Introduction: Coastal ecosystems play an important role in maintaining biodiversity and ecological balance, nutrient cycling, carbon storage, shoreline protection, and supporting local livelihoods. Mangrove forests are also important components of these ecosystems, providing habitats for diverse aquatic species and birds, stabilizing sediments, reducing erosion, and contributing to water-quality improvement. However, the expansion of human activities, particularly aquaculture, can affect coastal environments. Shrimp farming has expanded due to its economic importance and increasing demand, while effluents from shrimp farms may increase nutrient inputs and alter water quality. Chlorophyll-a concentration is an important indicator of phytoplankton biomass and the trophic status of aquatic systems, while changes in mangrove extent can provide insights into the ecological condition of coastal areas. Therefore, the main objective of this study was to investigate long-term changes in chlorophyll-a concentration, assess trends in mangrove forest extent, and examine the temporal coincidence of these changes with the development of shrimp farming in Govater Bay.
Materials and Methods: In this study, Landsat 5, Landsat 7, and Landsat 8 satellite imagery from 1990 to 2023 was used. Image processing was conducted in the Google Earth Engine (GEE) environment. Following cloud removal and the generation of annual composite images, chlorophyll-a (Chl-a) concentration was estimated using a model based on the reflectance of visible spectral bands. Mangrove cover was extracted using the Normalized Difference Vegetation Index (NDVI), the Normalized Difference Water Index (NDWI), appropriate spectral band combinations, and thresholding methods. To assess the accuracy of the classification, imagery from 2023 was used. After calculating NDVI and applying the determined threshold, mangrove-covered areas were delineated using a decision tree classification algorithm. The resulting classification was then compared with higher-resolution Google Earth imagery, and the spatial agreement between the classified areas and the visual reference data for mangrove cover was assessed. Subsequently, the spatial and temporal variations in chlorophyll-a concentration and mangrove extent were analyzed, and their trends were compared.
Results: Analysis of the chlorophyll-a time series from 1990 to 2023 revealed an overall increasing trend, with the highest concentrations recorded during 2020–2023. Spatially, higher chlorophyll-a concentrations were mainly observed in the inner and northern parts of the bay and near creeks and inflowing channels, whereas lower concentrations were generally found in the southern and seaward-facing areas. Mangrove forest extent also increased from 219.99 ha in 1993 to 923.64 ha in 2023, representing a net increase of 703.65 ha. Overall, the increase in chlorophyll-a concentration and mangrove extent occurred concurrently with the development of shrimp-farming activities in Govater Bay. Thus, although these changes occurred simultaneously during the study period, the observed patterns alone do not indicate a direct relationship among them.
Discussion: The results indicated that chlorophyll-a concentration in Govater Bay exhibited an increasing trend during the study period, coinciding with the gradual development of shrimp-farming ponds. This increase may reflect gradual changes in the trophic conditions of the water in parts of the bay; however, given the nature of the available data, the direct contribution of shrimp-farming activities to this change could not be determined. In contrast, mangrove forest extent also increased throughout the study period, and the concurrent development of shrimp-farming activities and expansion of mangrove cover indicates that aquaculture development at the regional scale was not accompanied by a net decline in mangrove extent. Overall, the findings indicate that the increase in chlorophyll-a concentration and the development of shrimp-farming activities occurred concurrently with an increase in mangrove extent.