Choosing One Restoration Measure for a Hypereutrophic Urban Lake
A 12-hectare urban lake receives treated sewage and storm runoff. Summer monitoring gives Secchi depth 0.4 m, total phosphorus 190 µg L−1, chlorophyll a 55 µg L−1, and hypolimnetic dissolved oxygen below 2 mg L−1. A fish survey shows that 70% of fish biomass is small planktivorous species, with few piscivores. Submerged macrophytes are absent and phytoplankton dominate. The municipal authority has funding for one intervention: (A) biomanipulation - remove planktivorous fish and stock piscivores; (B) alum addition to bind phosphorus in the sediment; (C) dredging the nutrient-rich sediment. Residents also use the lake for idol immersion, and the catchment still discharges untreated domestic waste during heavy rain. The engineer must recommend one option and state the conditions under which it would fail.
1. Classify the lake's trophic state using the supplied values and explain how phosphorus leads to low hypolimnetic oxygen.
Read the worked answer
Total phosphorus of 190 µg L−1 and chlorophyll a of 55 µg L−1 are well above the usual boundaries for eutrophy and place the lake in the hypereutrophic category; a Secchi depth of 0.4 m confirms severe turbidity. Phosphorus is the limiting nutrient in most freshwater systems, so the excess supports heavy phytoplankton growth.
The chain to low hypolimnetic oxygen runs as follows. High phosphorus → high primary production → high chlorophyll and turbidity → light attenuation and loss of submerged macrophytes. Dead algae and other organic matter settle into the hypolimnion. Bacterial decomposition consumes oxygen, and because the lake is stratified in summer the hypolimnion is cut off from the atmosphere and from oxygenated surface water. Dissolved oxygen therefore falls below 2 mg L−1. Low oxygen at the sediment surface can release iron-bound phosphorus back into the water, an internal loading feedback that sustains the problem even if external inputs are reduced. This is why the lake can remain turbid after obvious pollution sources are controlled.
2. Evaluate biomanipulation alone. Under what conditions would removing planktivorous fish reduce chlorophyll, and why might it fail here?
Read the worked answer
Biomanipulation rests on a trophic cascade. Removing small planktivorous fish allows large-bodied zooplankton such as Daphnia to become abundant. Those grazers filter phytoplankton, increasing water clarity, and clearer water allows submerged macrophytes to establish. Macrophytes then stabilise the clear-water state by competing for nutrients, reducing sediment resuspension and offering refuges to zooplankton. The manipulation must be substantial and sustained, because fish populations recover quickly.
Success is most likely in shallow, well-mixed lakes with low external phosphorus loading, an existing macrophyte seed bank and a fish community dominated by planktivores that can actually be removed. This lake fails several of those conditions. Untreated waste still enters during storms, and internal loading from the sediment is high, so phosphorus will continue to fuel phytoplankton even if grazing increases. Benthic fish such as carp can resuspend sediment and uproot plants. A one-off removal would probably produce a short-lived improvement followed by reversion. Biomanipulation is therefore an adjunct to nutrient control, not a substitute for it.
3. Which single intervention would you recommend and why? State two monitoring indicators that would tell you within two years whether it is working.
Read the worked answer
The first principle is that in-lake measures fail if external loading continues, so any recommendation must be conditional on catchment control of the storm discharges and idol-immersion waste. Among the three funded options, alum addition (B) is the most defensible single intervention. It binds phosphorus in the sediment and reduces internal loading, which is the feedback maintaining high chlorophyll. It is less costly and far less destructive than dredging, and it acts directly on the nutrient that limits phytoplankton.
Dredging (C) removes the sediment phosphorus store and would give longer-lasting relief, but it is expensive, disturbs benthic communities, requires safe disposal of contaminated sediment and still fails if external loading continues. Biomanipulation (A) alone is the weakest choice here because the phosphorus supply remains high. If the authority insisted on A, it should be paired with phosphorus control and repeated fish removal.
Two monitoring indicators: (1) summer total phosphorus and chlorophyll a, with Secchi depth as a cheap field check - success would be a sustained fall in phosphorus and chlorophyll and an increase in clarity; (2) hypolimnetic dissolved oxygen and zooplankton size structure, since rising oxygen and a shift towards large cladocerans would show that the internal loading and grazing pathways are responding. Fish biomass should also be tracked because planktivore recovery would undermine the result.