Ecological assessment in the fast lane …

Fifteen months ago, I reported on a new approach to rapid assessment of streams and rivers that I was developing with some colleagues (see “A RAPPER in da Lake District …”). Since then, we’ve continued developing and testing the method and, I am pleased to say, the results are very encouraging.

The idea behind RAPPER (“Rapid Assessment of PeriPhyton Ecology in Rivers”) is that a biologist can make a quick examination of the composition of larger algae which can then be used to assess the ecological status of the stream.   This involves recording the presence of different types of algae and, if necessary, checking the identity under a microscope. At the moment, this check needs to be done back in the laboratory, but it should also be possible to do this with a field microscope, which means that results will be available almost immediately.   The method allows sites to be classified as “high or good status” (which are both acceptable conditions according to the Water Framework Directive) or “moderate, poor or bad status” (which means that their condition needs to be improved). The former category is defined by the presence of algae which we know to be sensitive to pollution; the latter by high abundances of algae that we know are tolerant to pollution.

We managed to achieve agreement between RAPPER results and outcomes from more labour-intensive assessments in 72% of cases, which compares favourably with other situations where I have compared different types of algal-based assessments.   Of the remaining sites, 17% either had no appropriate indicator species or low abundances of tolerant species, whilst the remaining 11% had both sensitive and tolerant species present.   After some more analyses, we decided that these sites were probably in an intermediate state of risk.

The sensitivity of the RAPPER classification is highlighted by the charts below. The first shows the relationship between the three classes identified by RAPPER and the accompanying Trophic Diatom Index (TDI) value.   The first three classes show a nice clear separation, with low TDI values associated with the high and good status sites and higher TDI values associated with the more impacted sites. The final class (4_no_data), those for which we had no reliable indication of status, spanned a wide range of TDI values.   There are a number of possible reasons why these sites could not be classified, including a lack of suitable substrata for the algae to grow or heavy shade.  Maybe some extra refinements to RAPPER will mean that we can obtain more reliable classifications but it is important that we do no lose sight of the principle of “rapid assessment” in the process.   Better, perhaps, to reliably classify most sites, in order to free resources to look at those sites that are problematic in more detail?


Box-and-whisker plot showing difference in average TDI values for sites classified by RAPPER as high or good status (“1_HG”), maybe at risk of eutrophication (“2_both”) or moderate, poor or bad status (“3_MPB”) along with sites that could not be classified (“4_no_data”)

The second graph shows the relationship between the RAPPER classifications and soluble phosphorus and a similar pattern emerges, with sites classified as high or good status associated with low phosphorus concentrations, and those classified as impacted (3_MPB) associated with higher values.   Results for total phosphorus, total oxidised nitrogen and ammonia-nitrogen show similar trends, all of which suggest that the method is classifying sites correctly.


Box-and-whisker plot showing difference in median for soluble P for sites classified by RAPPER as high or good status (“1_HG”), maybe at risk of eutrophication (“2_both”) or moderate, poor or bad status (“3_MPB”) along with sites that could not be classified (“4_no_data”).

I’m particularly enthusiastic about RAPPER because it the outcome of convergent thinking between myself and colleagues in two separate agencies dealing with variants of the same need to cover a lot of ground with limited resources. The cost of full-scale ecological assessments is such that the sampling network is relatively thin.   This means that a diagnosis of a problem needs to be followed up by an investigation of the source of that problem in order to develop an appropriate programme of measures to address this. The idea of RAPPER and other rapid assessment approaches is not to replace the official methods, but to enable the operational biologists to collect information from a number of sites in a catchment that would then allow them to use resources more intensively.   RAPPER is, in other words, is part of an ecological “triage”.   And, with the public spending squeeze set to continue, being able to prioritise resources effectively continues to be one of our biggest challenges.