Dewy Pond Survey 2022
1st June saw the annual Dewy Pond survey carried out by volunteers and led by Kate Lake, herself an experienced volunteer in the Wood. Many thanks to Kate for leading the survey, collating the results and analysing them.
The results are similar to the last survey which was carried out on 18th August last year, though there was lower diversity in that slightly fewer species were recorded. Also there were no swimming mayfly nymphs, a common species that is semi-sensitive to oxygen levels and these have been observed during many years of surveys of invertebrates in the pond.
Dragonfly nymphs were not found whilst doing the survey, however after it was concluded, a volunteer located the best spot and found two large nymphs (see photo above). However, this did not make up for the fact that their numbers were very low. Also the only water boatmen found were when pondweed was being cleared and not during the survey.
The biotic index is an indication of low oxygen, usually due to eutrophication:
- Sensitive to pollutants/low oxygen - e.g. stonefly larvae (only found in fast flowing rivers)
- Semi-sensitive e.g. mayfly larvae, dragonfly larvae
- Semi-tolerant e.g. freshwater shrimps
- Tolerant e.g. water louse / red midge larvae
So not surprisingly, it looks like the pond is in need of phased dredging to raise oxygen levels to have a chance of improving diversity. This is a big job and it would ideally be phased to minimise disturbance to its ecosystem. Also, the recent drain problems in Crescent Wood Road and the consequent overflows into the Wood may have raised nitrate levels, although this is speculation.
So the results are displayed in the table below. It wasn't possible to show them in graph form given the huge range of totals for each species, water fleas being so numerous as to be impossible to count.
Organism name | Site 1 | Site 2 | Site 3 | Site 4 | Total | ||||||||
1 | 2 | 3 | 1 | 2 | 3 | 1 | 2 | 3 | 1 | 2 | 3 | ||
Water flea Daphnia | >50 | >50 | >50 | >20 | >50 | >50 | >10 | >10 | >50 | >20 | >360 | ||
Water flea Cyclops | >10 | >10 | >10 | >10 | >5 | >45 | |||||||
Worm | |||||||||||||
Phantom midge larva | 4 | 3 | 1 | 1 | 2 | 2 | 1 | 2 | 1 | 17 | |||
Non-biting red midge larva | 2 | 2 | 1 | 3 | 8 | ||||||||
Mosquito larva/ pupa | 2 | 5 | 4 | 2 | 1 | 5 | 1 | 1 | 21 | ||||
Rat-tailed maggot | |||||||||||||
Freshwater shrimp | 2 | 1 | 1 | 4 | |||||||||
Freshwater louse | 1 | 2 | 2 | 5 | |||||||||
Water mite | |||||||||||||
Swimming mayfly nymph | |||||||||||||
Flattened mayfly nymph | |||||||||||||
Diving beetle | |||||||||||||
Beetle larva (water tiger) | 1 | 1 | |||||||||||
Beetle larva other | |||||||||||||
Damselfly nymph | |||||||||||||
Dragonfly nymph | |||||||||||||
Greater water boatman |
It is interesting to compare these results to those of last year. Refer to Post of 21st August 2021.
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