Assessing the impact of nutrient enrichment in estuaries: susceptibility to eutrophication.

The main aim of this study was to develop a generic tool for assessing risks and impacts of nutrient enrichment in estuaries. A simple model was developed to predict the magnitude of primary production by phytoplankton in different estuaries from nutrient input (total available nitrogen and/or phosphorus) and to determine likely trophic status. In the model, primary production is strongly influenced by water residence times and relative light regimes. The model indicates that estuaries with low and moderate light levels are the least likely to show a biological response to nutrient inputs. Estuaries with a good light regime are likely to be sensitive to nutrient enrichment, and to show similar responses, mediated only by site-specific geomorphological features. Nixon's scale was used to describe the relative trophic status of estuaries, and to set nutrient and chlorophyll thresholds for assessing trophic status. Estuaries identified as being eutrophic may not show any signs of eutrophication. Additional attributes need to be considered to assess negative impacts. Here, likely detriment to the oxygen regime was considered, but is most applicable to areas of restricted exchange. Factors which limit phytoplankton growth under high nutrient conditions (water residence times and/or light availability) may favour the growth of other primary producers, such as macrophytes, which may have a negative impact on other biological communities. The assessment tool was developed for estuaries in England and Wales, based on a simple 3-category typology determined by geomorphology and relative light levels. Nixon's scale needs to be validated for estuaries in England and Wales, once more data are available on light levels and primary production.

Assessing the suitability of OSPAR EcoQOs for eutrophication vs ICES criteria for England and Wales.

This paper provides a preliminary evaluation of the suitability of OSPAR Ecological Quality objectives for eutrophication for coastal and offshore waters of England and Wales on the basis of ICES criteria. In principle, EcoQOs are easy to understand, but responses to nutrient enrichment are complex. Few studies provide unequivocal evidence of links between inputs and response. Monitoring is generally feasible and in place, but needs to be improved. The best EcoQO is winter nutrient concentrations. Chlorophyll concentrations are a good indicator in environments susceptible to nutrient enrichment. The EcoQO for zoobenthos/fish kills potentially meets all criteria for a good indicator; EcoQOs for phytoplankton indicator species, oxygen concentrations and zoobenthos changes do not. Greater emphasis needs to be placed on seasonal effects of nutrient inputs and phytoplankton response, natural susceptibility of different water bodies, differences between coastal and offshore environments, and developing longer time series of data.