Biofouling – the problem and the solution
If you are using a current profiler in certain types of marine environment, the accretion of organic molecules and organisms on the transducers – so-called biofouling – can severely impact the performance of the instruments.
However, the impact of biofouling can be reduced by using established methods such as the application of various substances, or the more recently developed technique of affixing silicone-based stickers over the transducers, making it difficult for organisms to adhere.
- ADCPs are unable to operate properly when covered with organic material.
- There are several methods of tackling biofouling on ADCPs, but not all are recommendable
- Silicone-based stickers are efficient and non-toxic, and last longer than most other options.
What and why
What is biofouling and why do ADCP users need to consider its implications?
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 that has been collected.
For this reason, it is imperative that the appropriate techniques are employed to combat biofouling, where it is a problem.
Who needs biofouling solutions?
The problem tends to be most pronounced in:
- Warmer, tropical waters.
- Still, low-energy environments.
- Shallow applications (light- and temperature-dependent).
- Areas with higher nutrient input.
Therefore, ADCP users deploying the instruments in such biofouling-prone environments should consider using a technique to reduce its impact.
Biofouling is a significant issue because data collection with ADCPs can be expensive. The costs of the instruments, batteries, deployment and recovery, travel (boat time etc.) and other factors add up to significant sums. Therefore, it makes sense to do everything possible to ensure that the data collected is trustworthy and of high quality. Silicone-based stickers will help avoid poor data caused by biofouling.
Additionally, in some areas, biofouling will interfere with the instrument’s ability to collect correct data, even before its power and/or data capacity has been reached. It seems wasteful to recover the instrument or send a diver underwater to clean the instrument when it should still have the power and data capacity to continue the deployment.
Choice of technique
There are several methods employed to tackle biofouling on ADCPs, but not all are recommendable.
One established method is to apply zinc oxide paste to the area around the transducers (see photo). 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. Be aware that zinc paste will wash away, especially in higher-energy environments. However, the paste works fairly well in some conditions.
Nortek Aquadopp profiler in late April 2018, before deployment in Galveston Bay by a team at Texas A&M University. The transducers are coated with zinc oxide paste.
Anti-biofouling paints are also available, but these often contain copper and can be damaging to benthic ecosystems and water quality in general. At the other end of the spectrum, some users creatively apply Vaseline with pepper, or even children’s lotion/cream, to prevent biofouling.
A more recent – and more sophisticated – solution is the application of adhesive anti-fouling stickers directly to the transducer windows (see photo). These allow the equipment to function normally, while providing a transparent, silicone-based surface to which it is difficult for micro-organisms 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 are covered with anti-biofouling stickers.
As stickers do not rely on biocides or poisons to combat biofouling, they are considered a less environmentally damaging solution.
Tests carried out by ADCP users suggest that stickers produce better results than using zinc paste in some areas where biofouling problems are severe, though these tests were not carried out in controlled conditions.
The same two Aquadopps (that had been deployed in Galveston Bay) on May 31, 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.
Silicone-based stickers are a very good option due to the following factors:
- Effectiveness in preventing biofouling
- Longevity (pastes only last a couple of weeks to a few months)
- Biodegradability and non-toxicity (many other paint-based solutions are harmful and/or even prohibited in some countries)
- Ease of use (the stickers are much easier to handle and don’t make a mess like the paste and paints)
See also this article about how to remove barnacles from instruments, and further advice on anti-fouling paint.
This set is suitable for Aquadopp 300 m, Aquadopp Profiler 1MHz and Aquadopp Profiler 2MHz
This set is suitable for Aquadopp Profiler Z-cell 1 MHz
This set is suitable for AWAC 1MHz
This set is suitable for AWAC 600kHz
This set is suitable for Signature1000
This set is suitable for Signature500