The Deeper We Go the Less We Know Revista De Biología Tropical, Vol
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Revista de Biología Tropical ISSN: 0034-7744 [email protected] Universidad de Costa Rica Costa Rica Menza, C.; Kendall, M.; Hile, S. The deeper we go the less we know Revista de Biología Tropical, vol. 56, núm. 1, mayo, 2008, pp. 11-24 Universidad de Costa Rica San Pedro de Montes de Oca, Costa Rica Available in: http://www.redalyc.org/articulo.oa?id=44920275002 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative The deeper we go the less we know C. Menza, M. Kendall & S. Hile National Oceanic and Atmospheric Administration, Center for Coastal Monitoring and Assessment, Biogeography Branch, 1305 East-West Highway Silver Spring MD 20910; [email protected] Received 15-VII-2007. Corrected 10-XI-2007. Accepted 13-II-2008. Abstract: The relatively recent and well-documented decline of coral cover among reefs around the world has stimulated numerous publications on coral reef ecology and in particular coral reef mortality. These studies have predominantly collected data from nearshore, shallow coral reefs. This concentration on shallow sites provides an incomplete and biased view of coral reefs because many reefs lie entirely or partially below this depth. In this report we quantified the bias of coral reef literature towards shallow reefs using a scientific literature survey. In addition, a case study based on data collected from a range of depths and distances from shore in the US Virgin Islands demonstrates that the deepest reefs farthest from shore have the highest live coral cover. Taken together, these data suggest that reefs with the most live coral cover are also the least studied. Rev. Biol. Trop. 56 (Suppl. 1): 11-24. Epub 2008 May 30. Key words: Coral Reefs, Deep, Bias, Remotely Operated Vehicles, US Virgin Islands. Broad-scale studies in many parts of the the impression that deep reefs could serve as world have shown dramatic declines of live potential refugia for shallow reef organisms coral cover the past three decades (Hughes during times of stress (Glynn 1996, Riegl and 1994, Rogers and Beets 2001, Gardner et Piller 2003, Bak et al. 2005). al. 2003). Implicated stressors include coast- Despite this speculation about the rela- al development, coral disease, coral bleach- tive condition of deep versus shallow reefs ing, hurricanes, and harmful fishing practices (Glynn 1996, Riegl and Piller 2003, Bak et (e.g., Rogers 1991, Rogers and Beets 2001, al. 2005), deep reefs have received relatively Bellwood et al. 2004, Chiappone et al. 2002), little scientific scrutiny. Most of the studies many of which are expected to be exacerbated documenting the decline of reefs are limited by population growth and global warming in to conspicuous and easily-accessible shallow the near future (Hoegh-Guldberg 1999). Some (< 30 m) reef ecosystems. In fact, much of studies have shown that deeper reefs may our understanding of coral reef ecology in be sheltered from many of the stressors that general is based on these relatively shallow shallow reef systems must endure (Bak and depths. Far less is known of mesophotic (30 Niewland 1995, Bak et al. 2005). For example, m – 100 m) and deep (>100 m) reefs due to deeper reefs may be insulated from hot, shal- the limits of basic scuba training and the costs low water masses that cause bleaching, and are associated with remote sensing technologies. generally farther offshore than shallow reefs While the proportion of studies in deep versus thus insulating them from many anthropogen- shallow systems is probably heavily skewed ic, land-based, stressors (Glynn 1996). These toward shallow areas, the extent of this pres- observations and conclusions have resulted in ent bias has not been quantified. Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 56 (Suppl. 1): 11-24, May 2008 11 The objectives of this report are to high- specific depths. Depth of study reefs were plot- light the knowledge gap for mesophotic and ted for each of the 50 articles in both literature deep reefs relative to shallow ones, and present searches. The total number studies were then new data characterizing reefs in the U.S. Virgin tallied within 5 m depth classes in histograms Islands from a range of depths and distances for both literature searches. from land. Caribbean Case Study: Estimates of live coral cover were determined for coral reefs MATERIALS AND METHODS on the insular shelf south of the islands of St. Literature Survey: We conducted a sur- Thomas and St. John, US Virgin Islands (see vey of recent peer-reviewed scientific articles Figure 1). This area is characterized by a net- to investigate the depths at which coral reef work of reefs which range in depths from 10 studies have been made. The survey used m to 70 m. keywords and subject category designations to A Spectrum Phantom S2 remotely oper- obtain a representative population of articles ated vehicle (ROV) deployed off the NOAA on coral ecology from the Science Citation ship Nancy Foster in February 2005 was used Index Expanded (ISI Web of Science, Thomson to obtain reef data. Forty-nine transects were Scientific). All articles in the citation index completed between St. John, St.Thomas, and (1984-2005) which included the topic keywords the insular shelf edge. Transects were placed “coral OR corals”, were in marine and freshwa- to obtain representative samples of the region’s ter biology, environmental science, ecology, or benthic habitats and include as many transitions zoology subject categories and did not include among habitats as possible. Distinct habitat the topic keyword “coral reef fish*” were used features and transitional areas were determined to generate the population of relevant articles. from spatial patterns in fine-scale multibeam An asterisk is used as a catchall term so that data acquired during the same mission. any letter or group letters can be included in the A high-resolution camera and strobe position of the asterisk. attached to the ROV acquired images of the A second more refined survey added the seafloor along transects. Still images were keywords “health OR mortal*” to the search acquired systematically every 30 seconds along criteria defined above. This was done in order the distance of each transect. The speed of to obtain a population of articles on coral ecol- the ROV was kept between 0.5 and 1 m s-1 ogy targeting coral health and mortality due to which resulted in images spaced approximately the heightened interest and importance of this every 16 m (± 1.7). Each image was estimated topic in recent literature. to cover 1 square meter area ± 50 cm. An Fifty articles were randomly chosen from ultra-short baseline system and differential each population of articles to obtain a represen- geographic positioning system were used to tative sample. These articles are presented in determine the geographic location of each Appendix A. If a selected article did not implic- image within approximately 5 m. A pressure itly gather data on coral reefs at a known depth, sensor mounted on the ROV was used to deter- the article was eliminated from the sample and mine depth. a new article was randomly chosen in its place. For each image, the relative area of live Individual studies consisted of one or several coral was estimated visually with the aid of coral reef sites listed at either discrete depths or a 10 X 10 grid superimposed on each image. through a range of depths. Depth of each study Images were used to estimate live coral cover was recorded as the depth of observations or on 13 areas of hermatypic reef (Figure 1). These measurements for in situ studies, coral collec- areas represented isolated reefs, surrounded by tion sites for lab studies, or transplant sites. All sand or rhodoliths, or portions of a single reef these contribute to knowledge of coral reefs at (e.g. mid-shelf ridge complex) separated by 12 Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 56 (Suppl. 1): 11-24, May 2008 St. Thomas St. John N 0 1 2 4 km Grammanik Bank Legend t%BUB$PMMFDUJPO4JUFT Study Area Bathmetry (m) High: -8.435 Low: -317.01 Fig. 1. Study area. The locations of investigated reef areas (A-M) are denoted by boxes. Data collection sites represent location of still images. more than 500 m. Estimates of live coral cover, shallower than 30 m (Figure 2). In both distance from shore and depth were calculated searches, an exponential decline in studies was by averaging data among still images on each observed with increasing depth strata. Studies reef area (Table 1). representing general coral reef ecology were Estimates of live coral cover were exam- proportionally few below 15 m and extremely ined in relation to average depth and distance rare below 30 m. Similarly, studies on coral from shore. Logarithmic trends were used to health or mortality below 30 m were also model relationships, because they explained extremely rare. more variance than alternative forms (e.g. lin- ear) and coral cover reached an asymptote at Caribbean Case Study: Analysis of live higher values of depth and distance. coral cover among the 13 investigated insular shelf reef areas shows live cover was generally RESULTS higher among deeper reefs and reefs further from land (Figure 3). Univariate logarithmic Literature Search: The general coral ecol- models for depth and distance from shore ogy and coral health/mortality literature search- explained 50% and 56% respectively of the es yielded 4920 and 844 articles, respectively.