Mar Menor at risk of algae bloom as oxygen levels plummet
Scientists warn that "there is a high risk" of marine life dying in the Mar Menor if conditions don’t improve
The Mar Menor is once again in environmental trouble and summer is only halfway through. Scientists monitoring the saltwater lagoon are sounding the alarm as the water shows clear signs of distress. According to the Spanish Institute of Oceanography (IEO), chlorophyll levels in the lagoon are now five times higher than what was measured just two years ago.
Chlorophyll is the green pigment that signals the presence of phytoplankton, microscopic organisms that form the base of the marine food web. While they are essential in small amounts, an overgrowth can be dangerous. The recent spike has made the water murkier and reduced sunlight reaching the seafloor, putting marine vegetation at risk.
Since June 30, light levels in the depths of the lagoon have dropped from 23.5% to just 5.8%.
Juan Manuel Ruiz, coordinator of the Belich monitoring project and principal investigator for the IEO, explained that “this percentage is a level that could be critical for bottom vegetation if the situation persists.”
So far, oxygen levels remain stable, and there are no signs of dying seagrass.
“Oxygen levels are optimal at this time of year, so in the short term there is no risk of partial or total oxygen loss in the Mar Menor,” Mr Ruiz said.
However, he warned that “there is a high risk that the lagoon ecosystem will be affected if the trends persist or intensify.”
The problem seems to be internal rather than coming from external nutrient inflows like runoff from the Albujón ravine. Mr Ruiz pointed to the exceptionally early heatwaves and high water temperatures of up to 32°C as a possible cause. This heat may have triggered microbial activity that releases nutrients from sediment at the bottom of the lagoon.
“We are experiencing a period of abnormally high temperatures, accumulated since practically the end of May, and this may have accelerated the microbial decomposition processes that release nutrients from the sediment,” he said.
These nutrients then feed the phytoplankton bloom, creating the dangerous conditions scientists are now tracking.
The situation is serious enough that the Secretary of State for the Environment, Hugo Morán, personally called regional Environment Minister Juan María Vázquez to brief him on the findings. The cause is likely the build-up of nutrients over decades in the lagoon's sediments and other compartments.
“Something has now altered that situation, and nutrients have been released into the water column,” the expert said.
The researchers are also considering other possible contributors, such as water from the underground aquifer discharging into the lagoon. This process has previously caused the formation of a white spot in the western area of the lagoon.
Mr Ruiz added: “Anoxia and the reduction of light at the bottom are the two factors that could put the ecosystem at greatest risk. Light is at levels that are already beginning to be critical for vegetation, but oxygen is at normal levels. In some places, there is a tendency to decrease, but always within normal values.”
At the same time, the Segura River Basin Authority is keeping a close eye on the groundwater beneath Campo de Cartagena, which feeds into the Mar Menor. Jesús García, head of Hydrological Planning, highlighted the “enormous complexity” of the aquifer system, noting that its water levels have been steadily dropping since 2019 by about 1.5 to 2 metres.
This decline has led to a decrease in nitrate-contaminated water flowing into the Albujón ravine.
“The levels aren't rising, they're falling, which doesn't mean we could not have an episode of intense rainfall (he cited spring 2022 as an example) that will raise the aquifer level again,” Mr García explained.
While this is good news for the lagoon in terms of nitrate pollution, it contrasts with the problems reported by local residents in towns like Torre Pacheco and San Javier. There, high groundwater levels are causing issues in garages and sewer systems.
Another development is the reduction in aquifer recharge from rain and irrigation, meaning that the long-term discharge of water - and with it, nutrients - into the lagoon could lessen.
However, the expert has warned that the upwelling of aquifer water through the Albujón will not stop entirely.
“It's something natural that will continue to occur,” he said.
“With the reduced water discharge, fewer nitrates also reach the stream, although this is a situation that will have to be reversed.”
Researchers are still trying to pinpoint the exact cause of the current crisis. They believe it is the result of long-term nutrient build-up combined with recent events that caused the nutrients to spread more widely through the lagoon.
Mr Ruiz emphasised the role of the recent heatwaves in driving the sudden increase in chlorophyll.
“The high temperatures, caused by the heat wave in the Mar Menor since May, which have cooled by 1°C since July 10, are the main cause of the increase in chlorophyll levels,” he surmised.
The situation is being watched closely and both the IEO and CHS will continue their work in the coming months to monitor changes in water quality and aquifer behaviour.
Also of interest: Three boats abandoned in the Mar Menor as illegal activities surge
Image: Archive
