Water pollution practice questions.

A shallow lake receives heavy fertiliser runoff. Which sequence puts the resulting changes in the correct order?

• oxygen falls, algal bloom, decomposers respire, rooted plants die, light blocked
• light blocked, oxygen falls, algal bloom, rooted plants die, decomposers respire
• algal bloom, light blocked, rooted plants die, decomposers respire, oxygen falls
• decomposers respire, oxygen falls, algal bloom, light is blocked, rooted plants die

Answer: algal bloom, light blocked, rooted plants die, decomposers respire, oxygen falls

Nutrient enrichment first raises producer growth at the surface; the oxygen crash comes last, when the dead algae and shaded macrophytes are broken down by aerobic decomposers. Putting the oxygen change early reverses cause and effect.

A kick-sample from a stream contains many Tubifex worms and no mayfly nymphs. This most strongly suggests

• clean water with a very high biochemical oxygen demand
• recent thermal pollution but no organic loading at all
• a high Simpson diversity index for the stream community
• heavy organic pollution and low dissolved oxygen

Answer: heavy organic pollution and low dissolved oxygen

Tubifex tolerates very low oxygen while mayfly nymphs are among the first species lost as oxygen falls, so the community composition itself indicates water quality. A biotic index uses exactly this difference in tolerance rather than a chemical measurement.

Nitrate reaching a river from fertiliser spread across an entire catchment is classified as

• non-point source pollution
• point source pollution
• secondary pollution formed within the water column
• acute point source pollution from a single outfall

Answer: non-point source pollution

Non-point sources are diffuse and enter over a wide area, which makes them far harder to monitor and regulate than a pipe with a measurable discharge. The pollutant is also primary: it is released in the form in which it causes harm.

At Minamata in Japan, methylmercury discharged into the bay reached harmful concentrations in people who ate large predatory fish. The process that made mercury concentrations in those fish far higher than in the smaller organisms they fed on is

• bioaccumulation within one fish over its lifetime
• biomagnification along the marine food chain
• eutrophication of the bay following the discharge
• dilution of the pollutant by tidal exchange with the open sea

Answer: biomagnification along the marine food chain

Biomagnification is the rise in concentration from one trophic level to the next, because a predator eats many contaminated prey but excretes little of the persistent pollutant. Bioaccumulation describes the build-up inside a single organism and does not on its own explain why top predators are worst affected.

After one afternoon of kick sampling at a single riffle just below a dairy outfall, a student finds only Chironomus larvae and Asellus, and concludes that the whole river is organically polluted. Identify the most serious flaw in her conclusion.

• One sample from one site on one day cannot describe the whole river.
• Chironomus and Asellus are pollution-sensitive taxa, so their presence points to clean water rather than to an organic load.
• Kick sampling collects fish rather than invertebrates, so no biotic index can be calculated from the catch.
• Organic loading can be judged only from a nitrate concentration, never from the invertebrates that are present.

Answer: One sample from one site on one day cannot describe the whole river.

Her identification is right — both taxa are tolerant of low oxygen — but a biotic index describes the reach it was taken from, so a single station on a single day says nothing about upstream or downstream water quality.

A technician records the dissolved oxygen of a river sample as 9.4 mg dm-3, leaves the stoppered bottle on a sunlit bench at 20 C for five days, records 9.9 mg dm-3, and reports a biochemical oxygen demand of -0.5 mg dm-3. Identify the error in the procedure.

• The bottle should have been incubated for ten days, so five days was far too short a period for any demand to register.
• The bottle must be incubated in darkness, because algae in the light photosynthesise and add oxygen.
• The incubation temperature should have been 5 C rather than 20 C for a standard determination.
• The dissolved oxygen should be measured only once, at the end of the incubation period.

Answer: The bottle must be incubated in darkness, because algae in the light photosynthesise and add oxygen.

The standard BOD determination is five days at 20 C in the dark. Light lets algae in the sample photosynthesise, so oxygen is produced as well as consumed and the fall that BOD measures is masked — here it is reversed altogether.