Satellite-based monitoring of Chlorophyll-a and turbidity dynamics in Negombo Lagoon, Sri Lanka

Abstract

Although water covers more than 70% of the Earth’s surface, the contradiction persists: issues such as freshwater shortage, contamination, and eutrophication remain among the most pressing environmental challenges we face today, particularly in sensitive coastal ecosystems. This study investigates water conditions, such as Chlorophyll-a and turbidity levels, and analyses the spatiotemporal dynamics of eutrophication. Moreover, the accuracy and suitability of Sentinel- 2 and Landsat-8 data were compared for water quality assessment and identification of seasonal variation in lagoon water contamination with rainfall, temperature, and surface runoff. Satellite data were obtained from ESA and USGS, resampled, and subset considering the study area. Ten sample pixels were randomly selected for IOPs extraction. Hourly climatic data were retrieved from the ERA5 database and aggregated into monthly averages. The C2RCC algorithm was employed to derive water quality parameters, while Carlson´s TSI was applied to assess eutrophication levels. To evaluate the accuracy of geospatial datasets, statistical metrics such as RMSE, MAE, R2, Bias, and NSE were utilised. Data processing and analysis were conducted using SNAP, ArcGIS Pro, Python, and Google Earth Pro software. The results indicate that Chl-a concentrations peaked during April–May and October–November (∼ 35 - 42μg/L), coinciding with the starting Southwest and second inter-monsoon periods (∼ 450 - 550mm per month). Turbidity levels were higher in June, from August to October, during the high rainfall months. Conversely, the lowest Chl-a and turbidity were observed in February and December, which recorded minimal rainfall in 2024. TSI results based on Sentinel-2 revealed consistent seasonal trends, with values mostly falling within eutrophic and hypertrophic ranges, indicating high nutrient levels and algal production. August exhibited significantly high eutrophic conditions across all sample points. Moreover, Landsat 8 data also reveal the same seasonal eutrophic levels. However, from April to May and from October to December, water quality was shown as oligotrophic. These findings underscore the need for sustainable coastal lagoon management strategies that must address eutrophication and ensure water quality improvements for coastal environments.