Lance Bode

The structure of reef fish metapopulations:modelling larval dispersal and retention patterns

An improved understanding of the dispersal patterns of marine organisms is a prerequisite for successful marine resource management. For species with dispersing larvae, regional–scale hydrodynamic models provide a means of obtaining results over relevant spatial and temporal scales. In an effort to better understand the role of the physical environment in dispersal, we simulated the transport of reef fish larvae among 321 reefs in and around the Cairns Section of the Great Barrier Reef Marine Park over a period of 20 years. Based on regional–scale hydrodynamics, our models predict the spatial and temporal frequency of significant self–recruitment of the larvae of certain species. Furthermore, the results suggest the importance of a select few local populations in ensuring the persistence of reef fish metapopulations over regional scales.

When to press on or turn back : dispersal strategies for reef fish larvae

Events that occur during the pelagic larval stage are thought to be important determinants of reef fish population dynamics. Recent research contradicts the early paradigm of larvae being advected as passive propagules and indicates that many late stage larvae have well‐developed sensory and locomotory capabilities. Whether and how larvae use these capabilities to influence their dispersal is unknown. We compare alternative hypotheses regarding larval behavior. Contrary to the trend in dispersal modeling, we focus on larval biology rather than physical oceanographic considerations. Specifically, we present two streams of models: one that describes a return‐based strategy and one in which dispersal is a central component. The models depend on different sets of behavioral assumptions for a pomacentrid species and for acanthurids, two groups with contrasting early life histories. Whether dispersal or return‐based strategies are favored depends on the efficiency and sustainability of larval swimming methods and the environmental conditions experienced during dispersal. We argue that dispersal models should consider a variety of behavioral hypotheses and that the sensitivity of results to the behavioral assumptions made should be quantified.

Effects of asymmetric dispersal on the coexistence of competing species

The global biodiversity crisis has made a priority of understanding biodiversity maintenance in ecological communities. It is increasingly apparent that dispersal patterns can have important effects on such maintenance processes. Nevertheless, most competition theory has focused on a small subset of the possible dispersal patterns in nature. Here, we show that spatially asymmetric dispersal, i.e. the disproportionate transport of propagules towards or away from particular habitat patches in a metacommunity, when it differs between species, can promote the coexistence of competing species even in the absence of environmental heterogeneity among habitat patches. Moreover, when asymmetric dispersal is present, changes in the self-recruitment of competitive dominants and subordinates have important, but fundamentally different, effects on species coexistence. Our results underscore the importance of the interplay between species interactions and dispersal patterns for understanding the effects of habitat fragmentation and for designing regional-scale conservation strategies, such as networks of protected areas.

The tides of the central Great Barrier Reef

Abstract The central third of the Great Barrier Reef is a diffuse matrix of platform reefs on the outer half of a 300 km long segment of continental shelf. The subset of constituents O1, P1, K1, N2, M2, S2 and K2 describes adequately both the vertical tides and tidal currents in the reef zone and lagoon. Sea level amplitudes and phases display quasi-linear changes in space throughout the region away from the coastal boundary, while sea level amplitude and phase gradients are very small along the shelf break. Cross-shelf sea level and phase gradients are relatively large, but decay equatorward, particularly where the shelf narrows from 120 to 60 km. This variation is reflected in current amplitudes, which range from spring maxima near 30 cm s−1 in the south to within the background noise in the north. The tidal data obtained in the field part of this study are used as boundary conditions to force a nonlinear numerical tidal model of the region, which is driven by the five principal constituents over a period of a month. This provides an overall picture of the dynamics, with results comparing well with measured data on a relatively coarse 5 nmi grid. Nowhere in this region are the reefs concentrated as an identifiable barrier, and it is found by comparative numerical experiments that, due to their scattered nature, they need not be modelled explicitly at this resolution. The M2 tides dominate and energy propagates across the shelf break in the south of this central section to turn poleward on the shelf. Elsewhere, the M2 energy flow is basically longshore and equatorward.

Different dispersal abilities allow reef fish to coexist

The coexistence of multiple species on a smaller number of limiting resources is an enduring ecological paradox. The mechanisms that maintain such biodiversity are of great interest to ecology and of central importance to conservation. We describe and prove a unique and robust mechanism for coexistence: Species that differ only in their dispersal abilities can coexist, if habitat patches are distributed at irregular distances. This mechanism is straightforward and ecologically intuitive, but can nevertheless create complex coexistence patterns that are robust to substantial environmental stochasticity. The Great Barrier Reef (GBR) is noted for its diversity of reef fish species and its complex arrangement of reef habitat. We demonstrate that this mechanism can allow fish species with different pelagic larval durations to stably coexist in the GBR. Further, coexisting species on the GBR often dominate different subregions, defined primarily by cross-shelf position. Interspecific differences in dispersal ability generate similar coexistence patterns when dispersal is influenced by larval behavior and variable oceanographic conditions. Many marine and terrestrial ecosystems are characterized by patchy habitat distributions and contain coexisting species that have different dispersal abilities. This coexistence mechanism is therefore likely to have ecological relevance beyond reef fish.

Reef parameterisation schemes with applications to tidal modelling

A variety of analytical models is used to investigate the effects on tidal propagation of a barrier reef system. These models specify reef geometry by two parameters. They can accommodate cases where water flows over reefs, as well as through inter-reef gaps, and also incorporate quadratic bottom friction. Although based on a one-dimensional approach, adaptations of a solution by Huthnance are used to account for the additional blockage effects associated with two-dimensional flow patterns near reef barriers. The present work adopts the philosophy that only a numerical approach can cope with the wide variations in reef geometry that are encountered in areas such as the Great Barrier Reef (GBR) region of Australia. Moreover, since typical model grids cannot resolve inter-reef gaps and other features with sufficient accuracy, a parameterised approach is needed to accommodate the conflicting demands of reef geometry and an economically feasible model resolution. The formulation of the analytical models is such that they can be applied immediately to standard numerical algorithms. Numerical experiments for flow in a channel, with a reef barrier across its centre, are used to test the parameterisation schemes. Comparison of the results for parameterised reefs with those obtained using extremely fine grids, shows convincing evidence of the success of the schemes. A separate method for automatically generating reef parameters has simplified the task of applying the methodology to real reefal systems. A tidal model of the Southern GBR, a region which exhibits unusual tidal behaviour, but which also has ample field data available for model testing, is used to demonstrate the accuracy that can be attained with the parameterised approach. Although tides are considered specifically in the present work, the formulation should be applicable with equal ease to the many other significant classes of low frequency motions in the GBR.

Modelled response of Gulf St Vincent (South Australia) to evaporation, heating and winds

In Gulf St Vincent, Australia, the salinity of the head waters can exceed 42 in summer when evaporation is maximum and the rainfall is minimal. A depth-integrated implicit finite difference model is extended to simulate the summer–autumn evolution of salinity, temperature, and density distributions, with climatological evaporation, rainfall, air temperature, and wind stress as inputs. Advection of salt and heat by the density- and wind-driven circulation is modelled by the QUICK scheme, whereas horizontal mixing by tidal circulation is parameterised by a dispersion coefficient related to the oscillatory vertical shear. Simulated distributions and seasonal variations compare well with available observations, which feature the flow of highly saline water along the eastern side of the gulf, while the western side is bathed by less saline shelf water. Model results show that, despite the increasing salinity gradients in summer, opposing temperature gradients can stifle the shelfward density currents in the southern parts of the region. Autumn cooling intensifies these density currents so that at the end of the season the flushing of highly saline water, accumulated in the gulf throughout the summer, is enhanced. It was found that the variability in the general circulation brought about by the directional variability in the prevailing winds is an important factor in maintaining the observed salinity distributions in the region.

The consequences of non-passive advection and directed motion for population dynamics

A. R. Robinson has proposed a method for studying population growth and advection in the sea. The assumption made by Robinson of a non–divergent velocity field for the biological tracer may not be valid for many situations. When the assumption is valid the biological dynamics of an advected patch are no different from those of a stationary patch, unless a greater degree of spatial structure is incorporated than he considers. The generality and applicability of the model are increased if the assumption of non–divergence can be relaxed. If the velocity field associated with a patch is convergent, as can occur for many biological tracers, then new dynamics arise, both in a Lagrangian sense and when working at the whole–patch level. These new dynamics are explored for four fundamental examples, which could provide new paradigms for spatially explicit models in mathematical ecology.

Poincare´waves obliquely incident to a continental shelf

Abstract The causes of large amplitude or resonant tides on continental shelves are many and complex. Here we examine the role played by the angle of incidence in determining the energy losses and the magnitude and frequency of resonant peaks for Poincare´waves obliquely incident to a step-shaped, frictional continental shelf. Peak shelf/incident wave amplitude ratios are substantially reduced for obliquely incident waves compared with normally incident waves, both with and without shelf friction. Peak values of shelf/ocean sea level ratios are increased marginally for obliquely incident waves travelling in the same alongshore direction as Kelvin waves, and the resonant frequency is reduced below that for normally incident waves. Obliquely incident waves travelling in the opposite alongshore direction have a reduced peak shelf/ocean sea level ratio with the resonant frequency increased. Peak shelf/incident wave amplitude ratios are associated with maximum tidal energy dissipation, while peak shelf/ocean sea level ratios are associated with the quarter-wave oscillator resonance.

ACCURATE MODELLING OP TWO-DIMENSIONAL MASS TRANSPORT

This report will update the coastal zone practitioner on the National Flood Insurance Program (NFIP) as it affects the implementation of manmade changes along the coastline. It is our intent to place in proper perspective this fast-changing and often difficult to interpret national program. Readers will achieve an overall understanding of the NFIP on the coast, and will be in a position to apply the programs requirements in their efforts. We will begin with a history of the application of the NFIP to the coastal zone. The history of the problems encountered will lead into current regulations, methodologies, and the changes the Federal Emergency Management Agency plans for the future.The spatial variability of the nearshore wave field is examined in terms of the coherence functions found between five closely spaced wave gages moored off the North Carolina coast in 17 meters depth. Coherence was found to rapidly decrease as the separation distance increased, particularly in the along-crest direction. This effect is expressed as nondimensional coherence contours which can be used to provide an estimate of the wave coherence expected between two spatial positions.Prediction of depositional patterns in estuaries is one of the primary concerns to coastal engineers planning major hydraulic works. For a well-mixed estuary where suspended load is the dominant transport mode, we propose to use the divergence of the distribution of the net suspended load to predict the depositional patterns. The method is applied to Hangzhou Bay, and the results agree well qualitatively with measured results while quantitatively they are also of the right order of magnitude.