Benthic development on large-scale engineered reefs: A comparison of communities among breakwaters of different age and natural reefs
Introduction
Man-made coastal defense structures such as breakwaters, jetties, and groynes now dominate the near-shore environment in many areas, in some cases more than doubling the length of the natural coastline (Bacchiocchi and Airoldi, 2003, Airoldi et al., 2005). Such structures represent large-scale unplanned artificial reefs on which marine communities develop (Svane and Peterson, 2001, Airoldi et al., 2005), with abundance and diversity of fish, corals, and other benthic organisms often exceeding that of nearby natural reefs (Pondella et al., 2002, Burt et al., 2009a, Burt et al., 2009b). Coastal defense structures, and other artificial reefs, are usually added to soft sediment environments and provide hard substrates for the attachment of organisms that usually recruit through the settlement of planktonic larvae. In most cases, epibenthic organisms do not seem to be recruitment limited (Svane and Peterson, 2001), so the addition of hard substrates may lead to an increase in epifaunal recruitment and biomass, providing food resources that may support increased regional fish production (Bohnsack, 1989). Fish and crustacean communities may also be influenced by the increased regional habitat complexity and heterogeneity provided by breakwaters and their associated benthic assemblages (Svane and Peterson, 2001). Although not designed for ecological purposes, the relatively large size and ubiquity of engineered coastal defenses suggests that they are likely to play an increasingly important ecological role in coastal marine ecosystems as human populations continue to grow, particularly given the increasing pressure on natural reef habitats (Jaap, 2000, Sale et al., 2010).
Most studies of community development on engineered coastal structures have focused on fish (Pondella et al., 2002, Guidetti et al., 2005, Clynick, 2006). However, the benthic community often contains species considered important to marine management for their aesthetic or nuisance value (Airoldi et al., 2005, Sheehy and Vik, 2009), and is of particular ecological importance in providing food, settlement habitat, and shelter for many reef associated organisms, including commercial fisheries species (Qian, 1999, Crossman et al., 2001, Burt et al., 2009a, Burt et al., 2009b). Despite this importance, few studies have examined benthic community development on coastal defense structures, particularly in tropical regions, and these have typically focused either on the very early stages of recruitment and colonization (Osman and Whitlatch, 2004, Bulleri, 2005a, Bulleri, 2005b, Burt et al., 2009a) or on mature communities (Moschella et al., 2005, Burt et al., 2009a, Burt et al., 2009b). Although there is some evidence to suggest that benthic communities on coastal defense structures can differ from those of natural reefs or rocky shores (Connell and Glasby, 1999, Knott et al., 2004, Bulleri, 2005b, Moschella et al., 2005, Burt et al., 2009a, Burt et al., 2009b, Burt et al., 2010), there is little knowledge of how these communities develop over time, nor whether they become increasingly similar to natural reefs as they mature (Bacchiocchi and Airoldi, 2003, Airoldi et al., 2005). Studies of sessile community development on artificial structures in temperate regions have observed contrasting patterns of community development. In some cases, there were rapid increases in species richness and abundance during the first year (Woodhead and Jacobson, 1985, Relini et al., 1994), followed by declines in richness as competitively dominant fauna excluded early colonists (Carter et al., 1985, Woodhead and Jacobson, 1985), while others observed increased species richness as communities developed over time (Chapman and Clynick, 2006). In other cases there were no consistent changes in biomass, abundance, or species richness with increasing artificial reef submergence time (Wendt et al., 1989). An understanding of the types of communities to expect and the sequence of development is essential for addressing the ecological and management implications of these large-scale artificial reefs.
Engineered coastal defense structures of differing age can be viewed as ‘natural experiments’ on which to observe the development of communities. Such structures are often designed with standardized materials, relief, and complexity, and are generally deployed in comparable near-shore environments within a region. Their staggered construction results in structures immersed for different lengths of time, providing an opportunity to infer temporal patterns of community development by comparing assemblages on structures of different known ages. Such natural experiments cannot account for confounding factors to the extent that is possible with true manipulative experiments, but they do allow examination of ecological processes at scales that would not be possible with a more controlled approach (Diamond, 1986). In this study, the benthic communities on six breakwater reefs ranging from 1 to 31 years of age were compared to provide an understanding of community development on large-scale artificial structures over time. These were compared to benthic communities on natural reefs to gain an understanding of community development on these artificial structures relative to mature natural habitats.
Section snippets
Materials and methods
This study was conducted in Dubai, United Arab Emirates, in the south-eastern basin of the Persian Gulf. Six rocky-reef breakwaters ranging from 1 to 31 years of age were selected for study (Fig. 1). Two sites were sampled on each breakwater to account for variability in benthic assemblages known to occur within structures (Burt et al., 2009b, Burt et al., 2010). All breakwaters were constructed of approximately 4–5 ton quarried rocks, with the exception of the Jebel Ali Port breakwater which
Results
Differences in benthic communities were indicated by separation of sites in NMS ordination (Fig. 2), where a two-dimensional representation reduced stress significantly compared with randomized data (Real 2-D stress: 3.8, Stress in randomized data: 15.6; p < 0.05), with no significant further reduction in stress in a third dimension. The first axis represented 88% of variation and was aligned with differences in benthic communities related to the age of breakwaters. There was a relatively tight
Discussion
Breakwaters represent large and ubiquitous engineered ecosystems in coastal urban areas, and the development of associated benthic communities can have positive or negative implications for marine management. Given the growing urbanization of coastal systems worldwide, developing an understanding of the sequence of community development on these structures is essential to allow prediction of if and when management implications are likely to occur, and to understand if and when communities on
Conclusions
Relatively few studies have examined the development of benthic communities on man-made coastal defense structures, and these have typically focused on young structures in temperate environments (Osman and Whitlatch, 2004, Airoldi et al., 2005, Bulleri, 2005a, Bulleri, 2005b). This study provides important data on benthic communities associated with tropical breakwaters across a wide range of ages (1–31 years), and contrasts these with communities associated with natural reefs. This information
Acknowledgements
The authors would like to thank A. Bauman, M. Bernardo, D. Feary, and P. Usseglio for assistance in the field, as well as K. M. Jenahi for assistance with image analysis. Nakheel PJSC provided funding and logistical support for this project while Major A. Al-Suwaidi of Emirates Marine Environmental Group provided field assistance; their support is appreciated. This study was part of the Nakheel – UNU-INWEH research program undertaken to understand the ecological dynamics of ecosystems in the
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Present address: Faculty of Science, New York University, Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates.