Recent findings from the Environmental Protection Agency (EPA) has shown that agricultural sources of nitrogen (N) were the main causes of algal blooms in rural areas.

While the EPA acknowledged that “agricultural sources of N were the biggest drivers of blooms in rural locations”, they added that effluent from wastewater treatment plants (WWTP) were “more relevant in urban areas”.

These algal blooms have negative impacts on local economies and biodiversity, the agency said, as they produce “massive mortalities of organisms” because of the reduction in dissolved oxygen concentrations in the water.

The EPA has contended through its research that nutrient release from agricultural land is already a “significant source of surface water pollution”, and is predicted to increase with an increasing demand for food.

The agency referred to the government’s Food Wise 2025, which is for “agricultural intensification to satisfy domestic demand and increase exports”.

The EPA has said this could be “at the expense of achieving water quality and greenhouse gas (GHG) emission targets.

“It is likely that this issue, in combination with other aspects of global change (i.e. seawater temperature increase, biological invasions, emerging contaminants), will result in increased eutrophication and hence increased incidence and severity of extraordinary macroalgal blooms.”

The EPA acknowledged that “little is known” about the impact of other emerging contaminants associated with agricultural activities (e.g. pesticides) on some seaweed species.

However, the EPA contended that “N and phosphorus (P) are considered the main limiting nutrients for freshwater and coastal ecosystems”.

Areas affected

The recently published EPA report is based on research carried out from 2018 to 2022, and it was conducted across various areas in Ireland.

The report detailed that the catchments of the Argideen and the Clonakilty estuaries in Co. Cork are “dominated by an agricultural landscape”, and in both cases, a WWTP is present.

In the case of the Argideen estuary, the EPA stated that nutrient loadings from the catchment are “mainly from agriculture”.

In Clonakilty, while there is a significant nutrient input from agriculture, nutrient loadings from the WWTPs are “relatively more relevant” than in the case of Argideen.

Major nutrient inputs in Dublin Bay are from the Ringsend WWTP, while the primary source of N in the River Liffey is wastewater, followed by emissions “from pasture”.

The nutrient enrichment of the Tolka river emanates from diffuse agriculture sources and contributes “significantly” to the total nutrient input in the bay.

Nutrients in aquatic ecosystems

N and P, and other agriculture related contaminants (e.g. pesticides and herbicides) are introduced into aquatic ecosystems via land run-off and urban and industrial wastewaters.

The EPA, through its research, has determined that the number of estuaries affected by nutrient over enrichment as a consequence of the intensification of agriculture in Ireland is “expected to increase”.

“Intensification of agriculture remains one of the most important pressures on aquatic ecosystems worldwide because of the run-off of nutrients and associated contaminant,” they added.

Another concern is over the impact of herbicides on aquatic ecosystems, of which the EPA has admitted “little is known about”, but that “on most occasions, agricultural run-off contains both nutrients and pesticides”.

The EPA has stated the widespread use of glyphosate and MCPA (a herbicide) has impacted water quality in Ireland and has contributed to the increasing contamination of surface waters and groundwaters.

Managing algal blooms

The EPA has said hydrological modifications (e.g. increased river flushing, reduction in water residence times, enhancing tidal currents, enhancing turbidity) or geoengineering actions (e.g. injection of aluminium in sediments) have been suggested as measures to reduce nutrient loadings to accelerate recovery from eutrophication.

However, the EPA has warned that these measures could have “unpredictable ecological consequences, and therefore extreme caution should be exercised.

“Reducing nutrient loadings through applying best agricultural practices, improving the efficiency of wastewater treatment facilities, and restoring key species involved in nutrient regulation, should be the preferred options for eutrophication abatement in Irish estuaries.”

The EPA has recommended to reduce nutrient loading from agricultural and urban sources, and to maintain N:P ratios at levels similar to those found in estuaries with little or low human activity.

In this scenario, the agency said it is important to reduce nitrogen loadings quicker than phosphorus loadings to avoid an imbalance in the N:P ratio, which could lead to harmful algal blooms.

The development of “best agriculture practices focused on reducing nitrogen loadings and improving the effectiveness of wastewater treatment facilities should be a priority,” the EPA added.

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eutrophication

EPA: Agriculture the 'biggest driver' of algal blooms in rural areas