How green is my river? A new paradigm of eutrophication in rivers

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Abstract

Although the process of eutrophication is reasonably well understood in lakes, there is currently no conceptual understanding of how eutrophication develops in rivers. This issue is addressed here. A review of the main processes controlling the development of eutrophication in lakes has been carried out as a precursor to considering the effect in rivers. The importance of hydraulic flushing in controlling algal growth suggests that short-retention-time rivers will show different effects compared to long retention-time, impounded rivers. The latter are likely to operate like lakes, moving from macrophyte domination to phytoplankton domination whereas the former move to benthic and filamentous algal domination. Subsequently, a conceptual model of the development of eutrophic conditions in short-retention-time rivers is developed. Although there is general agreement in the literature that an increase in nutrients, particularly phosphorus, is a pre-requisite for the eutrophic conditions to develop, there is little evidence in short-retention-time rivers that the plant (macro and micro) biomass is limited by nutrients and a good case can be made that the interaction of hydraulic drag with light limitation is the main controlling factor. The light limitation is brought about by the development of epiphytic algal films on the macrophyte leaves. The implications of this conceptual model are discussed and a series of observable effects are predicted, which should result if the model is correct.

Introduction

The Water Framework Directive of the European Community (WFD; Council of European Communities, 2000) provides one of the most significant environmental targets for improving surface water quality across Europe. While considering a host of pollutants, a key focus is placed on the role of nutrients in eutrophication. For example, in England and Wales alone, it has been estimated that the current costs of eutrophication in terms of increased costs of water treatment for public supply, loss of biodiversity and amenity value will be around £100 million per year and that the costs of remediation to address this damage amounts to about £55 million (Pretty et al., 2003). There is considerable debate over the relative importance of point (sewage) and diffuse (agriculture and, for nitrogen, atmospheric deposition) pollution with regards to river eutrophication, and climate variability complicates the situation further (Jarvie et al., in press, Neal and Jarvie, 2005). There are also questions as to the rate and extent to which freshwater systems might recover following nutrient reductions; reducing nutrient fluxes might not necessarily result in an ecological change back to pre-impacted conditions (Jarvie et al., 2004). A proper analysis and understanding of biological processing and ecosystem functioning within surface water systems is vital if environmental management strategies are to be based on sound science (Neal and Heathwaite, 2005, Wade, 2006). In this paper we will review our current understanding of the main processes governing eutrophication in lakes and use this as a basis for developing a conceptual model of eutrophication in rivers.

Section snippets

Background

In most of the developed world, pollution of waters brought about by the microbial breakdown of easily degraded organic matter, resulting in low oxygen concentrations, has been controlled by the introduction of effective sewage treatment facilities. However, there is a common acceptance that major increases in the primary production of water bodies, i.e. the excessive growth of plants, mainly in the form of algae and large rooted plants (macrophytes) due to high inputs of nutrients (mainly

A short review of the process of eutrophication in lakes

Lakes tend to become naturally more eutrophic with (geologic) time; as they become shallower due to silt inputs, the incoming nutrients are distributed amongst a smaller volume of water increasing their concentration and, hence, the resulting biomass of algae (Moss, 1980). However, the increased interest in eutrophication studies over the past few decades has been generated by the accelerated rates of eutrophication bought about by human activities and the need to re-establish a more

Eutrophication in rivers

Given the large amount of work which has been carried out on eutrophication in lakes, a good starting point in developing a mechanism of eutrophication in rivers is to consider the effect of the major factors shown to be important in lakes.

The conceptual model for the development of eutrophic conditions in short-retention-time rivers

On the basis of a comparison with lake systems and the behaviour of some of the other factors considered above, it is possible to propose a conceptual model of the development of eutrophic conditions in rivers, particularly short-retention-time rivers, which can explain most of the observed features. The key features are:

  • 1)

    Long retention-time rivers respond differently to short retention-time rivers when exposed to excess nutrients. The former move to dominance by phytoplankton, whereas the

Consequences for remediation

If this mechanism for the development of eutrophication in rivers is correct, then a number of consequences ensue. Any remedial management of eutrophic and hyper-eutrophic rivers will aim to return the river to macrophyte dominance. On the basis of the hypothesis outlined in this paper it is likely that, in many lowland British rivers, light rather than P is the main factor limiting growth, since the phosphorus concentrations are relatively high. The appropriate management approach would still

Conclusions and recommendations

A hypothesis of the development of eutrophic conditions in rivers has been developed, based on mechanisms known to be important in lakes but modified by additional processes known to be important in rivers. The proposed model is consistent with a number of observations in eutrophic rivers. In addition, the model predicts a number of effects which should be tested by experiment and field observation to add further weight to the arguments in favour (or otherwise) of this proposed model. The

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