A couple of years ago, on a snowy day in January, I gave a talk and ran a practical on algal-based ecological assessment for an MSc class at the University of Bristol. As part of the practical class, I asked my colleague, Marian Yallop, who organised the session, if she could set some algal cultures growing a few days ahead of my arrival, in order to stimulate some discussion amongst the students on what we understood by “eutrophication”.
I asked for four flasks, each with a standard algal growth media, into which an inoculum of algae was pipetted. I can’t remember what species of algae we used, except that it was a common green algae. The growth medium was naturally low in nutrients, so Marian augmented the media in two of the flasks with an extra squirt of nutrients, so we had two with “background” nutrients and two with high nutrients. Then she placed one of each pair on a windowsill, and the other in a refrigerator and left them for a few days.
Our eutrophication “thought experiment” in Bristol in January 2013
I had given the students the following definition of eutrophication, used in a key EU Directive (the Urban Waste Water Treatment Directive): “the enrichment of water by nutrients, especially compounds of nitrogen and/or phosphorus, causing an accelerated growth of algae and higher forms of plant life to produce an undesirable disturbance to the balance of organisms present in the water and to the quality of the water concerned.”
My question to the students was simple. Which of the four flasks is “eutrophic”? The first part of the definition says “the enrichment of water by nutrients …”, so we could have argued that both of the “plus nutrient” treatments were eutrophic. However, the definition then goes on to say “… causing accelerated growth of algae ….”. The plus nutrient treatment that was kept in the refrigerator did not fulfil this criterion; however, the one kept on the window ledge is the greenest of all the flasks. So could we claim that only the “plus nutrient, window ledge” flask was eutrophic?
What about the “minus nutrient window ledge” flask? That looked quite green, even though the nutrient levels were low. This illustrates a further important point: the quantity of phosphorus, in particular, that a plant needs to grow is very small and if other conditions are right (i.e. a windowsill in a centrally-heated laboratory), then you can get high biomasses of algae, even without excess nutrients (see “A brief excursion to Norway”). In nature, such natural abundance would not last for long: it would be scoured away by a flood or eaten by hungry invertebrates on the river or lake bed. Based on my experience of northern English rivers, I would only get concerned if the high biomass persisted beyond a few weeks.
The flask with added nutrients that was kept in the refrigerator offers another perspective. Even if we could argue that it is not strictly “eutrophic”, we should acknowledge that there is a risk of a high biomass developing. In our thought experiment, the cool dark environment of the refrigerator minimised the risk of a high biomass developing. However, for as long as there are elevated nutrient concentrations, we have to acknowledge that both “plus phosphorus” treatments represent a hazard to healthy ecosystem functioning.
A final, and slightly pedantic, interpretation was that none of these treatments fulfilled the final criterion in the definition of causing “… an undesirable disturbance to the balance of organisms …”. In this case, of course, there was only a single organism present, so it was impossible to demonstrate this particular criterion. However, a quick scan of the literature on eutrophication does show a strong bias towards establishing a link between nutrients and aquatic plants and algae, and the secondary effects are often neglected. These, however, provide the essential “so what?” to our arguments. I live in a world of algae-obsessed nerds and we sometimes need a sharp poke in the ribs to be reminded that most people need a better return on the expensive investment in better water treatment than just to be told that the algae have changed (see “So what?”).