One might think that adding nutrients to aquatic systems are a good thing, but it is in fact creating dead zones where most animal life die!

We generally learn that nutrients are a good thing for most ecosystems, and more nutrients lead to an increased biodiversity with a higher species richness and better living conditions for the animals that life there. This is true for many cases, but in aquatic systems the opposite tends to be true, especially when lots of nutrients are available.

Many aquatic systems has experienced eutrophication in the last 100 years, where a body of water is enriched with nutrients. These nutrients tend to come from agriculture, where excess nutrients are brought to a stream or river during rainfall. The river might pick up nutrients from lots of different farms before depositing it all to a lake or pond, where the nutrient concentration will be extremely high.

We see water bodies with eutrophication problems all over the world, and it is especially abundant in places that are close to large agricultural fields. Eutrophication can also happen in other areas, such as places close to sewage systems, or any place where discharged water from humans gets released into natural systems.

Phosphorus and nitrogen tend to be the biggest troublemaker, where phosphorus is the biggest threat for freshwater systems, and nitrogen for saltwater systems.

How increased nutrients lead to death of the animals

I bet some people find it counterintuitive that increased nutrients lead animal death and in some cases dead zones, so I will try my best to explain it. Once nutrients are leaked into the aquatic system, algae and plants will grow at an increased rate, and we can often even see algal blooms in places where they don’t typically occur. This can be great at first, but the increased algal biomass also means that there will be an increase of dead algae after a while.

Once the algae starts to die, they fall down to the ocean or lake bottom, and here bacteria will start to break them down to recycle nutrients. This is vital for the ecosystem, but with the increased algae population these bacteria will break down a lot more nutrients. The problem with this is that these bacteria needs to consume dissolved oxygen from the water to complete this process, and in systems with a lot of algae it might eventually lead to a complete oxygen starvation for the aquatic system!

Here is what an algal bloom could look like. Just imagine what happens when all these algae dies! Image by Dwight Burdette, posted with the Creative Commons Attribution 3.0 Unported license.

When the oxygen is almost completely gone, most fish, insects and other organisms will start to die, and only plants and algae will be able to live there. This condition is termed hypoxia, and if this condition is persistent for a while the area might turn in to a dead zone where no or very few animals are able to live.

We have dead zones in both the ocean and in freshwater lakes, and once an area is turned into a dead zone, getting it back to a natural state can be difficult. The best way to combat this is basically to remove as much nutrients as possible, and this is generally done by removing the first meter or so of the sediment at the lake floor. This will reduce the amount of dead algae that can be recycled, but doing this is both costly and ineffective. This treatment will need to be repeated a few times before the lake will begin to recover.

A much better strategy for deal with this problem would be to avoid it in the first place. It is much easier to get rid of excessive nutrients before they get into the lake, but this has do be done by the farmers themselves. Some farmers use nutrients that have less phosphorus, especially when feeding animals, while others use composting strategies to deal with the extra nutrients.

Despite the agriculture industry being the ones who are responsible, and the ones who need to take action, it affects all of us. Dead zones not only prevents any fish from living in the lake or the affected area, it also makes the water unsuitable for drinking, and can have effects on the ecosystem in which the water body is located. I don’t think this issue gets enough attention, but it’s actually a pretty common problem limnologists often have to face.

Thanks for reading!