Zebra Mussels, Clear Water, and Toxic Blooms
A lake used for drinking water and recreation has been monitored for eight years. In year 3, zebra mussels were first detected. Before the invasion, mean summer chlorophyll a was 22 micrograms per liter, Secchi depth was 1.2 m, and large Daphnia were abundant. After the invasion, chlorophyll a fell to a mean of 7 micrograms per liter, Secchi depth rose to 3.4 m, and large Daphnia declined by 70%. At the same time, dissolved phosphorus in the lake rose from 12 to 19 micrograms per liter, and the biovolume of the cyanobacterium Microcystis rose from 2 to 14 cubic millimeters per liter. Microcystis forms colonies that are difficult for Daphnia to eat and can produce toxins. The lake's bottom waters have shown lower dissolved oxygen in late summer, and the town has issued more beach closures. Some residents argue that because the water is clearer, the mussels have improved the lake. Others argue the lake is worse because of the blooms.
1. Explain how the zebra mussel invasion could produce clearer water at the same time that harmful cyanobacterial blooms increase. Include at least two mechanisms.
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The invasion can make the water clearer while making bloom risk worse because clarity and bloom risk measure different things. First, zebra mussels are filter feeders. They remove suspended algae and particles from the water, so chlorophyll a falls and Secchi depth increases. Second, the mussels excrete phosphorus and produce nutrient-rich pseudofeces. The measured rise in dissolved phosphorus from 12 to 19 micrograms per liter gives the cyanobacteria more nutrients.
A third mechanism is selective grazing. Mussels and Daphnia remove edible algae more effectively than they remove large, colonial Microcystis. As edible algae decline, Daphnia decline by 70%, and the grazing pressure on Microcystis is reduced. Clearer water also lets more light penetrate, which can favor cyanobacteria that regulate their buoyancy. Thus the lake can look cleaner while toxin-producing blooms become more severe.
2. Predict the effect of continued mussel invasion on late-summer dissolved oxygen and on coldwater fish. Explain the chain of reasoning.
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Late-summer dissolved oxygen in the bottom waters is likely to decrease further, and coldwater fish habitat is likely to shrink. The reasoning chain is: mussels release phosphorus and alter grazing, which favors more cyanobacterial biomass; the bloom eventually dies, and bacteria decompose the dead organic matter. Decomposition consumes oxygen. The lake is stratified in late summer, so bottom water is cut off from the atmosphere and cannot easily be reoxygenated. Mussels and other organisms also consume oxygen through respiration.
As bottom-water oxygen falls, coldwater fish such as trout or whitefish are squeezed into a thinner layer of cool, oxygenated water or must leave the area. If oxygen drops low enough, fish kills can occur. The town's observation of lower late-summer oxygen is therefore consistent with the bloom and mussel changes, even though the water is clearer.
3. The town proposes two options: (A) reduce external phosphorus inputs from farms and septic systems, or (B) stock a fish that eats zebra mussels. Evaluate both options using the data.
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Option A targets the driver of the blooms. The data show that dissolved phosphorus rose from 12 to 19 micrograms per liter and that Microcystis increased at the same time. Reducing external phosphorus lowers the lake's total phosphorus supply, which should reduce bloom intensity even if mussels continue to recycle some phosphorus internally. It is slow and requires cooperation across the watershed, but it addresses the cause rather than one symptom.
Option B is less predictable. A mussel-eating fish might reduce mussel density, but it could also disrupt the food web, harm native species, or simply shift the system in unexpected ways. Fewer mussels might reduce phosphorus excretion, but they would also filter less water, so clarity could decline. Stocking a non-native fish is itself a risk. On the evidence given, nutrient reduction is the stronger primary strategy, with continued monitoring and perhaps physical removal of mussels only in limited areas.