An environmental agency is monitoring a river system downstream from an agricultural runoff point source to assess the impact of organic pollution on the aquatic ecosystem.
Which of the following ecological observations at a sampling site downstream would provide the most reliable indication of severe organic pollution (and subsequent hypoxia), along with its correct biological rationale?
A high abundance of stonefly larvae (Plecoptera) and mayfly nymphs (Ephemeroptera), because their highly branched tracheal gills allow them to rapidly extract oxygen from oxygen-depleted waters.
A high abundance of tubifex worms (Tubifex tubifex) and midge larvae (Chironomus species), because they possess haemoglobin-like respiratory pigments that allow them to extract oxygen under hypoxic conditions.
A significant increase in overall macroinvertebrate species richness, because the influx of organic nutrients provides a wider variety of ecological niches for diverse colonizing species.
A decline in the population of bacterial decomposers, because the high biochemical oxygen demand (BOD) directly inhibits the metabolic activity of saprobionts.
Practise OCR GCSE Biology Monitoring and maintaining the environment with exam-style questions for Foundation and Higher tier. 27 questions, matched to the OCR GCSE Biology (J247) specification and written in the written papers style. Every question includes a full worked solution and mark scheme, so you can see where marks are awarded rather than just whether you got the answer right.