In July 1996, a survey of the macroinvertebrate fauna of the River Thames was undertaken in the vicinity of Didcot A Power Station, which is located on the right (south) bank of the river. The aim was to determine whether short-range biological effects could be detected due to abstraction and discharge of cooling water. Sampling zones were selected upstream of the abstraction point (zone A), between the abstraction and discharge point (zone B), and below the discharge point (zone C). Five pond-net and five dredge sampling units were taken from each bank in each of zones A, B and C and analyses were undertaken at BMWP family level. A total of 46 BMWP families were captured during the survey.
Marginal pond-net samples from the left bank had a significantly higher mean number of BMWP taxa than those from the right bank. These differences are thought to be due to the greater diversity of macrophytes and substrata on the left bank. In addition, there were lower numbers of BMWP taxa on the right bank of zone C compared with zone B (but not zone A), probably due to poor quality habitat.
When the fauna from dredge samples in 'control' zone A was compared with zone B, there was no evidence on either bank of a deleterious impact on the fauna in zone B. Comparison of dredge samples from zone C with those from zones A and B, revealed no statistically significant differences in the number of BMWP taxa between zones C and A or between C and B on either bank. However, the first two dredge sampling units from the right bank of zone C, immediately downstream of the discharge point, had just five and 12 BMWP taxa, respectively, compared with a range of 17-28 taxa in all other dredge samples (on either bank of the three zones). These results correlated with a local change in dominant substratum to sand and associated fine particles, and suggest a very localized effect of the discharge on river-bed morphology and hence on the fauna of the right bank in zone C.