Invertebrates telling water’s dirtiest secrets – L’Esteron River, Saint-Auban, France.

Invertebrates telling water’s dirtiest secrets – L’Esteron River, Saint-Auban, France.

As promised, here is a brief overview of the work I did in Saint-Auban, France on L’Esteron River. I decided to study the possibility of pollution from a water treatment works (WTW) that has recently been built entering the river.



Pollution – At its worst is a global killer. As our population increases so does pollution, potentially resulting in major disease and catastrophic environmental issues. This is not just an issue in cities, but also in the countryside as people look to less built up areas to live. In some cases this leads to pollution disturbing the environment we would otherwise describe as clean and natural.

So what part do invertebrates play in pollution? Each family of invertebrates differs in their ability to tolerate different types and different levels of pollution. This coupled with their high prevalence and ease of capture makes the invertebrates an ideal indicator of pollution. To quantify the water quality in a river system the Biological Monitoring Working Party (BMWP) score is used. This takes pollution tolerant families such as the worms and gives them a score of 1, and pollution intolerant families such as the mayflies and gives them a score of 10. The assumption made is that pollution intolerant families are found in non-polluted water, and that pollution tolerant families are found in polluted water. Every family present at a sampled site is given a score from 1-10, and the sum of those scores is used to relate to the BMWP to give an indication of water quality.

Hypothesis 1: The water quality is poorer downstream of the Water Treatment Works relative to upstream of the Water Treatment Works

Hypothesis 2: The water will be of better quality the further downstream of the Water Treatment Works is sampled.


Methodology & Results

Three sites were sampled, one upstream of the WTW two downstream and six kick samples taken

Figure 1

at each site (See Figure 1 – adjacent). The kick sample collects any sediment and invertebrates within that sediment into a net. Once each sample was transferred from the net to a

box, it was taken back to the lab for identification. The kick samples showed

which invertebrate families were present at each site. Each family was then given a score, and the scores combined to give the BMWP score.

From Figure 2 (bottom of post) it is clear two dominant families were found at sites 1 and 3 – the Freshwater Shrimp (Gammaridae) and the Mayfly nymphs (Ephemeridae). Both these families are indicative of non-polluted water. At site 2 the most dominant family is the worms (Oligochaeta).The worms are particularly indicative to polluted water. This would comply with hypothesis 1 in that the invertebrates found at sites 1 and 3 differ dramatically to those at site 2. Furthermore the invertebrates found at site 2 indicate more polluted waters compared to the water at site 1.

Site 3 also seems to have a mixture of the diversity found at sites 1 and 2. The BMWP score for site 1 was moderate water quality, site 2 was poor water quality and site 3 was moderate water quality. The mixture of diversity at 3 insinuates the water can support both pollution tolerant and intolerant invertebrates, and therefore there is a relative water quality recovery from site 2 to site 3.

A general observation made from the samples was that the size of freshwater shrimps, as well as the total number of pregnant females was much greater at site 1 compared to site 3. It could be hypothesised that this is because there are more appropriate nutrients at site 1 compared to site 3. For example there could be more calcium at site 1, which is required for the crustaceans’ exoskeleton. At site 3 it could then be speculated that this free calcium has combined with any nitrate or phosphate pollution from the WTW, making calcium nitrate or calcium phosphate, which is inaccessible to the crustaceans in this state.

A chi square statistical test was used to test the data obtained for significance. For a degree of freedom of 22, the critical value in the chi square table at the 1% level is 48.27, and the chi square value from the data obtained here is 125. Therefore the data obtained is significant at the 1% level, or the probability of the data being obtained randomly is less than 0.01.



It is important to note that while these results do show that there is more pollution in the river at site 2 compared to site 1, there is no evidence as to where this pollution has entered the river. It is true that the only human activity between site 1 and 2 is the WTW but there could be other unknown contributions to the poor water quality such as surface run-off, or dumping of rubbish and other human waste. Therefore the results from this report can justify claiming there is pollution entering the L’Esteron River between sites 1 and 2. The source of that pollution can be speculated to be the WTW, but ultimately there is no evidence to directly prove or disprove that claim.

It is of vital importance that the pollution entering the river is monitored at least annually to ensure the pollution does not get any worse. If it does worsen the findings from this report and the findings from future reports should be forwarded to the WTW and the local governing body to try and solve the problem.



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Dos Santos, D. A., C. Molineri, et al. (2011). “Which index is the best to assess stream health?” Ecological Indicators 11(2): 582-589.

Greenhalgh, M. and D. Ovenden (2007). Freshwater life. London, HarperCollins Publishers Ltd.

Martin, R. (2004). “Information on Distribution Maps and Histograms.”   Retrieved 05/07/2011, from

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Smith, J. G., C. C. Brandt, et al. (2011). “Long-Term Benthic Macroinvertebrate Community Monitoring to Assess Pollution Abatement Effectiveness.” Environmental Management 47(6): 1077-1095.

Figure 2
Figure 2