Lifting the veil: Impact of contaminants on coastal phytoplankton

Coastal waters are among the most productive ecosystems on the planet. Yet, they also suffer from a high contaminant load, resulting in a ‘grey veil’ of contaminants. Exposure of microalgal communities to these contaminants potentially results in shifts in microalgal community composition. As microalgae form the basis of the marine food webs, those shifts can ultimately affect the carrying capacity of coastal and estuarine ecosystems. However, neither the composition, nor the potential harmful effects of the ‘grey veil’ of contaminants on marine microalgae have been identified. This thesis aimed to ‘lift the veil’ by identifying contaminants hazardous to microalgae and to determine the hazard and risk of these contaminants. It was demonstrated that that of the numerous contaminants currently present in the Dutch coastal zone, six herbicides (atrazine, diuron, irgarol, isoproturon, terbutylazine, terbutryn) could be identified as the main contributors to the toxic pressure on phytoplankton in these waters. Although the risk of individual herbicides is currently low, mixture toxicity and multi-stress effects will modify the toxicity in the field, thereby creating a ‘margin of uncertainty’ of the ultimate toxic pressure. This margin of uncertainty should also be taken into account in environmental quality standard setting. Finally, the current monitoring system will benefit from an effect-based approach as demonstrate in the present study for phytoplankton. Such an integrated ecotoxicological and chemical monitoring of coastal waters will support the protection of the vital function of coastal ecosystems.