Using macro-invertebrate communities to assess environmental condition in UK freshwaters

Species like C. splendens have been used to formulate a range of new biotic indices responding to acidification, flow velocity, siltation, morphology and salinity or reflecting community conservation importance

Species like C. splendens have been used to formulate a range of new biotic indices responding to acidification, flow velocity, siltation, morphology and salinity or reflecting community conservation importance

Macro-invertebrates – animals without backbones that can be seen with the naked eye – have been widely used for many years to assess environmental condition. In many ways macro-invertebrates are ideal bio indicators, as they are relatively straightforward to identify; they are reasonably long lived, enabling conditions to be retrospectively assessed; they are relatively sedentary, allowing the determination of spatial impacts/events; and, perhaps most important of all, they exhibit a range of responses to different pressures. Mayflies, for example, are generally very sensitive to poor water quality whereas most leeches are not. Macro-invertebrates, in effect, integrate effects on their environments throughout their lifetimes.

Although individual species have specific tolerance ranges and habitat requirements, a much more robust way of summarising environmental health is to look at the combined response of the whole community. This can be done in a number of ways, but one of the most useful and informative approaches involves the calculation of biotic indices.

Biotic indices

Biotic indices (or metrics, as they are increasingly being called) all work in the same fundamental way – individual species, genera or families (taxa) in a sample are scored according to their sensitivity or tolerance to the pressure the index is designed to detect. These scores are then combined to produce a single number or letter which can be used to infer and interpret the current state of the environment. Some indices are simple, just using the presence or absence of taxa in their formulation. Others are more sophisticated and additionally consider the abundance of different members of the community. This latter approach is generally better, as it enables the detection of more subtle environmental pressure gradients.

Biotic indices have been developed for a number of different target groups, including higher plants, algae, protozoa, and fish, but methods based on macro-invertebrates are by far the most common and widely used.

A much fuller understanding of ecological response and the controls and pressures acting on aquatic organisms becomes possible

Early invertebrate indices focused exclusively on water quality, as this was the prevailing pressure affecting rivers through much of the 20th century. As early as 1902, Kolkwitz and Marrson developed the Saprobian system designed to detect organic pollution in Europe and this was followed by an increasing number of methods designed for local application. Water quality indices developed for use in the UK for example included the Trent Biotic Index, the Chandler Score and the Biological Monitoring Working Party (BMWP) score. Some of these methods are still in use today.

Additional pressures

While some river pollution problems persist, there has been a general improvement in water quality over the last three decades and this has led to the gradual recognition of other pressures affecting aquatic ecosystems. Aquatic macro-invertebrates are also very capable of detecting these additional pressures.

The presence of the damselfly larva Calopteryx splendens for example, indicates reasonably clean water, but what else can the occurrence of this species tell us? The answer is quite a lot. The species requires slow flowing water and a muddy bottom, so its presence will indicate slack flows and a silted river bed. It is also intolerant of acidification and raised salinity. The presence of emergent/floating vegetation and open meadows nearby will also be indicated, as these are needed for adults to display and feed. Species like C. splendens have now been used (along with other colonising invertebrates) to formulate a range of new biotic indices responding to acidification, flow velocity, siltation, morphology and salinity or reflecting community conservation importance.

A multiplicity of information consequently becomes available from a single invertebrate sample taken from a river and if this is considered alongside supporting data such as flow rates and habitat structure, then a much fuller understanding of ecological response and the controls and pressures acting on aquatic organisms becomes possible.

Furthermore, the calculation of new biotic indices has enabled an appreciation that responses to pressures are not simple, but are mediated by habitat structure. It can, for example, be shown that invertebrate communities living in physically modified rivers, are far less resilient and self-sustaining when subject to droughts and floods than populations colonising more natural rivers and streams.

A good summary of biological indicator methods can be found in: Conservation Monitoring in Freshwater Habitats (Editors Hurford, Schneider and Cowx) Springer 2010. ISBN 978-1-4020-9277-0, although some of the newer indices are not included in this overview.

Written by

  • , Ecology Analysis and Reporting Manager, The Environment Agency of England and Wales
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