Coral recruitment of a subtropical coral community at Yenliao Bay, northern Taiwan
© Ho and Dai; licensee Springer. 2014
Received: 14 August 2013
Accepted: 20 December 2013
Published: 24 January 2014
The subtropical coral community at Yenliao Bay (25° 3′ N, 121° 56′ E), northern Taiwan, is distinguished from tropical reefs by low species diversity, low coral cover, and limited reef-building activities. Coral recruitment in this community was hypothesized to be lower due to features of marginal environment.
We studied the seasonal variations of coral recruitment at Yenliao Bay from May 2006 to September 2009. Two groups of ceramic plates were deployed at three reefs with one group retrieved at 3-month intervals and the other at 1-year intervals. Coral recruits in Yenliao Bay were found only in summer and early autumn (from June to October) corresponding to the reproductive season of corals in this area. The taxonomic composition of coral recruits was dominated by Pocilloporidae (52% to 90%) and Acroporidae (10% to 41%). Recruitment rates varied greatly in four consecutive years, ranging from 8.0 ± 3.1 recruits m-2 (mean ± SE) in 2008 to 116.4 ± 42.4 recruits m-2 in 2006. The survival rate of coral recruits after 1 year was approximately 12%, and acroporid recruits had a higher survival rate. Most of the coral recruits settled on top and vertical surfaces in contrast to the preference of bottom surface in tropical reefs. Furthermore, there was a negative correlation between the number of recruits and depths.
Coral recruitment at Yenliao Bay occurred in summer and early autumn. The high recruitment rates in 2006 and 2007 were comparable with those of tropical reefs, suggesting that recruitment might not be a limiting factor for the maintenance and development of local coral communities. The number of coral recruits on top and vertical surfaces was negatively correlated with depths, suggesting that light intensity is possibly the primary factor controlling settlement and survival of coral recruits in subtropical coral communities.
Coral recruitment has been recognized as the critical process in determining the robustness and resilience of coral reef systems, especially when such systems are subjected to threats or damages (Gittings et al. 1988; Hughes and Tanner 2000; Kojis and Quinn 2001). Recruitment is also a key factor for understanding the population dynamics and community structures in these ecosystems (Caley et al. 1996; Quinn and Kojis 2003; Glassom et al. 2004). Hence, it is widely recognized that investigating the patterns of coral recruitment is essential for reef conservation and management practices. However, insufficient attention has been paid to understand the recruitment processes in marginal coral communities (Harriott and Banks 2002; Hoey et al. 2011).
Coral recruitments have been intensively studied in different geographic regions during the past three decades (e.g., Birkeland 1977; Harriott 1992; Hughes et al. 1999; Soong et al. 2003; Glassom et al. 2004; Nozawa et al. 2006; Adjeroud et al. 2007; Salinas-de-León et al. 2013). These studies have pointed out that recruitment rates are highly variable on spatial and temporal scales. Recruitment rates vary with geographical positions, depth, light availability, substrate topography, biotic interactions, and other biophysical factors (Birkeland 1977; Maida et al. 1995; Babcock and Mundy 1996; Hughes et al. 1999; Glassom et al. 2004). Hughes et al. (2002) reported that recruitment rates of corals decreased with increasing latitudes in the Great Barrier Reef. Other studies, however, showed that there was no consistent pattern in recruitment rates between subtropical and tropical reefs (Harriott 1992; Harriott and Simpson 1997; Glassom et al. 2006). Several studies have showed that recruitment rates on subtropical reefs are highly variable in both spatial and temporal scales (Harriott and Banks 1995; Tioho et al. 2001; Glassom et al. 2004). The recruitment of broadcast-spawning corals were reported to decline toward higher latitudes (Harriott 1999; Hughes et al. 2002), while the opposite trend was found among brooding corals, resulting in the dominance of brooders at higher latitudes (Glassom et al. 2004, 2006; Tioho et al. 2001; Nozawa et al. 2006).
Coral communities in Yenliao Bay are typical marginal coral communities characterized by low coral coverage and limited reef-building activity (Yang and Dai 1982). The main environmental conditions restricting reef development are possibly low sea temperature, strong northeast monsoon, and high turbidity in winter. Soong et al. (2003) showed that coral recruitment rate was very low in northern Taiwan based on the surveys conducted in March to May. This low recruitment rate may be responsible for the low species diversity and may constrain the resilience of this marginal coral community. However, Soong et al. (2003) did not examine coral recruitment in other months, especially during the spawning season of corals in northern Taiwan which was reported to occur in June and July (Dai et al. 1992). Thus, more comprehensive studies of coral recruitment in this area are needed. Furthermore, this subtropical coral community may become the refuge of tropical coral species under the impacts of climate change (Tsai et al. 2005). However, this will be mainly determined by the availability of coral recruitments in subtropical coral communities. In this study, we investigated the patterns of coral recruitment at Yenliao Bay in northern Taiwan with an aim to reveal the recruitment pattern and to examine whether the recruitment rate is the major limiting factor for the maintenance and development of coral communities.
All retrieved plates were preserved in 95% alcohol for further lab examination. Each plate was checked under a dissecting microscope for coral recruits, and all coral recruits were counted and identified to family level if possible (Baird and Babcock 2000; Babcock et al. 2003). Positions of coral recruits on the plate surfaces (top, lower, or vertical) were also recorded.
Statistical analyses were conducted using SPSS 16 (SPSS Inc., Chicago, IL, USA) and StatView 5.0.1 (SAS Institute Inc., Cary, NC, USA). The recruitment data were first tested for homogeneity and normality. In case of non-normality and heterogeneity of the data, a non-parametric Kruskal-Wallis rank test was applied to examine variations in number of coral recruits among years, sites, and plate surfaces. Mann–Whitney U test with Bonferroni correction as post hoc test was then used for the multiple comparisons since it is an efficient and robust method for heterogeneous data. In addition, the relationship between total number of coral recruits on different surfaces of settlement plates and depths was examined using correlation and linear regression analyses.
Results and discussion
Coral recruitment and timing
Summary of statistical tests for comparisons of recruitment rates
For long-term surveys, the recruitment rate on plates deployed from May 2006 to August 2007 was 13.7 ± 3.8 recruits m-2. This rate was only ca. 12% in comparison with that of the short-term survey in May to August 2006. The recruits recorded in the following two long-term surveys (August 2007 to September 2008 and September 2008 to August 2009) were regarded as newly settled recruits because the time of deployment was in early autumn and all the recruits were comparable or smaller than those recorded in short-term surveys. Therefore, these two surveys were excluded for further analysis.
Coral recruits on different surfaces
Summary of statistical tests for comparisons of recruitment rates
T vs. B
B vs. V
T vs. V
Recruitment rates and depths
Correlation analyses ( Z test) between depths and number of coral recruits on different surfaces of plates
Depth vs. top surface
Depth vs. bottom surface
Depth vs. vertical surface
Depth vs. total recruits
Coral recruitment rates
During the 4-year study period, coral recruitments peaked in summer and early autumn (June to October) in each year (Figure 5). This timing of coral recruits is consistent with the reproductive season of corals in northern Taiwan which has been showed to occur in early June and July (Dai et al. 1992; Fan and Dai 1995). Coral recruitment rates in this study were about five times higher than those reported by Soong et al. (2003) in northeastern Taiwan. This difference was mainly due to the timing of plate deployment since the survey of Soong et al. (2003) was conducted in April to May, about 2 months before the reproductive season of corals in northeastern Taiwan (Dai et al. 1992).
Comparison of coral recruitment rates on settlement plates at high-latitude or marginal locations around the world
Recruitment rate (number m-2year-1)
Nozawa et al. (2006)
Yenliao Bay, northern Taiwan
Solitary Islands, Eastern Australia
Harriott and Banks (1995)
Gneering Shoals, Australia
Banks and Harriott (1996)
Cubagua Island, Venezuela
Rodrǐguez et al. (2009)
Abelson et al. (2005)
Glassom et al. (2004)
Sodwana Bay, South Africa
Glassom et al. (2006)
The recruitment rates of coral communities at Yenliao Bay in 2006 and 2007 were comparable with those reported from other high-latitude or marginal areas of the world (Table 4). The lower recruitment rates in 2008 and 2009 were similar to those recorded in high-latitude coral communities in southern Japan (Tioho et al. 2001; Nozawa et al. 2006). Furthermore, the survival rate of coral recruits in Yenliao Bay from 2006 to 2007 was also comparable with that reported in southern Japan (Nozawa et al. 2006). Based on similar recruitment and survival rates, Harriott (1999) suggested that the low recruitment rates should be sufficient to maintain the high-latitude coral communities. Moreover, the recruitment rates of corals at Yenliao Bay in 2006 and 2007 were higher than that reported in a tropical reef in Nanwan Bay (32.5 spats m-2), southern Taiwan (Kuo and Soong 2010). Therefore, it seems that the recruitment rate is not a limiting factor for the maintenance and development of coral communities in Yenliao Bay.
Pocilloporid corals were the most abundant recruits during the study period (Figure 6), and most of the recruits were possibly Stylophora pistillata, the common pocilloporid coral in Yenliao Bay (Yang and Dai 1982). This is consistent with the pattern of high-latitude coral communities where brooding corals (mainly pocilloporids) are the most abundant recruits (Tioho et al. 2001; Hughes et al. 2002; Glassom et al. 2004, 2006; Nozawa et al. 2006). It seems that brooding pocilloporids release planulae only in summer and early autumn in northern Taiwan, while they have extended reproductive season throughout the year in southern Taiwan (Dai et al. 1992). The short pre-competent period of brooding larvae may result in high contribution to local recruitment of high-latitude coral communities (Harriott 1992; Tioho et al. 2001). The recruits of acroporid corals were less than those of pocilloporids in short-term surveys, while they became the most abundant in long-term surveys. This is possibly due to the higher survivorship of acroporid corals since they tend to form larger spats, and the survivorship of larger spats is often higher than that of smaller spats (Nozawa et al. 2006). However, the branching Acropora colonies were susceptible to storm disturbances; thus, their abundance was relatively low compared with the encrusting and foliaceous colonies of Montipora species in the coral communities at Yenliao Bay (Yang and Dai 1982).
Orientation of recruits
Coral recruitment patterns may represent the results of habitat selection of pre-settlement larvae (Babcock and Mundy 1996; Kuffner 2001; Baird et al. 2003) and post-settlement factors (Sato 1985; Smith 1997). Most studies on shallow tropical reefs demonstrated that coral recruits had a greater preference for the vertical and bottom surfaces (Harriott and Fisk 1987; Fisk and Harriott 1990; Harrison and Wallace 1990). It is suggested that grazing and sedimentation are the main factors inhibiting the settlement and survival of coral larvae on the top surface (Birkeland 1977; Carleton and Sammarco 1987; Fisk and Harriott 1990). However, studies on the settlement pattern of subtropical reefs showed that more coral recruits were found on top and vertical surfaces of the settlement plates (Harriott and Banks 1995; Banks and Harriott 1996). Harriott and Banks (1995) suggested that the preference of top surface might be due to low light intensity and competition of other benthos.
Light intensity and solar radiation has been suggested to be one of the important factors responsible for the pattern of coral recruitment (Maida et al. 1994; Harriott and Banks 1995). Coral recruits preferred to settle in the position that offered optimal light condition for spat development, and the survivorship and average size of coral recruits were both higher for those settled in the position of the plates with optimal light condition (Maida et al. 1994). The light intensity in subtropical reefs is often lower than that of tropical reefs mainly due to higher water turbidity and lower solar radiation (Harriott and Banks 1995; Harriott and Simpson 1997; Glassom et al. 2006). Under such an environment, light on the lower surface is likely too low to support the photosynthesis of coral larvae which is critical for their survival and growth. Therefore, coral larvae tend to colonize the top surface, and coral recruits that colonize the top surface have a higher survival rate (Maida et al. 1994).
Competition with other benthos for settlement space and post-settlement mortality due to overgrowth of other organisms may also limit coral recruitment on the bottom surface in subtropical reefs (Birkeland 1977; Harriott and Banks 1995). In this study, the bottom surface of settlement plates was frequently occupied by macrobenthos such as bryozoans, ascidians, polychaetes, and barnacles. These cryptic organisms often have higher growth rates and may quickly occupy the bottom surface, thus prohibiting or excluding the settlement of coral recruits (Birkeland 1977).
The number of coral recruits in Yenliao Bay showed a negative correlation with depths (Table 3). Moreover, the number of coral recruits on top and vertical surfaces decreased with increasing depths (Figure 8). Since water turbidity in Yenliao Bay is frequently high due to constant input of runoffs from adjacent rivers and resuspension of sediment, light intensity in the water column reduces rapidly with increasing depths. Light availability at different depths is likely the primary factor controlling the settlement pattern of coral recruits in Yenliao Bay.
We investigated the recruitment pattern of scleractinian corals of a subtropical coral community in northern Taiwan. The recruitment season occurred from summer to early autumn, and this was consistent with the reproductive season of corals in Yenliao Bay. The recruitment rates were highly variable among years and might be influenced by the annual variation of seawater temperatures. The high recruitment rates recorded in 2006 and 2007 were comparable with those of tropical reefs and other subtropical reefs, suggesting that coral recruitment might have the potential to support the maintenance and development of this marginal coral community. Pocilloporid corals were the most abundant recruits, while acroporids had a higher survival rate. Coral recruits settled mainly on the top and vertical surfaces of settlement plates, and the number of coral recruits was negatively correlated with depths, suggesting that light intensity might be the primary factor affecting the settlement and survival of coral recruits.
We are grateful to Dr. YR Cheng, Mr. S Chang, C Chou, J Chou, P Huang, C Liu, R Shiu, and members of Coral Lab at Institute of Oceanography, National Taiwan University for their assistance with field work. We would like to thank two anonymous reviewers for their helpful suggestions. This study was supported in part by a grant from Taiwan Power Company.
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