Fringing Reef Lagoon Research Articles

Reef Flat Flow Dynamics for a Nearly Closed Fringing Reef Lagoon: Ofu, American Samoa

AbstractWe discuss observations of tidally varying wave‐forced flows in the reef system on Ofu, American Samoa, a barrier reef and lagoon system that appears open at low tide and closed at high tide. At high tide, the free‐surface pressure gradient nearly balances the radiation stress gradient in the depth‐integrated momentum equation. At depth, there is an imbalance between these two forces, generating an undertow and flows that turn alongshore, and for some of the time, offshore, behavior similar to rip currents observed on beaches. At low tides, the wave forcing drives purely onshore flows. In general, wave transport is important to determining the total net transport. While the dynamically closed nature of the lagoon mostly suppresses cross‐reef transport, there is always some flow through the lagoon with the strongest flows occurring at high tides and when the wave forcing is strongest.

Intratidal and Residual Flows Around Inlets of a Reef Lagoon

Wave-driven circulation and inlet dynamics were investigated in a Caribbean fringing reef lagoon via velocity transect data collected with a towed ADCP along two lagoon-inlet connections over diurnal tidal cycles. The inlets sampled were Boca Grande inlet, 700 m wide, and to the north of Boca Chica, 250 m wide. Flow reconstructions with semidiurnal (D2) and diurnal (D1) tidal bands plus a residual showed that the strongest flows were seaward and located at the inlets. Maximum outflows were 0.6 m/s in the center of Boca Chica and 0.7 m/s at the southern end of Boca Grande. Differences in inlet width and varying bathymetry between the two inlets caused atypical flow circulation, including inflow and residual recirculation. Boca Chica featured converging outflow, with across-lagoon flows (East-West) dominating over along-lagoon flows (North-South). Boca Grande displayed a highly variable, complex vertical structure with 2-layer diverging flows. Other characteristics of Boca Grande included counter-clockwise vorticity appearing at the inlet and inflow at the north end of the inlet. Dynamics related to residual flows were Bernoulli-like, with advective accelerations predominantly balancing pressure gradients.

Seasonal variability of saltwater intrusion at a point‐source submarine groundwater discharge

AbstractSubmarine groundwater discharge provides freshwater and nutrients to coastal environments. In some places throughout the world, this direct connection between aquifers and oceans may also allow saltwater intrusion. Saltwater intrusion was studied at a submarine spring within a fringing reef lagoon on the eastern Yucatán Peninsula by observing its intratidal and synoptic‐scale variations during wet and dry periods. Saltwater intrusion was linked to wave‐driven setup, no rain, high tides, and sea‐level rise caused by remote forcing from Yucatán Current variability. Jet discharge velocities were inversely related to tidal oscillations, with maximum velocities at low tides. The wet period produced saltwater intrusion at high tides associated with three different conditions: syzygy tides, wave setup, and Yucatán Current weakening. During the dry period, saltwater intrusion occurred throughout most high tides and was aided by Yucatán Current weakening and wind‐driven setup within the lagoon. These results suggested that seasonal precipitation was most important in modulating spring discharge, followed by syzygy tides, Yucatán Current variability, and wave events. The spring discharge was modeled with a modified Bernoulli energy equation that included a friction term. Aquifer elevation and a friction factor were used as free parameters. The dry period produced the best model results because of infrequent rainfall that yielded a relatively steady aquifer level. Precipitation during the wet period most likely led to a more variable aquifer level, reducing the variance explained by the model that assumes a constant aquifer elevation. Nevertheless, the model predicted saltwater intrusion events reasonably well using simplified physics.

Salt intrusion at a submarine spring in a fringing reef lagoon

AbstractVariations in discharge and turbulent kinetic energy (TKE) were studied at a point‐source submarine groundwater discharge (SGD), within a fringing reef lagoon, from quadrature (neap) to syzygy (spring) tides. The principal factors affecting discharge and TKE variations were tides and waves. Field data indicated discharge, and TKE varied with high and low tides, and with quadrature and syzygy. Maximum discharge and TKE values were observed during low tides when the hydrostatic pressure over the jet was minimal, while the lowest discharge and TKE values were found at high tides. Syzygy tides produced consistent saltwater intrusion during high tides, while quadrature tides produced the greatest TKE values. In general, as the discharge intensified during low tides, jet temperatures decreased suggesting that waters originated further within the aquifer. At the same time jet salinities increased, suggesting a mixing of aquifer and seawater. To reconcile these two seemingly opposing views, it is proposed that the jet conduit is connected to a stratified chamber with seawater below brackish water. The greatest subtidal discharge occurred during quadrature tides. Syzygy produced low subtidal discharge driven by flow reversals (flow into the aquifer) observed throughout syzygy high tides in conjunction with the peak wave setup (>5 cm) observed during a storm. While tides were the primary driving force of the discharge, waves played a nonnegligible role. Wave effects on the discharge were most evident during syzygy high tides combined with a storm, when the subtidal spring discharge was weakest and salt intrusion developed.

HYDRODYNAMIC DRIVERS OF SEDIMENT TRANSPORT ACROSS A FRINGING REEF

Coral reefs are highly valuable ecosystems, which are under an increasing number of environmental pressures. Sedimentation and sediment transport patterns are among key physical drivers of coral reefs, so it is important to improve our understanding of these poorly studied dynamics on reefs. To this purpose, flume experiments were performed on a scaled fringing reef in the laboratory facilities of Deltares in Delft. The objective was to improve the understanding of hydrodynamic and sediment transport processes across fringing reefs. The water depth and bed roughness were shown to have influence on many processes such as short wave breaking, infragravity (IG) wave generation, IG wave transformation, reef flat seiching, wave-induced setup and wave reflection. The measurements showed that long waves dominate over the short waves at the back of the reef flat. The flow velocities in the rough cases were lower than those in the smooth cases as a result of the bed friction. Analysis of the third order velocity moment and the bed level changes indicates that both the short and the long waves play a role, but that the long waves appear to be the dominant factor in sediment transport and bed profile development - especially close to the beach. Bed roughness affected the shape of a swash bar, which was more pronounced for the smooth than for the rough cases. This demonstrates that the dominance of long waves in a fringing reef lagoon results in different sediment dynamics than, for example, on a regular sandy beach.

Geoelectrical signals of geologic and hydrologic processes in a fringing reef lagoon setting

Coastal groundwater may discharge into nearshore and offshore waters forced by terrestrial fluxes, controlled by local geology, and modulated by the hydrodynamics of littoral water. We investigated the electrical signature of these features with a dense, multiscale network of electrical resistivity tomography (ERT) surveys in the Muri Lagoon of Rarotonga, Cook Islands. The ERT surveys spanned from onshore to 400 m into the lagoon and used standard electrodes on land and across the foreshore, submerged electrodes in the shallow subtidal zone, and floating electrodes towed throughout the reef lagoon by a boat. ERT surveys on land mapped a typical freshwater lens underlain by a saltwater wedge, but with possible deviations from the classical model due to an adjacent tidal creek. Further inland, ERT surveys imaged a layer of lava flow deposits that is potentially a confining hydrogeologic unit; this unit was used to constrain the expected electrical resistivity of these deposits below the lagoon. ERT surveys across the intertidal zone and into the lagoon indicated fresh groundwater and porewater salinity patterns consistent with previous small-scale studies including the seaward extension of fresh groundwater pathways to the lagoon. Electrical resistivity (ER) variations in the lagoon subsurface highlighted heterogeneities in the lagoon structure that may focus submarine groundwater discharge (SGD) through previously unknown buried lava flow deposits in the lagoon. A transition to higher ER values near the reef crest is consistent with the ER signature of porosity reduction due to ongoing differential cementation of reef deposits across the lagoon. The imaged coastal hydrostratigraphic heterogeneity may thus control terrestrial and marine porewater mixing, support SGD, and provide the pathways for groundwater and the materials it transports into the lagoon. This hydrogeophysical investigation highlighted the spatial heterogeneity of submarine coastal geology and its hydrogeologic control in a reef lagoon setting, but is likely to occur in many similar coastal settings. Ignoring geologic complexity can result in mischaracterization of SGD and other coastal groundwater processes at many spatial scales.

Wave-induced extreme water levels in the Puerto Morelos fringing reef lagoon

Abstract. Wave-induced extreme water levels in the Puerto Morelos fringing reef lagoon are investigated by means of a phase-resolving non-hydrostatic wave model (SWASH). This model solves the nonlinear shallow water equations including non-hydrostatic pressure. The one-dimensional version of the model is implemented in order to investigate wave transformation in fringing reefs. Firstly, the numerical model is validated with (i) laboratory experiments conducted on a physical model (Demirbilek et al., 2007)and (ii) field observations (Coronado et al., 2007). Numerical results show good agreement with both experimental and field data. The comparison against the physical model results, for energetic wave conditions, indicates that high- and low-frequency wave transformation is well reproduced. Moreover, extreme water-level conditions measured during the passage of Hurricane Ivan in Puerto Morelos are also estimated by the numerical tool. Subsequently, the model is implemented at different along-reef locations in Puerto Morelos. Extreme water levels, wave-induced setup, and infragravity wave energy are estimated inside the reef lagoon for different storm wave conditions (Hs >2 m). The numerical results revealed a strong correlation between the offshore sea-swell wave energy and the setup. In contrast, infragravity waves are shown to be the result of a more complex pattern which heavily relies on the reef geometry. Indeed, the southern end of the reef lagoon provides evidence of resonance excitation, suggesting that the reef barrier may act as either a natural flood protection morphological feature, or as an inundation hazard enhancer depending on the incident wave conditions.

Habitat Specialization in Tropical Continental Shelf Demersal Fish Assemblages

The implications of shallow water impacts such as fishing and climate change on fish assemblages are generally considered in isolation from the distribution and abundance of these fish assemblages in adjacent deeper waters. We investigate the abundance and length of demersal fish assemblages across a section of tropical continental shelf at Ningaloo Reef, Western Australia, to identify fish and fish habitat relationships across steep gradients in depth and in different benthic habitat types. The assemblage composition of demersal fish were assessed from baited remote underwater stereo-video samples (n = 304) collected from 16 depth and habitat combinations. Samples were collected across a depth range poorly represented in the literature from the fringing reef lagoon (1–10 m depth), down the fore reef slope to the reef base (10–30 m depth) then across the adjacent continental shelf (30–110 m depth). Multivariate analyses showed that there were distinctive fish assemblages and different sized fish were associated with each habitat/depth category. Species richness, MaxN and diversity declined with depth, while average length and trophic level increased. The assemblage structure, diversity, size and trophic structure of demersal fishes changes from shallow inshore habitats to deeper water habitats. More habitat specialists (unique species per habitat/depth category) were associated with the reef slope and reef base than other habitats, but offshore sponge-dominated habitats and inshore coral-dominated reef also supported unique species. This suggests that marine protected areas in shallow coral-dominated reef habitats may not adequately protect those species whose depth distribution extends beyond shallow habitats, or other significant elements of demersal fish biodiversity. The ontogenetic habitat partitioning which is characteristic of many species, suggests that to maintain entire species life histories it is necessary to protect corridors of connected habitats through which fish can migrate.

Breathing of a coral cay: Tracing tidally driven seawater recirculation in permeable coral reef sediments

Coral reefs are characterized by high gross productivity in spite of low nutrient concentrations. This apparent paradox may be partially reconciled if seawater recirculation in permeable sediments over large (meters) and long (hours to days) scales is an important source of recycled nitrogen and phosphorus to coral reefs. In this paper we use radon (222Rn, a natural tracer) to quantify tidally driven pore water (or groundwater) exchange between (1) an offshore coral cay island and its fringing reef lagoon and (2) a reef lagoon and the surrounding ocean. As seawater infiltrates Heron Island at high tide, it acquires a radon signal that can be detected when pore waters emerge from carbonate sands at low tide. A nonsteady state model indicated that vertical pore water upwelling rates (or saline submarine groundwater discharge) were >40 cm/d within the reef lagoon and >100 cm/d outside the lagoon at low tide. Within the lagoon, tidal pumping and temperature‐driven convection were the main driving forces of pore water advection. At low tide, the reef lagoon level is about 1 m higher than the surrounding ocean. As a result, a steep hydraulic gradient develops at the reef edge, driving unidirectional filtration through the reef framework. Groundwaters were highly enriched in nitrate (average of 530 μmol, likely influenced by bird guano) relative to lagoon waters (1.9 μmol). Rough but conservative estimates indicated that groundwater‐derived nitrate fluxes (7.9 mmol/m2/d) can replace the entire lagoon nitrate inventory every <19 days. We speculate that as offshore coral islands “breath” (inhale seawater at high tide and exhale groundwater at low tide), they release nutrients that lead to sustained productivity within coral reefs.

On the circulation in the Puerto Morelos fringing reef lagoon

For a period of 22 months beginning in September 2003, an array of four current profilers were deployed on the Puerto Morelos fringing reef lagoon, a microtidal Caribbean environment characterised by the influence of the Yucatan Current (YC) and a Trade Wind regime. The dataset includes water currents, bottom pressure, and surface waves complemented with coastal meteorological data and surface currents from an acoustic Doppler current profiler moored 12 km offshore. Normal circulation conditions consisted of a surface wave-induced flow entering the lagoon over a shallow reef flat and strong flows exiting through northern and southern channels. This wave induced flow was modulated by a low-frequency sea level change related to a geostrophic response to the YC variability offshore, with tidal and direct wind forcing playing additional minor roles. Under extended summer low-wave height conditions, together with a decrease in sea level from the intensification of the offshore current, the exchange of the lagoon with the adjacent ocean was drastically reduced. Under normal wave conditions (H S = 0.8 ± 0.4 m, mean ± SD), water residence time was on average 3 h, whereas during Hurricane Ivan’s extreme swell (H S = 6 m) it decreased to 0.35 h.

Distribution and partitioning of nitrogen and phosphorus in a fringing reef lagoon of Ishigaki Island, northwestern Pacific

Concentration of dissolved and particulate carbon (C), nitrogen (N), and phosphorus (P) were measured daily and seasonally in the water column of a coral reef lagoon at Ishigaki Island. Concentrations of dissolved inorganic N and P (DIN, DIP), dissolved organic C and N (DOC, DON), particulate organic C, and particulate N and P (POC, PN, PP) were usually higher in the lagoon than at offshore stations, while dissolved organic P (DOP) was higher at the offshore stations than in the lagoon. Concentration ratios of DIN:DIP and PN:PP were near 16:1 in winter and >20:1 in summer. DON:DOP was always ≥30. Total P (DIP + DOP + PP) was fairly constant around 0.177 ± 0.025 μmol l–1 (mean ± SD), suggesting conservative behavior of water-column P around this reef. DIP and PP seemed to be produced at the expense of DOP, and PP rather than DIP increased when gross primary production (GPP) was high. In contrast, total N (DIN + DON + PN) showed large spatial and seasonal variation from 4.3 to 12.4 μmol l–1. DON and PN increased when GPP and DIN:DIP were high, which suggests that export production of organic N by the lagoon biota was enhanced by primary production and the availability of N relative to P. Estimation of C, N, and P exchange fluxes between the lagoon and the outer ocean using a simple hydrodynamic model showed that the lagoon ecosystem released N (especially DIN) but not P to the surrounding ocean. Oceanic DOP could be the major source for the P budget of the lagoon. While the lagoon was a net source of organic C to the outer ocean, the net export was only a minor fraction (<5%) of gross exchange of organic C between these 2 ecosystems.

Effect of water flow on the photosynthesis of three marine macrophytes from a fringing-reef lagoon

We studied the effect of water flow on the reduction of the photosynthetic electron trans- port rates (ETR) of the seagrass Thalassia testudinum and the brown algae Lobophora variegata and Padina sanctae-crucis. The main objective of this study was to contribute to the understanding of the magnitude of carbon limitation in a sheltered, shallow coral reef environment, while improving the use of the fluorescence technique. The results confirm that photosynthesis, in the 3 species, is sink limited at normal seawater pH. Considering that the actinic light intensity chosen for the experimen- tal design was close to saturation irradiance for the 3 species, stronger ETR declines are expected under larger carbon requirements at supersaturating irradiance. The results suggest that photo- synthesis may be strongly carbon limited in sheltered coral reef environments. Nevertheless, pharmacological experiments revealed the occurrence of carbon concentration mechanisms (CCM) related to: (1) the external presence of the enzyme carbonic anhydrase under very low hydrodyna- mics, when the diffusive boundary layers stabilize and enzyme losses are minimal (L. variegata and T. testudinum) and (2) the ability to take up HCO3 - (T. testudinum and P. sanctae-crucis). The possi- bility that the capacity to calcify may be associated with the CCM of P. sanctae-crucis needs further testing. These results suggest that photosynthetic organisms adapted to coral reef systems, where calm, cloudless days with minimal water turbulence and maximum light penetration are common, have developed efficient CCMs. The study also supports the utility of the fluorescence technique for the analysis of the photosynthetic response to hydrodynamics within the limits imposed by the linear- ity between oxygen-based and fluorescence-based photosynthesis determinations. It also empha- sizes the importance of light absorption determinations for accurate estimations of ETR values.

Form-function analysis of the effect of canopy morphology on leaf self-shading in the seagrass Thalassia testudinum

The variation in seagrass morphology and the magnitude of leaf self-shading within the canopy of Thalassia testudinum, were compared among nine sites in a fringing reef lagoon. We found a significant variation in the growth-form of T. testudinum reflected in a 5.4-fold variation in the attenuation coefficient (K (d)) within the canopy. The largest morphological variation was observed in shoot density. Leaf biomass, leaf area index (LAI), and shoot density were positively associated with canopy-K (d) and with the percentage of surface irradiance received by the top of the seagrass canopy (% Es). These results provide an explanation for the consistent pattern of depth reduction in seagrass leaf biomass and shoot density reported in the literature. Shoot density and shoot size are two descriptors of the growth-form of T. testudinum related to its clonal life-form. Shoot size was not significantly correlated with canopy-K (d), nevertheless, it showed a significant effect on the slope of the relationship between shoot density and canopy-K (d). According to this model, shoot size also contributes to light attenuation within the seagrass canopy by increasing the effect of shoot density. This form-function analysis suggests that light may have a relevant role in the regulation of the optimal plant balance between horizontal (variation in shoot density) and vertical (variation in shoot size) growth of seagrasses. Other environmental factors and interactions also need to be examined to fully understand the mechanistic bases of the morphological responses of seagrasses to the environment.

Growth rate, ultrastructure and sediment contribution of Halimeda incrassata and Halimeda monile, Nonsuch and Falmouth Bays, Antigua, W.I.

Halimeda incrassata and Halimeda monile, the two dominant rhipsalian Halimeda, were evaluated behind a bank barrier reef, in a fringing reef lagoon and in an open lagoon. Growth was calculated in number of segments, weight of segments and turnover rate. More than 1800 plants were stained with Alizarin Red-S dye, yielding average number of segments/plant/day and g CaCO3/m2/year for each of the above areas of 2.17/114, 1.43/65.7 and 1.6/56.9, respectively. Average weight CaCO3/segment was 4 mg. SEM revealed ultrastructure of short and long unoriented aragonite crystals forming in new segments within 24 h and an effective holdfast system with filaments partially coated with carbonate fragments. Greatest growth occurred within thin to medium density grass beds. In Nonsuch Bay sediment production from these two species alone was 0.057 mm/year or 1 1/2 orders of magnitude less than estimates of the total production from all Halimeda species (1.01 mm/year) over the past 6745 years.