The ecological status of the Mar Menor (little sea) lagoon will deteriorate with a 3ºC to 5°C rise in water temperature

As a result of an up to 5°C increase in water temperatures over the next few years, this pioneering study shows an increase in the regression rate of benthic primary producers, a deterioration in ecological status and the appearance of eutrophication processes in many coastal lagoons. Notable effects include the proliferation of jellyfish.

The work, recently published in the Estuarine Coastal and Shelf Science magazine and financed by the Euro-Mediterranean Institute of Water, represents the first data-based assessment of the vulnerability of the lagoon’s entire coastal ecosystem to a probable environmental change and eutrophication. According to the researchers, it is “essential” to know the interactions between the processes for identifying future impacts and establishing effective coastal planning and management measures.

“If climate change predictions come true, the current state of the Mar Menor lagoon could collapse due to proliferations of phytoplankton and floating macroalgae”, Javier Lloret, one of the study's researchers, explained to SINC. He talked about a profound deterioration of the entire ecosystem “through the appearance of eutrophication processes with high concentrations of nutrients”.

The research, applicable to other lagoons, forecasts that the global climate will have a “high” effect on coastal lagoons, which are considered “one of the most fragile marine environments to these changes”, Lloret pointed out. Among the most harmful effects, scientists highlight the increase in water temperature, a rise in sea level of at least a 50 cm, changes in the hydrodynamism of water masses and in the water’s salinity, as well as an increase in dissolved carbon dioxide, frequency of extreme climatic events and appearance of eutrophication processes.

Proliferation of jellyfish due to climate change

One of the main consequences of an increase in lagoon temperatures is the proliferation of jellyfish, which represent “an example of the alteration of the system’s trophic state and instability of parameters for the lagoon”, indicated the researcher from the Ecology and Hydrology Department at the University of Murcia.

In addition, the study highlighted that a loss of benthic macrophytes and appearance of eutrophication processes could result in “a substantial decrease in the quality of the lagoon’s habitat with unforeseen consequences for the biological diversity of its communities”. To this is added the possible reduction in the amount of light reaching the beds of the Mar Menor lagoon due to the proliferation of phytoplankton.

“This reduction is the result of the combined effect of the rise in sea level and decrease in the transparency of the water column caused by an increase in the entry of nutrients and dissolved solids”, Lloret added. The biomass of the Caulerpa prolifera macroalgae, which covers 91.7% of the lagoon's beds and is below 5 metres in depth, is responsible for maintaining a positive carbon balance. However, most of this biomass would be affected, even with death, due to a reduction in photosynthesis with an increase of water temperature over 30ºC.

The Mar Menor lagoon has ecological characteristics of high productivity and biological diversity as a result of being separated from the Mediterranean Sea by a 22 km long, 100 m to 1,200 m wide sand bar. Designated by the United Nations as a ‘Specially Protected Area of Mediterranean Importance’, the coastal lagoon is, however, vulnerable to eutrophication due to the rise in population along the coast and use of fertilisers for agriculture.