Discharge from a wastewater treatment plant may be a factor in increased levels of sludge from rotting seaweed in Dublin Bay, according to marine biologists at NUI Galway.
The dark, foul-smelling organic material has been accumulating on beaches in recent days, notably at Shelley Banks, which is located beside Ringsend Wastewater Treatment Plant.
Scientists say it may be having a negative impact on vital seagrasses in the area.
The scale of sludge has prompted concerns from local residents, bay users and city councillors, who have repeatedly highlighted the issue of frequent discharges from the Ringsend plant.
Dublin City Council said the sludge is arising because a seaweed species in the area known as Ectocarpus was rotting, and that it was not due to a sewage discharge.
Irish Water, which operates the plant, said last week there had been no discharge recently.
Ectocarpus is more prevalent in the summer months as it is attracted to warmer waters and benefits from extended daylight hours.
Betty Villamayor, a PhD student researching seagrasses at NUIG, said an abundance of nutrients, such as those found in the wastewate from the processing plant, may be accelerating the seaweed growth.
While it is not toxic to humans, the macroalgae from the seaweed is wreaking havoc on the local seagrass population, she said.
Dwindling
Seagrass forms in the shallow intertidal areas along the coastline and is greatly impacted by human activity. Dwindling seagrass populations can have wide-ranging negative impacts on the local marine ecosystems - from disrupting habitats to capturing less carbon dioxide from the atmosphere.
“We do know from numerous studies worldwide that increased nutrients from wastewater, agricultural activities and urban areas causes extensive algal blooms that smother seagrasses,” Ms Villamayor said.
Ms Villamayor has studied seagrasses for nearly a decade, and was in Dublin last week to take samples of the seaweed coating the beaches.
She said she is concerned by the observed increase of the Ectocarpus seaweed, which affects what she calls the “seagrass/seaweed balance.” It forms a solid mat on top of the seagrass, which reduces the sunlight that reaches the seagrass, impacting its ability to photosynthesise.
Dr Dagmar Stengel, head of Botany and Plant Science at NUIG, said she could not prove a definitive link between the wastewater flow into Dublin Bay and increased presence of Ectocarpus, but the influence of nutrients on seaweed is generally accepted by researchers.
“Certainly there is a link between nutrients and general increase in fast growing, opportunities (nuisance) seaweeds, which would include various species, most likely also Ectocarpus,” Dr Stengel added.
Seagrass plays a vital role in the marine ecosystem, according to the conservation group Project Seagrass, because it stabilises sediment, preventing erosion and provides a habitat and feeding ground for fish, shellfish and other marine organisms. Additionally, communities around the world depend on seagrass meadows for food security and livelihoods, it said.
In Dublin Bay, seagrass also provides vital nutrition for the migratory Brent goose, which is a protected species, according to Rita Hagan, a researcher on coastal and marine environments at NUIG.
But arguably its most important contribution is the amount of carbon it pulls out of the atmosphere, Ms Villamayor said. Seagrass is 35 times more efficient at capturing and storing carbon than tropical forests. Maintaining a healthy seagrass community around the world is significant for combating climate change and reaching the global target of net-zero carbon emissions.
In addition to wastewater seepage, Ms Hagan has seen overflows from storm water drains flowing into the bay. After a heavy rain in early August, she noticed a particularly foul-smelling overflow at Merrion strand.
“The water around the overflow was almost black, and smelled heavily of faeces,” she said. “Wet wipes and sanitary waste were clearly visible around the discharge and on the sand itself. People recoiled when they came in the vicinity of it.”
While people may recoil, this is a prime food source for the Ectocarpus. The nutrients flowing from the wastewater plant and storm drain overflows encourage even more seaweed, further throwing off the seagrass/seaweed balance and putting the seagrass population in danger.
“Loss of seagrass is correlated with a decrease in marine biodiversity,” Ms Villamayor said. “The public needs to be informed on how important these marine plants are, and the threats that these meadows are facing.”