A group of students investigated the effect of organic pollution on a river. They measured the percentage of dissolved oxygen at five different sampling sites (P to T) downstream of a farm wastewater outlet.
| Sampling site | Dissolved oxygen level (%) |
|---|---|
| Site P (upstream of outlet) | 92 |
| Site Q (at outlet) | 14 |
| Site R (10m downstream) | 36 |
| Site S (50m downstream) | 65 |
| Site T (100m downstream) | 88 |
Which indicator species is most likely to be found in the highest abundance at Site Q?
Tubifex worm
Mayfly nymph
Freshwater shrimp
Stonefly nymph
