All surfaces in aquatic environments are impacted by biofouling, which begins accumulating immediately when a surface is introduced into the water (Dafforn et al., 2011). The majority of fouling occurs when a surface is idle, especially in port or nearshore waters (Floerl and Inglis, 2003; Davidson et al., 2020; Floerl et al., 2021; Hopkins et al., 2021; Luoma et al., 2021). Microfouling consists of conditioning films (adsorbed chemicals), biofilms or “slime” composed of bacteria, fungi, diatoms and other microalgae. Macrofouling includes barnacles, tube worms, oysters, tunicates, and more (Vuong et al., 2023). These different types of fouling occur on differing time scales and interact with surfaces in various ways, making them more or less attractive to subsequent fouling (Weber and Esmaeili, 2023).

Maintaining surfaces free of fouling is critical for many industries; however, ships have received the most attention because of the scale of the problem. The calculated wetted surface area globally for vessels over 100 gross tons between 1999 and 2013 was 571 square kilometers, and for those entering the US between 2011 and 2014, it was 510 square kilometers annually (Miller et al., 2018; Weber and Esmaeili, 2023). Globally, shipping produces about 3% of anthropogenic emissions, which increased slightly from 2012 to 2018 (Balcombe et al., 2019; IMO, 2021). Despite this increase, the overall efficiency has improved (Balcombe et al., 2019; IMO, 2021; Weber and Esmaeili, 2023). Biofouling increases ships’ drag, fuel costs, greenhouse gas (GHG) emissions, and maintenance costs associated with engine wear and tear. It also increases the risk of transporting potentially invasive nonindigenous species (NIS) (Davidson et al., 2016; Balcombe et al., 2019; Luoma et al., 2021; Wu et al., 2022; Weber and Esmaeili, 2023). Additionally, fouling can increase corrosion, an issue affecting more than just ships (Wu et al., 2022; Davidson et al., 2023).

Niche areas may be defined as non-hull surfaces of a ship’s wetted surface and include rudders, propellers, intakes, sea chests, internal piping, sacrificial anodes, dock block areas and others that are not traditionally well protected by standard fouling prevention solutions (Davidson et al., 2016; Luoma et al., 2021). These areas may comprise up to 20% of a vessel’s total wetted surface area and have at least twice the biofouling accumulation of the rest of the hull (Davidson et al., 2016; Luoma et al., 2021). Propellers are a particular case in that they are not well protected by traditional antifouling coatings and are potentially significant contributors to drag and fuel costs (Davidson et al., 2016; Farkas et al., 2021). In addition to propellers, internal seawater systems (ISS) are another niche that is difficult to protect and prone to accumulation of high levels of fouling (Davidson et al., 2023). Impacts from fouling in niche areas include restricted flow, corrosion, loss of critical functions like heat exchange and increased maintenance. These are also areas at high risk for the transport of NIS (Davidson et al., 2023). The costs associated with necessary responses to fouled ISS can be substantial, greater than US$450,000 per vessel per month (Davidson et al., 2023).

Biofouling also impacts non-ship installations. Piping for power plants and other industries have been shown to have a 20% increase in the cost of pumping after one year and over 40% after two years in a heavily impacted system (de Souza et al., 2023). Additionally, biofouling is the primary factor limiting long-term monitoring studies using optical instruments. A thin biofilm on the sensor face can impact collected data quality (Matos et al., 2023). In a high fouling environment, macrofouling can begin accumulating on sensor faces within days, and some fouling can damage the surface coating on the sensors (pers obs). Cleaning the sensors to remove fouling can lead to damage and impact readings (Matos et al., 2023). Fouling on static structures can lead to corrosion and increased maintenance costs (Weber and Esmaeili, 2023).

The global antifouling market is estimated at US$5 billion annually and is expected to continue to grow, increasing the demand for novel antifouling solutions (Davidson et al., 2016). Historically, several methods of biofouling control have been commonly utilized, including inert, antifouling and fouling release coatings (Dafforn et al., 2011; Lejars et al., 2012). Fouling control coatings can fall short due to mismatches between paint type and duty cycle, presence of biocide tolerant organisms, improperly applied, old or damaged paint, and many other factors (Davidson et al., 2009; Dafforn et al., 2011; Sylvester et al., 2011; Davidson et al., 2016). Because these historically utilized methods of biofouling control are inadequate in ports where there are concentrations of sensitive anthropogenic structures like ships, pilings, seawalls, instruments and more, the development of novel antifouling methods is of critical importance not only to the shipping industry but to anyone wishing to protect submerged surfaces.

Ralston and Pringle (2023) introduced a novel antifouling method where the surface to be protected is enclosed or encapsulated in a wrap or bag while static. In this experiment, sleeves prevented macrofouling for more than 12 months in a location known for rapid and diverse fouling accumulation, even on biocidal surfaces (Ralston et al., 2022; Ralston and Pringle, 2023). Most antifouling coatings have higher biocide output while voyaging and are least vulnerable to fouling, creating a mismatch with higher fouling pressure in port (Balcombe et al., 2019).

Encapsulation may not be able to cover complex structures like boat hulls or instruments closely. In many cases, there may need to be a larger volume enclosed by the bag to encapsulate the entire surface. This experiment aimed to determine the impact of encapsulating larger volumes and the proximity of the fabric to the protected surfaces on the efficacy of the antifouling enclosures.

The bags were effective at preventing macrofouling for eight months across all bags ( Figure 2 ). At the eight-month assessment, the PVC in the 4’ bags began to accumulate a very light cover (<3%) of hydroids and tube worms. The hydroids were no longer present at the eleven-month immersion assessment. One replicate of the bronze in the 4’ bags began accumulating tube worm fouling at nine months of immersion. At the end of the experiment, after twelve months of immersion, only the settlement plates in the 4’ bags had accumulated macrofouling consisting of a very light cover of tube worms. The total cover of macrofouling in the bags never reached 5% cover. The settlement plates in the 1’ and 2’ bags never accumulated macrofouling through twelve months of immersion.

In contrast, control plates were heavily fouled after one month of immersion, with macrofouling cover at nearly 100% over the entire experiment. The fouling was significantly different among the bagged samples and the open controls. Fouling changed seasonally and consisted of hydroids, encrusting and arborescent bryozoans, barnacles, tube worms, oysters, sponges, tunicates and sea anemones ( Figure 3 ). The percent cover of biofilm on the samples in the bags never exceeded 50% cover. A PCA grouped panels into two loose groups, one consisting of all of the controls and the other comprised of the bagged samples ( Figure 4 ). Differences were primarily attributable to differences in barnacle cover (82.7%), biofilm (9.4%) and encrusting bryozoan (4.1%).

Water quality varied between the open readings and the bagged readings for DO and pH, while temperature and salinity never varied significantly ( Figure 5 ). Dissolved oxygen was always significantly lower in the bags than in the open readings. The pH varied at times throughout the year. It was significantly lower in the bags in months one (September), nine (May) and twelve (August). The pH was significantly lower in the 4’ bags than all other treatments in month five (January) and lower than the open in month seven (March). The pH was lower in the 2’ and 4’ bags in months eight (April) and ten (June) than in the open readings. The pH was not significantly different during other months.

Continuous monitoring of temperature and DO showed some interesting patterns. When using the data averaged over the month, temperature did not show significant differences between open and bagged readings. However, using the unaveraged data, the temperature change was slower in the bags than in the open during periods of rapid temperature change ( Figure 6 ). The buffering effect was more evident in the larger bags. When temperature change was more gradual, this apparent buffering disappeared. These differences were slight and nonsignificant statistically. Dissolved oxygen showed interesting changes during the first month of immersion and could be divided roughly into four zones ( Figure 7 ). When first immersed, little difference was observed in DO between the bags and open water. After about a week, the oxygen in the bags dropped to very low, hypoxic, and even approaching anoxic values. After around ten days, oxygen in the bags increased but was highly variable. Finally, DO settled into a balance where concentrations were lower than the open readings but followed normal daily trends. This trend continued throughout the remainder of the experiment ( Supplementary Materials ).

Best practice encourages reducing or preventing fouling to mitigate environmental impacts from transporting NIS and GHG emissions (Zabin et al., 2018; IMO, 2011; Bailey et al., 2020; Georgiades et al., 2020). Ralston and Pringle (2023) showed a novel encapsulation or wrapping method that protected surfaces from macrofouling for 13 months. In this experiment, bags ranging in size from 30.5 to 122 cm in diameter (66720 cm³ to 1.07 m³ enclosed volume) could protect inert PVC surfaces for 8 months and bronze surfaces for 9 months. The largest bags began accumulating macrofouling first, with the smaller bags never accumulating macrofouling. The results for the 1’ and 2’ bags were similar to the results seen in the original sleeves, which were about 2/3 of the size of the bags in this experiment (Ralston and Pringle, 2023).

As with the previous experiment, dissolved oxygen was always lower in the bags, regardless of size. Hypoxic and anoxic conditions have been shown to lower settlement rates of many invertebrates (Baker and Mann, 1992; Lagos et al., 2015; Jorissen and Nugues, 2021). Impacts on settlement have been seen even after conditions have returned to normoxic conditions or come out of the hypoxic range. This has been attributed to a larval response to cuing by biofilms that retain a record of prior low oxygen conditions (Cheung et al., 2014; Lagos et al., 2016). During formation, biofilm structure, composition and physiological activity is impacted by many factors, including DO (Antunes et al., 2019). Macrofouling larvae can sense changes in biofilms and may respond by settling or avoiding settlement based on biofilm characteristics (Dobretsov and Rittschof, 2020). In this experiment, DO varied and went through a cycling process where conditions became hypoxic and even anoxic for a time during initial deployment. This may have changed the biofilms, leaving a record of these detrimental, low-oxygen conditions. Biofilms were not explicitly examined in this experiment; therefore, no definitive statements can be made about the differences. However, visually, biofilms in the bags were thin and appeared different than those formed on the open controls.

Additional experiments have been conducted to examine the possible factors responsible for the non-fouling of the protected substrates. In addition to the changes in DO, pH and nutrient concentrations have been shown to vary between bagged and open samples. The bags do not exclude meroplanktonic larvae, nor do the biocides kill the larvae in the bag (Ralston et al., in prep). There are other potential cues and variables that vary between open and bagged treatments. This includes things such as light and water movement that would likely be decreased inside the bag (Abelson and Denny, 1997; Walters et al., 1999; Larsson et al., 2016; Wheeler et al., 2017). Competent larvae become negatively phototactic and often settle in dark or shaded areas (Rittschof et al., 1998; Ettinger-Epstein et al., 2008; Wheeler et al., 2017). Some species show a preference for lighted surfaces, contrary to the majority (Whalan et al., 2008). Flow could have many impacts on settlement and recruitment, including turbulence increasing the likelihood of contact with a surface, larval behavior at the surface and flow may mediate settlement cues or act as a settlement cue (Abelson and Denny, 1997; Larsson et al., 2016). Many organisms explore more or stay longer on surfaces under flow than static conditions (Walters et al., 1999).

Despite large differences in the volume of water enclosed by the bags and the spacing between antifouling fabric and protected surface, the technology protected inert PVC and propeller mimics for eight months to at least one year without any visible macrofouling. This result shows promise for real-world applications, where a bag or wrap may only be able to enclose a complex surface like a propeller or instrument with a large set-off distance.