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What may seem like a positive effect may not be, since ecosystems have a balance between all their parts, and if this is altered, the consequences can trigger a domino effect that alters the entire ecosystem. In this article we will explain what eutrophication is, its causes and its consequences.
Certain limits are good
In aquatic ecosystems, whether freshwater or saltwater, the base of the food chain is plants. Plants require three things for their development: nutrients, light and water. The limiting factor for plant growth in marine ecosystems is nutrients, specifically phosphorus and nitrogen.
Nitrogen can be fixed by some marine bacteria from atmospheric nitrogen, such as cyanobacteria (remember that 78% of the Earth’s atmosphere is nitrogen), or reach the sea through runoff waters. However, the bacterial contribution of nitrogen to marine ecosystems is less than 0.5% of total nitrogen.
Phosphorus also reaches the sea through the water courses that flow into it. Both the contribution of nitrogen and phosphorus to aquatic ecosystems are slow processes, which take long periods of time to vary substantially, so the ecosystem has time to adapt to any natural change in the contribution of these nutrients.
The problem here, as on many other occasions, appears when human beings interfere with the natural functioning of ecosystems.
What is eutrophication and its causes
Every eutrophication process (a term that comes from the Greek eutrophos , meaning “well fed”) begins with the addition of nutrients to a watercourse. More specifically, with the addition of phosphorus or (more frequently) nitrogen, since other nutrients are not limiting for plant growth.
The two most common ways this can happen are through improper use of nitrogen fertilizers (excess nitrogen reaches the underground aquifers, contaminating them, and from there it reaches the water courses) or negligent treatment of fecal waste, whether they are of human origin or from livestock farms, which are released directly into a river or the sea. Particularly harmful are waste from pig farms (the well-known slurry), as they have a high nitrogen content.
Once the ecosystem receives the extra supply of nutrients, algae and microalgae (phytoplankton, microscopic single-celled algae) begin to multiply.
First symptoms of eutrophication
One of the first symptoms of a eutrophication process is that the water turns green, and can become completely covered with small algae or aquatic ferns (if it is a freshwater course). Here the reader will ask “well, but why is this a problem?” If there are more algae, the water will have more oxygen and there will be more organisms that can feed on those algae.” This is partially true.
A problem in this stage of eutrophication is that the natural balance of the ecosystem is broken and the entry or proliferation of non-native organisms is favored, especially some aquatic plants such as Eichhornia crassipes or Azolla filiculoides in freshwater ecosystems. The presence of these invasive plants in aquatic ecosystems only worsens the eutrophication process.
One more step: phytoplankton self-shading
In aquatic ecosystems that suffer advanced eutrophication processes, a phenomenon called phytoplankton self-shading occurs. While nutrients are no longer a limiting factor for phytoplankton growth, light is.
As phytoplankton completely occupies the surface of the water, it blocks the passage of light to phytoplankton lower in the water column, causing them to die. This does not prevent phytoplankton from continuing to multiply, which is why it does not slow down its growth.
But the accumulation of organic matter at the bottom of the water also increases its decomposition rate. And, although phytoplankton, in addition to breathing, generate oxygen through photosynthesis, those organisms that carry out the decomposition of organic matter only consume oxygen, without producing it.
Once plant growth has run amok due to the increase in nutrients, if this is not stopped, the next step in eutrophication occurs, which is the accumulation of dead organic matter at the bottom of the water.
In lotic ecosystems, those where water is moving (such as rivers or streams) this is usually not a big problem, and eutrophication does not normally progress beyond this point.
But in lentic ecosystems, in which water is not in constant movement (such as in ponds, lakes or swamps), dead organic matter accumulates progressively. It should be noted that some invasive aquatic species, such as Alternanthera philoxeroides , can grow to the point of completely covering the surface of a river and transforming it into a swamp, with the water on the banks practically stagnant.
In this scenario of advanced eutrophication, the net oxygen consumption of the ecosystem increases exponentially, to the point where the oxygen in the water is depleted, and this causes death by suffocation of the animals and plants in the ecosystem. This is the final step in the eutrophication process, anoxia or hypoxia.