Juvenile Coral Density Research Articles

Coral community changes in response to a high sedimentation event: A case study in southern Hainan Island

A high sedimentation event caused by dredging and dumping of sediment was recorded on Xiaodonghai Reef in Yulin Bay, southern Hainan Island, China. Significantly high sedimentation and constant light shading were observed during the sediment dumping event (SD Event). Using long-term video transects, we quantified coral community changes and responses to the SD Event between 2008 and 2010. The SD Event caused severe coral mortality on Xiaodonghai Reef at a depth of 6 m, while corals at 3 m were less affected. Total live coral cover at 6 m decreased from 54.3% to 14.8%, and Diploastrea heliopora replaced Galaxea fascicularis as the dominant coral species at 6 and 9 m. The density of juvenile corals also declined after the SD Event, especially for the genera Galaxea and Platygyra. However, the density of juvenile Porites and Pocillopora spp. slightly increased. Monitoring for 11 months after the SD Event indicated no recovery of coral communities on Xiaodonghai Reef. Long-term video transect data also revealed that mean live coral cover dramatically declined, from 30.5% in 2008 to 9% in 2010, while the dominant corals in Yulin Bay shifted to more tolerant coral species, such as massive Porites spp. and D. heliopora. The rapid coral community degradation in Yulin Bay between 2008 and 2010 was probably caused by high sediment deposition resulting from intensive dredging and land-clearing activities. These results highlight the necessity for an integrated watershed management to control sediment deposition on near-shore coral reefs.

Predicting Coral Recruitment in Palau’s Complex Reef Archipelago

Reproduction and recruitment are key processes that replenish marine populations. Here we use the Palau archipelago, in the western Pacific Ocean, as a case study to examine scales of connectivity and to determine whether an oceanographic model, incorporating the complex reef architecture, is a useful predictor of coral recruitment. We tested the hypothesis that the reefs with the highest retention also had the highest densities of juvenile coral density from 80 field sites. Field comparisons showed a significant correlation between the densities of juvenile Acropora colonies and total larval recruitment derived from the model (i.e., calculated as the sum of the densities of larvae that self-seeded and recruited from the other reefs in the archipelago). Long-distance larval imports may be too infrequent to sustain coral populations, but are critical for recovery in times of extreme local stress.

Empirical relationships among resilience indicators on Micronesian reefs

A process-orientated understanding of ecosystems usually starts with an exploratory analysis of empirical relationships among potential drivers and state variables. While relationships among herbivory, algal cover, and coral recruitment, have been explored in the Caribbean, the nature of such relationships in the Pacific appears to be variable or unclear. Here, we examine potential drivers structuring the benthos and herbivorous fish assemblages of outer-shelf reefs in Micronesia (Palau, Guam and Pohnpei). Surveys were stratified by wave exposure and protection from fishing. High biomass of most herbivores was favoured by high wave exposure. High abundance of large-bodied scarids was associated with low turf abundance, high coral cover, and marine reserves. The remaining herbivores were more abundant in reefs with low coral cover, possibly because space and hence food limitation occur in high-coral-cover reefs. Rugosity had no detectable effect on herbivorous fish abundance once differences in exposure and coral cover were accounted for. At identical depths, high wave exposure was associated with greater volumes (cover × canopy height) of macroalgae and algal turfs, which most likely resulted from high primary productivity driven by flow. In exposed areas, macroalgal cover declined as the acanthurid biomass increased. The volume of algal turfs was negatively associated with coral cover and herbivore biomass. In turn, high coral cover and herbivore biomass are likely to intensify grazing. The density of juvenile corals was variable where macroalgal cover was low but was confined to lower densities where macroalgal cover was high. High coral cover and density of juvenile corals were favoured in sheltered habitats. While a weak positive relationship was found between scarid biomass and juvenile coral density, we hypothesise that high scarid densities may hinder juvenile density through increased corallivory. New hypotheses emerged that will help clarify the role of acanthurids, wave exposure, and corallivory in driving the recovery of Pacific coral communities.

High macroalgal cover and low coral recruitment undermines the potential resilience of the world's southernmost coral reef assemblages.

Coral reefs are under increasing pressure from anthropogenic and climate-induced stressors. The ability of reefs to reassemble and regenerate after disturbances (i.e., resilience) is largely dependent on the capacity of herbivores to prevent macroalgal expansion, and the replenishment of coral populations through larval recruitment. Currently there is a paucity of this information for higher latitude, subtropical reefs. To assess the potential resilience of the benthic reef assemblages of Lord Howe Island (31°32′S, 159°04′E), the worlds' southernmost coral reef, we quantified the benthic composition, densities of juvenile corals (as a proxy for coral recruitment), and herbivorous fish communities. Despite some variation among habitats and sites, benthic communities were dominated by live scleractinian corals (mean cover 37.4%) and fleshy macroalgae (20.9%). Live coral cover was higher than in most other subtropical reefs and directly comparable to lower latitude tropical reefs. Juvenile coral densities (0.8 ind.m−2), however, were 5–200 times lower than those reported for tropical reefs. Overall, macroalgal cover was negatively related to the cover of live coral and the density of juvenile corals, but displayed no relationship with herbivorous fish biomass. The biomass of herbivorous fishes was relatively low (204 kg.ha−1), and in marked contrast to tropical reefs was dominated by macroalgal browsing species (84.1%) with relatively few grazing species. Despite their extremely low biomass, grazing fishes were positively related to both the density of juvenile corals and the cover of bare substrata, suggesting that they may enhance the recruitment of corals through the provision of suitable settlement sites. Although Lord Howe Islands' reefs are currently coral-dominated, the high macroalgal cover, coupled with limited coral recruitment and low coral growth rates suggest these reefs may be extremely susceptible to future disturbances.

Conditions of coral communities in the Gulf of Thailand: a decade after the 1998 severe bleaching event

This paper documents the conditions of selected coral communities in the Gulf of Thailand with an insight into the degree of resistance and resilience to coral bleaching and localized anthropogenic disturbances. Live coral coverage at the study sites ranged from 5.2% at Koh Samui to 64.3% at Koh Lan. The most dominant coral species was Porites lutea. Only a few colonies of Acropora were found at the study sites. The most abundant macrobenthic animal was the sea urchin, Diadema setosum, with the highest population density 7.9 of individuals/m2 at Koh Khang Khao and D. setosum was not recorded in the belt transects at Koh Samui. Most coral colonies at Koh Lan showed partial mortality (77%), mainly pink spot and white band syndromes, and reflected potential impacts from coastal tourism. The highest recorded juvenile coral density was 8.3 colonies/m2 at Koh Kood while the lowest one was 1.1 colonies/m2 at Koh Khang Khao. The most abundant juvenile coral at all study sites was Porites spp. The occurrence of low live coral coverage and juvenile coral density in conjunction with high densities of the sea urchin D. setosum at Koh Khang Khao may partly explain why there are no structural reefs on the coasts of many islands in the inner part of the Gulf of Thailand. In addition, the present study documents macroalgae-dominated communities at Koh Samui and implies appropriate strategies and measures for coral reef restoration.

Spatial and temporal variability in juvenile coral densities, survivorship and recruitment in La Parguera, southwestern Puerto Rico

Juvenile coral survivorship (the proportion of juvenile colonies surviving from t1 to t2) was assessed by counting, photographing and mapping all juveniles found in 2003 within random permanent 0.25m2 quadrats at four depth intervals at each of four inshore and mid-shelf reefs and at deeper habitats (>18m) in two shelf-edge reefs (N=96/reef) in La Parguera, southwest coast of Puerto Rico. Quadrats were resurveyed and photographed in 2005 to evaluate surviving juveniles, mortality and new recruits. Total number of juveniles dropped from 718 in 31 scleractinian species in 2003 to 396 in 28 species in 2005, an average juvenile survivorship of 54.8%. Mean juvenile density decreased from 1.2 (± 0.06) colonies/0.25 m2 in 2003 to 0.7 (± 0.05) colonies/0.25 m2 in 2005. Juvenile coral composition, relative abundances, survivorship and recruitment varied significantly across depth intervals within reefs and among reefs. Some species with high relative abundances in 2003 showed high survivorship in 2005 [Siderastrea siderea (28.3% and 65% respectively), Porites astreoides (15.1% and 55.6%), and Diploria strigosa (7.5% and 45.2%)]. Other taxa had relative low abundances but high survivorship [Montastraea cavernosa (4.6% and 66.7%) and Stephanocoenia intersepta (4.6% and 48.3%)]. Survivorship was significantly higher in deeper habitats at three of the four fringing reefs. Furthermore, the semi-exposed inshore, highly sedimented reefs, showed higher juvenile survivorship than the mid-shelf and shelf-edge reefs. There was no clear relationship between survivorship and reproductive mode (brooding vs. broadcast spawning) of sexual reproduction. Overall, only 78 new recruits were found in 2005.

Spatial study of juvenile corals in the Northern region of the Mesoamerican Barrier Reef System (MBRS)

Juvenile coral abundance and community composition depend on the spatial scale studied. To investigate this, an evaluation was made of juvenile coral density with hierarchical spatial analysis in the northern Mesoamerican Barrier Reef System (MBRS) at ~10 m depth. Study scope included semi-protected and unprotected areas located in this region. A total of 19 juvenile coral taxa were found, including 10 scleractinian species, 8 scleractinian coral genera not identified to species, and 1 Millepora species (Hydrozoa-Milleporidae). In terms of relative abundance, Agaricia spp., Siderastrea spp., and Porites spp. were the main juvenile taxa in the coral community at the surveyed sites, reefs, and regions levels. Greater variance was seen at smaller scales, at site level for taxa richness, and at the transect level for juvenile density, and lower variance was seen at larger scales (reefs and regions). The variance component contribution from each scale likely differed from other studies because of the different factors affecting the community and the different extensions of each scale used in each study. Densities (1–6.4 juvenile corals/m2) and dominant taxa found in this study agree with other studies from the Western Atlantic. Detected variability was explained by different causal agents, such as low grazing rates by herbivorous organisms, turbidity, and/or sediment suffocation and some nearby or distant localized disturbance (human settlement and a hurricane).

Juvenile coral population dynamics track rising seawater temperature on a Caribbean reef

There is now widespread consensus that climate change-related increases in seawater temperature have become a leading cause of coral mortality by bleaching. However, despite widely reported and geographically extensive coral-bleaching episodes, little is known about the less con- spicuous demographic consequences of elevated temperature for coral populations. Here, I use 12 yr of daily temperature records and 8 yr of annual surveys of juvenile corals (≤40 mm diameter, a size range that includes recruits) from the US Virgin Islands to demonstrate the subtle effects of an unusu- ally rapid increase in seawater temperature (0.06°C per year since 1989) on coral community struc- ture in shallow water (5 to 9 m depth). The analysis reveals that the density of juvenile corals is cor- related positively with mean seawater temperature, but concurrently, in warm years, juvenile corals tend to grow slowly and appear to die faster, in a pattern leading to changes in relative generic abun- dance. Although perturbations such as these appear minor against the backdrop of widespread reef degradation, summation of these subtle effects could result in major changes, with far-reaching eco- logical consequences for the survival of coral reefs.

Recovery of Diadema antillarum reduces macroalgal cover and increases abundance of juvenile corals on a Caribbean reef.

The transition of many Caribbean reefs from coral to macroalgal dominance has been a prominent issue in coral reef ecology for more than 20 years. Alternative stable state theory predicts that these changes are reversible but, to date, there is little indication of this having occurred. Here we present evidence of the initiation of such a reversal in Jamaica, where shallow reefs at five sites along 8 km of coastline now are characterized by a sea urchin-grazed zone with a mean width of 60 m. In comparison to the seaward algal zone, macroalgae are rare in the urchin zone, where the density of Diadema antillarum is 10 times higher and the density of juvenile corals is up to 11 times higher. These densities are close to those recorded in the late 1970s and early 1980s and are in striking contrast to the decade-long recruitment failure for both Diadema and scleractinians. If these trends continue and expand spatially, reefs throughout the Caribbean may again become dominated by corals and algal turf.

Patterns in the distribution of juvenile corals and coral reef community structure in St. John, US Virgin Islands

There is increasing awareness that coral reefs exhibit a high degree of spatio-temporal variability, yet the processes that create these patterns are poorly known. In this study, benthic community structure and scleractinian recruitment on shallow reefs (5 m depth) were quantified at 18 sites along 10 km of the coast of St. John, US Virgin Islands. The goal was to test 2 mechanisms that could create spatio-temporal variation-coral recruitment and early life-history events-by addressing 3 questions: (1) Is the percent cover of scleractinians correlated with the density of juvenile corals? (2) How is the density of juvenile corals affected by coral reef community structure? (3) What are the rates of mortality and growth of juvenile corals and how do they map onto the patterns of variation in juvenile density? Community structure was assessed as the percentage cover of the 4 major substratum components, and coral recruitment was estimated from the abundance of juvenile scleractinians. Temporal variation in the population of juvenile corals was examined over 4 yr (1994 to 1997) at 5 sites. The same 5 sites were used to assess the growth and survivorship of juvenile corals over 1 yr. Overall, the results demonstrate that there is a high degree of spatio-temporal variability in the shallow reefs of St. John. The community structure varied significantly among sites, and the density and taxonomic composition of juvenile corals varied significantly among sites and years. However, the density of juvenile corals was not correlated with the percentage cover of scleractinians, and the overall community structure did not explain a significant portion of the variation in the density of juvenile corals. Similar results were obtained when the juvenile corals were separated by reproductive mode (brooders vs spawners). The variation in density of juvenile corals was unlikely to have resulted from differential growth, as growth rates did not vary among sites. Additionally, mortality was not correlated with density of juvenile corals, although it did vary among sites. Together, these results suggest that the community structure of coral reefs is related only loosely to the contemporaneous distribution of juvenile corals and early life-history events (i.e., survivorship and growth of juvenile corals). Studies on larger spatial (> 10 km) and temporal (> 4 yr) scales probably are necessary to quantify the relationships between coral recruitment and coral community structure.