NoteColony size-frequency distribution of pocilloporid juvenile corals along a natural environmental gradient in the Red Sea
Introduction
A key mechanism by which coral reef ecosystems are capable of recovering from natural or anthropogenic disturbances is their ability to replenish their populations through the successful recruitment of new individuals (Bak and Engel, 1979, Caley et al., 1996). The establishment and survival of juvenile corals are indicative of good conditions for the development and growth of corals, and consequently highlight the importance of studying demographic processes (Dunstan and Johnson, 1998, Babcock et al., 2003). Although many factors may influence recruitment and colonization patterns of reef organisms (Burt et al., 2009, Trapon et al., 2013a, Trapon et al., 2013b, Lozano-Cortés and Zapata, 2014a), there is little information about coral reproduction and demography in the Red Sea (Berumen et al., 2013). While there are a few accounts of inferred or observed spawning patterns from the Red Sea (Shlesinger and Loya, 1985, Shlesinger et al., 1998, Hanafy et al., 2010, Bouwmeester et al., 2011a, Bouwmeester et al., 2011b, Bouwmeester et al., 2015), published literature on coral recruitment in this region is restricted to the far northern area (Gulf of Aqaba) and is focused primarily on settlement in artificial plates (Glassom et al., 2004, Abelson et al., 2005, Glassom and Chadwick-Furman, 2006).
Bak and Meesters (1998); Meesters et al. (2001) and Vermeij and Bak (2002) used colony size-frequency data and a lognormal size distribution model of corals in the Caribbean Sea to quantify characteristics of the coral populations (e.g., coefficient of variation, mode, and skewness) and related these measurements to reef environment (e.g., degradation level). These authors used skewness to compare the proportion of small vs. large colonies as a proxy for the establishment of new juvenile corals in reefs with different degrees of pollution. They found differences in these size-frequency variables among- and within-species in different reef localities and suggested that these characters could provide a tool to more generally estimate the response of coral populations to changes in the environment. These studies reported a relationship between negative values of skewness and more degraded reefs. Similar results were found by Crabbe (2009) and Lozano-Cortés and Zapata (2014b) in other Caribbean coral reefs that had been impacted by bleaching events and tourism (Hawkins et al., 1999), as well as by Bauman et al. (2013) in reefs under extreme environmental conditions (high temperature and salinity) in the Arabian Gulf.
The Red Sea is characterized by a latitudinal gradient in physicochemical variables such as salinity (42–37 psu), temperature (20–32 °C), turbidity and primary productivity (low–high), creating distinct habitats in the north-central region from the southern region (Raitsos et al., 2011, Raitsos et al., 2013, Nanninga et al., 2014). The separation between these zones occurs around 20°N where there is a notable shift in both reef development patterns and oceanographic conditions (i.e., such as a shift from clear and oligotrophic waters in the north-central region to more turbid, warmer, nutrient-rich waters with less reef development in the south [Roberts et al., 1992]). This north–south latitudinal gradient is also strongly seasonal. For example, the water temperature changes from 26 to 20 °C (summer–winter) in the north and from 32 to 28 °C in the south. Similarly, Raitsos et al. (2013) documented a seasonal pattern for Chl-a with the highest concentrations during the winter and lowest during the summer. These authors attributed this seasonality to processes of vertical mixing (north) and high-nutrient water intrusion (south) during winter months, and the occurrence of strong water stratification in summer time. This marked environmental shift has also been associated with changes in biological factors of reef organisms such as abundance, community composition, and richness, as well as restrictions in gene flow between regions (Roberts et al., 1992, Froukh and Kochzius, 2007, Nanninga et al., 2014, Sawall et al., 2014a, Giles et al., 2015 but see Robitzch et al., 2015). More recently, differences in zooplankton diversity and zooxanthellae density in corals have been documented along the aforementioned gradient (Pearman et al., 2014, Sawall et al., 2014b). Finally, directly inverse relationships have been reported elsewhere between juvenile coral density and fish density or algal cover (Box and Mumby, 2007, Mumby et al., 2007, Trapon et al., 2013a). Although herbivory and grazing are generally considered to be beneficial for reef health (Mumby et al., 2006, Mumby, 2009), grazing fishes may be a major source of juvenile coral mortality via incidental removal or predation while feeding (Penin et al., 2010, Doropoulos et al., 2012). On the other hand, negative interactions between algal cover and coral juvenile density are expected due to competition for space. As such, high algal cover is typically associated with low coral recruitment (Hughes et al., 2007, Mumby et al., 2007, Hoey et al., 2011).
In this study, we quantified the abundance and size of pocilloporid juvenile corals in nine coral reefs in the central and southern regions of the Saudi Arabian Red Sea. The studied reefs experience different oceanographic regimes and levels of disturbances, including biological variables such as density of grazing fishes and algal cover. By assessing the size-frequency distributions of juvenile populations, we aimed to explore the relationship, if any, between these demographic variables and reef conditions. As negatively skewed populations have been previously associated with less favorable environmental conditions, we expected to find more negative values in the southern Red Sea in comparison with the central region where coral populations are potentially under suboptimal conditions for development (e.g., reduced somatic growth and less successful reproduction). The Red Sea's natural environmental gradient represents an excellent opportunity to study the variation of juvenile colony size-frequency and its relationship with the reef environment.
Section snippets
Study sites and species
A total of nine coral reefs (central and southern regions) were surveyed in the Saudi Arabian Red Sea spanning 670 km of coastline (Fig. 1, Table 1). In the central region, six reefs (one inshore, three mid-shelf, and two offshore) near Thuwal and Al Lith were surveyed between October 2013 and May 2014. Both sampling months in our study can be considered to be in the Red Sea summer season (May is early summer while October is late summer). Published studies on coral recruitment in the Red Sea
Juvenile corals
Juvenile corals (N = 454) ranged in size from 0.5 to 50 mm maximum diameter with an overall mean diameter of 29.43 mm (SD ± 10.24 mm; Table 1). Statistical differences were detected in the size of juvenile corals among reefs (K–W, H(8, N = 454) = 19.067, p = 0.0145), with smaller colonies in the offshore central reefs (Abu Madafi and Shib Kabir, M–W, U = 14,169.50, p = 0.000973; Table 1), but not between regions (M–W, U = 18,847.00, p = 0.146). Juvenile coral density (mean ± SD) differed significantly among reefs (W,
Discussion
We found that the pocilloporid juvenile coral size-frequency distribution, independent of the region, was more negatively skewed for reefs under unfavorable environmental conditions (i.e., recently bleached) or under ongoing biological stress (i.e., high parrotfish density and algal cover). Coral size-frequency distribution has been used as a proxy for reef conditions and past disturbances (Bak and Meesters, 1998, Meesters et al., 2001, Vermeij and Bak, 2002 Crabbe, 2009, Bauman et al., 2013,
Submission declaration and verification
The authors have declared that the work described in this paper has not been published previously and it is not under consideration for publication elsewhere. All the authors approved the final version of this paper and its submission to Marine Environmental Research.
Contributors
Conceived and designed the experiments: DLC MLB. Performed the experiments: DLC. Analyzed the data: DLC. Contributed reagents/materials/analysis tools: MLB DLC. Wrote the paper: DLC MLB.
Conflict of interest
The authors have declared that no competing interests exist.
Acknowledgments
This work was supported in part by baseline funding from King Abdullah University of Science and Technology (KAUST) to MLB, with logistical support from the KAUST Coastal and Marine Resources Core Lab and the crew of the R/V Thuwal. We thank the members of the Reef Ecology Lab at KAUST, especially Jessica Bouwmeester, Remy Gatins, Maha Khalil, Alex Kattan, May Roberts, and Marcela Herrera-Sarrias, as well as Eva Salas (University of California Santa Cruz) and Océane Salles (Université de
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