Monitoring recommended as algae blooms expected to increase
Algae blooms have been spotted in Clonakilty Bay, West Cork. Picture: Andy Gibson.
Green and red macroalgal blooms in Irish estuaries are predicted to increase, in a new report prepared for the Environmental Protection Agency, which confirmed that agricultural sources were the biggest drivers of these blooms in rural locations, while effluent from wastewater treatment plants was more relevant in urban areas.
These seaweed tides or seaweed blooms damage local economies and biodiversity, with "massive mortalities of organisms of commercial interest" and visual and odour impacts. They peak in the spring and summer each year, boosted by nutrients and the increased light and temperature.
The experts who authored the report predicted increased eutrophication and hence increased incidence and severity of extraordinary macroalgal blooms, and blamed the Food Wise 2025 plan for agricultural intensification, in combination with seawater temperature increase, biological invasions, and emerging contaminants.
They warned that agricultural intensification may be at the expense of achieving national water quality and greenhouse gas emission targets.
The team of 13 experts included two from Spain, one from Sweden, one from Germany, and nine from Ireland. They investigated blooms at Clonakilty and Courtmacsherry in Co Cork (which can cover a total of 793 hectares); in Co Waterford at the Lower Blackwater Estuary and at Dungarvan (totalling 1,527 ha); at Bannow Bay, Co Wexford (985 ha); and in Co Dublin at Tolka, Malahide, and Rogerstown (totalling 970 ha) The experts recommended that nutrient loading from agricultural and urban sources be reduced, and specifically that nitrogen to phosphorous ratios be maintained at natural levels. They said it is important to reduce nitrogen loadings quicker than phosphorus loadings, to avoid an imbalance. Best agriculture practices and improved effectiveness of wastewater treatment facilities should get priority. Restoration of oyster beds and seagrass meadows, and harvesting of seaweed, could also help.
They recommended biomonitoring nitrogen and phosphorus content in bloom-forming species. The nitrogen isotopic composition identifies the likely sources of nutrient enrichment (such as agriculture or wastewater), for establishing more effective mitigation strategies. This can be a forensic tool for identifying specific localised pressures.
They also recommended use of satellite imagery for monitoring macroalgal blooms, alongside traditional field monitoring.
And they called for further thorough investigation of the impacts and effects of herbicides such as glyphosate and MCPA, which are likely to be found in agricultural run-off.
The project also included mapping a Pilayella littoralis bloom (golden tide) affecting Killybegs Harbour in Co Donegal. This area is dominated by peat bog and agriculture, plus wastewater discharge from the local population and the fish processing industry.
The experts noted that the catchments of the Argideen and Clonakilty estuaries are dominated by agriculture, and in both cases a wastewater treatment plant is also present. In the case of the Argideen estuary and associated rivers, nutrient loadings from the catchment are mainly from agriculture. In Clonakilty, while there is a significant nutrient input from agriculture, nutrient loadings from the wastewater treatment plant are more relevant than at Argideen.
In Dublin Bay, the major nutrient inputs were found to be from the Ringsend wastewater treatment plant, followed by inputs from the Liffey and other rivers. The primary source of nitrogen in the Liffey was found to be wastewater, followed by emissions from pasture. The Tolka and Dodder rivers also contribute significantly to the total nutrient input in the bay, with nutrient enrichment of the Tolka estuary emanating from diffuse agriculture sources.
Eutrophication persists as one of the most important threats to aquatic ecosystems, and reduction of nutrient loading remains the main restoration measure, according to the report.
The authors are Ricardo Bermejo, Nessa Golden, Sara Haro, Sita Karki, Michéal MacMonagail, Sara García-Poza, Teresa Navarrete-Fernández, Benedikt Brunner, Kay Knöller,Mark Healy, Owen Fenton, Per-Erik Mellander, and Liam Morrison.
