Restoring a shallow eutrophic lake
A 40-hectare shallow lake has a mean depth of 2.5 m. Summer total phosphorus (TP) is 65 µg/L, chlorophyll a is 45 µg/L, and cyanobacteria make up 70% of phytoplankton biovolume. Summer hypolimnetic oxygen falls to 2 mg/L. External phosphorus loading from a wastewater plant is 120 kg/year; sediment-release estimates add 80 kg/year. Nutrient-addition bioassays show that adding phosphorus alone raises chlorophyll a 3.0-fold, adding nitrogen alone raises it 1.1-fold, and adding both raises it 3.2-fold. Managers propose three options: (A) upgrade the plant to cut external phosphorus by 90%; (B) stock pike to reduce zooplanktivorous fish; (C) add alum to bind sediment phosphorus. Monitoring includes two years before and two years after any intervention, with a nearby reference lake.
1. Evaluate the claim that phosphorus is the sole limiting nutrient in this lake. Use the bioassay and the standing nutrient concentrations to explain what the data do and do not show.
Read the worked answer
The bioassay shows that phosphorus addition alone produces a 3.0-fold chlorophyll increase, nitrogen alone only 1.1-fold, and both 3.2-fold. That is strong evidence that phosphorus is the primary limiting nutrient and that nitrogen is not limiting on its own. But 'sole' is too strong: the N+P response is slightly above P alone, which can indicate secondary co-limitation or relief of a minor nitrogen constraint; the difference is small and needs replication. Standing TP of 65 µg/L is high, so the lake is not phosphorus-poor; the limitation is relative to other resources. At high biomass, light limitation and grazing can also control chlorophyll, and cyanobacteria can fix nitrogen, weakening nitrogen limitation. The bioassay uses small bottles, which exclude fish and large grazers and can overestimate phytoplankton response. A better conclusion is that phosphorus is the main manageable limiting nutrient, not that nitrogen and light are irrelevant.
2. Calculate the effect of a 90% cut in external phosphorus loading on total phosphorus load, assuming internal loading stays at 80 kg/year. Predict whether summer TP will fall to the target of 30 µg/L within two years, and explain.
Read the worked answer
Current total load is 120 + 80 = 200 kg/year. A 90% cut reduces external load to 12 kg/year, so total load becomes 92 kg/year, a 54% reduction, not 90%. The sediment still releases 80 kg/year, which is now 87% of the load. Because internal loading is large, summer TP is unlikely to fall quickly to 30 µg/L; the lake will keep recycling phosphorus from sediments, especially under low oxygen at the sediment surface. The response may also lag because phosphorus stored in biomass and sediments turns over over years. The target might be reached eventually if the sediment gradually depletes, but two years is too short to assume it. Alum (option C) would directly reduce internal loading and could be paired with the external cut. Monitoring should measure TP, chlorophyll, oxygen, and sediment release, not just the plant's effluent.
3. Assess the pike-stocking proposal. What ecological conditions would make it succeed or fail, and how would you design the monitoring to test it?
Read the worked answer
Pike stocking aims at a trophic cascade: fewer zooplanktivorous fish, more large zooplankton, lower phytoplankton, and higher transparency. It can work in some lakes, but shallow, eutrophic lakes with cyanobacterial blooms often fail because cyanobacteria are poorly grazed by zooplankton, and because high internal phosphorus loading keeps supporting blooms. Success also requires pike recruitment, suitable spawning habitat, and refuge for zooplankton; if the lake has no submerged macrophytes, zooplanktivores may still find prey and pike may not control them. The two-year before-after design with a reference lake is a good start, but it needs multiple control and impact sites, pre-intervention years, and measurement of fish, zooplankton, chlorophyll, TP, and transparency. A before-after-control-impact analysis can separate lakewide weather effects from the treatment. If external and internal phosphorus are not controlled, any cascade effect is likely temporary.