Catchment News

The impact of nutrient reduction measures on the health of Irish estuarine systems

| in News, Science

Nutrient over-enrichment in estuaries and coasts due to human activity is a widely recognised global phenomenon. These ecosystems can respond to this disturbance through enhanced growth of phytoplankton and opportunistic seaweeds, resulting in the formation of algal blooms. Some phytoplankton bloom species can produce toxins and be harmful to plants and animals while opportunistic macroalgal blooms (such as sea lettuce) can be unsightly and produce odour problems as they decay which may impact on tourism and local communities. In Ireland, high phytoplankton biomass and increased frequency of bloom events, due to nutrient enrichment, have contributed to estuarine waters being classed eutrophic or potentially eutrophic. At present, 36 % of Irish transitional and 67 % of coastal waters monitored by the Environmental Protection Agency (Water Quality report 2012) are considered ‘healthy’ (good or high status) leaving a large proportion of areas requiring improvement.

The amount of nutrients entering estuaries and coasts is dependent on the nutrient source which can either be diffuse (e.g. run-off from agriculture, forestry etc) or point (e.g. waste water treatment plants, industrial sources). In Ireland today the amount of nitrogen and phosphorus from each source will vary considerably in each catchment. For example, while phosphorus will originate largely from agricultural sources in some catchments, phosphorus largely emanates from waste water treatment plants in catchments which have large cities or towns.

In Ireland, blanket mitigation measures have been applied to reduce nutrient losses from agriculture, domestic and industrial sources. These measures stem from a number of European Directives, and National Regulations which have been implemented in recent years. The EPA has been monitoring estuaries and coasts for the last 30 years allowing us to identify whether measures have improved water quality since their implementation.

The main objective of this research project was to trace the historic nitrogen and phosphorus flows from their source to the coastal zone to determine the effectiveness of recent mitigation measures. The study focused on 17 Irish catchments and looked at how nutrient loads have changed from 2000 to 2013, how this has impacted river nutrient delivery to the estuary, and how the estuary itself has responded in terms of nutrient concentrations and phytoplankton biomass.

Load Apportionment Modelling indicated that in most systems the greatest overall contributor to nutrient loads are diffuse sources. However, reductions in both diffuse and point sources have resulted in considerable reductions in phosphorus loads to coastal systems, highlighting the effectiveness of measures which have been applied. Reductions in nitrogen loads have been more modest and are largely related to agricultural improvements. In each catchment the type of land use (agriculture, forestry etc) and the proximity of large towns to the estuary or coast greatly influenced the effectiveness of measures. The greater decrease in phosphorus inputs meant that ratios of nitrogen to phosphorus increased in some estuaries. This indicated an imbalance in nutrient reduction which, while improving estuarine ecosystem health, may cause problems downstream in the outer coastal zone.

Phosphorus inputs, measured on each river just before it enters the estuary, showed a significant reduction in 15 cases, but only 4 rivers showed a concurrent reduction in nitrogen. Significant parallel improvements in estuarine water quality were evident in only 8 of the downstream systems, highlighting that the response of estuaries to nutrient load reductions can be complex.

This response is often determined by nutrient cycling processes within the estuary itself and factors such as light and residence time which can ‘dampen’ the impacts of nutrient enrichment or reduction. In some systems light availability may be low due to large amount of suspended material in the water (e.g. sediment, organic material). This will reduce the growth of the phytoplankton when nutrients increase. In these types of systems blooms will be less frequent or dampened as conditions are not be optimal for growth.

This occurs for example in the Upper Slaney Estuary which has high nutrient concentrations but does not have phytoplankton blooms. Some Irish estuaries have short residence times as river flow rates can be high or the nutrients and phytoplankton will be flushed out with each tidal cycle. In these cases phytoplankton do not have time to bloom before they are flushed out to sea. Often in these types of systems we may see green opportunistic seaweed blooms, for example in the Tolka Estuary in Dublin.

The complexity of estuarine systems and how this impacts their response to nutrient increases or reductions dictates that a portfolio of separate, but complimentary, management approaches may be required to address eutrophication in each estuary. In addition the influence of measures, cycling and physical controls will evolve through the estuarine continuum from fresh to marine water, highlighting the need to consider the impact of measures on each river-estuarine system in the context of these control shifts.

The results of the study will inform decision makers on the effectiveness of measures to date and the possible response of systems to future controls in the light of targets set out under the Water Framework Directive.

Sorcha Ní Longphuirt, EPA Cork

Read more in EPA Research Report 184: Assessing Recent Trends in Nutrient Inputs to Estuarine Waters and Their Ecological Effect http://bit.ly/eparesearch184

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