The use of remote sensing and GIS-models in assessing the impact of levels of suspended solids in coastal waters on environmental quality and marine life

Abstract

The aim of the present work was to apply remote sensing and GIS-models in the assessment of suspended solids in coastal surface waters, as arising from non-point land-based sources. Furthermore, the effect of turbidity arising from suspended solids on the marine benthos in particular, Posidonia oceanica (L.) Delile meadows was investigated. The methodology was based upon modelling the route by which suspended solids can reach coastal surface waters from land. This involved creating a digital elevation map using triangulated irregular networks and deriving a baseline runoff model from it. The baseline runoff model and regression analysis for specific land covers were used in order to determine the sediment loading in the coastal areas. The water quality of coastal surface waters was mapped using archived Landsat TM data. Two different approaches were adopted: empirical analysis and the use of artificial neural networks. Benthic habitats in shallow coastal waters were mapped using archived Landsat TM data. This involved the use of principal components analysis and the use of artificial neural networks. Multi-criteria evaluation was used in order to identify coastal areas vulnerable to accumulation of solids and transferring them to the marine environment. This study showed that when annual total suspended solids loadings were taken into consideration, the highest loading was always recorded at Salina Bay whilst for storm events with precipitation levels lower than 17 mm, the highest suspended solids loading was recorded in Birzebbugia. More than 2 million kg of suspended solids can be discharged into the marine environment during a 15-year return period flash flood of 200 mm. These results were coherent with the results of multi-criteria evaluation which determined that the areas mostly vulnerable to accumulation of suspended solids and conveying them to the marine environment were localised to mouths of valleys. These areas corresponded to marine areas which showed significant levels of turbidity as established from Landsat TM imagery. Overlaying maps showing areas vulnerable to pollution due to turbidity on the benthic habitats maps showed that seagrasses are the benthic community found in shallow coastal waters mostly exposed to significantly high levels of suspended solids. This study indicated that reduced water transparency can affect Posidonfo oceanica meadows. The ecological significance of this was discussed.