Chlorophyll-a estimation and evaluating the effect of land use change in a Ramsar Site of North East India

Abstract

Chlorophyll concentration could potentially measure the relative productivity and health of lakes and ecosystem. The phytoplankton activities in the water bodies are measured by determining the amount of chlorophyll concentrations, which are used as a proxy for primary production and eutrophication. In addition, satellite imageries provide spatial and temporal changes that would indicate the health of the ecosystems. In this study, we have employed the Normalized Difference Chlorophyll Index (NDCI) algorithm to estimate the Chl-a concentrations based on the bands of Landsat-8 satellite imageries in the Deepor beel a Ramsar site, in Northeast India, Assam. The advantage of NDCI is that it can be used to detect algal bloom and qualitatively infer Chl-a concentration ranges, when the ground data is not available. Two spectral bands at 530 to 590 nm (Green Band) and 640 to 670 nm (Red Band) were selected to develop the index. We also performed the land use land cover (LULC) classifications from 2015-2021 within the lake using the supervised approach. The results indicated that the settlements within the area have increased due to human habitats with decrease in the marshy land, forest and vegetation cover. The runoffs from the settlements and nearby areas resulted in algal blooms, which could potentially result in reduced water quality for survival of aquatic habitats. Therefore, seasonal variability of the Chl-a concentrations during pre-monsoon and post monsoon period was intercompared over the years. It was observed that the Chl-a concentration undergoes both spatial and temporal variation. We found the values to be significantly high during the post monsoon period compared to the pre-monsoon period. Based on the analysis, the study would be of significant importance in evaluating the nutrient loading in lakes, where the fertilizer spill or toxicity levels may be an important aspect under consideration.

Keywords: Chlorophyll-a, Normalized Difference Chlorophyll Index, LULC