A paper has just been published that should be required reading for anyone interested in the management of nutrients in in ecology. It is a follow-up of a 2006 paper with the catchy title “How green is my river” that set out to provide a conceptual framework for how rivers responded to enrichment by nutrients. That original paper contained several good ideas but, crucially, not all of them were underpinned by evidence. A decade on, several of the predictions and statements made in that original paper have been tested, and the time has come to re-examine and modify that original conceptual model.
My reaction to the 2006 paper was that it was very interesting but not fully reflective of the rivers in my part of Britain, whose rougher topography produced quite different responses to nutrient enrichment than that proposed in their original model. That criticism has been addressed in the revised version, which places greater emphasis on the physical habitat template, which means that it is more broadly applicable than the original version. But that, in turn, got me wondering about the continued relevance of a term such as “eutrophication” to rivers.
People have been using the term “eutrophic” to describe lakes with high concentrations of nutrients since early in the 20th century. As the century progressed, evidence of a causal relationship between inorganic nutrients and algal biomass, and the consequences for other components of lake ecosystems grew. With this foundation, it has then become possible to predict the benefits of reducing nutrients and there are plenty of case studies, particularly from deep lakes, that demonstrate real improvements as nutrient concentrations have declined.
Attempts to apply the same rationale to rivers have, however, met with far less success. Legislation to reduce nutrients in rivers has been in force in Europe since 1991 (the Urban Wastewater Treatment Directive, followed by the Water Framework Directive) and whilst this has led to reductions in concentrations of phosphorus in rivers (see “The state of things, part 2”), there has, in most cases, not been a corresponding improvement in ecology. There are a number of reasons for this but, at the heart, there was a failure to understand that the tight coupling between nutrients and biology that was the case in deep lakes did not also occur in running waters. What was needed was recognition of fundamental differences between lakes and rivers, and “How green is my river?” and, now, this new paper have both contributed to this.
However, one consequence of recognising the importance of the physical habitat template alongside nutrients is to challenge the relevance of the term “eutrophic” when describing rivers. “Eutrophic” literally means “well-nourished” so is appropriate in situations where high nutrients cause high plant or algal biomass. This high biomass (strictly speaking, the primary production arising from this biomass) then creates problems for the rest of the ecosystem (night-time anoxia caused by plants consuming oxygen being a good example). If high biomass can arise due to, let’s say, removal of bankside shade or alteration to the flow regime, perhaps (but not always) in combination with nutrients, then perhaps we need a term that does not imply a naïve cause-effect relationship with a single pressure?
My suggestion is to shift the focus from nutrients to plant growth by using the term “euphytic” (“too many plants”) as this would shift the emphasis from simply driving down nutrient concentrations (expensive and not always successful) towards reducing secondary effects. It is possible that strategies such as planting more bankside trees, for example, or altering the flow regime or channel morphology (see “An embarrassment of riches …”) may be just as beneficial, in some cases, as reducing nutrient concentrations. That said, we also have to bear in mind that nutrients may have an effect well downstream, so focus on amelioration of effects within a particular stream segment will never be a complete solution.
I should emphasise that a lot of work has been done in recent years to understand the concentrations of nutrients that should be expected in undisturbed conditions, and also to understand the nutrient concentrations that lead to changes in community structure in both macrophytes and algae. These show that many rivers around Europe do have elevated concentrations of nutrients and I am not trying to side-step these issues. I do, however, think it is important that regulators can prioritise those rivers in greatest need of remediation and, in most cases, they do this without considering the risk of secondary effects.
It is, largely, a matter of semantics. I have been involved in many conversations over the past couple of decades about how to improve the state of our rivers. Many of those have centred on the importance of reducing nutrient concentrations (which would be, indisputably, a major step towards healthier rivers). But there is more to it than that. And Mattie O’Hare and colleagues are helping to open up some new vistas in this paper.
Note: the photograph at the top of this post shows the River Wear at Wolsingham. This stretch of the river captures many of the challenges facing river ecologists: nutrient concentrations are relatively low and there is good bankside shade. However, the flow of the river is highly altered due to impoundments upstream and a major water transfer scheme. How do all these factors interact to create the often prolific algal growths that can be seen here, particularly in winter and spring?
Hilton, J., O’Hare, M., Bowes, M.J. & Jones, J.I. (2006). How green is my river? A new paradigm of eutrophication in rivers. Science of the Total Environment 365: 66-83.
O’Hare, M.T., Baattrup-Pedersen, A., Baumgarte, I., Freeman, A., Gunn, I.D.M., Lázár, A.N., Wade, A.J. & Bowes, M.J. (2018). Responses of aquatic plants to eutrophication in rivers: a revised conceptual model. Frontiers in Plant Science. 9: 451