Clearer Isn't Always Better: Zebra Mussels in a Training Lake

Use seven years of lake records to argue how a new species changed the water and the lake's populations.

Answer Key

The water in Granite Lake

Read the passage and study Table 1. Then answer the questions.

Training in Granite Lake

1Every summer, thirteen-year-old Beatriz Almeida trains for open-water races in Granite Lake with her swim team. Over the last few years she noticed two changes. The water became so clear that she could see rocks on the bottom several meters below her, and thick beds of underwater plants began to brush the swimmers' legs near the old starting dock. Coach Tyrone Pierce had to move the practice course farther out.

2Beatriz asked Dr. Nkechi Obi, a biologist with the lake association, about the changes. Dr. Obi explained that zebra mussels, small striped mussels that are not native to North America, were first found in Granite Lake in 2020. They most likely arrived on a boat from another lake. Zebra mussels attach in huge numbers to hard surfaces such as docks, rocks and the shells of the lake's native mussels. Like native mussels, they feed by filtering tiny algae and other particles out of the water. One adult zebra mussel can filter about 1 L of water a day.

3The lake association checks the lake each July. Volunteers lower a Secchi disk, a black-and-white disk on a rope, until they can no longer see it; the deeper the disk can be seen, the clearer the water. They also measure the amount of algae in the water and the water temperature, and they count the zebra mussels on one section of the dock (Table 1).

4Every other year, divers survey the lake bottom. They count native mussels in 10 plots, note what percent of them have zebra mussels attached to their shells and estimate how much of the bottom between 2 m and 5 m deep is covered by rooted plants (Table 2). Rooted plants grow only where enough sunlight reaches their leaves. Young yellow perch in the lake feed mostly on water fleas, tiny animals that eat algae.

Table 1. Granite Lake Each July (algae in micrograms per liter)
Year	Zebra mussels per m²	Secchi depth (m)	Algae (µg/L)	Water temp. (°C)
2018	0	2	12	24
2019	0	2.1	11	23
2020	40	2.3	10	24
2021	900	3.4	6	24
2022	3,200	4.8	3	23
2023	4,000	5.2	3	24
2024	4,100	5.3	3	24

How many meters deeper could the Secchi disk be seen in 2024 than in 2019? 3.2 m

Which argument about 2019 to 2024 do the passage and Table 1 support best?

A. The clear water drew zebra mussels to the lake, so their numbers grew.
B. Algae increased as zebra mussels spread, and that made the water clear.
C. As zebra mussels spread, they filtered out algae, so the water cleared.
D. Warmer water each year killed the algae, so the lake became clearer.

Why does the water temperature column matter for this argument?

A. It shows that the Secchi disk can be seen deeper in colder water than in warm water.
B. It shows the lake warmed by several degrees, which explains why the water cleared.
C. It stayed near 23–24 °C, so a temperature change likely does not explain the drop in algae.
D. It shows that zebra mussels spread fastest in the years when the lake was coldest.

Use paragraph 2 and Table 1. Suppose each zebra mussel on the dock filters about 1 L of water a day.

About how many liters of water a day would the zebra mussels on 1 m² of dock filter in 2024? 4100 L

Mussels, plants and fish

Use paragraphs 2 and 4, Table 2 and Table 1 to answer the questions.

Table 2. Diver Surveys of the Lake Bottom
Year	Native mussels per plot	With zebra mussels (%)	Plant cover, 2–5 m (%)
2018	42	0	10
2020	40	5	12
2022	15	70	38
2024	7	85	46

By how many native mussels per plot did the count drop from 2018 to 2024? 35 mussels per plot

Which explanation of the drop in native mussels fits the passage and both tables best?

A. Zebra mussels covered their shells and filtered out much of their food.
B. Clear water let sunlight heat the lake bottom until it was too hot for them.
C. The spreading rooted plants filtered the algae the mussels feed on.
D. The native mussels ate the zebra mussels and were harmed by their shells.

Plant cover between 2 m and 5 m deep rose from 10% to 46%. Which explanation fits the data and paragraph 4 best?

A. Zebra mussels carried plant seeds down into the deeper parts of the lake.
B. Clearer water let more sunlight reach the bottom, so plants grew deeper.
C. Cooler water at 2 to 5 m deep helped the plants grow faster than before.
D. The plants fed on the dead native mussels lying on the bottom of the lake.

Which chain of cause and effect fits the order of events in both tables best?

A. Plants grow deeper → algae drop → zebra mussels arrive → water clears
B. Zebra mussels arrive → algae drop → water clears → plants grow deeper
C. Water warms → algae drop → zebra mussels die off → native mussels drop
D. Water clears → zebra mussels arrive → algae increase → native mussels increase

Use paragraph 4. Which prediction about the lake's young yellow perch does the evidence support best?

A. They likely have less to eat, since less algae can feed fewer water fleas.
B. They likely gain from the drop in algae, since they eat algae directly.
C. They likely change little, since young perch feed on rooted plants.
D. They likely have more to eat, since water fleas feed on zebra mussels.

10.

A teammate says, “The clearer water means the lake is healthier for all of its living things.” Which response uses the evidence best?

A. Disagree: the water temperature rose each year, which harms lake animals.
B. Agree: the water is clearer, and plant cover rose from 10% to 46%.
C. Agree: algae fell from 12 to 3 µg/L, so less algae is left to harm the fish.
D. Disagree: rooted plants spread, but native mussels fell from 42 to 7 per plot.

11.

Coach Pierce wonders whether the lake changed because of the weather or because of the zebra mussels. Write an argument: make a claim, use evidence from Table 1 and Table 2, and explain how the change affected at least two populations in the lake.

Claim: the arrival of zebra mussels, not the weather, most likely caused the changes in Granite Lake's populations. Evidence: zebra mussels went from 0 per m² in 2019 to 4,100 in 2024. Over the same years, algae fell from 11 to 3 µg/L and the Secchi depth rose from 2.1 m to 5.3 m, while the July water temperature stayed at 23 to 24 °C. Native mussels fell from 42 to 7 per plot, and by 2024, 85% of them had zebra mussels on their shells. Rooted plants covered 10% of the bottom from 2 to 5 m in 2018 but 46% in 2024. Reasoning: zebra mussels filter algae out of the water, so there was less algae and the water became clearer. Native mussels lost much of their food to the zebra mussels and were covered by them, so their population dropped. The clearer water let sunlight reach deeper, so the rooted plant population grew. Because the temperature did not change, the weather is not a good explanation for these changes.