What is biofouling?
Biofouling affects a wide range of marine activities, hindering operations and potentially driving up costs.
The term refers to the accumulation of marine organisms on submerged artificial surfaces, such as the underside of ships, underwater structures and equipment such as current profilers. The biofouling process starts with the adsorption of organic molecules to a surface, which leads to the attachment of further layers of microorganisms, such as bacteria or algae.
This biofilm in turn may attract larger organisms, including marine invertebrates such as barnacles, mussels and sponges, and then crabs, shrimps, snails and other mobile organisms.
If the transducers on acoustic Doppler current profilers (ADCPs) become covered in a thick layer of organic material, they are unable to operate properly. Biological growth may affect the acoustic signal and decrease the instrument’s range and the quality of the data collected. For this reason, it is imperative that the appropriate techniques are employed to combat biofouling where it is a problem.
The problem of biofouling tends to be most pronounced in:
- Warmer, tropical waters
- Still, low-energy environments
- Shallow applications (light- and temperature-dependent)
- Areas with higher nutrient input
Why is biofouling a problem when measuring waves and currents?
Biofouling is a significant issue because it limits the deployment time and increases the cost of a measurement campaign.
Even though the current velocity is not normally affected, the profiling range of an ADCP will decrease as the build-up on the transducer elements increases. Also, in some conditions, mussels and barnacles etc. can grow on or close to the instrument. In long-term deployments, rubber connectors may eventually be destroyed as a result.
Thus, biofouling can interfere with the instrument’s ability to collect correct data, even before its power and/or data capacity has been reached. The costs of the instruments, batteries, deployment and recovery, travel (boat time etc.) and other factors add up to significant sums. It seems wasteful to recover the instrument or send a diver underwater to clean it when it should still have the power and data capacity to continue the deployment.
Therefore, it makes sense to do everything possible to combat biofouling to ensure that the deployment can be extended for as long as possible, while ensuring that the data collected is trustworthy and of high quality.
How can we reduce biofouling on ADCPs?
There are many methods in use to reduce biofouling. Some are “classical” methods and have been used for many decades. Other methods are new and have only been in use over the last few years.
Using paints to reduce biofouling on ADCPs
Ships, outboard engines, etc. are usually painted to protect against fouling. While some of these paints can be quite effective in hindering growth, some are no longer legal, some can destroy the instrument, and some reduce the effective profiling range.
- Ships, outboard engines, etc. are usually painted to protect against fouling. While some of these paints can be quite effective in hindering growth, some are no longer legal for environmental reasons, some can destroy the instrument, and some reduce the effective profiling range. The most effective paint contained organotin compounds acting as a biocide. These compounds, such as TBT, are now banned in antifouling paint by the International Maritime Organization’s International Convention on the Control of Harmful Anti-fouling Systems on Ships. Some US companies continued to sell paints containing TBT even after the ban, but this came to an end when the paint makers were convicted and given prison sentences.
- Other antifouling paints, containing copper, have also been sold, but these are not recommended for use with ADCPs for three reasons: (1) the paints can be damaging to benthic ecosystems and water quality in general; (2) some of the paints include large flakes of copper that can diffuse the acoustic radiation patterns; and (3) others have been found to be acidic and can damage the polyurethane surface used in most transducer designs.
- For ADCPs, two homespun antifouling methods have also been used. The first involves applying zinc oxide paste (sold as skin protection for babies) to the area around the transducers (see photo below). This paste contains a zinc-based biocide that is poisonous to the small marine organisms that want to colonize the substrate, thereby reducing the amount of organic material able to attach itself to the instrument. Note that zinc paste will wash away, especially in higher-energy environments, but it works fairly well in some conditions. The other creative method is to mix Vaseline with cayenne pepper and smear it onto the device to discourage organisms from attaching themselves to the transducers. Both methods have only anecdotal success, and we are not aware of any tests having been carried out to systematically measure the increase in deployment time.
Nortek Aquadopp profiler in late April 2018, before deployment in Galveston Bay by a team at Texas A&M University. The transducers have been coated with zinc oxide paste.
Using a protective film/coating to reduce biofouling on ADCPs
A more recent – and more sophisticated – solution is the application of a protective film or coating. There are at least two providers at present, and they use competing approaches to the problem of application.
Severn Marine Technology (USA)
Severn Marine Technology use what they refer to as the ClearSignal® Biofouling control system, where the entire sensor or vehicle is covered following a special process. ClearSignal® is a clear, non-toxic coating that resists biofouling as a result of its non-stick properties. The product is a permanent coating that is designed to last the lifespan of the platform or instrument it is protecting. To apply ClearSignal® to your instrument or sensor, contact Severn Marine directly.
Nitto have developed adhesive antifouling stickers that users can attach directly to the transducers. This allows the equipment to function normally, while providing a transparent, silicone-based surface to which it is difficult for microorganisms to adhere. The stickers are particularly effective where water movement helps wash the organisms away, and can be peeled off after use.
Nortek Aquadopp profiler in late April 2018, before deployment in Galveston Bay by a team at Texas A&M University. The transducers have been covered with anti-biofouling stickers.
The stickers are made of silicone resin, polyester film and acrylic adhesive. As the stickers do not rely on biocides or poisons to combat biofouling, they are considered a less environmentally damaging solution than paints.
Nortek has worked with multiple customers to test the Nitto patches, and the results suggest that the stickers produce good results even in areas where biofouling problems are severe. The stickers are fairly simple to apply and remove. The stickers for many of Nortek’s coastal instruments can be purchased in the Nortek web store (limitations apply).
See this support article to learn more about how to clean instruments and how to prevent biofouling.
The same two Aquadopps that had been deployed in Galveston Bay, on 31 May 2018 after they had dried off in the lab. The transducers protected by stickers are relatively clear, while those covered with zinc paste are covered in organic material.