Two Lakes, Same Phosphorus: Choosing a Management Action
North Lake covers 40 hectares and receives runoff from surrounding farm fields. Total phosphorus measures 85 µg/L; chlorophyll a is 45 µg/L; Secchi depth is 0.8 m. Net samples contain fewer than 2 large Daphnia pulicaria per liter but about 60 small Ceriodaphnia per liter. Bluegill are extremely abundant. Largemouth bass have been absent since a 2019 winterkill that followed two summers of heavy algal decay. Ten kilometers away, South Lake has total phosphorus of 80 µg/L but chlorophyll a of 8 µg/L, Secchi depth of 2.6 m, and about 40 Daphnia pulicaria per liter, with a healthy bass population and few bluegill. The lake association has $200,000 and three proposals on the table: (1) an alum treatment to bind phosphorus in the sediment, $180,000; (2) stocking 1,200 largemouth bass, $9,000; (3) mechanical harvesting of shoreline macrophytes, $60,000.
1. Explain, as a step-by-step chain of cause and effect, how two lakes with nearly identical phosphorus can differ more than fivefold in chlorophyll a. Name the ecological process involved and identify which lake has a food web altered at the top.
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Name of the process: a trophic cascade. Predators at the top change the abundance of the middle level, which changes the abundance of the base - an effect that alternates in sign as you move down the chain.
South Lake, step by step: abundant bass → few bluegill, and the bluegill that remain are pushed into sheltered cover → large-bodied Daphnia pulicaria survive and reach about 40 per liter → heavy grazing on algae → chlorophyll only 8 µg/L → light penetrates, Secchi 2.6 m.
North Lake, step by step: no bass since the winterkill → bluegill are abundant and can forage freely in open water → large Daphnia are eaten preferentially, because big zooplankton are the most profitable prey → only small Ceriodaphnia remain → small-bodied zooplankton cannot graze large algal cells efficiently → chlorophyll climbs to 45 µg/L and Secchi falls to 0.8 m.
The key insight: phosphorus sets the potential for algal biomass, but grazing sets how much of that potential is realized. North Lake is not more polluted in any simple sense; it is 5 µg/L different in phosphorus and entirely different in who is doing the eating. North Lake's food web has lost its top level.
2. Evaluate proposal 2 (stocking 1,200 bass). Describe two ways it could fail, and one way it could produce a new problem even if the bass survive and eat bluegill.
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Way it could fail - the cause of the original winterkill is still present. The bass disappeared after winters of heavy algal decay, which consumes oxygen and can suffocate fish under ice. If North Lake still produces 45 µg/L of chlorophyll each summer, the low-oxygen conditions that killed bass in 2019 will recur, and the stocked fish will die the same way. The proposal treats a symptom while leaving the mechanism that produced it.
Way it could fail - a predator may not control this prey. Bluegill are abundant, spawn repeatedly, and can hide in dense cover; adding predators to a lake with limited bass spawning habitat and heavy prey may not shift the ratio much. Without monitoring, the association would not know for years.
How it could backfire even if it works: the cascade could overshoot. If bluegill are suppressed and large Daphnia flourish, the water could clear enough for sunlight to reach the bottom across a wider area, fueling dense rooted plants or filamentous algae - trading a green water problem for a weedy one. Clearer water is not automatically a healthy lake.
Cost note: $9,000 is cheap, which makes it tempting. Cheap and ineffective is still a lost year, and repeated stocking is not free over time.
3. Which proposal best addresses the underlying cause rather than a symptom, and what single piece of information gathered before any money is spent would most change your ranking? Justify both answers.
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Cause-directed option: proposal 1, the alum treatment, because it targets phosphorus - the variable that sets the ceiling on algal biomass and that neither bass nor harvesting touches. Proposal 2 changes who eats the algae; proposal 3 removes nutrients locked in plant tissue but only temporarily, since harvested plants regrow and the nutrients in the sediment remain.
The single most important piece of information: how much phosphorus is still entering the lake from its watershed each year. Alum binds phosphorus in the sediment, but it has a finite capacity. If farm runoff continues to deliver phosphorus at a high rate, the sediment will simply reload and the treatment's effect will fade within a few years, after $180,000 has been spent. If external loading has already been reduced and the lake's problem is internal release from sediment accumulated over decades, alum is well matched to the cause and could work for a decade or more.
Supporting measurements worth taking at the same time: dissolved oxygen and phosphorus concentrations just above the bottom in late summer, which reveal internal loading directly. If oxygen is low and phosphorus rises near the bottom, internal release is confirmed and proposal 1 moves to the front. If not, the money should go to watershed work first.