For many, the history of the Mar Menor is the chronicle of an announced death: long before the collapse of 2016, the crystal clear waters of the lagoon were already enduring a multitude of impacts until there came a time when, simply, the ecosystem could not hold out more.
The famous ‘green soup’ that the Mar Menor became has its origin in a process called eutrophication, which is the result of the massive contribution of nutrients to a body of water . “The excess of nutrients is absorbed by the phytoplankton in the water, which proliferates to form a very dense mesh on the surface that prevents light from reaching the bottom of the lagoon,” explains Professor of Ecology at the University of Murcia Miguel Ángel Esteve. “As a consequence, all the plants that were naturally anchored at the bottom of the lagoon (phytobenthos) stop receiving light to carry out photosynthesis and end up dying. At the same time, the dead phytoplankton is deposited on the bottom, and all that organic matter together demands a lot of oxygen, an element that is increasingly scarce since the phytobenthos are no longer photosynthesizing ”. A kind of vicious circle that generates, at the bottom of the lagoon, the famous anoxic conditions that we have heard so much about. “These anoxic conditions, depending on the stratification of the water, can extend to the entire lagoon”, Esteve explains. “Without going into very technical details, the result is that when this lack of oxygen extends to the entire lagoon for long periods of time, it can be deadly for the organisms that breathe in the water. If the anoxia is generalized, as it has happened in the Mar Menor, the mortality is massive ”.
Why does eutrophication occur?
For many years, the Mar Menor suffered the impacts of heavy metal mining and urban activity, but currently the pressures come mainly from intensive agriculture. Esteve has published several dozen papers on the Mar Menor and its ecogeographic environment and explains that his group has developed mathematical models to determine the relative weight of all possible causes of eutrophication in the lagoon: “We have observed that intensive agricultural activity ( especially agriculture and, more secondarily, livestock) can account for around 85% of all the nutrients that enter the Mar Menor. The other 15% comes from urban sources, but the origin of the collapse is fundamentally agrarian ”, he points out.
The researcher explains that agricultural discharges consist, fundamentally, of nitrates, and that phosphates only enter with contributions of animal origin or with large avenues. As for urban discharges, “they played a prominent role in the Mar Menor until 25 years ago, both due to poor design of the treatment plants and breakages and fugitive losses, but this now has a secondary role.”
The role of DANA and heat waves
When the first mass fish mortality event occurred, in October 2019, many attributed its origin to the heavy rains that had taken place weeks ago. And, in the same way, the episodes of 2021 have been related to heat. Are they determining factors, or rather the last straw in an ecosystem that is already out of balance? For most experts, it would be rather the latter: “Everything helps, but the sick person is already sick. The Mar Menor has a chronic disease due to all that amount of nutrients that it receives on a daily basis. The DANA supposes a massive entrance of more nutrients, it acts as a broom and sweeps all the fertilizers that were on the surface of the crops. It is an extra supply of nutrients and usually produces stratification, so that the deep part of the Mar Menor is disconnected from the surface due to the change in salinity and a huge bag of anoxia is generated at the bottom ”, indicates Esteve. “In 2019 there was already this anoxia pocket, but this process was so intense that the sulfur bacteria were activated and the sulfates in the water became sulfides, which are very toxic . Due to the winds, those putrid waters at the bottom, without oxygen and loaded with sulfides, emerged on the surface and killed the fish that we saw jumping towards the banks in a kind of assisted suicide.
The ecosystem services of the Mar Menor
On one occasion, Esteve stated that ” the Mar Menor gave us more than two decades of extra ecosystem services, due to its resilience capacity .” The lagoon had been enduring all kinds of pressures and impacts for years, but thanks to its ability to buffer this massive supply of nutrients, the collapse was delayed until 2016. “On the one hand, the peripheral wetlands would absorb a large part of the nutrients, but above all The role of phytobenthos is remarkable, of all those algae and vascular plants at the bottom of the lagoon that grew a lot and absorbed most of all the nutrients that came from the Campo de Cartagena. In a very secondary way, also the huge explosion of jellyfish would help, since many have zooxanthellae that filter nitrogen ”, explains the expert. “In any case, the water of the Mar Menor remained crystalline for a long time, until a time came when all these mechanisms were not enough to support the high load of nutrients, and the phytoplankton explosion occurred”. With the famous green soup, the Mar Menor went from having crystalline waters to turbid waters, “the eutrophication process culminated with this crisis and with the collapse of the historical functioning of the Mar Menor. This operation has been turned around like a tortilla: it is no longer controlled by the plants at the bottom (phytobenthos), but by the unicellular plants of the body of water (phytoplankton) ”.
Is there going back?
One of the big questions about the Mar Menor crisis is the reversibility of the process: has the ecosystem reached a point of no return? Although there are doubts, Esteve tells us that an optimistic position prevails in the debates: “ if the cause – the extra nutrient inputs – were eliminated , the historical resilience capacity of the Mar Menor would become a restoration capacity and the phytobenthos would recover the control of the lagoon ”.
The solutions that his team proposes go to the root of the problem, which is the excess of intensive agricultural activity. “The figure is not exactly known, but of the approximately 60,000 irrigation systems that exist around the lagoon, about 10,000 would be illegal. If you eliminate the latter, you already remove a source of compost and fertilizers. Logically, it would also be necessary to regulate the entry of fertilizers that come from the legal agricultural activity, it is necessary to lower the load of nutrients ”.
Renature the Campo de Cartagena
As a complement, the experts insist on the need to re-naturalize the Campo de Cartagena: “Around 7% of its surface should be occupied by natural hedges, and the boulevards that are currently cleared for agricultural activity must be revegetated. In this way we would have a kind of natural mesh attached to the crops that extracts nutrients, evapotranspires water and, in addition, also acts as a retention when large avenues arrive ”. Esteve insists on the important role of wetlands: “they function as kidneys of the system by extracting part of the nutrients before reaching the water. We must restore the salt marshes and wetlands, create a belt around the entire periphery of the internal lagoon that retains the water from the avenues and does denitrification work ”. Regarding the waters of the Albujón Rambla, which is currently the main collector of all the waters that end up pouring into the Mar Menor, the researcher indicates that it would be necessary to try, with bioreactors and with the role of the wetlands themselves, to reduce the load of nutrients they carry.
Implementing all these measures is a complex challenge, but experts are confident that, if achieved, the process could be reversed to regain balance and the historic crystal clear waters of this unique ecosystem in Europe.
