Ecological Appraisal of Geotextiles in Coastal Erosion Protection Engineering Jebarathnam Prince Prakash Jebakumar* , Ganesan Nandhagopal, Bose RajanBabu, Shunmugavel Ragumaran, Appukuttan Santhakumari Kiran, and Vijaya Ravichandran Coastal Environmental Engineering Division National Institute of Ocean Technology Pallikaranai, Chennai Tamil Nadu, India ABSTRACT Jebakumar, J.P.P.; Nandhagopal, G.; RajanBabu, B.; Ragumaran, S.; Kiran, A.S., and Ravichandran, V., 2022. Ecological appraisal of geotextiles in coastal erosion protection engineering. Journal of Coastal Research, 38(2), 345–354. Coconut Creek (Florida), ISSN 0749-0208. The emergence of geotextiles in coastal erosion protection as an alternative material to stone boulders and concrete tetrapods is evident in the recent past. The submergence of geotextiles being colonized by epibiotic organisms on their surface is called marine growth, similar to all other materials inundated in seawater. It includes organisms of the primary producers of algal populations, secondary consumers, and filter feeders of benthic-fouling communities crusting geotextiles’ surface. In-situ experimental studies of more than a year of research at a fishing harbour along Chennai coast, India, projected a typical pattern of epibiotic encrustation and its impact on tensile strength nonwoven geotextiles fabrics. Results demonstrated a patchy-mosaic design of marine growth on the geotextile comprising 41 species of sessile invertebrates, including seaweeds, showing a natural succession of biotic density and diversity. Noted distinct stages in colonization were commencing with the domination of deposit feeders, followed by filter feeders, secondary consumers, and primary producers, where later filter feeders subjugated them. The incidence of fundamental ecological succession based on food availability, nature of trophic consumers, and intra/interspecific competition lead to dynamic population shifts on geotextiles. The monolayer settlement pattern of marine growth on the surface is dominated by the benthic filter-feeding community, forming an inverted trophic pyramid structure. Significant differences in tensile strength between geotextile with and without marine growth (p ¼ 0.042) and the progressive improvement in tensile strength indicated no adverse impact on the erosion protection structure’s stability. Thus, a harmonious relationship between population density and tensile strength (r ¼ 0.8) represents a mutual relationship between encrusting biota and geotextile substrate, resulting in camouflaging with the coastal background ecosystems as an innovative ecodesign for coastal erosion protection engineering. ADDITIONAL INDEX WORDS: Coastal erosion protection, marine growth, epibiota, novel habitats, inverted trophic pyramid, ecosystem services, tensile strength, coastal erosion protection. INTRODUCTION Recent considerations of environmental approaches and the limited resources of natural rocks in specific regions led to the adaptation of geotextile sandbags, tubes, and containers to create offshore breakwaters as a coastal protection measure (Kiran et al., 2016; Lee et al., 2014). The flexible nature of the structure by geotextile containers facilitates versatile applications szsdsssssuch as surfing and artificial reefs, taking into account particular stakeholders’ interest (Heerten et al., 2000; Jackson and Corbett, 2007; Jackson et al., 2007; Jackson and Hornsey, 2002; Restall et al., 2002). In addition, natural hard substrates provide stable substrates in dynamic coastal environments, offering habitats for the biotic assemblage of diverse, sessile, and semi-sessile organ- isms (Wetzel, Wiegmann, and Koop, 2011). These organisms’ tendency also extends to artificial structures in the coastal environment because they provide a stable substrate for these organisms and rapid colonization. However, limited knowledge of biotic assemblages (marine growth) on human- made coastal structures is recently evident (Bacchiocchi and Airoldi, 2003; Jebakumar, Ravichandran, and Nandhagopal, 2014; Jebakumar et al., 2015a). Apart from these, marine growth on plastic litres (Zettler et al., 2013) and offshore windmills were also reported (Langhamer, Wilhelmsson, and Engstr¨ om, 2009). Further, marine growth colonization on artificial structures recorded exotic species’ incidence (Tyrrell and Byers, 2007) where such increases added weight, changing the geometry and roughness of these submerged structures. But primarily, the ability of colonization depends on the texture of the surface (rough and smooth surface), playing a vital role in affecting the character of epibenthic communities on underwater surfaces (Anderson and Underwood, 1994; Berntsson et al., 2000; Commito and Rusignuolo, 2000; de Nys and Steinberg, 2002; Glasby, 1999; Herbert and Hawkins, 2006; Mullineaux and Garland, 1993). Amidst this, the substantial problem of biofouling organism settle- ment on the ship hulls of merchant fleets creates a significant economic loss worldwide; however, the fouler barnacle settlement can be subdued by altering the macro- texture of settling surfaces (Berntsson et al., 2000). DOI: 10.2112/JCOASTRES-D-21-00067.1 received 26 April 2021; accepted in revision 12 August 2021; corrected proofs received 14 September 2021. *Corresponding author: prince@niot.res.in Ó Coastal Education and Research Foundation, Inc. 2022 Journal of Coastal Research 38 2 345–354 Coconut Creek, Florida March 2022