Effects of monsoon-driven wave action on coral reefs of Guam and implications for coral recruitment

Benthic cyanobacteria can respond rapidly to favorable environmental conditions, overgrow a variety of reef organisms, and dominate benthic marine communities; however, little is known about the dynamics and consequences of such cyanobacterial blooms in coral reef ecosystems. In this study, the benthic community was quantified at the time of coral spawnings in Guam to assess the substrate that coral larvae would encounter when attempting settlement. Transects at 9, 18, and 25-m depths were surveyed at two reef sites before and after heavy wave action driven by westerly monsoon winds. Communities differed significantly between sites and depths, but major changes in benthic community structure were associated with wave action driven by monsoon winds. A shift from cyanobacteria to crustose coralline algae (CCA) accounted for 44% of this change. Coral recruitment on Guam may be limited by substrate availability if cyanobacteria cover large areas of the reef at the time of settlement, and consequently recruitment may in part depend upon wave action from annual monsoon winds and tropical storms which remove cyanobacteria, thereby exposing underlying CCA and other substrate suitable for coral settlement.     

Connectivity,regime shifts and the resilience of coral reefs

Connectivity of larvae among metapopulations in open marine systems can be a double-edged sword, allowing for the colonization and replenishment of both desirable and undesirable elements of interacting species-rich assemblages. This article studies the effect of recruitment by coral and macroalgae on the resilience of grazed reef ecosystems. In particular, we focus on how larval connectivity affects regime shifts between alternative assemblages that are dominated either by corals or by macroalgae. Using a model with bistability dynamics, we show that recruitment of coral larvae erodes the resilience of a macroalgae-dominated ecosystem when grazing is high, but has negligible effect when grazing is low. Conversely, recruitment by macroalgae erodes the resilience of a coral-dominated ecosystem when grazing is low, leading to a regime shift to macroalgae. Thus, spillover of coral recruits from highly protected areas will not restore coral cover or prevent flips to macroalgae in the surrounding seascape if grazing levels in these areas are depleted, but may be pivotal for re-building coral populations if grazing is high. Fishing restrictions and the re-introduction of herbivores should therefore be a prime conservation objective for preventing undesirable regime shifts. Connectivity by some components of coral reef assemblages (e.g., macroalgae, pathogens, crown-of-thorns starfish) may be detrimental to sustaining reefs, especially where overfishing and other drivers have eroded their resilience, making them more vulnerable to a regime shift.     

Phase shifts, herbivory, and the resilience of coral reefs to climate change

Many coral reefs worldwide have undergone phase shifts to alternate, degraded assemblages because of the combined effects of over-fishing, declining water quality, and the direct and indirect impacts of climate change. Here, we experimentally manipulated the density of large herbivorous fishes to test their influence on the resilience of coral assemblages in the aftermath of regional-scale bleaching in 1998, the largest coral mortality event recorded to date. The experiment was undertaken on the Great Barrier Reef, within a no-fishing reserve where coral abundances and diversity had been sharply reduced by bleaching. In control areas, where fishes were abundant, algal abundance remained low, whereas coral cover almost doubled (to 20%) over a 3 year period, primarily because of recruitment of species that had been locally extirpated by bleaching. In contrast, exclusion of large herbivorous fishes caused a dramatic explosion of macroalgae, which suppressed the fecundity, recruitment, and survival of corals. Consequently, management of fish stocks is a key component in preventing phase shifts and managing reef resilience. Importantly, local stewardship of fishing effort is a tractable goal for conservation of reefs, and this local action can also provide some insurance against larger-scale disturbances such as mass bleaching, which are impractical to manage directly.     

Restricted gene flow in the Caribbean staghorn coral Acropora cervicornis: implications for the recovery of endangered reefs

Coral reef conservation requires information about the distance over which healthy reefs can rescue damaged reefs through input of coral larvae. This information is desperately needed in the Caribbean where the 2 dominant shallow water corals Acropora cervicornis and Acropora palmata have suffered unprecedented declines. Here we compare the population genetic structure in the staghorn coral A. cervicornis across the greater Caribbean using DNA sequence data from 1 mitochondrial and 3 nuclear genes. Data from 160 individuals from 22 populations and 9 regions show that A. cervicornis exhibits significant population genetic structure across the greater Caribbean in both the mitochondrial (Phi(st) = 0.130) and nuclear data (Phi(st) = 0.067). The highest population structure was observed in the species' own, native mtDNA haplotypes (Phi(st) = 0.235). Introgressed alleles from A. palmata tempered higher population structure in A. cervicornis over regional scales but in some cases generated highly localized "introgression hot spots" and fine-scale genetic structure among reefs separated by as few as 2 km. These data show that larval dispersal over moderate or long distances (>500 km) is limited for this threatened species and in some cases locally limited as well. Thus, the endangered Caribbean staghorn corals require local source populations for their recovery and targeted conservation efforts over spatial scales much smaller than the hundreds to thousands of kilometers usually proposed for marine reserves.     

Coral bleaching, reef fish community phase shifts and the resilience of coral reefs

The 1998 global coral bleaching event was the largest recorded historical disturbance of coral reefs and resulted in extensive habitat loss. Annual censuses of reef fish community structure over a 12-year period spanning the bleaching event revealed a marked phase shift from a prebleach to postbleach assemblage. Surprisingly, we found that the bleaching event had no detectable effect on the abundance, diversity or species richness of a local cryptobenthic reef fish community. Furthermore, there is no evidence of regeneration even after 5–35 generations of these short-lived species. These results have significant implications for our understanding of the response of coral reef ecosystems to global warming and highlight the importance of selecting appropriate criteria for evaluating reef resilience.     

Phase shifts and the role of herbivory in the resilience of coral reefs

Cousin Island marine reserve (Seychelles) has been an effectively protected no-take marine protected area (MPA) since 1968 and was shown in 1994 to support a healthy herbivorous fish assemblage. In 1998 Cousin Island reefs suffered extensive coral mortality following a coral bleaching event, and a phase shift from coral to algal dominance ensued. By 2005 mean coral cover was <1%, structural complexity had fallen and there had been a substantial increase in macroalgal cover, up to 40% in some areas. No clear trends were apparent in the overall numerical abundance and biomass of herbivorous fishes between 1994 and 2005, although smaller individuals became relatively scarce, most likely due to the loss of reef structure. Analysis of the feeding habits of six abundant and representative herbivorous fish species around Cousin Island in 2006 demonstrated that epilithic algae were the preferred food resource of all species and that macroalgae were avoided. Given the current dominance of macroalgae and the apparent absence of macroalgal consumers, it is suggested that the increasing abundance of macroalgae is reducing the probability of the system reverting to a coral dominated state.     

The biology and ecology of coral rubble and implications for the future of coral reefs

Coral Reefs - Structural complexity provided by the living coral reef framework is the basis of the rich and dynamic biodiversity in coral reefs. In many cases today, the reduction in habitat...     

Limited functional redundancy in high diversity systems: resilience and ecosystem function on coral reefs

Abstract Biodiversity is frequently associated with functional redundancy. Indo‐Pacific coral reefs incorporate some of the most diverse ecosystems on the globe with over 3000 species of fishes recorded from the region. Despite this diversity, we document changes in ecosystem function on coral reefs at regional biogeographical scales as a result of overfishing of just one species, the giant humphead parrotfish (Bolbometopon muricatum). Each parrotfish ingests over 5 tonnes of structural reef carbonates per year, almost half being living corals. On relatively unexploited oceanic reefs, total ingestion rates per m² balance estimated rates of reef growth. However, human activity and ecosystem disruption are strongly correlated, regardless of local fish biodiversity. The results emphasize the need to consider the functional role of species when formulating management strategies and the potential weakness of the link between biodiversity and ecosystem resilience.     

三亚珊瑚礁分布海区浮游生物的群落结构

为了更好地了解珊瑚礁区生物群落应对环境变化的生态响应机制,以及浮游生物群落结构与珊瑚礁发展发育的关系,我们于2006年10月26日至11月10日对三亚珊瑚礁保护区9个有珊瑚礁分布的站点进行了浮游生物群落结构的调查.共鉴定出浮游植物种类61属130种(包括变种、变型),其中硅藻门48属101种,甲藻门10属25种,蓝藻门2属3种,金藻门1属1种.硅藻门的角毛藻属(Chaetoceros)种类最多,根管藻属(Rhizosolenia)的种类次之.调查海区浮游植物的细胞丰度范围为348-11,320个/L,平均为3,247个/L.在浮游植物群落中硅藻占绝对优势,平均丰度为3,230个/L,占总密度的99.5％.调查海区共鉴定出浮游动物76种,其中桡足类29种,水母类17种,浮游幼虫10种,毛颚类7种,被囊类6种,浮游腹足类4种,十足类、多毛类和介形类各1种.调查海区浮游动物的密度范围为43-190个/m3,平均为114个/m3.优势类群为桡足类、各类幼虫和毛颚类,平均分别占浮游动物总密度的28.5％,27.7％和13.6％.各站位浮游植物的多样性指数和均匀度平均分别为3.98和0.70,浮游动物的多样性指数和均匀度平均分别为4.37和0.87.鹿回头和大东海海域的浮游植物密度大,而生物多样性指数低.活的造礁石珊瑚种数和覆盖率高的站点的浮游生物多样性也较高.     

Influence of sewage discharge on nitrogen fixation and nitrogen flux from coral reefs in Kaneohe Bay, Hawaii.

Nitrogen fixation was investigated in Kaneohe Bay, Oahu, Hawaii, a subtropical eutrophic estuary, by using the acetylene reduction technique on algal samples. No active, planktonic, N2-fixing blue-green algae or bacteria were observed. However, Calothrix and Nostoc capable of fixing N2 were cultured from navigational buoys and dead coral heads. Nitrogen fixation associated with these structures was greater in the middle sector than in the south and north sectors of the estuary. Experiments demonstrated that the fixation was photosynthetically dependent. Examination of the data showed that there was no significant correlation between rates of nitrogen fixation and concentration of combined nitrogen compounds in the Bay water. Fixation was significantly correlated to the inorganic N/P (atomic) ratio in the south and middle sectors but not in the north sector. The nutrient data indicate there was a flux of combined nitrogen, but not phosphate, from the reef flats.     

Spatial variation in mechanical properties of coral reef substrate and implications for coral colony integrity

The physical structure of coral reefs plays a critical role as a barrier to storm waves and tsunamis and as a habitat for living reef-building and reef-associated organisms. However, the mechanical properties of reef substrate (i.e. the non-living benthos) are largely unknown, despite the fact that substrate properties may ultimately determine where organisms can persist. We used a geo-mechanical technique to measure substrate material density and strength over a reef hydrodynamic gradient. Contrary to expectation, we found a weak relationship between substrate strength and wave-induced water flow: flow rates decline sharply at the reef crest, whereas substrate properties are relatively constant over much of the reef before declining by almost an order of magnitude at the reef back. These gradients generate a novel hump-shaped pattern in resistance to mechanical disturbances for live corals, where colonies closer to the back reef are prone to dislodgement because of poorly cemented substrate. Our results help explain an intermediate zone of higher taxonomic and morphological diversity bounded by lower diversity exposed reef crest and unstable reef back zones.     

Influence of sewage discharge on nitrogen fixation and nitrogen flux from coral reefs in Kaneohe Bay, Hawaii.

Nitrogen fixation was investigated in Kaneohe Bay, Oahu, Hawaii, a subtropical eutrophic estuary, by using the acetylene reduction technique on algal samples. No active, planktonic, N2-fixing blue-green algae or bacteria were observed. However, Calothrix and Nostoc capable of fixing N2 were cultured from navigational buoys and dead coral heads. Nitrogen fixation associated with these structures was greater in the middle sector than in the south and north sectors of the estuary. Experiments demonstrated that the fixation was photosynthetically dependent. Examination of the data showed that there was no significant correlation between rates of nitrogen fixation and concentration of combined nitrogen compounds in the Bay water. Fixation was significantly correlated to the inorganic N/P (atomic) ratio in the south and middle sectors but not in the north sector. The nutrient data indicate there was a flux of combined nitrogen, but not phosphate, from the reef flats.     

Sediment addition drives declines in algal turf yield to herbivorous coral reef fishes: implications for reefs and reef fisheries

Coral Reefs - Coral reefs around the world are changing rapidly, with overfishing of herbivorous fishes and increased sediment inputs being two of the major local-scale stressors. We therefore...     

Bathymetry,water optical properties,and benthic classification of coral reefs using hyperspectral remote sensing imagery

The complexity and heterogeneity of shallow coastal waters over small spatial scales provides a challenging environment for mapping and monitoring benthic habitats using remote sensing imagery. Additionally, changes in coral reef community structure are occurring on unprecedented temporal scales that require large-scale synoptic coverage and monitoring of coral reefs. A variety of sensors and analyses have been employed for monitoring coral reefs: this study applied a spectrum-matching and look-up-table methodology to the analysis of hyperspectral imagery of a shallow coral reef in the Bahamas. In unconstrained retrievals the retrieved bathymetry was on average within 5% of that measured acoustically, and 92% of pixels had retrieved depths within 25% of the acoustic depth. Retrieved absorption coefficients had less than 20% errors observed at blue wavelengths. The reef scale benthic classification derived by analysis of the imagery was consistent with the percent cover of specific coral reef habitat classes obtained by conventional line transects over the reef, and the inversions were robust as the results were similar when the benthic classification retrieval was constrained by measurements of bathymetry or water column optical properties. These results support the use of calibrated hyperspectral imagery for the rapid determination of bathymetry, water optical properties, and the classification of important habitat classes common to coral reefs.     

Facilitation in Caribbean coral reefs: high densities of staghorn coral foster greater coral condition and reef fish composition

Recovery of the threatened staghorn coral (Acropora cervicornis) is posited to play a key role in Caribbean reef resilience. At four Caribbean locations (including one restored and three extant populations), we quantified characteristics of contemporary staghorn coral across increasing conspecific densities, and investigated a hypothesis of facilitation between staghorn coral and reef fishes. High staghorn densities in the Dry Tortugas exhibited significantly less partial mortality, higher branch growth, and supported greater fish abundances compared to lower densities within the same population. In contrast, partial mortality, branch growth, and fish community composition did not vary with staghorn density at the three other study locations where staghorn densities were lower overall. This suggests that density-dependent effects between the coral and fish community may only manifest at high staghorn densities. We then evaluated one facilitative mechanism for such density-dependence, whereby abundant fishes sheltering in dense staghorn aggregations deliver nutrients back to the coral, fueling faster coral growth, thereby creating more fish habitat. Indeed, dense staghorn aggregations within the Dry Tortugas exhibited significantly higher growth rates, tissue nitrogen, and zooxanthellae densities than sparse aggregations. Similarly, higher tissue nitrogen was induced in a macroalgae bioassay outplanted into the same dense and sparse aggregations, confirming greater bioavailability of nutrients at high staghorn densities. Our findings inform staghorn restoration efforts, suggesting that the most effective targets may be higher coral densities than previously thought. These coral-dense aggregations may reap the benefits of positive facilitation between the staghorn and fish community, favoring the growth and survivorship of this threatened species.     

Coral reefs as drivers of cladogenesis: expanding coral reefs, cryptic extinction events, and the development of biodiversity hotspots

Diversification rates within four conspicuous coral reef fish families (Labridae, Chaetodontidae, Pomacentridae and Apogonidae) were estimated using Bayesian inference. Lineage through time plots revealed a possible late Eocene/early Oligocene cryptic extinction event coinciding with the collapse of the ancestral Tethyan/Arabian hotspot. Rates of diversification analysis revealed elevated cladogenesis in all families in the Oligocene/Miocene. Throughout the Miocene, lineages with a high percentage of coral reef-associated taxa display significantly higher net diversification rates than expected. The development of a complex mosaic of reef habitats in the Indo-Australian Archipelago (IAA) during the Oligocene/Miocene appears to have been a significant driver of cladogenesis. Patterns of diversification suggest that coral reefs acted as a refuge from high extinction, as reef taxa are able to sustain diversification at high extinction rates. The IAA appears to support both cladogenesis and survival in associated lineages, laying the foundation for the recent IAA marine biodiversity hotspot.     

Regenerative functions and microbial ecology of coral reefs: Labelled bacteria in a coral reef microcosm

Representative coral reef organisms and substrata assembled in a laboratory microcosm removed radioactively labelled bacteria from water circulated over them. A similar experiment with a reef clam and its algal-encrusted base gave similar results. Biochemical fractionation of selected organisms in these experiments suggested digestion and possible assimilation of bacterial proteins. In view of previous results concerning the microbial ecology of coral reefs, it is suggested that reef infaunal metazoa are adapted to utilize internal sedimentary processes and regenerative functioning through suspension- (and deposit-)feeding mechanisms. A model ecosystem is presented to suggest the possible feedback of these mechanisms as they operate within a reef.     

The functional roles of surgeonfishes on coral reefs: past,present and future

Reviews in Fish Biology and Fisheries - Surgeonfishes have had a long evolutionary history that has been closely linked with coral reef ecosystems. Today they are a key component of reef fish...