Factors affecting the accumulation of phytoplankton biomass in Irish estuaries and nearshore coastal waters: A conceptual model

• PCA was used to examine the response of phytoplankton to nutrient enrichment.
• Residence time and light availability were found to be the main modulating factors.
• In most estuaries these factors operated on a mutually exclusive basis.
• Enrichment by one nutrient caused limitation of phytoplankton growth by the other.
• A conceptual model of the susceptibility of these waters to enrichment is presented.

A multivariate statistical approach was used to investigate the response of phytoplankton in Irish estuaries and nearshore coastal waters to nutrient enrichment and to examine the factors which modulate this response. The analysis suggests that while many estuaries are nutrient-enriched, relatively few display phytoplankton-related symptoms of eutrophication as the response to nutrients is primarily affected by insufficient retention time, in some by inadequate light availability, and only rarely by both factors acting together. Nearshore coastal waters are nitrogen (N) and silica (Si) limited in summer, but in some nearshore waters along the south coast, where N is elevated, phosphorus (P) is potentially limiting. The reduction in P loadings to estuarine waters is likely to lead to an improvement in the eutrophication status of these mainly P-limited waters. The disproportionate reduction in loadings of P compared to N (52% versus 24%, since the early 1990s), and the potential weakening of the estuarine N filter, as eutrophication symptoms lessen, may result in the downstream movement of nitrogen to N-limited coastal waters. These findings support the view that an integrated dual-nutrient reduction strategy is required to address eutrophication along the freshwater-marine continuum. The outcome of the analysis is a conceptual model which is of direct value and use to water managers in determining the relative susceptibility of these waters to nutrient enrichment. This understanding can in turn be used to develop informed programmes of measures which are targeted and ultimately cost effective.

Conceptual model of factors affecting phytoplankton growth in Irish estuaries and coastal waters.Figure optionsDownload high-quality image (167 K)Download as PowerPoint slide