Clipperton Atoll (eastern Pacific): oceanography,geomorphology, reef-building coral ecology and biogeography

Coral reef geomorphology and community composition were investigated in the tropical northeastern Pacific during April 1994. Three areas were surveyed in the Revillagigedo Islands (Mexico), and an intensive study was conducted on Clipperton Atoll (1,300 km SW of Acapulco), including macro-scale surface circulation, sea surface temperature (SST) climatology, geomorphology, coral community structure, zonation, and biogeography. Satellite-tracked drifter buoys from 1979–1993 demonstrated complex patterns of surface circulation with dominantly easterly flow (North Equatorial Counter Current, NECC), but also westerly currents (South Equatorial Current, SEC) that could transport propagules to Clipperton from both central and eastern Pacific regions. The northernmost latitude reached by the NECC is not influenced by El Niño-Southern Oscillation (ENSO) events, but easterly flow velocity evidently is accelerated at such times. Maximum NECC flow rates indicate that the eastern Pacific barrier can be bridged in 60 to 120 days. SST anomalies at Clipperton occur during ENSO events and were greater at Clipperton in 1987 than during 1982–1983. Shallow (15–18 m)and deep (50–58 m) terraces are present around most of Clipperton, probably representing Modern and late Pleistocene sea level stands. Although Clipperton is a well developed atoll with high coral cover, the reef-building fauna is depauperate, consisting of only 7 species of scleractinian corals belonging to the generaPocillopora, Porites, Pavona andLeptoseris, and 1 species of hydrocoral in the genusMillepora. The identities of the one Pocilpopora species and one of the twoPorites species are still unknown. Two of the remaining scleractinians (Pavona minuta, Leptoseris scabra) and the hydrocoral (Millepora exaesa), all formerly known from central and western Pacific localities, represent new eastern Pacific records. Scleractinian corals predominate (10–100% cover) over insular shelf depths of 8 to 60m, and crustose coralline algae are dominant (5–40% cover) from 0.5 to 7m. Spur and groove features, constructed of alternating frameworks ofPocillopora andPorites, and veneered with crustose coralline algae, are generally well developed around most atoll exposures. Although crustose coralline algae predominate in the breaker zone (with up to 100% cover), a prominent algal ridge is absent with only a slight buildup (ca. 10 cm) to seaward. Frequent grazing by the pufferfishArothron meleagris results in the removal of large amounts of live tissue and skeleton fromPorites lobata. Acanthaster planci is present, but rare. The grazing of large diadematid sea urchins, (2 species each ofDiadema andEchinothrix) on dead corals cause extensive erosion in some areas. Large numbers of corals on the 15–18 m terrace had recently suffered partial (P. lobata, 60–70% maximum of all colonies sampled) or total (Pocillopora sp., 80% maximum) mortality. The lengths of regenerating knobs and the rates of linear skeletal growth inP. lobata, determined by sclerochronologic analysis, indicated a period of stress during 1987. Massive skeletal growth is significantly higher at intermediate (16–17 m) than shallow (6–8 m) depths with mean extension rates of 1.5 mm yr^–1 inP. lobata and 1.4 mm yr^–1 inP. minuta at intermediate depths. Skeletal growth inP. lobata was depressed during the 1987 El Nifio event at Clipperton. The branching coralPocillopora sp. demonstrated high and similar skeletal growth rates at both shallow (25.4 mm yr^–1) and intermediate (26.5 mm yr^–1) depths. The presence of widely distributed Indo-Pacific zooxanthellate corals at Clipperton and the Revillagigedo Islands indicates that these NE Pacific Islands probably serve as a stepping stone for dispersal into the far eastern Pacific region.     

Damage and recovery of coral reefs affected by El Niño related seawater warming in the Thousand Islands,Indonesia

Extensive coral bleaching occurred during sea-water warming (as a result of the 1982/3 El Niño Southern Oscillation event) in 1983 on the shallow reefs in the Java Sea. Mean seawater temperatures rose by 2–3° C over a six month period with values greater than 33° C being recorded between 1200–1500 h. As many as 80–90% of corals died on the reef flats at the study sites, with the major casualties being branching species in the genera Acropora and Pocillopora. Five years after the event the community structure of the study sites has recovered significantly, though coral cover is still 50% of its former level. Contrasting patterns of recovery at two selected sites, in close proximity to each other, are discussed.     

History and dynamics of the crown-of-thorns starfish Acanthaster planci (L.) in the Suva area,Fiji

Annual recruitment of Acanthaster planci over a 13 year period in a known juvenile refuge substratum on Suva Reef was very patchy in time and space. Macroscale recruitment events resulted in outbreaks over many thousands of hectares over 100 km of coastline whereas localised but intensive mesoscale events resulted in concenrated aggregations over areas of several tens of hectares. A very low level of recruitment occurred in most years but three intensive, macroscale recruitment events occurred in 1977, 1984 and 1987, and two localised but intensive recruitment events occurred in 1982 and 1983. Annual recruitment of A. planci between 1975–89 had no overt relationship with rainfall events, as proposed in the terrestrial run-off hypothesis on the cause of primary out-breaks. Growth rates within monitored cohorts were highly variable, but mean growth rates of two cohorts 1984 and 1987 were relatively similar. Mortality rates of three monitored cohorts were variable. Longevity of the 1977 cohort was 7–8 years, while that of the 1984 cohort was 2–3 years. Mortality of the 1987 cohort has been low to date (i.e. 4 years of age). Disease, attributed to an undescribed sporozoan pathogen, was responsible for two mass mortalities in the 1984 cohort, in 1984 and in 1986, and eventually resulted in the extinction of this cohort. Anecdotal reports indicate that outbreaks of A. planci are increasing in frequency and severity. According to Fijian reef fishermen, the starfish were low to moderate in abundance in the Suva area from the 1920s to the 1960s. A progressive build up occurred on some reefs in 1963–67 leading to a major outbreak episode from 1967–1971. The outbreaks were in a chronic phase between 1977–1989. Our findings demonstrate that there is no invariable pattern or process involved in the population dynamics of A. planci and that a single explanation of the A. planci phenomenon is therefore unlikely.     

Archipelago-wide episodic recruitment of the file fish <Emphasis Type="Italic">Pervagor spilosoma </Emphasis> in the Hawaiian Islands as revealed in long-term records

The endemic filefish, Pervagor spilosoma, has long been known to recruit infrequently but in large numbers to shallow water habitats in Hawaii. When it does recruit, as it did in 1944, 1975 and 1982–1987, it becomes the most abundant or one of the most abundant species on Hawaiian reefs. At these times of high abundance, juvenile individuals also become abundant in the off-shore pelagic environment and in deeper benthic habitats. The results of two previously published studies and this one show that the last heavy recruitment occurred throughout the Hawaiian Island chain in the period from 1982–1988. The earliest strong recruitment in this interval occurred on the leeward, or northwest Hawaiian Islands in 1982, then on Oahu in 1983, then on the island of Hawaii, at the southeast end of the island chain in 1985. At the Kaneohe Bay, Oahu, sampling site, strong recruitment occurred in 1983–1987. Fish that had recruited in the summer of a year survived less than a year at these shallow (<15 m) sites or moved off the reefs. No other fish species among the 30 species censused in a 20 year period at Kaneohe Bay has shown the extremes of abundance displayed by this species, nor did any other species recruit so strongly in the 1983–1987 period. The latter event suggests that the pattern of recruitment in this species was not solely due to a particular pattern of ocean circulation, because if that were the case, other species would also show strong recruitment. The rarity of large recruitment events in this species, the high density of the recruits, the length of individuals at recruitment, the observation by others of juveniles and possibly young adults in the water column, and the sequential nature of the recruitment down the length of the island chain may in part be the result of metamorphosis in the pelagic environment and a pelagic juvenile stage in this species.     

Recovery of the coral Montastrea annularis in the Florida Keys after the 1987 Caribbean “bleaching event”

Many reef-building corals and other cnidarians lost photosynthetic pigments and symbiotic algae (zooxanthellae) during the coral bleaching event in the Caribbean in 1987. The Florida Reef Tract included some of the first documented cases, with widespread bleaching of the massive coral Montastrea annularis beginning in late August. Phototransects at Carysfort Reef showed discoloration of >90% of colonies of this species in March 1988 compared to 0% in July 1986; however no mortality was observed between 1986 and 1988. Samples of corals collected in February and June 1988 had zooxanthellae densities ranging from 0.1 in the most lightly colored corals, to 1.6x10⁶ cells/cm² in the darker corals. Minimum densities increased to 0.5x10⁶ cells/cm² by August 1989. Chlorophyll-a content of zooxanthellae and zooxanthellar mitotic indices were significantly higher in corals with lower densities of zooxanthellae, suggesting that zooxanthellar at low densities may be more nutrientsufficient than those in unbleached corals. Ash-free dry weight of coral tissue was positively correlated with zooxanthellae density at all sample times and was significantly lower in June 1988 compared to August 1989. Proteins and lipids per cm² were significantly higher in August 1989 than in February or June, 1988. Although recovery of zooxanthellae density and coral pigmentation to normal levels may occur in less than one year, regrowth of tissue biomass and energy stores lost during the period of low symbiont densities may take significantly longer.     

Bleaching of reef coelenterates in the San Blas Islands,Panama

Starting in June 1983, 25 species of hermatypic corals, gorgonians, hydrocorals, anemones and zoanthids in the San Blas Islands, Panama, began showing signs of a loss of colour leading in some cases to a white bleached appearance. Histologic examination of six coral species indicated that bleaching was associated with drastic reductions in the density of zooxanthellae and with the atrophy and necrosis of the animal tissue. The severity of the bleaching varied among species and many species were unaffected. The species most extensively affected were: Agaricia spp., which became completely bleached and frequently died; Montastraea annularis which bleached and continued to survive; and Millepora spp. which bleached white but quickly regained their colouration. Shallow reefs dominated by Agaricia spp. suffered the most extensive bleaching. At one site, Pico Feo, 99% of the Agaricia (32% of the living cover) was bleached. On fore reers, which were dominated by Agaricia spp. and M. annularis, the proportion of M. annularis bleached ranged from 18 to 100% and that of Agaricia spp. from 30 to 53%. Transects at Sail Rock and House Reef were surveyed in August 1983 and January 1984. At those sites, 53% of the Agaricia cover died between August and January. The remaining living cover of Agaricia and of all other species exhibited normal colouration in January. Salinity and temperature were monitored every second day at 4 m depth between May 10 and August 28, 1983 at one of the localities. Bleaching was first observed within two weeks of a 2 °C rise in temperature which occurred in late May 1983. Temperatures remained at or above 31.5 °C for the following 3 weeks and were at or above 30 °C for an additional 4 weeks. The bleaching of corals in the San Blas was most likely due to those elevanted temperatures.     

Quantitative defense theory and patterns of feeding by oak insects

Summary Patterns of herbivory over a two year period on Quercus emoryi (Fagaceae) were correlated with seasonal and yearly changes in tannin and protein content. Quantitative defense theory predicts that tannin and protein content in apparent plants should be negatively and positively correlated, respectively, with degree of herbivory. Most herbivory occurred early in the growing season, but the pattern varied between the two years. Tannin and protein content sometimes varied negatively and sometimes positively with degree of herbivory; they did not consistently covary with herbivory. Protein content was positively correlated with herbivory in 1981–1982 but not in 1982–1983. Condensed tannin content was negatively correlated with herbivory in 1981–1982 but not in 1982–1983. Hydrolysable tannin content was positively correlated with herbivory activity. Multiple regression analyses indicated these phytochemical variables explained either no significant variation in herbivory (1982–1983) or did so in a fashion opposite (1981–1982) to the predictions of the theory of quantitative defense.Feeding by oak insects was not solely a function of seasonal changes in quantitative defenses and nutrients. Obviously, population dynamics of the insects are sensitive to factors other than phytochemistry of the trees and I discuss other factors that can influence patterns of herbivory.     

Effects of ambient Lake Mohave temperatures on development,oxygen consumption,and hatching success of the razorback sucker

Synopsis Spawning of razorback suckers,Xyrauchen texanus, in Lake Mohave occurred from 10–22°C and larvae were collected at water temperatures from 10–15°C in 1982 and 1983. In the laboratory, hatching success was similar from 12–20°C, but reduced hatching success was found at 10°C while none hatched a 8°C. Development rate and oxygen consumption were positively related to incubation temperature. Direct effects of ambient Lake Mohave water temperatures on hatching success of razorback sucker embryos are considered minimal. Historical spawning temperatures for the species are hypothesized based upon successful incubation temperatures and comparison to the white sucker,Catostomus commersoni.     

Spatial Variation in Abundance,Size and Orientation of Juvenile Corals Related to the Biomass of Parrotfishes on the Great Barrier Reef,Australia

For species with complex life histories such as scleractinian corals, processes occurring early in life can greatly influence the number of individuals entering the adult population. A plethora of studies have examined settlement patterns of coral larvae, mostly on artificial substrata, and the composition of adult corals across multiple spatial and temporal scales. However, relatively few studies have examined the spatial distribution of small (≤50 mm diameter) sexually immature corals on natural reef substrata. We, therefore, quantified the variation in the abundance, composition and size of juvenile corals (≤50 mm diameter) among 27 sites, nine reefs, and three latitudes spanning over 1000 km on Australia’s Great Barrier Reef. Overall, 2801 juveniles were recorded with a mean density of 6.9 (±0.3 SE) ind.m⁻², with Acropora, Pocillopora, and Porites accounting for 84.1% of all juvenile corals surveyed. Size-class structure, orientation on the substrate and taxonomic composition of juvenile corals varied significantly among latitudinal sectors. The abundance of juvenile corals varied both within (6–13 ind.m⁻²) and among reefs (2.8–11.1 ind.m⁻²) but was fairly similar among latitudes (6.1–8.2 ind.m⁻²), despite marked latitudinal variation in larval supply and settlement rates previously found at this scale. Furthermore, the density of juvenile corals was negatively correlated with the biomass of scraping and excavating parrotfishes across all sites, revealing a potentially important role of parrotfishes in determining distribution patterns of juvenile corals on the Great Barrier Reef. While numerous studies have advocated the importance of parrotfishes for clearing space on the substrate to facilitate coral settlement, our results suggest that at high biomass they may have a detrimental effect on juvenile coral assemblages. There is, however, a clear need to directly quantify rates of mortality and growth of juvenile corals to understand the relative importance of these mechanisms in shaping juvenile, and consequently adult, coral assemblages.     

Changes in the coral reefs of San Blas,Caribbean Panama: 1983 to 1990

Between 1983–1990 large changes in abundances of corals and macroalgae occurred on shallow (1–5m) lagoonal reefs in the San Blas Islands of Panama. In 1983 these reefs were dominated by the vertical plate forms of the coral generaAgaricia andMillepora. By 1990 we observed the following major changes: (1) loss of approximately one-half of the initial live coral cover, primarily during 1983–1986, and almost completely due to a decline in the abundance of Agaricia. Corals only occupied 12–26% of the reef area by 1990. (2) Macroalgae (mostlyDictyota andHalimeda) increased from 2% cover in 1983 to 28% cover in 1990. (3) Microalgal cover increased two to ten-fold between 1983 and 1986, then declined to 50% greater than the initial values by 1990. There are at least three contributors to these changes in the benthic community: (1) a coral bleaching event in 1983; which disproportionately affectedAgaricia; (2) the mass mortality ofDiadema antillarum in 1983, which led to decreases in grazing pressure on algae; and (3) possible increases in sediment and nutrient loads due to runoff from deforested mountainsides. Temporal patterns and observations of interactions suggest that the decrease inDiadema herbivory is a major factor in this shift in coral and algal populations.     

Regional and local variability in recovery of shallow coral communities: Moorea,French Polynesia and central Great Barrier Reef

Coral communities at Moorea, French Polynesia, and on the Great Barrier Reef (GBR), Australia, were severely depleted by disturbances early in the 1980s. Corals were killed by the predatory starfish Acanthaster planci, by cyclones, and/or by depressed sea level. This study compares benthic community structure and coral population structures on three disturbed reefs (Vaipahu-Moorea; Rib and John Brewer Reefs-GBR) and one undisturbed reef (Davies Reef-GBR) in 1987–89. Moorea barrier reefs had been invaded by tall macrophytes Turbinaria ornata and Sargassum sp., whereas the damaged GBR reefs were colonised by a diverse mixture of short macrophytes, turfs and coralline algae. The disturbed areas had broadly similar patterns of living and dead standing coral, and similar progress in recolonisation, which suggests their structure may converge towards that of undisturbed Davies Reef. Corals occupying denuded areas at Vaipahu, Rib and John Brewer were small (median diameter 5 cm in each case) and sparse (means 4–8 m^-2) compared to longer established corals at Davies Reef (median diameter 9 cm; mean 18 m^-2). At Moorea, damselfish and sea urchins interacted with corals in ways not observed in the GBR reefs. Territories of the damselfish Stegastes nigricans covered much of Moorea's shallow reef top. They had significantly higher diversity and density of post-disturbance corals than areas outside of territories, suggesting that the damselfish exerts some influences on coral community dynamics. Sea urchins on Moorea (Diadema setosum Echinometra mathaei, Echinotrix calamaris) were causing widespread destruction of dead standing coral skeletons. Overall, it appears that the future direction and speed of change in the communities will be explicable more in terms of local than regional processes.     

Larval development of certain gamete-spawning scleractinian corals

Embryogenesis and larval development were documented in 19 species of hermatypic scleractinians which release gametes during the summer coral spawning season on the Great Barrier Reef. Cleavage of fertilized eggs began approximately 2 h after spawning in all species, and gave rise to blastulae after 7–10 h. Endoderm formation in Platygyra sinensis was by invagination, and this appeared to occur in all species studied. All species observed at 36 h after spawing were mobile and full mobility was reached by 48 h. Settlement of planulae placed in aquaria occurred between 4 and 7 days after fertilization. These results suggest that larval corals produced by most gamete-releasing coral species are likely to be dispersed away from the parent reef.     

Annual cycle of gametogenesis and spawning in a tropical zoanthid,Protopalythoa sp.

Gametogenesis and spawning have been studied since 1986 in a population of Protopalythoa sp. (Anthozoa, Zoanthidea) from Orpheus Island, Great Barrier Reef. Fertile zooids were gonochoric or hermaphroditic with oocytes and sperm vesicles intermingled in the same mesentery; colonies were single sex or mixed. Oocytes arose in macrocnemes below the actinopharynx and were first observed in July. Their mean diameter increased from < 50 µm to 160–240 µm by November, by which time they contained zooxanthellae. Female zooids each produced 800–2400 ova. No testicular vesicles were visible before October but spermatozoa were present in November and December. Spawning in each of three years occurred in November (4th to 6th nights after full moon) simultaneously with mass spawning of scleractinian corals. The buoyant eggs (along with sperm in the hermaphroditic zooids) were released in bundles. Some large oocytes and mature sperm were still present in December.     

Abundance and distribution of pelagic rotifers in a cold,deep oligotrophic alpine lake (Königssee)

Rotifer community structure of the oligotrophic, alpine lake KGnigssee was investigated from October 1982 to October 1983. Twenty different species were found, including several cold-stenothermal and oligotrophic species. Polyarthra vulgaris/dolichoptera, Kellicottia longispina and Keratella cochlearis were the most abundant species throughout the year, comprising 90% of the total rotifer community (2.8 × 10⁶ individuals m^–2, maximum). The remainder of the rotifer community was represented by as many as eight species which occured for a short time in summer and autumn. The rotifer community was limited to the upper 50 m (85–100%) in which most (60–80%) of the rotifers preferred the trophogenic layer (0–20 m). Species-specific depth preferences were observed. Rotifer abundance and distribution are discussed in terms of the specific environmental conditions in Königssee. The rotifer community of Konigssee is compared to that of Lake Constance in its former oligotrophic state.     

Post-stocking perturbations in a population of subarctic whitefish,Coregonus lavaretus (L.): effects on growth,condition and cestode infection

Lake Kilpisjärvi was stocked with a total number of 440 000 migratory and benthic whitefish fry during the period of 1959–1964. Thereafter, the Diphyllobothrium ditremum plerocercoid infection became a problem decreasing the feasibility and commercial exploitation of the whitefish stock. Since dense fish populations may cause stunting, all stockings in the study area were stopped in an attempt to enhance the fisheries. The aim of the present study was to explore the effect of the cessation of the stocking on the growth, condition, catches and D. ditremum infection of the whitefish population. A total of 1594 whitefish were sampled with benthic gill nets. Growth was estimated from length-at-age and mass–length relationship was estimated with a non-linear regression in 1982–1983, 1992–1993 and 1997. Parasite infection was studied in 1992–1993 and 1997. Growth and condition were poor in the beginning of the 1980s. In 1992–1993, the growth and condition of the fish improved as the numerical catch per unit effort decreased from 1982 to 1992. The mean catches of other fish species were low. There were no significant changes in whitefish growth or condition from 1992–1993 to 1997. From the 1970s to 1982–1983, during slow growth and poor condition of the fish, the median gillraker count was decreasing to 19 (min–max 14–26). In 1992–1993, the gillraker count had increased significantly to 23 (min–max 19–26). However, the improved growth and condition did not cause decrease in the D. ditremum burden. On the contrary, based on our own results and literature notes, the parasite abundance seems to increase during the observation period of 1964–1997.     

The effects of prolonged “bleaching” on the tissue biomass and reproduction of the reef coral Montastrea annularis

Colonies of Montastrea annularis from Carysfort Reef, Florida, that remained bleached seven months after the 1987 Caribbean bleaching event were studied to determine the long term effects of bleaching on coral physiology. Two types of bleached colonies were found: colonies with low numbers of zooxanthellae with normal pigment content, and a colony with high densities of lowpigment zooxanthellae. In both types, the zooxanthellae had an abnormal distribution within polyp tissues: highest densities were observed in basal endoderm and in mesenteries where zooxanthellae are not normally found. Bleached corals had 30% less tissue carbon and 44% less tissue nitrogen biomass per skeletal surface area, but the same tissue C:N ratio as other colonies that either did not bleach (normal) or that bleached and regained their zooxanthellae (recovered). Bleached corals were not able to complete gametogenesis during the reproductive season following the bleaching, while recovered corals were able to follow a normal gametogenic cycle. It appears that bleached corals were able to survive the prolonged period without nutritional contribution from their zooxanthellae by consuming their own structural materials for maintenance, but then, did not have the resources necessary for reproduction. The recovered corals, on the other hand, must have regained their zooxanthellae soon after the bleaching event since neither their tissue biomass nor their ability to reproduce were impaired.     

Diversity of Scleractinia and Octocorallia in the mesophotic zone of the Great Barrier Reef, Australia

Mesophotic coral reefs in the Indo-West Pacific, the most diverse coral reef region on earth, are among the least documented. This study provides the first detailed investigation of the diversity of Scleractinia and Octocorallia of the mesophotic Great Barrier Reef (GBR). Specimens were collected by 100-m rock dredge tows at 47–163 m depth on 23 sites in four regions (15.3°–19.7° latitude South). Twenty-nine hard coral species from 19 families were recorded, with the greatest diversity found at <60 m depth, and no specimen was found >102 m. Many of these species are also commonly observed at shallower depths, particularly in inshore areas. Twenty-seven octocoral genera were collected, 25 of which represented azooxanthellate genera. Generic richness of octocorals was highest at depths >60 m. Sixteen of the 25 azooxanthellate genera were either absent or very rare at <18 m, and only five azooxanthellate genera were common on both shallow and mesophotic reefs. Species-area models indicated that the total diversity of hard corals on the deep mesophotic reefs sampled during this study was ~84 species while octocorals were represented by ~37 genera; however, the wide 95% confidence limits indicates that more intensive sampling effort is required to improve the accuracy of these estimates. Nonetheless, these results show that the taxonomic richness, particularly of hard corals, on mesophotic reefs may be much higher than previously thought, a finding that has implications for the comprehensive and adequate protection of the full range of biodiversity of the GBR.     

Submerged reefs and terraces on the shelf edge of the Great Barrier Reef,Australia

The morphology and distribution of seabed features on the shelf edge and upper slope adjacent to the Great Barrier Reef, Australia, has been examined using shallow seismic profiling, side-scan sonar and precision echo sounding data, supplemented by submersible investigations. The data reveal a submerged barrier reef system at different locations between 15° 45 S and 21° 00 S. At two locations, an extensive offshore platform rising above the 50 m isobath and extending for over 20 km parallel to the shelf edge is backed by a relict lagoon at an average depth of 75 m. In addition, outer shelf and upper slope terraces are found at many depths; however, only some occur consistently throughout the region while most others occur only locally. Frequency distributions indicate the greatest occurrence of features at depths of 44–46, 60–66, 72–78, 80–84, 102–106 and 146–148 m. Caution should be exercised when interpreting these features with respect to specific lower sea level stands.     

Bathymetric Variation in Recruitment and Relative Importance of Pre- and Post-Settlement Processes in Coral Assemblages at Lyudao (Green Island), Taiwan

Studies on coral communities have typically been conducted in shallow waters (∼5 m). However, in the face of climate change, and as shallow coral communities become degraded, a greater understanding of deeper coral communities is needed as they become the main reef remnants, playing a central role in the future of coral reefs. To understand the dynamics of deeper coral assemblages, the recruitment and taxonomic composition of different life-stages at 5 and 15 m depths were compared at three locations in Lyudao, southeastern Taiwan in 2010. Coral recruits (<1 cm diameter, <4 months old) were examined using settlement plates. Juvenile corals (1–5 cm, several years old) were examined with quadrats, and adult corals (>5 cm, several years to decades old) were examined using transect lines. Pocilloporid and poritid corals had similar and higher numbers of recruits at 5 m compared to 15 m, whereas acroporid recruits were more abundant at 15 m. The primary cause for the former may be larval behavior, such that they position themselves in shallow waters, while that for the latter may be the dominance of brooding acroporid species (Isopora spp.) at 15 m. The taxonomic composition, especially between recruits and juveniles/adults, was more similar at 15 m than at 5 m. These results suggest a change in the relative importance of pre- and post-settlement processes in assemblage determinants with depth; coral assemblages in shallow habitats (more disturbed) are more influenced by post-settlement processes (mortality events), while those in deeper habitats (more protected) are more influenced by pre-settlement processes (larval supply).     

Chemical and Physical Environmental Conditions Underneath Mat- and Canopy-Forming Macroalgae,and Their Effects on Understorey Corals

Disturbed coral reefs are often dominated by dense mat- or canopy-forming assemblages of macroalgae. This study investigated how such dense macroalgal assemblages change the chemical and physical microenvironment for understorey corals, and how the altered environmental conditions affect the physiological performance of corals. Field measurements were conducted on macroalgal-dominated inshore reefs in the Great Barrier Reef in quadrats with macroalgal biomass ranging from 235 to 1029 g DW m⁻² dry weight. Underneath mat-forming assemblages, the mean concentration of dissolved oxygen was reduced by 26% and irradiance by 96% compared with conditions above the mat, while concentrations of dissolved organic carbon and soluble reactive phosphorous increased by 26% and 267%, respectively. The difference was significant but less pronounced under canopy-forming assemblages. Dissolved oxygen declined and dissolved inorganic carbon and alkalinity increased with increasing algal biomass underneath mat-forming but not under canopy-forming assemblages. The responses of corals to conditions similar to those found underneath algal assemblages were investigated in an aquarium experiment. Coral nubbins of the species Acropora millepora showed reduced photosynthetic yields and increased RNA/DNA ratios when exposed to conditions simulating those underneath assemblages (pre-incubating seawater with macroalgae, and shading). The magnitude of these stress responses increased with increasing proportion of pre-incubated algal water. Our study shows that mat-forming and, to a lesser extent, canopy-forming macroalgal assemblages alter the physical and chemical microenvironment sufficiently to directly and detrimentally affect the metabolism of corals, potentially impeding reef recovery from algal to coral-dominated states after disturbance. Macroalgal dominance on coral reefs therefore simultaneously represents a consequence and cause of coral reef degradation.