Larvae Dispersion in Coral Reefs and Mangroves

Eric Wolanski & Joe Sarsenski. American Scientist. Volume 85, Issue 3. May/Jun 1997.

Topographically, coral reefs and mangroves are dauntingly complex. The largest of the earth’s coral reefs, Australia’s Great Barrier Reef, consists of a dense matrix of more than 2,000 individual reefs of various sizes and shapes separated by channels and shoals, all spread along the continental shelf of northeast Australia—a 2,600 kilometer-long, 350,000-square-kilometer area containing bits of land and much water 60 meters or less deep. Within this lovely hodgepodge of sand and coral resides a legendary abundance of life.

Mangroves, heavily vegetated forests in tidal floodplains along estuaries, are even more prevalent than coral reefs on a global scale and are analogous to them in terms of physical complexity. Mangroves are likely to develop on any protected, shallow shore where sediment is plentiful, frosts are absent, and tides and currents allow water to circulate. They are particularly well developed in the part of the world we study—in Southeast Asia and along the coast of the Great Barrier Reef where their importance to fisheries has come under increasing notice.

Coral reefs and mangroves have in common the fact that they interfere with prevailing water currents. On a large scale, reefs and mangroves may be seen to block and steer the prevailing oceanic currents. But on a smaller scale they also generate complex flows—including eddies, jets and stagnation zones. Larvae of the diverse reef and mangrove fauna are carried along on these flows, and the large—and small—scale effects combine to determine both the distribution and the success of many species. Not surprisingly, then, understanding the links between the physics and the biology of coral reefs and mangroves has been seen as a laudable goal. But at the same time, it has been recognized as a nearly impossible task—a problem akin to understanding the biology in individual eddies of a turbulent flow.

To learn how physics and biology link in blue-water fisheries oceanography necessitates multidisciplinary research. More often than not, however, oceanographers, chemists and biologists speak different languages. Thus to date multidisciplinary research has been the exception rather than the rule, and little scientific cross-fertilization has taken place. The topographic maze of coral reefs and mangroves only serves to magnify such difficulties. Here the water circulation is at first glance too complex for a physicist to offer a biologist information that can be used in fruitful ways. The result has been discipline—specific literature that resists synthesis and prevents marine resources managers, who also speak their own language, from extracting useful information.

Recent advances in computer technology are helping to resolve these problems. By enabling the calculation and visualization of complex flows and their chemical and biological implications, such technology facilitates multidisciplinary marine research. Our work integrating field data from the Great Barrier Reef and Australian and Southeast Asian mangroves with computer models has shown that the interaction between the complex bathymetry, or bottom profile, and the currents in coral reefs and mangroves controls the processes of dispersion. This research also reveals that the recruitment (the process by which larvae find a permanent home) of prawn, fish and coral larvae depends on the interaction between such water currents and bathymetry. Currents through the Great Barrier Reef Tides and wind are the predominant forces driving currents through the Great Barrier Reef. Along the reef’s outer edge, oceanic forcing generates prevailing longshore currents, which in turn are topographically steered and in some cases blocked by the reef matrix. Reef flows thus vary spatially and temporally, and resemble turbulent flows with eddies, jets, and convergence and stagnation zones. To understand the details of these flows, we carefully calibrated computer models against field data. Computer visualization then allowed us to explore the complexity of the data from the field studies and the model output.

One of us (Wolanski) began collecting one such field data set, the most extensive set worldwide for flows around coral reefs, in the early 1980s, using 26 current meters moored in the lee of a small island on the Great Barrier Reef, Rattray Island. These meters measured the dynamics of island “wakes”—the eddies shed by an island interfering with a swift tidal current.

Actual measurements of water flows behind real reefs are important because laboratory models fail to provide accurate or detailed information about them. Island eddies have different physics from eddies formed behind blunt bodies in tank experiments in fluid-mechanics laboratories. By necessity there is a significant difference in the ratio of the water depth to the width of the obstacle in the field versus in the laboratory. Rattray Island is about 1,500 meters wide, and the surrounding waters are 20 to 30 meters deep. Building a physical model that adheres to such a ratio is not practical, so laboratory experiments modify it—perhaps to a ratio very much larger than 1:1. Flows around a coral reef, of course, always have depth-to-width ratios much smaller than 1:1. The implication is that in the field, bottom friction is important, whereas laboratory models ignore it.

In addition, water tanks cannot be scaled up to mimic the intense turbulence in the separation layers that form behind a reef. Separation layers are the thin sheets of water separating the waters in the free stream from the eddy waters. Their importance lies in the intense small-scale turbulence they manifest. Field studies show that separation layers are too thin to be explicitly calculated in numerical models. But when parameters for this turbulence are explicitly added to numerical models, the models are capable of correctly reproducing the strong three-dimensional flows in the lee of a coral reef. Such flows include upwelling in the bulk of the eddy and downwelling in a thin, near-circular layer along the edges of the eddy Vertical velocities are typically 20 meters per hour—enormous values compared to those found in a classical coastal upwelling, which typically are from 5 to 20 meters per day

Water circulation through the 2,000 obstacles in the Great Barrier Reef thus includes extremely complex, three-dimensional eddies or jets, each with the horizontal dimensions of the individual reefs, typically a few kilometers in diameter, and 30 to 60 meters deep. Similar complex tidal flows prevail through mangrove forests, where the scale of the eddies is according to the size of the vegetation obstructing the flow, typically centimeters to tens of centimeters.

Biological Implications

Water currents are important because they determine the destination of waterborne particles, including natural ones such as coral eggs and the larvae of prawn and fish, as well as pollutants such as mud from runoff and dredging, pesticides and nutrients from farm runoff, and sewage from treatment plants.

The upwelling in topographically steered currents around coral reefs removes the finest mud particles from the seafloor in the island’s wake, thereby providing a different substrate for benthic, or bottom-dwelling, organisms. The downwelling, in turn, aggregates floating matter along the outer edges of the eddy. This aggregation can readily be observed following the mass spawning of corals, an event that occurs annually on the Great Barrier Reef. After release into the water column, the coral eggs float to the surface and are initially distributed widely on the surface all over the submerged coral reef. A day later, however, the eggs and larvae are found aggregated in lines that can be several hundreds of meters long but only a few meters wide. The distribution of coral eggs is thus very patchy within a few hours after mass spawning. Aggregation and patchiness prevail even though there are generally negligible vertical gradients of salinity and temperature on the Great Barrier Reef.

This aggregation is contrary to what oceanic-dispersion models predict—that the size of the patch should continuously increase as a result of diffusion. In reality, the three-dimensional flows should be calculated to account for aggregation, which acts functionally as negative diffusion. However, since the three-dimensional flows are small-scale processes, either a very small pixel (mesh) size (typically 20 meters per pixel) must be used in the models, at great computing cost, or the processes must be explicitly added to the dispersion models, with a pixel size of typically 200 meters. We chose to parameterize processes—that is, add equations representing processes that occur on a scale smaller than the pixel size—in order to provide accuracy at a reasonable computing cost.

By the second day after spawning, aggregations are no longer visible, making reliable measurements of coral-larvae concentration difficult. An over—or underestimate of concentration is possible using current field—sampling techniques, which rely on a limited number of nets towed underwater at a number of sites. Accurate estimates require a large number of sites and repeated net tows. The only data set worldwide that meets this standard was collected at Bowden Reef, on the Great Barrier Reef. The Australian Institute of Marine Science (AIMS) enjoys access to this data base, which shows that the numbers of coral larvae in the reef vicinity decrease daily after spawning and that the bulk of the coral larvae are flushed away from their natal reefs in a few days.

Numerical models of currents and of advection-dispersion of coral eggs can be calibrated against such data. Such models predict that a plume of coral larvae forms in the lee of the coral reef. The plume shows great spatial and temporal patchiness. Patterns are visible in the patchiness, but these change constantly with time, making field calibration very difficult. Computer animation facilitates this step.

Coral reefs thus depend for recruitment on oceanic dispersion of larvae emanating from upstream reefs. Of course, which reef is upstream and which is downstream depends on the prevailing currents during the dispersal period (lasting two to three weeks). Because of variations in wind and oceanic forcing, these prevailing currents can vary from year to year during the spawning season. Computer modeling, calibrated on historical data, can be used to determine this interannual variability, which enables a calculation of source reefs and sink reefs for individual years-a very important piece of information for resource managers.

Fish larvae also drift with the currents at early stages of their life, making it possible to use them as a tracer to measure their recruitment on reefs. Once again AIMS enjoys access to the most extensive field data base on the concentration of prerecruitment fish larvae around two coral reefs, Helix and Bowden reefs. The data were obtained by Peter Doherty using fish traps equipped with a light attractor. They display significant complex temporal and spatial variability that requires computer animation for interpretation. Based on an analysis of the computer animation, the fish larvae are not initially resident around a particular coral reef. Arriving from upstream on water currents, the smallest fish larvae are concentrated in hydrodynamic shadow zones on the lee side of coral reefs, and the largest fish larvae aggregate in front.

Classical oceanographic fisheries models assume that larvae are carried by simple water currents alone. When applied to the Bowden Reef site, these models predict unrealistic larvae-distribution patterns with no aggregation and patchiness. Direct SCUBA observations by J. Leis of the Australian Museum reveal that the fish larvae actually swim horizontally toward a reef. It is possible for computer models to quantitatively reproduce observed aggregations if swimming is incorporated within a 2,000-meter radius of the reef.

The importance of this behavioral pattern had never been suspected.

Most larvae recruit on coral reefs and are not dispersed widely when the active-swimming hypothesis is incorporated in the AIMS model for the entire Great Barrier Reef. In contrast, the classical “no swimming” hypothesis predicts that most larvae die at sea. Thus for the Great Barrier Reef the evidence suggests a direct link between adult fish stock and larval recruitment. This discovery reveals that the Great Barrier Reef is likely to be locally sensitive to overfishing.

Mangroves Fisheries

Mangroves play a vital role as producers of nutrients, forest resources and animal species of economic value. Here we look at one example, and a less-wellknown one at that: the link between the health of a mangrove ecosystem and prawn fisheries. Mangrove-fringed, shallow coastal waters support rich prawn fisheries, and although the spawning grounds are located in coastal waters, the nursery grounds are in the mangroves. On their way to the mangroves, the drifting prawn larvae traverse shallow coastal waters. Field data collected by V C. Chong show great spatial and temporal patchiness in the larvae during this phase. This phenomenon is best understood by exploring the data through computer visualization, which reveals seasonal and spatial gradients as well as interspecies variability.

Hydrodynamic models of mangrove-fringed coastal waters have been formulated and calibrated against the field data. Model runs incorporating two-week-old, prerecruitment larvae at concentrations and sites determined from the field data reveal that mangroves sustain prawn fisheries by providing a hydrodynamic trap. Recruitment is enhanced at spring tides, when the tidal range is large, and trapping increases at neap tides, when the tidal range is at its minimum. Thus the quality of water and the water level in mangroves appear vital to successful prawn fisheries.

Information for Resource Management Computer technology for modeling dispersion in mangroves and coral reefs provides managers with a practical tool that has a solid scientific foundation. The problems such technology can address are pressing, because these ecosystems are rapidly disappearing worldwide.

Coral reefs face anthropogenic and natural threats, and combinations of the two, that cannot be successfully addressed unless we understand the physics and the biology of the reefs. For example, our work suggests that the Great Barrier Reef may be more sensitive to overfishing than previously was believed. This has important management implications, as it now appears possible to determine scientifically the key source reefs for larvae. These reefs need higher protection levels to conserve the adult-fish stock.

For the case of mangroves, computer models and visualization of the prawnfisheries recruitment processes allow marine-resources managers to assess strategies for managing mangroves and their fisheries in view of pressure for converting mangrove land to other uses.

Computer technology is essential for advancing multidisciplinary research in marine science. It offers a practical way for physicists, chemists and biologists to merge their data sets from point measurements of currents, of concentrations of larvae of shrimp, fish and corals, of suspended sediment, and of chemical constituents (for example, hydrocarbons). Computer visualization can turn data that are patchy in time and space and practically unusable by standard statistical techniques into useful tools. It enables a scientist to understand which are the dominant physical, chemical and biological processes that must be taken into account in models of coral reefs and mangroves. These processes can then be incorporated in mathematical models of processes such as corallarvae dispersion, fisheries recruitment, siltation and pollution.

Galileo Galilei wrote, “Measure what is measurable, and make measurable what is not so.” The computer may not make measurable that which is not, but it does allow us to visualize computer data and model what for practical purposes is unmeasurable.


The authors thank the Australian Institute of Marine Science, the IBM International Foundation, Japan’s Port and Harbor Research Insitute and Kansai Electric Power Company-Kansai Environmental Engineering Company, the CRC Reef Research, and Brian King, Peter Doherty, Jamie Oliver, Ving Chin Chong and Ian Gardner.