Posidonia seagrass meadow. | Manu San Felix

Posidonia oceanica meadows are the treasure of the Mediterranean Sea.

As we explained in a previous article entitled “Posidonia, a treasure under pressure”, this seagrass provides white beaches and transparent waters acting as underwater forests providing oxygen and biodiversity while being important carbon sinks. However, this treasure is under threat from anchors and wastewater.

Today we will talk about impacts of wastewater inputs to the sea; a future article will focus on possible solutions to preserve our submarine forests and reduce water pollution.
The water going out from our sinks, showers and toilets, travels a long way before ending in the sea.

Wastewater treatment plants remove nutrients and organic matter before the treated water is returned to the environment. In the Balearic Islands, an important portion of the treated water (38 %) is dumped into the sea, through pipelines known as effluents. Most marine effluents are discharged at about 20 m depth, many of them being sited over posidonia meadows.

In some areas Posidonia meadows have disappeared because of these nutrient and organic matter inputs of treated sewage.


Some of the main problems regarding to wastewater treatment plants and their inputs to the marine environment are related to its functioning:Residential sewage is often mixed with rainwater, significantly increasing the flow of water arriving to the wastewater treatment plants in episodes of heavy rain.

Most wastewater treatment plants were constructed (or improved) in the 90s; when the resident and touristic population was much lower than today. So, the treatment capacity is not currently adapted to summer population peaks, where about 2 million people are living in the Balearic Islands at the same time.

Inputs of treated sewage add nutrients and organic matter to the marine environment. These inputs result mainly in 4 impacts to marine ecosystems:

  • Increase algal biomass and production, resulting in a change in watercolour, becoming green and increasing turbidity, and decreasing light penetration in the water column.This, in turn, results in a decrease in light availability at the sea bottom, with negative consequences for seagrasses and macroalgae that need light to survive.
  • When algal blooms die, they fall into the bottom, where they are respired by bacteria, in a process that consumes oxygen and produces carbon dioxide, depleting oxygen from the sea bottom. Oxygen is required for most marine organisms to survive, so the lack of oxygen creates a dead zone where most life is removed.
  • Organic matter inputs to marine sediments increases the presence of a very toxic compound: sulphide. Sulphide is extremely toxic to most organisms, and kills posidonia plants at low concentrations.
  • Nutrients and organic matter inputs also favour the proliferation of organisms growing on top of posidonia leaves (epiphytes). These organisms cover the leaves, impeding photosynthetic activity, thus compromising the plant performance.

At Marilles Foundation we advocate for protecting and recovering seagrass meadows through an integrated management of coastal areas and an appropriate management of sewage.

We encourage immediate action, promoting solutions to protect the marine environment.