Water quality assessment in the Nha Trang Bay (Vietnam) by using in-situ and remotely sensed data

The Nha Trang Bay (Vietnam) is an international marine protected area with significant economic, natural and recreational values. Considerable economic development is expected in particularly for tourism, navigation and aquaculture. However, in recent years the environmental quality of the Bay has been degraded by human activities and impacts. Even though the conditions of the Bay are monitored with in-situ measurements in a marine station, the environmental data, measured only in half-yearly intervals, are not sufficient for comprehensively assessing environmental quality at various temporal and spatial scales. Therefore, it is necessary to seek for complementary data sources to assess, control and manage the marine environment of Nha Trang Bay. We conducted three field trips in Nha Trang Bay at 23 stations, including 6 off-shore stations that are not influenced by freshwater from land, and measured the distribution of marine optical properties. These properties include the light extinction coefficient (K) of photosynthetically active radiation (PAR) in the surface layer (i.e. 1m depth) and in water columns. K ranged from 0.099 m-1 off-shore to 0.409 m-1 in coastal waters. In addition, light absorption at various wavelengths were used to assess the concentration of chlorophyll-a, total suspended sediments and primary production. Further, we collected and used spatially explicit data using the OC4 method and the method of band 1, 2 and 3 ratios of the Landsat TM satellite. A data comparison confirms a significant correlation between the different sources of in-situ and remotely sensed data of light absorption. However, the predicted values at stations with a water depth of less than 5 m are significantly affected by sea bottom reflection of, for example, mud, sand, coral reefs and algae. The results indicate that it is appropriate to use remote sensing methods to derive spatially explicit distributed variables of optical properties and derived products for an environmental assessment of coastal waters