Icky green goo3rd Feb 2020
It’s great having a dam, pond, lake or river on an estate, but when it turns green overnight, and things start dying in it, it’s more of a liability than an asset. So, as estate managers, we need to understand – firstly – how this happens, and then how to deal with it.
What is that icky green stuff?
The technical term for what’s happened to your fabulous water feature is eutrophication, which means, basically, that your aquatic system (and any water feature is just an artificial aquatic system) has too many nutrients. Now nutrients are critical to the ability of the water body to sustain life, and wherever there is water, there is also an abundance of biological life. This is why water features are so important, because at primordial level the human brain associates them with abundance of life and therefore a good place to be. So, intuitively, more nutrients sounds like a good thing. But – as with many things – you can have too much of a good thing.
Surely nutrients are a good thing
The direct relationship between the availability of nutrients in forms that can be used to sustain biological life and the presence of biological life is known as the trophic status of a water body. The lower the nutrient level, the less biological life can be supported within the water column itself, therefore the lower the trophic status. A system with very low nutrient levels is known as an oligotrophic system. In such systems the water is crystal clear, so large predators can hunt by sight. Here we find otters and fast-swimming predatory fish like the tiger fish and trout. Oligotrophic systems have a specific range of biodiversity created by the clear water habitat. Clear water, by definition, means the absence of algae.
The opposite of an oligotrophic system is a eutrophic one. In a eutrophic system there is a high level of nutrient availability, and therefore a larger biomass can be sustained within the water column. Eutrophic systems support algae, so they have low visibility and are therefore unable to host predators that rely on sight for finding food. In a eutrophic system we find slow-moving fish like barbel and carp, which find food by smell or touch rather than sight.
Somewhere between these two we find mesotrophic water systems, which are characterised by relatively clear water with a healthy population of algae – mostly growing on the bottom.
At the absolute extreme we find hypertrophic systems that are choked with icky globs of algae (technically called algal blooms) that can use up all the oxygen so that fish and other water creatures die – and even produce deadly toxins that can affect people or animals that swim in or drink the water.
The attack of the blue-green monsters
Most cases of eutrophication in fresh water involve a primitive organism that we call blue-green algae. It is – as far as we have been able to ascertain – one of the first living organisms to have evolved on the ancient Earth, and it’s been doing pretty well ever since. They love elevated temperatures in nutrient-rich waters, and they can survive ultraviolet light at levels that would kill other living creatures. Their presence is an indicator of a disrupted or distressed ecosystem. But before you start sending out metaphorical hate mail, these primitive organisms were essential to the evolution of life as we know it. It’s only through their slow accumulative increase of oxygen levels on ancient Earth that more complex life forms (like roses, baobabs, impalas and humans) could evolve.
How does eutrophication happen?
The origin of those nutrients is usually in the return flow of water that has been used for something else like irrigation, car washing or toilet flushing. The most important nutrients are nitrogen and phosphate. Both are manifest in different forms and are found in a range of products like soap, detergent, fertiliser and sewerage discharge. In fact, one of the main sources of phosphates is laundry detergent, and domestic grey water contains vast amounts of this nutrient. (This is why grey water is so good for lawns and other garden plants.)
What this indicates is that most eutrophication is as a result of human activity. This is why we need to be so careful about the levels (and types) of fertiliser we use on our gardens and golf courses, and why the management of waste water is such an important issue.
So what do we do about it?
This is one instance in which prevention is so much better, cheaper and more effective than cure. Management of grey water and – obviously – sewerage, and careful monitoring of run-off of fertilisers is the first step. But water features do not exist in isolation, and their management is not solely the responsibility of the estate – as the coastal Imhoff’s Gift estate in Cape Town, which offers residents fabulous views of the beautiful Wildevoëlvlei, discovered. Because of a complex relationship between the nearby municipal wastewater treatment plant and increased population in the area, the Wildevoëlvlei became dangerously and toxically hypertrophic in 2010. Treatment was complex and expensive, and included dropping vast amounts of salt in the vlei from helicopters. But, in the absence of addressing the causes, even these drastic measures would have been no more than a band-aid. Really – prevention is far, far better than cure.