Surface composition and orientation interact to affect subtidal epibiota

Abstract

Settlement panels were used to evaluate the effects of composition of the substratum (sandstone, concrete, wood) and orientation (vertical, horizontal undersides) on subtidal epibiota. It was predicted that both factors would influence the development of epibiotic assemblages, but that differences due to composition would be less marked on horizontal undersides compared to vertical panels. Differences in assemblages among sandstone, concrete and wooden panels orientated vertically were predicted to be similar to those described previously among vertical surfaces of sandstone rocky reefs and concrete and wooden urban structures (pilings and pontoons). Multivariate analyses indicated that assemblages were influenced greatly by orientation, whereas the effects of surface composition differed for the two orientations and among sites. Assemblages on wood were always significantly different from those on sandstone or concrete — patterns between the latter two surfaces depended on the orientation of the panels. The taxa that dominated these surfaces were not similar in identity nor abundance to those on urban structures of the same composition. The covers of most taxa were influenced by orientation alone or by surface composition for just one orientation. This study demonstrates the need for caution in generalizing about effects of orientation and surface composition because they may interact with each other and/or with other factors and they are certainly quite different for different taxa and among sites.

Introduction

Fouling organisms have been studied for many decades and some of the earliest research assessed the influences of surface characteristics on the settlement and growth of these sessile epibiotic species. Various surface characteristics have been found, at least in some experiments, to influence the numbers and types of organisms that settle on artificial surfaces, e.g. texture (Pomerat and Weiss, 1946, Crisp and Ryland, 1960, Harlin and Lindbergh, 1977), complexity (Hixon and Brostoff, 1985, Bourget et al., 1994, Lemire and Bourget, 1996, Walters and Wethey, 1996), size (Jackson, 1977, Keough, 1984a, Butler, 1991), composition (Caffey, 1982, Raimondi, 1988, McGuinness, 1989, Anderson and Underwood, 1994) and colour (Pomerat and Reiner, 1942, Wisely, 1959, James and Underwood, 1994) of the substratum. Artificial surfaces are continuously being added to waterways all over the world due to the rapid urbanization of coastal regions. Epibiota on pilings have been studied in detail (e.g. Karlson, 1978, Kay and Keough, 1981, Kay and Butler, 1983, Butler, 1986, Butler and Connolly, 1996, Butler and Connolly, 1999), but far less information is available about epibiota on other urban structures. Recent work indicates that assemblages on vertical surfaces of pilings, pontoons and retaining walls (made of wood, concrete and sandstone) are quite different from those on nearby natural rocky reefs (Connell and Glasby, 1999, Glasby, 1999a).

Some of these differences in assemblages are attributable to differences in shading and proximity to the seafloor (Glasby, 1999b, Glasby, 1999c). It is not known to what extent surface composition may be responsible for differences in assemblages among natural and artificial structures. Although numerous studies have compared fouling assemblages on settlement panels made of different materials, few have compared materials that are common in urbanized waterways (but see McGuinness, 1989). Not only do urban structures provide surfaces of different compositions, they may also provide surfaces of various orientations. Most notably, floating pontoons provide large shaded undersurfaces in addition to smaller vertical sides. Very different types of epibiotic assemblages have been shown to occur on surfaces of different orientations (e.g. upper vs. lower surfaces: Fuller, 1946, Withers and Thorp, 1977, Todd and Turner, 1986; vertical vs. horizontal surfaces: Harris and Irons, 1982, Wendt et al., 1989, Hurlbut, 1991a, Baynes, 1999, Connell, 1999). It is likely, therefore, that the combined effects of surface composition and orientation may greatly influence the development of epibiotic assemblages.

In order to begin to understand the reasons for differences between assemblages on urban structures and natural substrata, it will almost certainly be necessary to design multifactorial experiments which can test for interactive effects (Underwood, 1981, Harris and Irons, 1982, Glasby, 1999c). Moreover, without an understanding of interactive effects, it may often be assumed incorrectly that the effects of various factors are consistent across a range of situations. This may explain, in part, some of the conflicting results from studies of the effects of surface characteristics on epibiota (Harris and Irons, 1982). Studies that compared similar factors were rarely done in the same manner by different researchers and, amongst other differences, settlement panels were often orientated differently in experiments.

The study described here aimed to elucidate the roles of orientation and surface composition in the development of subtidal epibiotic assemblages. In a previous study, assemblages on the sides of floating pontoons were shown to be very different from those on nearby vertical surfaces of permanently submerged rocky reefs (Connell and Glasby, 1999). Quantitative comparisons between the undersides of pontoons and naturally occurring rocky overhangs were not made because of the rarity and small size of overhangs in local estuaries. Nevertheless, assemblages on the undersides of pontoons appear to be similar to those under rocky overhangs and not as different as assemblages on vertical surfaces of the two structures (personal observation). It was proposed, therefore, that factors associated only with undersides of surfaces (perhaps increased shading, different patterns of water flow, etc.) can over-ride (at least partially) or obscure any differences that occur between assemblages on vertical surfaces of natural and artificial structures. Thus, different assemblages were predicted to develop on vertical surfaces of different compositions, but not on horizontal undersides. The differences among vertical sandstone, concrete and wooden surfaces were predicted to be similar to those described previously (Connell and Glasby, 1999) among sandstone reef and wooden and concrete urban structures.