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NUI Galway Study Finds High levels of Green Tides in Cork and Dublin Bays

Researchers from Earth and Ocean Sciences and the Ryan Institute at NUI Galway have carried out a study on seaweed blooms that occur in Irish estuaries on an annual basis as a result of nutrient enrichment.

Nitrates and phosphates from the land flow into the sea, which can lead to the growth and proliferation of green seaweeds, commonly known as Sea Lettuce (mainly Ulvoid species). In affected bays and estuaries the shore becomes so green that these seaweed blooms are referred to as “green tides”. In Ireland, The Tolka Estuary in Dublin Bay and Courtmacsherry and Clonakilty Bays in west Cork are heavily affected by these green tides. In this study, Dr Liam Morrison and Dr Ricardo Bermejo sampled several seaweed blooms on a seasonal basis, assessing the two types of sea lettuce morphologies present and extracting the DNA to identify the range of species present.

Courtmacsherry, Co. Cork

Nutrient enrichment in our marine waters has increased worldwide as a consequence of the growing human population, especially in urban centres along the coastal zone. Nutrient enrichment in European coastal waters has been identified as a key pressure on water under the Water Framework Directive (WFD), and the reduction of nutrient loads from agricultural practices and wastewater treatment are the main restoration measures. Although green tides are not generally toxic to humans their occurrence, by virtue of their sheer size, impacts shore-based activities including navigation, tourism, fisheries and the recreational use of our coastal amenities.

Lead researcher of the study, Dr Liam Morrison, Earth and Ocean Sciences, Ryan Institute, NUI Galway, said: “Ireland is famous for its green countryside but this should not include our beaches too. Green tides present a large and costly problem for humans and coastal ecosystems. We need to work harder on trying to keep nutrients out of the water achieving a good balance between agronomic and environmental gains, especially with the removal of milk quotas and increased intensifciation in dairy farming in Ireland.”

Dr Morrison, added: “In terms of the circular economy, more research is needed into the re-use of this seaweed biomass for example as potential fish meals or for energy production. This could create the potential to consider these blooms as a resource.”

This research was part of an Environmental Protection Agency funded ‘Sea-MAT Project’ (http://www.seamatproject.net) and was carried out over two years. These results were recently published in the journal Harmful Algae. Results revealed that these seaweed blooms contain several different species, with Ulva prolifera, Ulva compressa and Ulva rigida the most frequent species. The species composition changes over the year and this species succession was common to both estuaries. The blooms are dominated by anchored species during spring and early summer, changing to more floating morphologies during late summer and autumn.

Nutrient enrichment of estuarine and coastal waters (e.g. from direct discharges such as wastewater treatment plants and diffuse sources such as agricultural runoff) is considered a key factor for the development of green tides. While different locations can have similar nutrient inputs, the way the green tides form and develop can be quite different. This complicates the ability to predict these events based on information about nutrient status alone. This research has provided key information on the development of these blooms over their growing season and gives managers better information for understanding their scale and impacts. The results showed that the amount of green seaweed in the Tolka Estuary in Dublin Bay appeared to have increased over the last 20-30 years.

These findings should be considered for the development of management and monitoring strategies since the different types of seaweed may play an important role in the balance of nutrients and biomass in the estuary or determine the response to pollutant exposure. Furthermore, the presence of different seaweed species with different ecological requirements could favour the duration and extension of the bloom.

The EPA Research Programme is a Government of Ireland initiative funded by the Department of Communications, Climate Action and Environment.