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.
25 exam-style questions on OCR GCSE Biology 6.1 Monitoring and maintaining the environment, covering 6.1.1 Field investigation of distribution and abundance, 6.1.2 Human interactions and biodiversity, 6.1.3 Maintaining biodiversity, and 6.1.4 Environmental change and distribution of organisms. Each one has a worked solution and a mark scheme showing where the marks go.