Seasonality and dynamics in coral reef macroalgae: variation in condition and susceptibility to herbivory

Seasonal variation in coral reef macroalgal size and condition is well documented, yet seasonal variability of herbivory on macroalgae by coral reef fishes is unknown. Herbivore feeding intensity was quantified monthly on an inner-shelf reef on the Great Barrier Reef, using Sargassum bioassays. Removal rates of transplants displayed high levels of variation with significantly higher rates of removal during the summer months. Differences in Sargassum plant size and condition suggest that the variability in herbivore feeding intensity is attributed primarily to the variation in the condition of the macroalgae, especially epiphyte loads. The dramatic changes in macroalgal removal reveal a considerable decrease in herbivore activity in the winter. This highlights the clear distinction between ‘summer’ and ‘winter’ months in terms of reef processes, emphasizing the high seasonal variation in macroalgal removal rates at different time of the year.     

Modelling coral reef ecosystems with limited observational data

Ecosystem models represent potentially powerful tools for coral reef ecosystem managers. They can provide insight into ecosystem dynamics not achievable through alternative means allowing coral reef managers to assess the potential outcome of any given management decision. One of the main limitations in the applicability of ecosystem models is that they often require detailed empirical data and this can restrict their applicability to ecosystems that are either currently well studied or have the resources available to collect the required data. This study describes the development of a coral reef ecosystem model that can be calibrated to an ecosystem with limited empirical data. Based on the assumption that coral reef ecological structure is generic across all tropical coral reefs and that the magnitude of the interactions between ecological components is reef specific, the dynamics of the ecosystem can be replicated based on limited empirical data. The model successfully replicated the dynamics of three individual reef systems including an inshore and oceanic reef within the Great Barrier Reef and a Caribbean reef system. It highlighted the importance of understanding the specific dynamics of a given reef and that a positive management intervention in one system may result in a negative outcome for another. The model was also used to assess the importance of various interactions within coral reef ecosystems. It identified the interactions between hard corals and other non-algal benthic components as being an important (but currently understudied) facet of coral reef ecology. The development of this modelling approach provides access to ecosystem modelling tools for coral reef managers previously excluded due to a lack of resources or technical expertise.     

Nitrogen- versus phosphorus-limited growth and sources of nutrients for coral reef macroalgae

Recent investigations of nutrient-limited productivity in coral reef macroalgae have led to the conclusion that phosphorus, rather than nitrogen, is the primary limiting nutrient. In this study, comparison of the dissolved inorganic nitrogen:phosphorus ratio in the water column of Kaneohe Bay, Hawaii, with tissue nitrogen:phosphorus ratios in macroalgae from Kaneohe Bay suggested that nitrogen, rather than phosphorus, generally limits productivity in this system. Results of nutrient-enrichment experiments in a flow-through culture system indicated that inorganic nitrogen limited the growth rates of 8 out of 9 macroalgae species tested. In 6 of the species tested, specific growth rates of thalli cultured in unenriched seawater from the Kaneohe Bay water column were zero or negative after 12 d. These results suggest that, in order to persist in low-nutrient coral reef systems, some macroalgae require high rates of nutrient advection or access to benthic nutrient sources in addition to nutrients in the overlying water column. Nutrient concentrations in water samples collected from the microenvironments inhabited or created by macroalgae were compared to nutrient concentrations in the overlying water column. On protected reef flats, inorganic nitrogen concentrations within dense mats of Gracilaria salicornia and Kappaphycus alvarezii, and inorganic nitrogen and phosphate concentrations in sediment porewater near the rhizophytic algae Caulerpa racemosa and C. sertularioides were significantly higher than in the water column. The sediments associated with these mat-forming and rhizophytic species appear to function as localized nutrient sources, making sustained growth possible despite the oligotrophic water column. In wave-exposed habitats such as the Kaneohe Bay Barrier Reef flat, water motion is higher than at protected sites, sediment nutrient concentrations are low, and zones of high nutrient concentrations do not develop near or beneath macroalgae, including dense Sargassum echinocarpum canopies. Under these conditions, macroalgae evidently depend on rapid advection of low-nutrient water from the water column, rather than benthic nutrient sources, to sustain growth. Received: 1 December 1997 / Accepted: 9 July 1998     

Thalassia testudinum leaf dynamics in a Mexican Caribbean coral reef lagoon

Shoot density, leaf growth, initiation, biomass and primary production in Thalassia testudinum (Banks ex König) were monitored at monthly intervals from August 1990 until January 1992 at three stations in the tropical coral reef of Puerto Morelos lagoon, Mexico. Leaf growth decreased with increasing leaf length, declining rapidly once the tips of leaves had started to decay; however, the leaves continued to grow until complete senescence. Maximum potential leaf age was>90 d. Leaf growth, biomass and primary production were highest at the station in the vicinity of mangrove discharges, intermediate at the nearshore fringe of the seagrass meadow, and lowest at the back-reef station. Leaf growth, leaf initiation, biomass and primary production were minimum in the winter months and maximum in the summer. Leaf growth and primary production were significantly correlated with water temperature or/and the hours of daylight. This is the first report of temperature-or/and hours of daylight-related seasonal variability in T. testudinum production from the tropical Caribbean.     

Modelling dependence structures of extreme wind speed using bivariate distribution: a Bayesian approach

When investigating extremes of weather variables, it is seldom that a single weather station determines the damage, and extremes may be caused from the combined behaviour of several weather stations. To investigate joint dependence of extreme wind speed, a bivariate generalised extreme value distribution (BGEVD) was considered from frequentist and Bayesian approaches to analyse the extremes of component-wise monthly maximum wind speed at selected weather stations in South Africa. In the frequentist approach, the parameters of extreme value distributions (EVDs) were estimated with maximum likelihood, whereas in the Bayesian approach the Markov Chain Monte Carlo (MCMC) technique was used with the Metropolis–Hastings algorithm. The results showed that when fitted to component-wise maxima of extreme weather variables, the BGEVD provided apparent benefits over the univariate method, which allowed information to be pooled across stations and resulted in improved precision of the estimates for the parameters and return levels of the distributions. The paper also discusses a method to construct informative priors empirically using historical data of the underlying process from weather characteristics of four pairs of surrounding weather stations at various distances. The results from the Bayesian analysis showed that posterior inference might be affected by the choice of priors that were used to formulate the informative priors. From the results, it could be inferred that the Bayesian approach provides a satisfactory estimation strategy in terms of precision, compared with the frequentist approach, because it accounts for uncertainty in parameters and return level estimations.     

Nitrogen fixation on a coral reef

Acetylene reduction was used to assess nitrogen fixation on all major substrates at all major areas over a period of 1 to 6 yr (1980–1986) at One Tree Reef (southern Great Barrier Reef). Experiments using ¹⁵N₂ gave a ratio of 3.45:1.0 for C₂H₂ reduced:N₂ fixed. Acetylene reduction was largely light-dependent, saturated at 0.15 ml C₂H₂ per ml seawater, and linear over 6 h. High fixation was associated with two emergent cyanophyte associations, Calothrix crustacea and Scytonema hofmannii, of limited distribution. Subtidally, the major contribution to nitrogen fixation came from well-grazed limestone substrates with an epilithic algal community in the reef flat and patch reefs (3 to 15 nmol C₂H₄ cm^-2 h^-1). Similar substrates from the outer reef slope showed lower rates. Nitrogen fixation on beach rock, intertidal coral rubble, reef crest and lagoon sand was relatively small (0.3 to 1.0 nmol C₂H₄ cm^-2 h^-1). Seasonal changes in light-saturated rates were small, with slight reduction only in winter. Rates are also reported for experimental coral blocks (13 to 39 nmol cm^-2 h^-1) and for branching coral inside and outside territories of gardening damselfish (3 to 28 nmol cm^-2 h^-1). This work supports the hypothesis that the high nitrogen fixation on the reef flat and patch reefs of the lagoon (34 to 68 kg N ha^-1 yr^-1) is because these subtidal areas support highly disturbed communities with the greatest abundance of nitrogen-fixing cyanophyte algae. It is calculated from a budget of all areas that One Tree Reef has an annual nitrogen fixation rate of 8 to 16 kg N ha^-1 yr^-1.     

Detrital pathways in a coral reef lagoon

Coral reef lagoons are generally regarded as zones of net heterotrophy reliant on organic detritus generated in more productive parts of the reef system, such as the seaward reef flat. The abundance and biomass of sediment infauna were measured seasonally for one year (1986) within the lagoon of Davies Reef, central Great Barrier Reef, to test the hypothesis that macrofaunal biomass and production of coral reef lagoons would decrease with distance from the reef flat and would change seasonally. In general, there were no simple relationships between infaunal standing stock or production and distance from the reef flat or season. Bioturbation by callianassid shrimps negatively affected the abundance of smaller infauna, suggesting a community limited by biogenic disturbance rather than by supply of organic material. Polychaetes and crustaceans were dominant amongst the smaller infauna (0.5 to 2mm) while larger animals (> 2 mm) were mostly polychaetes and molluscs. Mean biomass of infauna at both sites and all seasons was 3 181 mg C m^?2. The smaller animals (0.5 to 2 mm) contributed about 40% of total macrofaunal respiration and production although they represented only 15% of the total macrofaunal biomass. The biomass of macrofauna was about equal to that of the bacteria and meiofauna, while respiration represented 10 to 20% of total community respiration. Consumption by macrofauna accounts for only 3 to 11% of total organic inputs to sediment, with a further 14 to 17% being lost by macrofaunal respiration.     

Detrital pathways in a coral reef lagoon

Coral reef lagoons have generally been regarded as sinks for organic matter exported from more productive reef front and reef flat zones. The object of this study was to examine the importance of detritus as a carbon source for benthic communities in the lagoon at Davies Reef, central Great Barrier Reef. We report the results of seasonal measurements, taken in 1986, of bacterial numbers and production, protozoan numbers, community primary production and respiration in the sediments of Davies Reef lagoon. Deposition rates of organic matter in the lagoon were also measured. Deposition rates (±1 SE) of carbon ranged from 9.2 (±1.5) to 140.7 (±10.3) mg Cm^-2d^-1. Deposition rates were highest in winter and spring, lowest in summer. Rates of bacterial production ranged from 4.7 (±0.2) pmol thymidine incorporated g^-1 dry wt (DW) h^-1 in winter to 23.5 (±1.0) pmol thymidine incorporated g^-1 DW h^-1 in spring. The number of ciliates ranged from 65 (±10) to 356 (±50) cm^-3 through the year and the number of large (20 m) flagellates from 38 (±7) to 108 (±16) cm^-3. There were no clear relationships between the sediment organic content, detrital input or temperature and the rates of bacterial processes, community metabolism or the standing stocks of microbes in the lagoon. The relative significance of detritus and in situ primary production as sources of carbon in the lagoon varied with season. In summer and autumn, detritus was less important than primary production as a source of carbon (4 to 27% of total carbon input). In winter and spring, detritus input became more significant in supply of carbon to the sediments (32 to 67% of the total carbon input). The lagoon does not simply act as a sink for carbon exported from the reef flat. We calculate that only 5% of the net reef flat primary production reached lagoon sediments in summer, but nearly 40% in winter.     

Life history characteristics of a coral reef sponge

Study of the life history characteristics of the common Red Sea sponge Mycale fistulifera (Poecilosclerida: Demospongiae) reveals several traits which may categorize it as an opportunistic species: (1) at least part of the population is reproductively active throughout the year, providing a continuous supply of larvae for settlement; (2) sponges may produce and release a large number (152±39 larvae d^-1) of brooded larvae; (3) released larvae are capable of fast settlement and metamorphosis (minutes to 30 hours post-release); (4) the turnover of the population is high and population size varies with time. From 48 initial sponges plus 94 recruits, only 25 remained alive after 14 months of observation; (5) most members of the population (>70%) have a small body size (<30 cm²); (6) sexual maturity may occur at an early age. These traits facilitate continuous establishment of M. fistulifera in new spaces on the reef; (7) M. fistulifera, preferred substrate is another opportunistic species, the coral Stylophora pistillata which it overgrows.     

Steps towards the evaluation of coral reef restoration by using small branch fragments

“Gardening” of denuded coral reef habitats is a novel restoration approach in which sexual and asexual recruits are used. The present study aimed at the evaluation of the potentiality for restoration use of different types of small fragments subcloned from the Red Sea coral species Stylophora pistillata. In situ short-term (24 h, ⁴⁵Ca method) and long-term (1 year, alizarin Red S vital staining) experiments revealed high variation (up to 70%) in growth rates between up-growing branches of a specific genet, and that tip ratios in dichotomous branches (n = 880) differ significantly between newly formed and older branches, further emphasizing the within-colony genetic background for spatial configuration. Small, isolated branches (<4 cm) revealed high survivorship (up to 90%, 1 year) and up to 20–30% (1 year, single- vs. dichotomous-tip branches, respectively) growth, showing that small-sized branches are suitable for restoration purposes. Results differed significantly between genets. Total length added for dichotomous-tip branches was in general at least twice that recorded for single tips of a specific genet. Restoration protocols may be applied either by sacrificing whole large colonies via pruning high numbers of small fragments or, by pruning only a few small branches from each one of many genets. An in situ “nursery period” of approximately 8 years is predicted for S. pistillata small fragments. Received: 17 August 1999 / Accepted: 15 February 2000     

Cohabitation of a coral reef sponge and a colonial scyphozoan

A sensitive experimental protocol using cloned corals (hereafter microcolonies) of the branching scleractinian coral Stylophora pistillata and ⁴⁵Ca has been developed to enable reproducible measurements of physiological and biochemical mechanisms involved in calcium transport and compartmentalization during coral calcification. Cloned S. pistillata microcolonies were propagated in the laboratory from small fragments of parent colonies collected in 1990 in the Gulf of Aqaba, Jordan. Cloned microcolonies have several intrinsic properties that help to reduce unwanted biological variability: (1) same genotype; (2) similar sizes and shapes; and (3) absence of macroscopic boring organisms. Errors specifically associated with long-standing problems to do with isotopic exchange were further reduced by producing microcolonies with no skeletal surfaces exposed to the radioisotope-labelled incubation medium. The value of the technique resides principally in its superior ability to elucidate transportation pathways and processes and not in its ability to quantitatively estimate calcium deposition by corals in nature. We describe here a rapidly exchangeable calcium pool in which up to 90% of the radioactive label taken up during incubations is located. This pool (72.9±1.4 nmol Ca mg^-1 protein) is presumably located within the coelenteric cavity as suggested by the following: (1) it has 4-min half-time saturation kinetics; (2) the accumulation of calcium is linearly correlated with the calcium concentration of sea-water; and (3) its insensitivity to metabolic and ion transport inhibitors indicate that membranes do not isolate this compartment. Washout of this large extracellular pool greatly improved estimates of calcium deposition as evidenced by 10 to 40% reduction in coefficients of variation when compared with previous ⁴⁵Ca²⁺ methods described in the literature. Comparisons of calcification measurements simultaneously carried out using the alkalinity anomaly technique and the ⁴⁵Ca protocol described here show that the correlation coefficient of both techniques is close to 1. Unlike previous reports, our ⁴⁵Ca²⁺-derived measurements are slightly lower than those computed from the alkalinity depletion technique.     

Quantifying parameter uncertainty in a coral reef model using Metropolis-Coupled Markov Chain Monte Carlo

Coral reefs are threatened ecosystems, so it is important to have predictive models of their dynamics. Most current models of coral reefs fall into two categories. The first is simple heuristic models which provide an abstract understanding of the possible behaviour of reefs in general, but do not describe real reefs. The second is complex simulations whose parameters are obtained from a range of sources such as literature estimates. We cannot estimate the parameters of these models from a single data set, and we have little idea of the uncertainty in their predictions.We have developed a compromise between these two extremes, which is complex enough to describe real reef data, but simple enough that we can estimate parameters for a specific reef from a time series. In previous work, we fitted this model to a long-term data set from Heron Island, Australia, using maximum likelihood methods. To evaluate predictions from this model, we need estimates of the uncertainty in our parameters. Here, we obtain such estimates using Bayesian Metropolis-Coupled Markov Chain Monte Carlo. We do this for versions of the model in which corals are aggregated into a single state variable (the three-state model), and in which corals are separated into four state variables (the six-state model), in order to determine the appropriate level of aggregation. We also estimate the posterior distribution of predicted trajectories in each case.In both cases, the fitted trajectories were close to the observed data, but we had doubts about the biological plausibility of some parameter estimates. We suggest that informative prior distributions incorporating expert knowledge may resolve this problem. In the six-state model, the posterior distribution of state frequencies after 40 years contained two divergent community types, one dominated by free space and soft corals, and one dominated by acroporid, pocilloporid, and massive corals. The three-state model predicts only a single community type. We conclude that the three-state model hides too much biological heterogeneity, but we need more data if we are to obtain reliable predictions from the six-state model. It is likely that there will be similarly large, but currently unevaluated, uncertainty in the predictions of other coral reef models, many of which are much more complex and harder to fit to real data.     

Diel patterns of abundance of presettlement reef fishes and pelagic larvae on a coral reef

Most presettlement reef fish settled at night at One Tree Island, Great Barrier Reef. Fish were sampled day and night using channel nets located on the reef crest, and a plankton-mesh purse-seine net in the lagoon (1992–1994). Catches of fish at night were generally tens to hundreds of times greater than those taken during the day. Preflexion fish, as well as postflexion and pelagic juveniles, were taken in greater numbers at night. Preflexion forms were a combination of those that had hatched from demersal eggs and later stages that had been transported over the reef crest. Highest numbers of postflexion and pelagic juvenile forms of Apogonidae, Blenniidae, Gobiesocidae, Gobiidae, Labridae, Lutjanidae, Mugiloididae, Mullidae, Pomacentridae, Scaridae, Serranidae and Tripterygiidae were found at night. Observations, while SCUBA diving, and purse-seine samples in the lagoon indicated that the only resident larvae were of the genera Spratelloides and Hypoatherina; most of the fishes caught in nets, therefore, were immigrants. Patch reefs, sampled for new settlers early in the morning and late in the day, indicated that the majority of apogonids (Apogon doederleini, >95%) settled at night. Although greater numbers of pomacentrids were found in morning counts (e.g. Pomacentrus wardi), if data were converted to an hourly rate, many pomacentrids showed a similar hourly rate of settlement day and night. Depth-stratified sampling in waters near One Tree Island (to 20 m) indicated that some taxa rise to the surface at night. This behaviour, perhaps combined with avoidance of diurnal predators may explain on-reef movement of potential settlers soon after dark. Studies on settlement cues, therefore, need to focus on night-related phenomena. Received: 3 March 2000 / Accepted: 20 June 2000     

ReefBahia, an integrated GIS approach for coral reef conservation in Bahia, Brazil

Coral reefs around the world are facing serious threats. These fragile ecosystems are in need for conservation. The coastal state of Bahia hosts the most extensive and richest area of coral reefs in the South Atlantic Ocean. Assessment, planning and management of coral reef ecosystems are particularly challenging tasks. This work shows how the creation of a GIS improves the process of management, monitoring and conservation of the Bahian reef environments The initial data input started by the vectorization of 1) bathymetric data from the Bureau of Hydrography and Navigation (DHN), 2) shoreline and mangrove areas from Landsat 7 ETM + images, 3) near surface reefs from Quickbird images, and 4) coastal and marine protected areas of federal, state and local administrations. Geological, physical, biological and social information was then included in order to create a suitable marine GIS for conservation aims. The data includes information on sediment granulometry and transport patterns, rocky substrate outcrops, sea surface temperature, wave direction, rain precipitation, major contributing river discharge, artisanal fishery, benthic cover and bleaching data. ReefBahia GIS has provided essential information for a better understanding of coral reefs of the state of Bahia geological and ecological characteristics such as mapping, representation, connectivity and biodiversity of coral reefs, geological facies, Quaternary sedimentation, numeric modeling of wave refraction and monitoring of bleaching events.     

Microplastics in mangroves and coral reef ecosystems: a review

Microplastic pollution has recently been identified as a major issue for the health of ecosystems. Microplastics have typically sizes of less than 5 mm and occur in various forms, such as pellets, fibres, fragments, films, and granules. Mangroves and coral reefs are sensitive and restricted ecosystems that provide free ecological services such as coastal protection, maintaining natural cycles, hotspots of biodiversity and economically valuable goods. However, urbanization and industrial activities have started contaminating even these preserved ecosystems. Here we review sources, occurrence, and toxicity of microplastics in the trophic levels of mangrove and coral reef ecosystems. We present detection methods, such as microscopic identification and spectroscopy. We discuss mitigating measures that prevent the entry of microplastics into the marine environment.

     

Energy allocation in a reef coral under varying resource availability

An organism’s pattern of resource allocation to reproduction and growth over time critically impacts on its lifetime reproductive success. During times of low resource availability, there are two fundamental, mutually exclusive strategies of energy investment: maintenance of somatic tissues to support survival and later reproduction or investment into an immediate reproductive event at the risk of subsequent death. Here, we examine energy allocation patterns in the coral Montipora digitata to determine whether energy investment during periods of resource shortage favours growth or reproduction in a sessile, modular marine species. We manipulated light regimes (two levels of shading) on plots within a shallow reef flat habitat (Orpheus Island, Great Barrier Reef, Australia) and quantified energy uptake (rates of net photosynthesis), energy investment into reproduction (E _R ), tissue growth per unit surface area (E _T ) and energy channelled into calcification (E _C ). With declining resource availability (i.e. reduced photosynthesis), relative energy investment shifted from high (~80%) allocation to tissue growth (E _R :E _T :E _C  = 11:81:8%) to an increasing proportion channelled into reproduction and skeletal growth (20:31:49%). At the lowest light regime, calcification was maintained but reproduction was halted and thus energy content per unit surface area of tissue declined, although no mortality was observed. The changing hierarchy in energy allocation among life functions with increasing resource limitation found here for an autotrophic coral, culminating in cessation of reproduction when limitations are severe, stands in contrast to observations from annual plants. However, the strategy may be optimal for maximising fitness components (growth, reproduction and survival) through time in marine modular animals.