Mussels in the Carrowmere Intake: A Biofouling Case Study

The diver came up from the fictional Carrowmere Lake with a fistful of shells. Each one was striped, thin, and no bigger than a thumbnail. She had scraped them off the inside of the intake pipe that feeds the town of Haleworth. The pipe is 48 inches across, and in four years the mussels had built a crust thick enough to leave an opening of only 42 inches.

The animal is a quagga mussel, a freshwater relative of the mussels people eat from the ocean, and it did not reach Carrowmere Lake on its own. Quagga mussels arrived in North American waters in the ballast tanks of ocean ships, and once here they moved inland on boat hulls, in live wells, and in the water left standing in engines. A trailer that leaves one lake on Saturday can launch into another lake the next day, which is how most new lakes become infested.

Their life cycle explains the speed of the spread. A single female can release close to a million eggs in a season, and the young hatch as microscopic larvae called veligers that drift with the current for two or three weeks. Because a veliger goes wherever the water goes, anything that moves water can move veligers, including a bilge, a live well, or a bait bucket. When a veliger finds a hard surface it settles, grows a shell, and anchors itself with strands of protein called byssal threads, which grip concrete, steel, plastic, and other mussels.

An intake pipe is close to ideal habitat, because water moves past it all day and moving water delivers food. The pipe never dries out, and few predators reach inside it. In the Carrowmere intake, biologists counted about 8,000 mussels per square meter on the pipe wall. Utility crews call this kind of growth biofouling.

Biofouling costs the plant in three ways. The first is friction. A narrower pipe with a rough lining takes more energy to push water through, and the fictional Haleworth plant measured a nine percent rise in pumping energy over four years. The second is blockage. Shells break loose in mats and jam screens, valves, and pump impellers. The third is water quality. When a large colony dies, the decaying tissue releases taste and odor compounds, and the plant hears about it from customers within a day.

The mussels change the lake as well. Each adult filters roughly one liter of water per day, straining out the tiny drifting plants and animals called plankton, and a dense colony can filter a large share of a lake's volume every week. The water gets clearer as a result. Sunlight then reaches deeper, so nuisance algae grow thickly on the lake bottom, while the plankton that young fish depend on becomes scarce and native mussels are smothered under layers of quagga shells. The food web does not collapse, but it shifts toward species that no one selected.

Utilities have several tools for the pipe itself. Some feed an oxidant such as chlorine at the intake during the weeks when veligers are settling. A low, steady dose keeps larvae from attaching without treating the whole lake. Some plants run hot water or steam through a section of pipe to kill the colony inside it. Others coat new pipe with smooth or copper-bearing surfaces that mussels grip poorly. Crews also send cleaning devices, called pigs, through the pipe to scrape the walls. Each method costs money and each has limits: chemicals raise the question of disinfection byproducts, heat treatment takes the intake out of service, and coatings wear off.

None of these tools remove the mussels from the lake itself. Once a lake is infested, the population is there for good, and the utility is managing a permanent condition rather than solving a problem. This is why the response at the boat ramp matters more than the response at the intake. Inspection stations check trailers for shells and standing water. Decontamination units wash boats with hot water. Some states require a drain-and-dry period before a boat may launch in a new lake. These programs are far cheaper than pipe cleaning, and they protect lakes that are still clean.

The board of the fictional Carrowmere Regional Water Authority now faces both questions at once. It must decide how to keep its own intake open this year, and it must decide whether to help pay for inspection stations on the two lakes upstream that remain free of mussels. The first decision protects this year's supply, while the second protects water that has not yet been colonized and that eventually flows toward Haleworth.

1. According to the passage, how does a quagga mussel attach itself to a hard surface, and what are those attachment strands called?
2. What is a veliger, and how does it travel from one place to another before it settles?
3. The intake pipe was 48 inches across before the mussels grew, and the opening measured 42 inches after four years. Using area equals 0.785 times diameter squared, calculate the percent of the original opening that remains. Round to the nearest tenth.
4. The passage says the lake water became clearer. Explain why clearer water in this case is not evidence that the lake is healthier.
5. Explain why a utility would feed an oxidant at the intake only during the weeks when veligers are settling rather than all year.
6. Compare treating the intake with inspecting boats at the ramp. Which one addresses the cause of the problem, and which one manages the result?
7. Using the passage, explain what biofouling means, then describe one other structure that could be biofouled and how you would expect it to be affected.
8. The water authority must decide whether to help pay for inspection stations on two lakes that are still free of mussels. Recommend a decision and support it with two specific details from the passage.