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. 14
