The colour of the Wadden Sea
While many tourists were spending their holidays on its islands and beaches this summer, the Wadden Sea was added to the UNESCO world heritage list. To keep the site as beautiful as it is, its water quality is regularly monitored. However, this needs expensive and time consuming expeditions. Is it possible to monitor its water quality by means of satellite images instead of water samples?
The Wadden Sea has been a nature reserve of its three bordering countries (Denmark, Germany, and the Netherlands) for a long time. On June 26th the World Heritage Committee added the Dutch and German parts to the UNESCO World Heritage List (http://whc.unesco.org/en/list/1314); the Danish part might follow. The Wadden Sea’s tidal flat areas are the largest in the world (~10 000 km2) and are a breeding place for many fish, shellfish, birds etc.
Maintenance of the new heritage site includes water quality monitoring. This is regularly done by national and local governments by means of water sampling and (chemical) analysis. The monitoring might be easier and cheaper should satellite images be used.
Not all parameters of interest (e.g. chemical pollution) can be measured via satellite data. However, the parameters that can be derived from optical data can tell much about the underwater climate. One of the easiest parameters to extract from optical satellite data is chlorophyll concentration, which is a measure for the amount of algal cells in the water. In spring water is often green (and might be smelly) due to high concentrations of algae. The green colour is caused by a high absorption of red light by chlorophyll. Algae, in turn, can be used as indicators for other parameters of interest such as degree of eutrophication or toxic chemical pollution (in which case they disappear rapidly). Satellite data can also be used to measure turbidity, an important environmental condition for many aquatic species such as fish and plants.
Algorithms to derive water quality properties from optical data are available for the open ocean and are also becoming operational for more turbid areas such as the North Sea. In very turbid areas, such as the Wadden Sea, the light that reflects back from the water to the satellite is influenced by various substances (suspended sediment, algae, coloured dissolved organic matter) simultaneously, which makes it more complex to identify and quantify them.
To be able to use satellite data to monitor water quality in the Wadden Sea first the specific absorption and scattering properties of the substances in the area have to be known. These properties are different in each coastal area. For example, the specific absorption of the suspended sediment depends on what material the bottom comprises, and the specific absorption by algae depends on which algae species are found in the area.
The specific (light influencing) properties of the substances in the Wadden Sea were measured during various field campaigns by IVM in cooperation with the Royal Netherlands Institute for Sea Research (NIOZ). The ranges in concentrations of substances influencing reflected light, the processes influencing changes in concentrations and the water colour were examined. The colour in the Wadden Sea appears to be highly variable, with tidal, seasonal and weather related variations and large spatial differences. Results of these in-situ measurements were presented in a paper that appeared in Estuarine, Coastal and Shelf Sciences in spring 2009.
Current research focuses on modelling the correlations between the colour of the water in the Wadden Sea and the concentrations of chlorophyll, suspended sediment and coloured dissolved organic matter.
More information: Steef Peters
Hommersom, A., Peters, S., Wernand, M.R., De Boer, J. (2009). Spatial and temporal variability in bio-optical properties of the Wadden Sea. Estuarine, Coastal and Shelf Sciences, in press, doi:10.1016/j.ecss.2009.03.042