alkalinity) due to fast and large waves that occurred from September 10 th to 14 th (MARN, 2021). Hsieh et al (2021) also, found that pH increased in areas close to oceanic inlet in a tropical coastal lagoon at Sri Lanka, suggesting this influence on pH above 7.0.…”

“…Correlations were negligible (p > .05) when comparing this parameter and bacteriological loads, however the authors found a low positive strength (r = .39) for enterococci at Barra Salada (Table IV). Despite of these mixed results, some authors have suggested that temperature is an important factor for intestinal bacteria abundance and survival in coastal marine environments at different latitudes (Mote et al 2012, Larrea-Murrel et al 2013, Adeiniji et al 2019, Hsieh et al 2021.…”

A Concise Study on Essential Parameters for the Sustainability of Lagoon Waters in Terms of Scientific Literature

“…pH > 8.5) were observed in coastal plumes of Jaffna, Batticaloa, and Puttalam lagoons since heavy rainfall was experienced due to the Southwest monsoon in the first two weeks of July 2022. However, the pH magnitudes during the wet season in Sri Lanka from August to December would be even higher than the measured pH magnitudes in the dry season (Hsieh et al, 2021). There are two possible explanations for such unusual pH change after a heavy rainfall at dry season as observed at experimental recordings.…”

“…Furthermore, overgrowth of algae species releases toxic substances into lagoon waters, depletes deep water oxygen, decreases water column transparency, reduces health and size of corals, mitigates the overall aesthetic value in lagoons, conceals economic repercussions, and declines the existence, biomass, and diversity of aquatic plants (Smith, 2003). When the effluent discharges to the lagoon are in high intensity, algal blooms happen more in the offshores compared to the lagoon water (Hsieh et al, 2021) due to high turbidity of interior lagoon water and because of the short residence time of algae.…”