Long-Term Field Monitoring of Biofouling Characteristics in the Yellow Sea off Jeju Island, South Korea

Abstract

Biofouling on offshore wind farm substructures threatens operational reliability and raises maintenance demands, underscoring the need for effective antifouling strategies. This study presents a 27-month evaluation of fouling development on a conventional non-antifouling coating and self-polishing copolymer (SPC) systems at a South Korean offshore wind farm. Biofouling coverage was assessed through long-term image analysis, and surface energy was characterized via contact angle measurements. Species analyses identified successional communities dominated by barnacles, coralline algae, and bryozoa. The conventional coating showed rapid colonization, exceeding 90% coverage within 10 months, whereas the SPC systems exhibited superior performance by suppressing settlement, though their effectiveness declined over time. Quantitative analysis revealed a clear relationship between higher surface energy and increased fouling rates, highlighting material properties as a key factor in colonization. This study provides one of the first long-term quantitative datasets from South Korean waters, advancing understanding of biofouling dynamics and informing antifouling strategies for offshore wind infrastructure.

1. Introduction

Global efforts to mitigate climate change and achieve carbon neutrality have accelerated the transition toward renewable energy, with offshore wind power emerging as one of the fastest-growing sectors supported by technological innovation, large-scale investment, and policy frameworks [1]. Unlike other renewable sources, offshore wind combines high energy density with relatively stable wind resources, making it a cornerstone technology for meeting mid- to long-term decarbonization targets. Offshore wind capacity is projected to continue expanding worldwide, particularly in Europe and East Asia, where shallow coastal shelves and supportive government initiatives create favorable conditions for deployment.

However, offshore wind infrastructure faces unique challenges compared with land-based systems. Substructures remain continuously submerged for decades, exposing them to marine colonization processes that inevitably lead to biofouling—the attachment and growth of bacteria, algae, barnacles, mussels, and other marine organisms on artificial surfaces [2,3,4,5]. Biofouling is not merely a superficial phenomenon; it drives wide-ranging ecological and engineering consequences.

On the positive side, fouling assemblages can function as artificial reefs, enhancing biodiversity, providing habitat for fish and invertebrates, and in some contexts fostering coexistence with fisheries and aquaculture [6,7]. Such ecological functions are increasingly recognized in the broader discussion of marine ecosystem services. Conversely, from an engineering and operational perspective, biofouling is highly problematic. The development of dense and substantially complex fouling layers significantly alters hydrodynamic loading, increases drag coefficients, accelerates structural fatigue, and elevates maintenance burdens [8,9,10]. These processes undermine operational reliability and increase lifecycle costs, with field reports highlighting additional logistical challenges during decommissioning [11] and heightened maintenance demands for marine renewable energy converters [12]. In parallel, advances in inspection and cleaning technologies are being explored to reduce operational costs and safety risks [13,14]. Recent assessments emphasize that biofouling management represents a major driver of operation and maintenance (O&M) costs in offshore wind projects, with case studies reporting multi-million-pound expenditures over a single project lifetime [15,16].

Long-term ecological surveys in European offshore wind farms have demonstrated that fouling succession is not static but rather a dynamic, evolving process. Rather than converging toward a stable climax community, assemblages exhibit temporal shifts in biodiversity and abundance that can persist over decadal timescales [17]. Colonization patterns are also shaped by depth-related zonation and seasonal hydrographic variability, producing distinct vertical profiles of community composition across subtidal gradients [18,19]. While these findings underscore the complex interplay between environmental forcing and biological succession, they are largely restricted to European waters. This geographic bias highlights a critical knowledge gap: equivalent long-term datasets from East Asia, including South Korea, remain sparse despite the region’s rapid expansion of offshore wind capacity. Filling this gap is essential for understanding how regional oceanographic and ecological conditions modulate fouling processes.

Among available mitigation strategies, self-polishing copolymer (SPC) coatings are widely employed to alleviate biofouling by gradually eroding at their surface upon prolonged seawater exposure. This self-polishing action renews the coating layer and provides antifouling effects through the controlled release of biocidal agents [20,21]. In addition, recent studies have explored alternative antifouling strategies, including silicone-based fouling-release coatings, biomimetic surface designs, and hybrid ceramic–polymer multilayer systems, which aim to enhance durability and reduce environmental impact under long-term offshore conditions [22,23]. However, most antifouling systems, most existing evaluations focus on short-term trials or controlled experiments. Robust field-based evidence regarding their durability, biocide release kinetics, and antifouling performance under multi-year offshore conditions remains limited. This lack of empirical validation restricts the ability to predict long-term efficacy and compromises coating selection strategies for wind turbine substructures exposed to highly dynamic marine conditions.

Surface properties are another key determinant of fouling settlement. Contact angle measurements and derived surface free energy (SFE) parameters are widely used as proxies for the adhesion potential of different materials. Numerous experimental studies have shown that surfaces with lower SFE values, especially those with reduced polar components, generally hinder the adsorption of organic macromolecules and the subsequent attachment of algal spores and invertebrate larvae. Conversely, elevated polar contributions are associated with stronger adhesive interactions and higher colonization rates. Laboratory investigations employing chemically defined substrata have consistently reported a positive correlation between surface wettability and the adhesive strength of Ulva spores, underscoring the mechanistic role of SFE in the early stages of fouling [24,25]. Despite these insights, a persistent gap remains: very few field-based investigations have attempted to correlate SFE measurements with the long-term progression of fouling communities on offshore wind substructures.

The present study addresses this gap by presenting a 27-month field monitoring dataset from an offshore wind farm located in South Korea. Polyvinyl chloride (PVC) plates were coated with two representative systems—a conventional non-antifouling paint and SPC-based antifouling coatings—and mounted on the substructure of a wind turbine for systematic observation. Over the monitoring period, panels were imaged periodically to quantify biofouling coverage, fouling communities were identified taxonomically, and surface properties were characterized using contact angle measurements and SFE analysis. By explicitly linking long-term fouling development with surface properties under real offshore conditions in South Korean waters, this study provides new evidence on coating durability, successional dynamics, and the mechanistic drivers of fouling. These findings are intended to inform antifouling strategies, improve maintenance planning, and contribute to the sustainable operation of offshore wind infrastructure in the region.

2. Materials and Methods

2.1. Study Site

The field investigation was conducted over a 27-month period (2022–2024) at an offshore wind farm located off Jeju Island, South Korea (33°21′ N, 126°10′ E) (Figure 1). The site is located within the Yellow Sea off Jeju Island, a region that exhibits pronounced seasonal variation in seawater properties.

To establish the environmental context of the study, oceanographic data for 2022–2024 published by the Annual Report on Marine Environment Monitoring in Korea (Ministry of Oceans and Fisheries) [26,27,28] were used. These surveys were conducted seasonally (February, May, August, and November) at surface and bottom layers, and included temperature, salinity, dissolved oxygen (DO), nutrients (total nitrogen, total phosphorus), chlorophyll-a, and transparency. The three-year mean values of these parameters are summarized in

Table 1. Three-year (2022–2024) mean oceanographic data at the research site. Data are derived from seasonal measurements (February, May, August, November) at surface and bottom layers.

Month	Depth (m)	Temperature (°C)		Salinity (‰)		Dissolved Oxygen (mg/L)		Total Nitrogen (μg/L)		Total Phosphorus (μg/L)		Chlorophyll a (μg/L)		Transparency (m)
		Sur. *	Bot. **	Sur.	Bot.	Sur.	Bot.	Sur.	Bot.	Sur.	Bot.	Sur.	Bot.
2 February	27.33	14.63	14.57	34.38	34.40	8.57	8.58	261.30	164.13	20.10	18.30	0.35	0.32	10.33
5 May	21.00	17.34	17.05	34.24	34.27	8.24	8.14	123.93	100.80	13.70	14.47	0.70	0.61	7.50
8 August	26.33	28.47	24.29	29.67	32.44	6.91	6.64	167.17	129.57	11.43	9.63	1.04	0.35	11.83
11 November	21.10	20.47	20.39	34.18	34.19	7.29	7.30	113.03	141.53	16.97	16.87	0.46	0.55	8.50

* Sur. = Surface, ** Bot. = Bottom.

as baseline environmental conditions of the research site, representing multi-year averages rather than one-off measurements.

Because biofouling is often influenced by oceanographic variability, a statistical analysis was conducted on four parameters widely recognized as relevant to fouling processes—temperature, dissolved oxygen, chlorophyll-a, and transparency. These parameters are particularly relevant to fouling dynamics, as temperature regulates metabolic activity and settlement rates of fouling organisms; dissolved oxygen determines respiration and survival; chlorophyll-a serves as a proxy for phytoplankton biomass that supports higher trophic colonizers; and transparency reflects light availability critical for algal and biofilm growth. The three-year mean values of these variables were 20.7 °C, 7.7 mg/L, 0.54 µg/L, and 9.0 m, respectively, and all parameters exhibited the largest standard deviations in August (Figure 2).

2.2. Sample Preparation and Field Exposure

Test samples were prepared to evaluate biofouling development under different coating conditions. The coatings were applied onto PVC plates (10 × 10 cm) that served as substrates. Three coating categories were selected: the first was a non-antifouling coating currently applied to offshore wind turbine substructures in South Korea, serving as a realistic baseline for natural fouling development. The second and third were SPC coatings, selected as representative antifouling systems for comparison. All coatings were applied with a uniform thickness of 100 µm (

Table 2. Specifications of coating samples used for field exposure.

Sample	Category	Functionality	Substrate	Thickness (μm)
#01	Self-polishing copolymer (SPC)—A	Antifouling (self-polishing)	PVC	100
#02	Self-polishing copolymer (SPC)—B
#03	Non-antifouling coating	Baseline (no antifouling)

). Each condition was prepared in triplicate, resulting in a total of nine samples.

SPC coatings function through a controlled polishing process during seawater exposure, gradually renewing the surface and limiting the settlement of fouling organisms. In contrast, the non-antifouling coating served as a baseline, representing conventional protective paints that provide corrosion resistance but lack antifouling functionality.

After curing, the coated samples were secured to a stainless-steel pipe frame fabricated for installation. The frame was installed at the bottom section of the boat-landing ladder of the turbine substructure, below the Approximate Lowest Low Water (ALLW) level (Figure 3). This location was selected to provide consistent exposure to natural seawater conditions and falls within the depth zone where hard fouling organisms such as barnacles and mussels commonly dominate offshore turbine structures.

Long-term monitoring of biofouling development was conducted for 27-month, during which images of the samples were acquired using underwater drones: FIFISH V6 Plus (QYSEA Technology Co., Ltd., Shenzhen, China) and diver-operated DSLR cameras: Nikon D800 (Nikon Corporation, Tokyo, Japan). The image datasets were processed with OriginPro 2018 (OriginLab Corp., Northampton, MA, USA), where the images were converted into matrix data and analyzed through profile extraction and histogram thresholding techniques. This procedure enabled a quantitative assessment of biofouling coverage over time.

2.3. Surface Properties

The surface properties of the coating samples were characterized prior to offshore installation by static contact angle measurements using two standard test liquids, deionized (DI) water and diiodomethane. Measurements were performed at room temperature on each coating type, with triplicate measurements per sample to ensure reproducibility. Contact angles were determined using a commercial contact angle goniometer, and mean values were calculated for subsequent analysis.

SFE was evaluated from the contact angle data using the Owens–Wendt–Rabel–Kaelble (OWRK) method. In this approach, the total SFE (${\gamma }_s$) is expressed as the sum of dispersive (${\gamma }_s^d$) and polar (${\gamma }_s^p$) components. The interfacial free energy between the solid surface and the test liquid was derived from Young’s equation, and the following relationship was applied:

$${\gamma }_l(1+\mathit{cos}\theta )=2(\sqrt{{\gamma }_s^d{\gamma }_l^d}+\sqrt{{\gamma }_s^p{\gamma }_l^p})$$

(1)

$${\gamma }_s={\gamma }_s^d+{\gamma }_s^p$$

(2)

where ${\gamma }_l$ denotes the liquid surface tension, $\theta$ the measured contact angle, and ${\gamma }_l^d$ and ${\gamma }_l^p$ the dispersive and polar components of the liquid surface tension, respectively. From these calculations, the surface energy of each coating type was obtained and quantitatively compared to evaluate differences in wettability among the coating systems.

3. Results

3.1. Depth-Related Biofouling Characteristics

The composition of fouling communities exhibited clear depth-related variation at the research site (Figure 4). At the shallow depth (0.5 m), assemblages were dominated by mussels and barnacles, consistent with the strong recruitment of hard foulers commonly reported in intertidal and shallow subtidal zones. At the mid depth (7.5 m), barnacles remained abundant, and co-occurred with sea anemones, resulting in assemblages that combined calcifying invertebrates with soft-bodied taxa. This coexistence suggests a transitional community structure relative to the shallow layer, reflecting the ability of different functional groups to establish under intermediate conditions. Near the seabed (18.7 m), coralline algae (Lithophyllum) dominated the fouling layer. The prevalence of encrusting algae at this depth contrasted with the invertebrate-dominated assemblages in shallower layers, indicating a depth-related shift in community composition under reduced light availability.

3.2. Temporal Development of Biofouling Coverage

The progression of biofouling coverage on the coating samples was monitored throughout the 27-month exposure period (Figure 5). Time-lapse images of the installed panels illustrated gradual accumulation of fouling organisms and clear contrasts among coating types. The non-antifouling coating (sample #03) exhibited dense colonization from the early stages, whereas both SPC coatings (sample #01 and #02) suppressed initial settlement.

Taxonomic identification was performed at all monitoring intervals using high-resolution DSLR cameras and drone images to document visible community composition. For accurate species-level identification, one replicate from each coating condition was retrieved at the 10-month mark and subjected to detailed taxonomic examination (Figure 6). The collected specimens revealed diverse communities including barnacles (Megabalanus), bryozoans (Membranipora), coralline algae (Lithophyllum, Corallina), polychaetes (Serpula), chlorophyta (Ulva), and biofilm-forming microorganisms. The simultaneous occurrence of calcifying taxa and soft-bodied organisms indicated that successional community development was already underway, with more diverse assemblages observed on the non-antifouling surfaces.

To illustrate the temporal progression of fouling under each coating condition, representative panels were selected and their monitoring images compiled (Figure 7). The non-antifouling coating (sample #03) displayed rapid colonization from the early immersion stages. In contrast, both SPC coatings delayed settlement with distinct responses. SPC-A (sample #01) initially reduced settlement but the coating was rapidly abraded within the first two months, after which fouling developed directly on the exposed PVC substrate. SPC-B (sample #02) maintained relatively low levels of fouling up to the 10-month observation, but progressively accumulated organisms after 13 months of immersion.

Based on these observations, the biofouling coverage was quantitatively analyzed to evaluate temporal changes under each coating condition (Figure 8). For the non-antifouling coating (sample #03), coverage exceeded 89% by 4 months and approached near-complete fouling by 10 months, with replicate values diverging (98% and 76%) but averaging 87%. SPC-A (sample #01) rapidly lost function after coating abrasion, while SPC-B (sample #02) maintained low fouling levels up to the 10-month observation before gradually declining in performance, reaching 88% coverage by 27-month. Although only two replicates were available per condition, their highly consistent patterns confirmed the reliability of the observed fouling progression.

3.3. Contact Angle and Surface Energy Analysis

Static contact angles measured with DI water and diiodomethane revealed distinct differences in wettability among the three coating systems (Figure 9a). SPC-B (sample #02) exhibited the highest hydrophobicity with mean contact angles of 103.23° (DI water) and 86.01° (diiodomethane). SPC-A (sample #01) showed intermediate values (77.13° and 58.36°), while the non-antifouling coating (sample #03) was comparatively more wettable (66.00° and 57.32°).

SFE values, calculated by the OWRK method from triplicate contact angle measurements, further clarified these contrasts (Figure 9b). The mean total SFE values followed the order SPC-B (16.7 mN/m) < SPC-A (36.8 mN/m) < non-antifouling (43.0 mN/m). Component analysis showed that SPC-B (sample #02) had the lowest polar contribution (${\gamma }_s^p$ = 2.1 mN/m) and a relatively small dispersive component (${\gamma }_s^d$ = 14.6 mN/m), consistent with a low-adhesion surface. SPC-A (sample #01) displayed intermediate energy values (${\gamma }_s^d$ = 29.5 mN/m, ${\gamma }_s^p$ = 7.2 mN/m), while the non-antifouling coating (sample #03) exhibited the highest total SFE, with a particularly large polar fraction (${\gamma }_s^d$ = 30.1 mN/m, ${\gamma }_s^p$ = 12.9 mN/m).

These distinctions directly affect early-stage settlement. Surfaces with higher total SFE—and especially larger polar components—tend to promote adhesion of conditioning films (adsorbed organic macromolecules) and propagules, thereby accelerating colonization. Accordingly, the non-antifouling coating (sample #03) provided the most favorable physicochemical environment for biofouling, whereas SPC-B (sample #02) represented the least favorable surface.

4. Discussion

The comparative evaluation of biofouling coverage progression and surface properties highlights the dual role of SPC properties and SFE in shaping colonization dynamics under long-term field exposure. SPC-B (sample #02), which exhibited the lowest mean SFE values, effectively suppressed early settlement and maintained low fouling levels through the first year. SPC-A (sample #01), with intermediate SFE, provided partial resistance but was less effective than SPC-B, whereas the non-antifouling coating (sample #03), characterized by the highest polar contribution, rapidly accumulated fouling communities from the onset. These patterns confirm that low SFE, particularly reduced polar components, diminishes the physicochemical affinity for conditioning films (adsorbed organic macromolecules) and propagules, thereby delaying initial attachment. Similar correlations between SFE and settlement resistance have been reported in marine coatings research, where fouling-release polymers with reduced SFE significantly inhibited microbial adhesion and macrofouler recruitment [29].

Despite these advantages, long-term immersion revealed that antifouling efficacy declined progressively. Both SPC systems exhibited increasing biofouling coverage beyond 13 months, reflecting the finite duration of self-polishing mechanisms. Previous reviews of antifouling coatings have reported that SPC systems generally sustain performance for more than a year before fouling pressure accelerates, after which coverage increases despite initial suppression [30]. This aligns well with our findings and supports the understanding that SPC technology is effective in delaying colonization but cannot indefinitely prevent it. In addition, the long-term use of SPC coatings may contribute to microplastic generation through continuous polishing and erosion. Although SPCs are widely considered among the most environmentally acceptable antifouling systems, the potential release of polymeric debris warrants further investigation in the context of offshore renewable infrastructure.

Offshore wind turbine substructures can act as artificial reefs, supporting the development of novel ecosystems that enhance biodiversity and provide habitat opportunities for diverse marine organisms. When compared qualitatively with long-term datasets from European offshore wind farms, notable regional contrasts emerge. For example, De Mesel et al. [17] and Krone et al. [19] reported successional pathways dominated by mussels and hydroids during mid- to late-stage immersion, whereas our South Korean dataset showed accelerated early colonization by barnacles and bryozoans. Although direct quantitative comparison is not possible due to methodological and environmental differences, these regional contrasts highlight the novelty of our 27-month dataset and underscore the need for geographically diverse monitoring programs.

An additional consideration is surface evolution under exposure. Although SPC-A (sample #01) initially exhibited intermediate SFE values indicative of reduced adhesion potential, the coating was depleted within ~2 months due to its accelerated self-polishing rate, which exposed the PVC substrate and altered the effective surface properties. This shift explains the subsequent fouling accumulation on SPC-A (sample #01), underscoring that long-term antifouling performance depends not only on SFE but also on coating durability in situ. The importance of coating retention and abrasion resistance has been emphasized in prior evaluations of SPC mechanisms, where physical erosion of the polymer matrix was identified as a dominant factor enabling rapid colonization once the active layer was depleted [20,31]. In particular, the performance decline was primarily attributable to accelerated self-polishing and coating layer depletion. To overcome these limitations, recent reviews highlight that hybrid coating systems and multilayer architectures are promising strategies for improving durability and sustaining antifouling efficacy in marine environments [32,33].

Collectively, these findings demonstrate that SPC coatings and low-SFE surfaces provide meaningful protection during the early and mid-stages of immersion, effectively reducing colonization pressure and extending maintenance intervals in offshore wind farms. However, during the 27-month deployment, even SPC systems showed progressive fouling accumulation, particularly once the coatings were fully polished away. Thus, the long-term mitigation of biofouling requires strategies that combine surface property optimization with improved coating retention. In practical terms, non-antifouling coatings require intensified inspection and mechanical fouling removal within the first year, whereas SPC coatings may require re-coating after 13–15 month or adjustment of initial coating thickness to extend their antifouling service life under offshore conditions. The present study provides one of the first quantitative long-term datasets from a South Korean offshore wind farm, contributing to the global understanding of biofouling dynamics in offshore wind environments and offering practical guidance for maintenance planning. In addition, future work will aim to secure more reliable datasets on biofouling by increasing the number of tested samples, thereby improving the accuracy and generalizability of the findings.

5. Conclusions

This study provides one of the first comprehensive, long-term datasets on a biofouling development in a South Korean offshore wind farm, offering valuable insight into the performance of conventional and antifouling coatings under real operational conditions. The 27-month field deployment revealed distinct differences in fouling trajectories among coating systems: the non-antifouling coating exhibited rapid colonization, while the SPC systems effectively delayed settlement during the early immersion period. SPC-B (sample #02), in particular, showed superior performance associated with lower surface free energy (SFE), whereas SPC-A (sample #01) was rapidly abraded, exposing the underlying PVC substrate and accelerating fouling accumulation thereafter. These outcomes demonstrate that antifouling efficacy is governed not only by surface properties but also by the durability and retention of the coating matrix in situ.

The integration of contact angle analysis and surface energy evaluation with long-term fouling observations provides direct evidence linking surface properties to community-level colonization processes. This highlights the critical role of SFE reduction in minimizing initial attachment. It also underscores that long-term protection requires coatings capable of maintaining durability while controlling self-polishing rates under offshore conditions. The present findings provide practical insight for maintenance scheduling and coating selection in offshore wind farms, reinforcing the need for durable and reliable antifouling strategies tailored to offshore renewable infrastructures.