Antifouling coatings and ecological control in marinas

The progressive increase of boat tourism and the consequent development of marina activities gives rise to a series of problems related to safeguarding the natural environment. Therefore, there is a need for appropriate monitoring of port facilities and water quality, as well as the development of new technologies dealing with yachting activities, suitable to minimizing their impact on biological communities. The peculiar ecological characteristics of marinas for their mono-functionality allow the accurate assessment of the different effects of specific contaminants on marine organisms. In fact, marinas constitute a valuable observation point from which pollution can be assessed on the basis of the responses of natural populations and communities, and make easier the control of the various pollutants, such as waste fuels, sewage discharges from craft and municipal waters and release of biocide from antifouling coatings. In particular, these paints are one of the main causes of concern and require careful assessment, in order to avoid deleterious effects on the natural environment. Biocide-based antifouling paints are a significant localized source of trace elements (in particular copper and zinc) and organic biocide in the water. In industrial ports the effects of antifouling paints on the biological component can be hardly distinguished from other sources of biocides, such as those generated by industrial activities, commercial shipping and agriculture. Therefore, taking advantage of marinas’ peculiarities in order to assess the effects of the different antifouling paints on marine organisms is an intriguing task. The need to use antifouling coatings is due to the occurrence of fouling organisms, such as algae, barnacles, and tube worms, which recruit and grow on any submerged surface, greatly increasing drag and reducing speed and fuel economy of boats. In the last decades, many biocides, such as tributyltin (TBT) copper- and zinc-based compounds, were introduced in order to restrict the recruitment and growth of fouling organisms on ship and boat hulls. TBT has been referred to as perhaps the most effective antifouling biocide. Nevertheless, due to its negative effects on non-target organisms, it was banned from 2001 onwards, according to the decisions taken by the International Convention on the Control of Harmful Antifouling Systems on Ships, adopted by the International Maritime Organization (IMO). Subsequently, the removal of over-coating of TBT antifouling paints became mandatory from 2008 (IMO, 2001). However, due to the high level of effectiveness of TBT paints, the risk of illegal use is present, even though it should be of minor concern in marinas with respect to commercial and industrial ports. Copper in the form of cuprous oxide continues to be a mainstay antifouling biocide but not necessarily the most effective. It remains the most commonly used biocide in antifouling paints for recreational vessels. Schiff et al. (2004) demonstrated that these paints, which may have 20–76% of copper content (such as cuprous oxide), leach approximately 4.0 g per cm2 per day or roughly 25 g per month for a typical 9 m power boat. This is a non-negligible quantity that can heavily affect biological communities. Recent studies dealing with the chemical monitoring of sediments showed the occurrence of high concentrations of dissolved copper. Species react to this chemical on the basis of their degree of adaptability giving rise to populations capable to live in waters with high concentration of cupric ions, by modulating the responses of detoxification systems at transcriptional and translational levels. The knowledge of these mechanisms and the structure of port biological communities can be of considerable aid in assessing the degree of pollution and its consequences on the environmental quality of adjacent areas. The biological response occurs at different levels of biological organisation, from cellular to community level. Molecular techniques may offer a powerful approach to assess contaminant-induced changes in the genetic architecture of populations and species. Direct surveys of genetic adaptation can be very effective in the assessment the deleterious population-level effects of contaminant exposure, even though often they are difficult to accomplish with most field-exposed organisms. There is the need, therefore, to identify suitable target organisms for this kind of analysis. Other analyses include the response to contaminant exposure at cellular and individual (biomarker) or community levels. The array of these analyses may offer an effective toolbox to assess marinas’ sustainability and monitor the effects of their impact on biological communities. On the basis of this knowledge, in recent years, attention was paid to new non-toxic antifouling systems in order to find replacement solutions overcoming the biocide-based technology. New technologies based on substances that make the surface smoother have been developed in order to obtain a low degree of bioadhesion. Non-stick, fouling-release coatings are an attempt to prevent the adhesion of fouling organisms by providing a low-friction, ultra-smooth surface, on which organisms have great difficulties in settling (Yebra et al., 2004). Many studies carried out to elucidate the properties that a coating should possess to counteract adhesion, pointed out that these properties are mainly possessed by two families of materials: fluoropolymers and silicones (Brady and Singer, 2000). Fluoropolymers form non-porous, very low surface-free energy surfaces with good non-stick characteristics (Brady and Singer, 2000). Silicones, which are applied in thick (6 mm) layers, markedly improved the non-stick efficiency of fluoropolymers. Poly(dimethylsiloxane)-based fouling-release coatings are the most widely used today due to their low surface energy, low microroughness, high elastic modulus and low glass transition temperature (Yebra et al., 2004). These surfaces present “moving targets” to the functional groups of marine adhesives, due to their conformationally mobile surfaces (Brady and Singer, 2000). The mechanical locking of biological glues is minimised and slippage and fouling-release are enhanced. Polysiloxanes substituted by fluorine can be considered attractive candidates for surfaces with low bioadhesion. In the Mediterranean many marinas are located in proximity to aquaculture plants or even included within the borders of marine reserves. The simultaneous presence of activities with contrasting effects on natural environment needs monitoring in order to minimize the impacts and to plan appropriate prevention measures. On the other hand, recreational harbouring assumes great importance in the context of the functionality of multiuse marine protected areas; hence, marinas should be favoured for both tourism and scientific control, taking all possible care, in order to minimise their impact on the natural environment. Therefore, the research on non-toxic antifouling coatings should be stimulated, implemented and refined. These new technologies may provide a valuable contribution to a sustainable coexistence of productive activities and nature conservation.