Discussion

Discussion

It is still early to expect any trends to show up as there are have only been two years consistent sampling undertaken so far and strictly speaking only the same seasons data is comparable. The results suggest an increase in WHPT scores from the first sample occasion (May 2019 or August 2018 for Above Drayton Farm), but with a decline from the intervening samples.

This illustrates the natural variation between seasons and years, rather than any significant decline in water quality. The variation in scores is accounted for by relatively insignificant changes in animals captured. For example 76% of the difference in the scores for Manor Farm, the site which shows the greatest fall between 2019 and 2020, is accounted for by just 4 individual animals (2 dance flies and one each of two caddis species). WHPT scores

220 210 200 190 180 170 160 150 140 130

120

Manor Farm Above Drayton Below Drayton Bishops Sutton

A change in the relative cover of one of the habitats present can account for the shift to lower scoring families which reduces the WHPT and ASPT scores. For example the increase in the marginal vegetation at Manor Farm allows many less oxygen sensitive species to be included as well as reducing the percentage of the most sensitive species from the gravel.

Gammarus counts have all increased from September 2019 to September 2020, which may be due to the warmer weather of 2020. Indeed the warmer weather will cause an increase in water temperatures which lead to a decrease in dissolved oxygen. Neither of these parameters are monitored in these surveys, so this can only be inferred. However, the effects would be to lower the occurrence of the more sensitive, higher scoring families which is seen in these results.

Overall it is concluded that two factors are the major influence on these results: Normal variation in sampling efficiency which has been suggested as representing 9 – 12 % between experienced practitioners (Clarke et al., 2002).  Natural year on year variation in abundance and species presence. Weather patterns and weed growth (and management) are likely to be major influences. It is expected that there would be at the very least 10% variation between years.  Together an annual variation of 20-25% seems a reasonable expectation and these results fall well within this.

One curious record is that for a beetle Dactylosternum abdominale which is apparently colonising the UK from Madeira. This terrestrial beetle is associated with organic rubbish and has been found in silage clamps, compost and animal manure so its presence in a chalk stream is difficult to account for, unless it is associated with the works on the far side of the river where some dredged material has been dumped onto the bank by the landowner (not TWC).

Photo 5. Dactylosternum abdominale