Tropical Coral Reef System Research Articles

An efficient diazotroph‐derived nitrogen transfer pathway in coral reef system

AbstractThe underlying mechanism for sustaining the high productivity of coral reefs in nutrient deplete water, known as the Darwin Paradox, has long been debated. The input of new nitrogen, for example, derived from N2 fixation, and efficient nitrogen recycling are hypothesized as two critical factors in resolving the paradox. However, the source of new nitrogen and its connection to nitrogen recycling in the coral reef system remain poorly understood. We measured N2 fixation rates and investigated the diazotroph community in different components of a tropical coral reef system. Our results show that the reef‐building crustose coralline algae demonstrate a significantly higher N2 fixation rate than the overlying water or the coral holobiont, representing a large source of N2 fixation to the system. The associated diazotroph community was dominated by the non‐cyanobacterial Rhizobiales. 15N2‐based pulse‐chase incubation experiments revealed ~ 50% of the diazotroph‐derived nitrogen was released from the crustose coralline algae holobiont to the overlying water and coral holobiont within 48 h, which was five fold higher than the fraction of nitrogen released from nitrate‐based assimilation by crustose coralline algae. Furthermore, the released diazotroph‐derived nitrogen was rapidly transferred to the coral holobiont through grazing and was subsequently released back to the overlying water as dissolved organic nitrogen, revealing an efficient pathway that connects the new nitrogen input with the nitrogen recycling in the coral reef ecosystem. Our results provide new insights into the sources and fates of new nitrogen in the coral reefs, helping to resolve the Darwin Paradox in this unique ecosystem.

Subinertial flow patterns in a tropical coral reef system of the southwestern gulf of Mexico

One-year subinertial flow profiles and near-bottom temperatures were recorded in a tropical coral reef of the southwestern Gulf of Mexico to determine whether baroclinicity affects flow distributions. The subinertial flow at four sites was predominantly influenced by three empirical modes of variability. The first mode (explaining 78% of the variance) was mostly associated to wind stress (r = 0.62). Its temporal variability had greatest energy during the northerly wind season (October–May), related to atmospheric cold fronts, at periods of 2–10 days and at 20–30 days. Mode 2 (with 6% of the variance) was related to sea-level slopes, linked also to northerly wind events; this mode induced an anticyclonic circulation around the reefs, although a less intense cyclonic circulation was also observed. Mode 3 (with 5% of the variance) was associated to a baroclinic pressure gradient that drove bidirectional flow profiles: Near-surface flows moving in opposite direction to near-bottom flows. Mode 3 dominated in the vertically stratified summer (June–September), the rainy season, but it was also occasionally observed during northerly winds. Near-bottom summer intrusions of relative cold water were baroclinically driven, linked to Mode 3, and unrelated to Ekman dynamics. Modes 2 and 3 of variability described additional flow distributions relative to the behavior already proposed for the reefs on the southwestern continental shelf of the Gulf of Mexico.

Hydrographic variation in a tropical coral reef system: The Veracruz Reef System, Gulf of Mexico

Three thousand forty-one profiles of temperature, salinity, density, dissolved oxygen, nitrogen and chlorophyll-a were used to study their seasonal variation on a tropical coral reef system, located in the central part, of the reef corridor of the southwestern Gulf of Mexico. The results revealed three seasons according to their hydrographic variations; the northerly wind season from September to April; the dry season from May to June; and the rainy season from July to August. The results of the density ratio during the dry season were ∼1.25 on average, while during the rainy season it had an average value of ∼0.62. Thus, the pycnocline was more influenced by the halocline during the rainy season and by the thermocline during the dry season. There was also an evident variation in chlorophyll-a concentration over the water column, which was not evident in the surface layer. During the summer (rainy season), dissolved oxygen was related to chlorophyll-a concentration; while, during the winter (northern wind season), these values were related to the vertical mixing of the water column due to wind stress. There was evidence of cooler ocean water intrusion into the Veracruz Reef System during the spring-summer season below ∼10 m. Finally, a second halocline, pycnocline, and nitrocline were found near ∼30 m depth during the rainy season.

Climatic and local stressor interactions threaten tropical forests and coral reefs.

Tropical forests and coral reefs host a disproportionately large share of global biodiversity and provide ecosystem functions and services used by millions of people. Yet, ongoing climate change is leading to an increase in frequency and magnitude of extreme climatic events in the tropics, which, in combination with other local human disturbances, is leading to unprecedented negative ecological consequences for tropical forests and coral reefs. Here, we provide an overview of how and where climate extremes are affecting the most biodiverse ecosystems on Earth and summarize how interactions between global, regional and local stressors are affecting tropical forest and coral reef systems through impacts on biodiversity and ecosystem resilience. We also discuss some key challenges and opportunities to promote mitigation and adaptation to a changing climate at local and global scales. This article is part of the theme issue 'Climate change and ecosystems: threats, opportunities and solutions'.

Physical dynamics of the reef flat, channel, and fore reef areas of a fringing reef embayment: An oceanographic study of Pago Bay, Guam

Long-term observations of oceanographic patterns and processes provide necessary context for integrative ecological studies and for assessing and mitigating anthropogenic impacts to coastal ecosystems. The oceanographic patterns and processes of Pago Bay, Guam, a tropical coral reef system with a small estuary, were observed for one year. An array of 50 sensors including current profilers, temperature and dissolved oxygen loggers, and water quality sensors were deployed throughout Pago Bay from 0–26 m depth. Water flow was primarily driven by wave height in the reef flat and channel. In addition, wind and wave directions that were directly across-shore contributed to faster flow speeds throughout the bay. On the fore reef, wave height was the strongest predictor of current speed, but wind direction had a strong influence on current direction. Flow in the channel and on the reef flat was more tightly tied to environmental factors of wind and waves than flow on the fore reef. Extreme daily temperature variations on the reef flat were observed. In deeper water, cold pulses were observed, indicating that large internal waves may propagate around the slopes of Guam. Cold pulses may deliver nutrients to shallower ecosystems and periodically cool deeper areas of the fore reef. In response to heavy rain events, the estimated backscatter in the channel increased sharply along with significant changes in other water quality parameters. Rain events of this magnitude and associated backscatter increases occur frequently. Along with soil-destabilizing land use practices in the Pago River watershed, the current flow and water quality observations presented here indicate that sedimentation may be a risk for the Pago Bay fore reef coral habitat. The understanding of circulation, thermal environment, and rainstorm impacts in this fringing reef system provides a baseline for understanding the ecology, coral reef resilience, and future oceanographic changes in Pago Bay.

New coral reef structures in a tropical coral reef system

A multibeam sonar combined with an Acoustic Doppler Current Profiler (ADCP) were used at the Veracruz Reef System (VRS), Gulf of Mexico, during the spawning period of August 2016 in order to elucidate plankton trajectories within the study area. The new high-resolution bathymetry provided the location of 50 coral reefs, 27 more reefs than known at the VRS. Most of those reefs are submerged reefs located at depths greater than 40 m. The total coral reef area of the VRS was calculated in 70.1557 km 2 . Only ~10% of the total area corresponds to submerged reefs. Forty-eight species were identified, seven more than known species at the VRS, 45 of the order of Escleractinia and 3 of the order of Anthoathecatae. Acropora prolifera , a hybrid, was also identified in most reefs. All species were observed in the emerged and submerged reefs. The distance at which the three local river discharges (Jamapa, La Antigua, and Actopan) brought sediments to the VRS was calculated. Those are inappropriate areas for coral settlement or development due to sediment transport and temperature and salinity fluctuation. Finally, light penetration was measured at 19 m depth near one reef structure during August 2016 suggesting that even during cloud coverage and rain periods there was a light bioavailability at the sampling point.

Monthly surface hydrographical variability in a coral reef system under the influence of river discharges

The annual variation of total nitrogen, dissolved oxygen, salinity, temperature, Chlorophyll-a and density time series in the surface layer (1 and 5 m depth) were analyzed in a tropical coral reef system, under the influence of city drainage and river discharges. It is important to fully understand the temporal variation of such parameters throughout the world since they are related to biological processes. Furthermore, most tropical coral reef systems, such as the Veracruz Reef System (VRS, Mexico), developed in areas where the reefs are under stress due to the influence of fresh water input during the rainy season, and sediments coverage (vertical mixing) during the northern winds (cold front) season. In addition to the atmospheric temporal variability at mesoscales in the western Gulf of Mexico (dry, rainy and northern wind seasons), the VRS exhibited two transition months between seasons, in contrast to the three months transition period expected from the atmospheric variability. Finally the system was found to be more productive than its surrounding areas due to nutrient input from rivers and city drainage.

Flow modifications over a tropical coral reef system

Hydraulic jumps, transitions from supercritical to subcritical flows, tend to maintain suspended particles in the water column and enhance biological productivity near the surface. Transitions occur from flow adjustments as the flow moves through a cross-section reduction, such as a reef crest. Hydraulic jumps can also inhibit the settlement of coral larvae and bivalve mollusks, but could favor connectivity among reefs by dispersing planktonic organisms to surrounding areas. The Veracruz Reef System (VRS) was chosen as a case study of hydraulic jumps produced over reef crests. This system is located in the southwestern part of the Gulf of Mexico (GoM) and features 50 coral reef structures. The area is influenced by vertical stratification that modifies its dynamics. Hydrographic and yearlong current data collections in the area during 2009 identified fronts and a cyclonic recirculation. On the other hand, the presence of a hydraulic jump weakened the cyclonic recirculation on the leeward side of the reefs. The vertical gradient of the horizontal velocity suggested the influence of vertical eddies at least 35% of the time throughout the entire year. Moreover, the current velocity profiles showed a bidirectional behavior in spring and summer conditions of stratified waters. These bidirectional flows contrasted the unidirectional structure caused by frictional dynamics during northerly winds.

Wind forcing of sea level variability in a tropical coral reef area in the western Gulf of Mexico

Sea level, water temperature, and wind stress data were obtained from the Western Gulf of Mexico to elucidate the relationship between sea level changes and meteorological variables in a tropical coral reef system. Sea level and water temperature data were measured with a series of acoustic Doppler current profilers anchored at a depth of approximately 13 m in the Veracruz Reef System. The barometric and wind stress data were obtained from an automatic coastal weather station. Principal Component Analysis was applied to the series to determine the relative influence of the different meteorological variables on sea level. The seasonal variability of sea level is evident in the year-long data (September 2008 to April 2010) with a thermal expansion during the summer. Barometric pressure was found to be the second strongest forcing mechanism of sea level variability and not the first one. Opposite to what would be expected in protected coastal areas where the wind forcing mechanism on sea level may not be as important. The local winds were the main forcing mechanism of the sea level variability from March to September, while the meridional winds were more important than local winds from October to February.

Annual hydrological variation and hypoxic zone in a tropical coral reef system

The “Parque Nacional Sistema Arrecifal Veracruzano” (PNSAV), a coastal reef system located in the corridor of the southwestern Gulf of Mexico, is an ecosystem with different environmental characteristics when compared to most tropical coral reefs. The current study aims to describe the hydrological variation of temperature, salinity, density, oxygen, nitrogen and chlorophyll-a from March, 2011 to March, 2012. A total of 13 sampling stations distributed along the PNSAV were used to describe the temporal variation of the physicochemical characteristic of the area. There was a good correlation between the physicochemical and biological variables according to a multivariate statistical analysis (cluster and PCA) performed using all variables. The correlation value among the physicochemical and biological variables was about the same for all stations despite the differences in coral coverage. It was found that the PNSAV had different seasons (dry, rainy, northern and transition seasons or southern and northern wind seasons) depending on the variables used to perform the analysis. Finally, some stations depicted a hypoxic area in front of the Jamapa River owing to bacterial activity, which maybe one of the explanation of the absence of coral reef structures in this area.

Tropical coral reef system hydrodynamics in the western Gulf of Mexico

The acoustic Doppler current profiler, temperature, salinity and density data set were recorded monthly over the course of a year in a tropical coral reef system to elucidate the effects of abrupt bathymetric changes on current variability and surface temperature over time. A minimum of five transect repetitions were performed during one diurnal tidal cycle each month during the year 2008. According to the empirical orthogonal functions, the dominant terms were advection and friction due to the shallowness of the system and the relatively short continental shelf located in front of the Port of Veracruz (Mexico). The data showed the dominance of the northwest-southeast current velocity components (parallel to the coast) attributed to the winds. The southeastward current velocity was the dominant component throughout the year, followed by the northwestward current velocity component. The data suggested that coral reefs produce current rectification near the shallow reef areas (1 m depth). No correlation existed between surface temperature and the chlorophyll-a levels throughout the year. Residual current velocities throughout the year never reached more than 50 cm –1 during the sampling periods, and the surface temperature varied from 21 to 30°C, with the highest temperature being observed near the coast and reefs. Finally there was a direct correlation of the northeasterly and strong northwesterly winds with the well-mixed cold-salty water column in the reef area.

Heat balances and thermally driven lagoon‐ocean exchangeson a tropical coral reef system (Moorea, French Polynesia)

AbstractThe role of surface and advective heat fluxes on buoyancy‐driven circulation was examined within a tropical coral reef system. Measurements of local meteorological conditions as well as water temperature and velocity were made at six lagoon locations for 2 months during the austral summer. We found that temperature rather than salinity dominated buoyancy in this system. The data were used to calculate diurnally phase‐averaged thermal balances. A one‐dimensional momentum balance developed for a portion of the lagoon indicates that the diurnal heating pattern and consistent spatial gradients in surface heat fluxes create a baroclinic pressure gradient that is dynamically important in driving the observed circulation. The baroclinic and barotropic pressure gradients make up 90% of the momentum budget in part of the system; thus, when the baroclinic pressure gradient decreases 20% during the day due to changes in temperature gradient, this substantially changes the circulation, with different flow patterns occurring during night and day. Thermal balances computed across the entire lagoon show that the spatial heating patterns and resulting buoyancy‐driven circulation are important in maintaining a persistent advective export of heat from the lagoon and for enhancing ocean‐lagoon exchange.

Benthic Primary Production Budget of a Caribbean Reef Lagoon (Puerto Morelos, Mexico)

High photosynthetic benthic primary production (P) represents a key ecosystem service provided by tropical coral reef systems. However, benthic P budgets of specific ecosystem compartments such as macrophyte-dominated reef lagoons are still scarce. To address this, we quantified individual and lagoon-wide net (Pn) and gross (Pg) primary production by all dominant functional groups of benthic primary producers in a typical macrophyte-dominated Caribbean reef lagoon near Puerto Morelos (Mexico) via measurement of O2 fluxes in incubation experiments. The photosynthetically active 3D lagoon surface area was quantified using conversion factors to allow extrapolation to lagoon-wide P budgets. Findings revealed that lagoon 2D benthic cover was primarily composed of sand-associated microphytobenthos (40%), seagrasses (29%) and macroalgae (27%), while seagrasses dominated the lagoon 3D surface area (84%). Individual Pg was highest for macroalgae and scleractinian corals (87 and 86 mmol O2 m−2 specimen area d−1, respectively), however seagrasses contributed highest (59%) to the lagoon-wide Pg. Macroalgae exhibited highest individual Pn rates, but seagrasses generated the largest fraction (51%) of lagoon-wide Pn. Individual R was highest for scleractinian corals and macroalgae, whereas seagrasses again provided the major lagoon-wide share (68%). These findings characterise the investigated lagoon as a net autotrophic coral reef ecosystem compartment revealing similar P compared to other macrophyte-dominated coastal environments such as seagrass meadows and macroalgae beds. Further, high lagoon-wide P (Pg: 488 and Pn: 181 mmol O2 m−2 lagoon area d−1) and overall Pg:R (1.6) indicate substantial benthic excess production within the Puerto Morelos reef lagoon and suggest the export of newly synthesised organic matter to surrounding ecosystems.

Inorganic carbon turnover caused by digestion of carbonate sands and metabolic activity of holothurians

Recent measurements have shown that holothurians (sea cucumbers) may play an important role in the cycling of CaCO3 in tropical coral reef systems through ingestion and processing of carbonate sediment. In this report, we present estimates of inorganic carbon turnover rates determined from laboratory incubations of Holothuria atra, Holothuria leucospilota and Stichopus herrmanni. The pH values of the gut lumen ranged from 7.0 to 7.6 when digestive tracts were filled with sediment compared with 6.1–6.7 in animals with empty digestive tracts. Empty gut volume estimates for H. atra and S. herrmanni were 36 ± 4 mL and 151 ± 14 mL, respectively. Based on these measurements and the density and porosity of carbonate sediments of coral reefs, it is estimated that these species process 19 ± 2 kg and 80 ± 7 kg CaCO3 sand yr−1 per individual, respectively. The annual CaCO3 dissolution rates per H. atra and S. herrmanni individual are estimated to be 6.5 ± 1.9 g and 9.6 ± 1.4 g, respectively, suggesting that 0.05 ± 0.02% and 0.1 ± 0.02% of the CaCO3 processed through their gut annually is dissolved. During incubations the CaCO3 dissolution of the fecal casts was 0.07 ± 0.01%, 0.04 ± 0.01% and 0.21 ± 0.05% for H. atra, H. leucospilota and S. herrmanni, respectively. The CaCO3 saturation state in the incubation seawater decreased markedly due to a greater increase in dissolved inorganic carbon (DIC) relative to total alkalinity (AT) as a result of respiration by the animals. Our results support the hypothesis that deposit feeders such as sea cucumbers play an important ecological role in the coral reef CaCO3 cycle.

Current pattern and coral larval dispersion in a tropical coral reef system

Acoustic Doppler current profiler data from a towing ADCP were obtained from a tropical coral reef system during a diurnal tidal cycle. Nine transect repetitions were performed during the coral spawning period of August 2008 to elucidate the possible larvae trajectories within the study area. The principal current velocity component was parallel to the coast (northward direction). The currents were mainly induced by surface friction (winds) and advection. A divergent area was observed near the northern coral reef area, which decreased the probabilities for coral larvae fixation. However, the divergence was only observed during the flood period. During the ebb tide period, the sea level decreased and constrained the denser sea water to a depth of approximately 5m from the bottom; as a result, the coral larvae had a greater probability of fixating to the hard sea floor. The coral larvae trajectories were determined from the vertical water velocity, which was calculated using the continuity equation, the surface temperature, and the biovolumes of zooplankton, which were calculated from the backscatter intensity of the ADCP. In addition, the residence time of the system (2.41 days) suggests that some of the larvae likely fixate in less than 3 days, although other larvae may be able to drift off the area for up to 40 days. Three coral reefs were identified as the providers of coral larvae within the northern Veracruz Reef System. The dominant northward current velocity was always less than 30cms−1.

Seasonal absolute acoustic intensity, atmospheric forcing and currents in a tropical coral reef system

The seasonal patterns of marine circulation and biovolume were obtained from time-series measurements carried out in the “Parque Nacional Sistema Arrecifal Veracruzano” (PNSAV), located in the western continental shelf of the Gulf of Mexico, from June 2008 to September 2009. Two mechanisms were depicted as the responsible for the current pattern observed in the PNSAV and not only one as suggested in large-scale studies. The first mechanism is the wind generated currents. This mechanism by itself is responsible for up to 78% of total variation of the seasonal circulation in the PNSAV as estimated with the first mode of the EOF’s (Empirical Orthogonal Functions), which was correlated (Normalized Lagged Correlation) with the north-south wind component. Therefore, the wind and the first mode were highly correlated for most of the year (r > 0.7). The second mode was attributed to the low frequency current, associated to the meso-scale circulation of the Gulf of Mexico, owing to the cyclonic eddy of the Campeche Bay. Both mechanisms were mostly observed throughout the year. Nevertheless, the cyclonic eddy of the Campeche Bay (meso-scale) was the first responsible for the current fluctuations observed during the summer of 2008 and 2009. The absolute acoustic intensity (plankton biovolumes) was highly correlated to currents, showing high spatial variability, attributed to advection produced by the meso-scale circulation and to river discharges, but also by eddy diffusion produced by atmospheric and coastal water fronts.

Current rectification in a tropical coral reef system

Acoustic Doppler current profiles (ADCP) and conductivity-temperature-depth (CTD) profiles were recorded in the Veracruz Reef System (VRS) to elucidate the effect of topography on current circulation and zooplankton distribution. Measurements showed the baroclinic behavior of the tropical coral reef system with vertical temperature and salinity gradients of 0.4°C m−1 and 0.5 psu m−1, respectively. Under sustained southerly wind conditions, a cyclonic eddy generated by subtidal and tidal current rectification was observed between the two groups of coral colonies studied. Current rectification was attributed to the shallowness of the coral colonies and to a small cape close to Anton Lizardo Village. The cyclonic eddy, with low surface temperature, produced an increment in the vertically integrated acoustic scattering volume (biovolume), suggesting that this is a high-productivity area.

Geological Approaches to Coral Reef Ecology

Tropical coral reef systems cover an estimated 284,300 square kilometers of the Earth's surface and are considered among the most valuable ecosystems on Earth. The reef systems are zones of high biological diversity, habitat for about one quarter of all known marine species, and important components of the global carbon cycle. In addition, they provide the physical foundation for a number of mid‐ocean nation states. Coral reefs worldwide are considered to be in serious ecological decline due to anthropogenic impacts, natural stresses, and climate change. However, these gloomy projections for coral reefs are based largely on analysis of short‐term changes in their ecological condition.

Seasonal variation in groundfish habitat associations in the Gulf of Maine–Georges Bank region

Fish distributions are related to several habitat factors. We explored how the distribution of a 24 species assemblage is related to depth, temperature, substrate, season, and time-block using a 35 yr time series in the Gulf of Maine-Georges Bank region. We examined the relative importance of each factor, how it changes with season, and how individual species shift their relative distribution along environmental gradients on a seasonal basis. Distribution patterns were more strongly related to depth and temperature than to substrate type in both fall and spring. We observed 4 major pat- terns: (1) some species remained in relatively deep waters in both fall and spring; (2) some remained in relatively shallow habitats in both seasons and experienced wide temperature fluctuations as a result; (3) some moved from warmer shallow areas in the fall to warmer deep areas in the spring; (4) some traveled from the cool deep portion of the region in the fall to the cool shallow portion of the region in the spring. Of the 24 species examined, 19 declined in biomass over the study period in response to exploitation. The relationships between abundance and substrate type previously established for some species at local scales were weak at more synoptic spatial scales, although some trends in sub- strate associations were observed. Defining habitat at broad spatial scales remains a unique chal- lenge. Compared to temperate systems, more refined habitat delineations for demersal marine fish have been established in tropical coral reef systems. Accordingly, much of our theory and the methodologies for applied spatial management have been derived from tropical systems. Differences between temperate and tropical systems necessitate modified approaches for temperate systems.

Are vertical distribution patterns of scleractinian corals maintained by pre- or post-settlement processes? A case study of three contrasting species

Vertical zonation of hard corals in tropical coral reef systems is a well documented and predictable pattern in the organisation of tropical coral communities. However, the mechanisms that underly vertical zonation of hard corals are poorly understood. Two alternative conceptual models for the maintenance of vertical zonation are considered; depth-dependent settlement of larvae, or indiscriminant settlement of larvae with distributions determined by differential post-settlement mortality with depth. These alternative models were evaluated by undertaking field translocation experiments using 10 d old juveniles of 3 species with contrasting adult distributions: Goniastrea aspera (Verrill)-restricted to shallow reef fiat habitats; Oxypora lacera (Verrill)-restricted to low light, lower slope habitats; and Platygyra daedalea (Ellis and Solander)-a species with a broad depth range. Laboratory-raised larvae of each species were settled onto terracotta settlement plates and the plates translocated to 3 depths in the field: reef flat (0 m), mid slope (5 m), lower slope (10 m). Survival of the laboratory-settled juveniles was monitored at 3, 6 and 12 mo after settlement. For each of the 3 species, juveniles survived equally well at all 3 depths over the 12 mo study period. The pattern of survivorship of G. aspera and P. daedalea among depths, however, was dependent on plate surface (top, bottom). Given the absence of depth-dependent survival of juveniles from the 2 zone-specific species G. aspera and O. lacera, we reject the hypothesis that adult distribution patterns of these 2 species is determined by indiscriminant settlement of larvae followed by differential early post-settlement mortality with depth.