Long term study of sedimentation and biofouling at Cascadia Basin, the site of the Pacific Ocean Neutrino Experiment

This study evaluates the long-term impact of biofouling and sedimentation on the optical transparency of pathfinder instruments at the Cascadia Basin, revealing significant degradation on upward-facing surfaces over five years while confirming that downward-facing surfaces remain largely unaffected, thereby informing future mitigation strategies for the Pacific Ocean Neutrino Experiment.

The Gist

The Big Picture: Building a Telescope in the Deep Blue

Imagine scientists want to build a giant, underwater telescope to catch "ghost particles" called neutrinos. These particles zip through the universe almost without stopping. To catch them, scientists need a massive detector in the deep ocean (specifically, the Cascadia Basin off the coast of Canada).

But there's a problem: The ocean is dirty.

Just like your car gets covered in mud if you leave it in a rainstorm, or your bathroom mirror gets foggy and slimy if you don't wipe it, anything you leave underwater gets covered in muck. This muck comes in two flavors:

1. Sediment: Like dust and sand slowly raining down from the surface.
2. Biofouling: Like barnacles, algae, and tiny sea creatures growing on the surface.

If this muck covers the telescope's "eyes" (sensors), the telescope goes blind. This paper is a report card on how dirty the ocean got over 5 years and what that means for the future telescope.

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The Experiment: The "Test Dummies" (STRAW)

Before building the massive telescope (called P-ONE), the team sent down two smaller, cheaper "test dummies" called STRAW-a and STRAW-b. Think of these as the canaries in the coal mine. They were dropped to the bottom of the ocean in 2018 and 2020 to see how fast they would get dirty.

They stayed down there for about 5 years. When they were pulled back up in 2023, the scientists took a good look at them.

The Findings: The "Up vs. Down" Problem

The most important discovery was about gravity and orientation.

• The "Upside-Down" Problem: The sensors facing up (toward the surface) got absolutely covered in slime and sediment. It was like leaving a plate on the kitchen counter; gravity pulls the dust and bugs right onto it.
  • The Result: After about 2.5 years, these upward-facing sensors started getting significantly darker. By the end of 5 years, they were anywhere from 35% as clear as new, or completely blocked out.
• The "Right-Side-Up" Advantage: The sensors facing down (toward the ocean floor) stayed surprisingly clean.
  • The Result: Most of them looked brand new! It's like holding a plate upside down under a table; the dust falls off it, not on it.

The Analogy: Imagine you are walking through a snowstorm.

• If you hold your umbrella up, it catches all the snow and gets heavy and clogged.
• If you hold your umbrella down, the snow falls past you, and the umbrella stays clean.

The "Growth Curve": When did it get bad?

The scientists didn't just look at the photos; they measured how much light the sensors could see over time. They found a pattern:

1. The Honeymoon Phase: For the first 2.5 years, the sensors were fine.
2. The Tipping Point: Around the 2.5-year mark, the "slime" started growing exponentially. It wasn't a slow drip; it was a sudden avalanche of muck.
3. The Future: If they don't do anything, the upward-facing sensors could eventually become completely opaque (like a window covered in thick paint).

The Microscopic Party: Who is living there?

The team scraped some of the slime off and looked at it under a microscope (and sequenced its DNA). They found a party of tiny bacteria and microbes.

• The Guests: The most common guests were families of bacteria like Flavobacteriaceae. These are "heterotrophs," which is a fancy way of saying they eat the dead organic matter (like dead plankton) that rains down from the surface.
• The Lesson: Since these bugs love eating the "snow" falling from above, the best way to stop them is to stop the "snow" from landing on the sensors in the first place.

What Does This Mean for the Real Telescope?

The real telescope, P-ONE, is currently being built. The lessons from the test dummies are changing the design:

1. Tilt the Sensors: Instead of having sensors point straight up (like a bucket catching rain), the new sensors will be tilted at an angle. This way, the "snow" (sediment) slides right off, just like rain sliding off a roof.
2. The "Non-Stick" Coating: They are testing a special coating (like Teflon for the ocean) on some sensors. This coating makes it hard for the barnacles and slime to stick, so the ocean currents can wash them away.
3. Focus on the Downward View: Since the sensors facing down stay clean, the telescope will rely heavily on those to catch the neutrinos.

The Bottom Line

The ocean is a messy place. If you leave a glass window facing up in the deep sea, it will eventually turn into a rock covered in algae. But, by understanding how it gets dirty, scientists can design a telescope that stays clean, tilts its head to the side, and uses special "non-stick" paint to keep its eyes open for the next 10 years.

Technical Summary

1. Problem Statement

The Pacific Ocean Neutrino Experiment (P-ONE) is a planned cubic-kilometer-scale neutrino telescope to be deployed in the Cascadia Basin (North Pacific Ocean) at a depth of 2,660 meters. Neutrino detection relies on observing Cherenkov light emitted by secondary charged particles in water using Photomultiplier Tubes (PMTs) housed in optical modules.

The primary challenge addressed in this study is marine fouling, specifically sedimentation (settling of inorganic/organic particles) and biofouling (colonization by living organisms). These processes accumulate on the glass housings of optical modules, reducing optical transmission efficiency. This effect is most severe on upward-facing surfaces, which are critical for detecting astrophysical neutrinos (via up-going events). Without mitigation, fouling could significantly degrade the detector's sensitivity or completely obscure the sensors over the experiment's multi-year lifespan.

2. Methodology

The study utilized data from two pathfinder instruments, STRAW-a and STRAW-b, deployed in the Cascadia Basin to characterize the site for P-ONE.

• Instrumentation:
  • STRAW-a: Deployed in June 2018 and recovered in summer 2023 (5 years total). It consisted of two mooring lines with 8 modules total: four Precision Optical CAlibration Modules (POCAMs, acting as light beacons) and four STRAW Digital Optical Modules (sDOMs). Each sDOM contained one upward-facing and one downward-facing PMT.
  • STRAW-b: Deployed in 2020 and recovered in 2023.
• Data Acquisition:
  • Low-Precision Mode: Continuously recorded PMT trigger rates (background light) from March 2019 to July 2023. This data was used to track ambient bioluminescence and radioisotope decay.
  • High-Precision Mode: Used POCAM flashers (LEDs at 465 nm) to measure light collection efficiency directly.
• Analysis Techniques:
  • Transparency Estimation: The ratio of light rates between upward-facing and downward-facing PMTs ($R = \langle N_{light} \rangle_{upper} / \langle N_{light} \rangle_{lower}$) was used as a proxy for the transmission efficiency of upward-facing surfaces, assuming downward-facing surfaces remained clean (verified by ROV surveys).
  • Modeling: The decline in transmission efficiency was fitted against biological growth models from the Richards family, specifically the Logistic and Gompertz models. These models characterize growth in three phases: lag, exponential, and asymptotic.
  • Biological Sampling: During recovery, biofouling samples were collected via ROV suction samplers and deck collection. DNA was extracted, and 16S rRNA amplicon sequencing (Illumina MiSeq) was performed to identify microbial diversity.

3. Key Contributions

• Quantification of Long-Term Fouling: Provided the first 5-year continuous dataset quantifying the degradation of optical transparency in the deep North Pacific.
• Differentiation of Fouling Effects: Demonstrated that downward-facing surfaces remain largely free of visible biofouling, while upward-facing surfaces suffer significant accumulation.
• Predictive Modeling: Applied biological growth models to optical data to predict the "critical time" (onset of rapid decline) and the asymptotic limit of transparency loss.
• Microbial Characterization: Identified the specific microbial families comprising the biofilm, offering a basis for targeted mitigation strategies.
• Design Validation: Validated the site suitability for P-ONE while highlighting the necessity of specific design modifications (e.g., angling sensors, anti-fouling coatings).

4. Results

• Optical Transmission Decline:
  • Upward-facing modules showed a steady decline in transparency starting approximately 2.5 years after deployment (the "critical time").
  • The decline was attributed to a combination of sedimentation and biofouling.
  • Extrapolation: Based on the fitted models, the long-term transparency of upward-facing surfaces is predicted to range from 35% of the original value to complete obscuration (100% loss) over the detector's lifetime.
  • Fouling Rate: The maximum annual loss in transparency is estimated at roughly 25% per year shortly after the critical time is reached.
• Visual Confirmation:
  • ROV surveys confirmed that upward-facing surfaces developed macro-fouling populations (primarily hydroids, Cnidaria: Hydrozoa) by 2023.
  • Downward-facing surfaces on modules at 70m, 110m, and 30m above the seafloor showed little to no visible fouling.
• Microbial Diversity:
  • The biofilms were highly heterogeneous but dominated by heterotrophic microbes.
  • The most abundant families were Flavobacteriaceae (17%), Rhodobacteraceae (5%), Colwelliaceae (5%), and Shewanellaceae (5%).
  • The composition was similar to deep-sea biofilms in the Mediterranean, suggesting common deep-ocean microbial communities regardless of geography.
• Current Effects: Occasional "cleaning" events were observed where currents temporarily removed loose sediment, causing transient spikes in efficiency, but the long-term trend remained negative.

5. Significance and Implications for P-ONE

• Detector Design: The findings confirm that P-ONE must prioritize the protection of upward-facing sensors. The study supports the design choice to angle PMTs away from the vertical in the P-ONE-1 prototype and future strings to reduce direct sediment accumulation.
• Mitigation Strategies:
  • The study validates the use of fouling-release coatings (specifically silicone-based, e.g., ClearSignal™) as a primary mitigation strategy. These coatings weaken adhesion, allowing currents to remove organisms.
  • A monitoring threshold is proposed: if transparency drops below 90% of the initial value, it indicates the onset of rapid biofouling, triggering maintenance or design adjustments.
• Site Characterization: The study confirms the Cascadia Basin is a viable site for a neutrino telescope, provided that optical module orientation and anti-fouling measures are implemented. The water attenuation length was confirmed to be stable (~28m at 450nm), and background light levels are manageable.
• Future Operations: The data suggests that for a 10-year mission, the first string (closest to the seabed) will face the most severe fouling, while higher modules will be less affected. This informs the deployment strategy and maintenance schedule for the full-scale array.

In conclusion, this paper provides a critical empirical foundation for the engineering of P-ONE, transforming biofouling from a theoretical risk into a quantified parameter with specific mitigation pathways.