Abstract
Batticaloa Lagoon is one of the estuaries in the country which is frequently affected by floating aquatic plants; mainly Eichhornia crassipes. The present study aimed to develop a relationship between field measured and satellite derived biomass that can satisfactorily estimate the spatial distribution of green and dry biomass of the floating aquatic plants in Batticaloa Lagoon. Cloud free six Sentinel-2A images were acquired for the period of March 2017 to February 2018. Real time field measurements of biomass of floating aquatic plants were obtained in 12 locations in two weeks interval. A buffer zone of 3 km was created around the lagoon to obtain Land Use/Land Cover (LULC) distribution to study the influence of surrounding LULC on floating aquatic plants. A number of band ratios and indices were developed using Sentinel-2A images to establish relationships with the field estimated biomass. The LULC analysis revealed that paddy was the abundant land use in the study area and the cultivation was highly seasonal which impacts the distribution of floating aquatic plants in dry and wet seasons. Among 21 tested band ratios and indices, normalized difference red edge index (NDREI, r2=0.78) and band ratio B8/B (r2=0.67) for the green biomass and band ratio B3/B4 (r2=0.73) and NDREI-Narrow (r2=0.61) for the dry biomass in dry and wet season showed strong positive correlation with field biomass. The temporal distribution of the estimated biomass also confirmed the potential of Sentinel-2A images to be used as a source of data for monitoring of floating aquatic plants in the lagoon due to high spatial and spectral resolution of NIR and Red edge bands. These estimated biomass maps can be used to identify the locations which are affected by aquatic plants in order to take proper control measures.
Highlights
Coastal and inland shallow aquatic ecosystems provide habitats for a wide range of macrophytes including floating, submerged and emergent aquatic plants
The present study aimed to develop a relationship between field measured and satellite derived biomass that can satisfactorily estimate the spatial distribution of green and dry biomass of the floating aquatic plants in Batticaloa Lagoon
A buffer zone of 3 km was created around the lagoon to obtain Land Use/Land Cover (LULC) distribution to study the influence of surrounding LULC on floating aquatic plants
Summary
Coastal and inland shallow aquatic ecosystems provide habitats for a wide range of macrophytes including floating, submerged and emergent aquatic plants. Estuaries subjected to significant pollution with the spreading of invasive alien floating aquatic plant, the Water Hyacinth (Eichhornia crassipes). Spreading of this invasive alien species is an indication of sedimentation and eutrophication of the lagoon as a result of degradation of the ecosystem due to unplanned development works in the lagoon area (IUCN and CEA, 2006). Vegetation on shallow aquatic system can be determined and monitored by field surveys and in situ measurements in terms of Biomass (BM) which is defined as “the mass per unit area of living plant material” (Downing and Andreson, 1985; Madsen, 1993). Field estimation of aquatic plants provides reliable results, the methods are time consuming, labour intensive and cost inefficient, especially when the water bodies are with large surface areas.
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