Blooms like it hot, but mussels do not: Influence of invasive quagga mussels on cyanobacteria during summer

This study demonstrates that while invasive quagga mussels can effectively reduce harmful cyanobacteria blooms in temperate lakes, their filtration capacity and survival are critically limited by high summer temperatures, suggesting that global warming may exacerbate algal blooms by simultaneously boosting cyanobacteria growth and suppressing mussel grazing.

The Gist

The Big Picture: A Tale of Two Invaders

Imagine a lake as a giant, swimming pool. Recently, two "invaders" have moved in, and they are having a very strange relationship:

1. The Cyanobacteria (The Bad Guys): These are toxic algae that turn the water green and slimy. They are like weeds in a garden, but they can poison fish and make people sick. They love the heat; the hotter the summer, the faster they grow.
2. The Quagga Mussels (The Gardeners): These are invasive mussels that act like underwater vacuum cleaners. They filter the water, eating the algae and keeping the lake clear.

For a long time, scientists thought the mussels were the heroes, keeping the toxic algae in check. But this paper asks a simple question: What happens when the summer gets too hot?

The Experiment: A Heat-Test for the Gardeners

The researchers set up a laboratory "kitchen" to test how these mussels behave. They fed the mussels different types of algae at different temperatures, ranging from a cool 24°C (75°F) to a scorching 33°C (91°F).

The Results:

• The Menu Matters: The mussels were picky eaters. They happily gobbled up one type of algae (Dolichospermum), which they considered a tasty snack. However, they mostly ignored or spit out other types of algae (like Microcystis) that formed sticky clumps or had tough shells. It's like a kid who loves apples but refuses to eat broccoli.
• The Heat Limit: The mussels worked great in the cool weather. But as the water got hotter, their "vacuum cleaner" power started to sputter.
  • Around 28.9°C (84°F), the mussels hit a wall. Their filtration rate dropped sharply.
  • By 32°C (90°F), the mussels were dead. They literally cooked in their own shells.

The Analogy: Think of the mussels like a marathon runner. In cool weather, they can run fast and clear the track (eat the algae). But if the temperature rises too high, they get heatstroke, slow down, and eventually collapse.

The Real-World Test: A 20-Year Look at Lake Müggelsee

The researchers didn't just stop at the lab. They looked at 20 years of data from a real lake in Germany (Lake Müggelsee). They compared the 10 years before the mussels arrived with the 10 years after.

What they found:

• Before the mussels: The lake was often full of toxic algae blooms.
• After the mussels (but only when it was cool): The mussels arrived and ate up the "tasty" algae. The water got clearer, and the bad blooms disappeared. The mussels were doing their job as gardeners.
• The Catch (The Heatwave): However, whenever the summer got really hot (above that critical 27.7°C / 82°F mark), the mussels stopped eating. Suddenly, the "tasty" algae exploded in numbers again.

The Metaphor: Imagine the mussels are security guards at a club. When the weather is mild, they stand at the door and stop the bad guys (algae) from getting in. But when the heatwave hits, the guards pass out from the heat. Suddenly, the bad guys rush in and take over the club.

Why This Matters for the Future

The paper warns us about a "perfect storm" caused by climate change:

1. The Bad Guys Love the Heat: Toxic algae grow faster and bigger as the planet warms.
2. The Good Guys Hate the Heat: The mussels that usually eat the algae stop working or die when the water gets too warm.

The Conclusion:
In the past, we thought invasive mussels might save us from toxic algae. But this study shows that global warming is breaking the mussels' ability to help.

As summers get hotter and heatwaves become more common, the "critical thermal window" (the temperature where mussels stop working) will be crossed more often. This means that in many lakes, the mussels will stop cleaning the water just when the toxic algae are growing the fastest.

In short: The algae are "blooming like it's hot," but the mussels "do not." As the world gets warmer, we can expect more toxic algae blooms because our underwater gardeners are too hot to work.

Technical Summary

1. Problem Statement

Cyanobacterial harmful algal blooms (HABs) are a critical environmental issue in freshwater systems, exacerbated by climate change and rising water temperatures. While invasive quagga mussels (Dreissena rostriformis bugensis) are known ecosystem engineers that filter phytoplankton and can potentially control HABs, their efficacy is uncertain under specific conditions.

• The Gap: It is unclear how quagga mussels interact with different cyanobacteria species (varying in morphology and toxicity) and how rising summer water temperatures affect their filtration capacity.
• The Hypothesis: The authors hypothesized that:
  1. Filtration rates vary among cyanobacteria species.
  2. Filtration rates decline above a critical temperature threshold.
  3. In the field, mussels reduce the biovolume of palatable cyanobacteria.
  4. This reduction effect disappears when water temperatures exceed a critical thermal window.

2. Methodology

The study employed a dual approach combining controlled laboratory experiments with long-term field data analysis from Lake Müggelsee, a shallow temperate lake in Berlin, Germany.

A. Laboratory Filtration Experiments

• Subjects: Adult quagga mussels (13–27 mm shell length) collected from Lake Müggelsee.
• Prey: Four common bloom-forming cyanobacteria species (Dolichospermum flos-aquae, Aphanizomenon flos-aquae, Anabaenopsis elenkinii, Microcystis aeruginosa), two green algae (Chlorella vulgaris, Acutodesmus obliquus), and mixed lake samples.
• Conditions: Experiments were conducted at temperatures ranging from 24°C to 33°C (in 1°C or 2°C increments).
• Procedure: Mussels were exposed to algae at ~20 µg L⁻¹ chlorophyll a for 3 hours in the dark. Filtration rates (clearance rates) were calculated based on the reduction in chlorophyll a concentration relative to controls.
• Analysis: Generalized Linear Models (GLM) were used to test species and temperature effects. Segmented regression was used to identify the "breakpoint" (BP) temperature where filtration rates significantly declined.

B. Long-Term Field Data Analysis

• Site: Lake Müggelsee (7.3 km², mean depth 4.9 m).
• Timeline: 10 years pre-invasion (2002–2011) vs. 10 years post-invasion (2012–2021). Quagga mussels invaded around 2012, reaching ~97% of the dreissenid population by 2017.
• Data: Weekly summer (July–September) water samples for nutrients (N, P), phytoplankton biovolume, and water temperature (0.5 m depth).
• Statistical Approach:
  • Zero-inflated Generalized Linear Mixed Models (GLMM) to analyze cyanobacteria biovolume changes relative to invasion status and palatability.
  • Segmented regression and Davies tests to identify critical temperature breakpoints in field data.

3. Key Contributions

• Species-Specific Palatability: The study definitively categorizes cyanobacteria based on quagga mussel preference, distinguishing between "palatable" and "less-palatable" species based on morphology (colony formation vs. single filaments).
• Critical Thermal Window: The research quantifies a specific thermal threshold (~28°C) above which quagga mussels lose their ability to filter cyanobacteria, a finding previously lacking for quagga mussels (though noted for zebra mussels).
• Climate Change Interaction: It provides empirical evidence that global warming creates a "double jeopardy" scenario: it promotes cyanobacterial growth while simultaneously disabling the primary biological control mechanism (mussel filtration).

4. Key Results

Laboratory Findings

• Palatability:
  • Palatable: Dolichospermum flos-aquae (non-colony forming) and green algae were filtered at high rates (~4.0 mL mg⁻¹ AFDW⁻¹ h⁻¹ at 24°C).
  • Less-Palatable: Aphanizomenon flos-aquae, Anabaenopsis elenkinii, and Microcystis aeruginosa (colony-forming or raft-forming) were filtered at negligible rates (<1.0 mL mg⁻¹ AFDW⁻¹ h⁻¹).
• Temperature Threshold:
  • Filtration rates remained stable up to ~28°C.
  • A breakpoint (BP) was identified at 28.9°C (95% CI: 27.6–30.2°C), where filtration rates sharply declined.
  • At 32°C, all mussels died within 4 hours.

Field Findings (Lake Müggelsee)

• Nutrient Shifts: Post-invasion, Total Phosphorus (TP), Soluble Reactive Phosphorus (SRP), and Total Nitrogen (TN) decreased significantly, likely due to mussel filtration and sedimentation.
• Biovolume Reduction:
  • Palatable Cyanobacteria: Dolichospermum biovolume significantly decreased after invasion, but only when water temperatures were below the critical threshold.
  • Less-Palatable Cyanobacteria: Biovolumes of Microcystis, Aphanizomenon, and Anabaenopsis showed no significant change post-invasion.
• Thermal Breakpoint in Nature: Segmented regression of field data identified a critical temperature of 27.7°C (95% CI: 26.9–28.4°C). Above this temperature, the suppressive effect of mussels on palatable cyanobacteria disappeared, and blooms re-emerged.
• Heatwave Impact: In years where summer temperatures exceeded 27.7°C, palatable cyanobacteria blooms occurred despite the presence of high mussel densities.

5. Significance and Implications

• Ecosystem Management: The study challenges the assumption that invasive mussels universally control HABs. Their effectiveness is strictly limited to "cool" summers and specific cyanobacteria morphologies.
• Climate Change Feedback Loop: As global warming increases the frequency and intensity of lake heatwaves:
  1. Cyanobacteria growth rates increase (optimal >25°C).
  2. Quagga mussel filtration rates drop to zero above ~28°C.
  3. This leads to a loss of top-down control, potentially causing more frequent and severe HABs in invaded lakes.
• Predictive Modeling: Future models predicting HABs in invaded lakes must incorporate this "critical thermal window." Without accounting for the temperature-dependent failure of mussel filtration, models may underestimate future bloom risks.
• Public Health: The inability of mussels to control blooms during heatwaves increases the risk of exposure to toxins (e.g., microcystins, cylindrospermopsin) in recreational and drinking water sources.

In conclusion, the paper demonstrates that while quagga mussels can act as a biological control for specific cyanobacteria, climate warming is dismantling this control mechanism, potentially leading to a paradox where invasive species fail to mitigate the very blooms they are often credited with suppressing.