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Home , Great Barrier Reef, Tide

Estilarme, Coastal and Shelf Science (2000) 50,27-32 Article No. ecss.1999.0528, available online at http://www.idealibrary.com on IDEM @ Sticky in the Great Barrier

E. Wolanski" and S. Spagnol

Australian Institute of Marine Science, PM B No. 3, M C, Qld. 4810, A ustralia

Received 1 September 1998 and accepted in revised form 1 March 1999

The is a mosaic of regions of high and low reef density. meter observations upstream from a region of high reef density revealed that the tidal and low-frequency currents were steered away from the region during but not during neap tides. A mathematical model suggests that this effect was due to both tidal friction and to the dissipation of by eddies behind reefs at spring tides. For a high reef density region, this results in a longer residence at spring tides than at neap tides. Conversely, this effect also diminishes connectivity between regions of high and low reef density at spring tides. This process may affect the recruitment and of fish and other larvae in the Great Barrier Reef. It may also invalidate the use of satellite altimetry and tidal harmonic analysis for currents in the Great Barrier Reef. (C) 2000 Academic Press

Keywords: circulation; tidal friction, ; reefs; Great Barrier Reef;

Introduction from neap tides to near-spring tides (Ullman & Wilson, 1998). In the upper reaches of the Fly , The bottom friction stress is a non-linear function of the same effect results in the mean level being velocity (Nihoul et al., 1989). This results in a non­ higher, by typically 10 cm at spring tides (3-5 m tides linear interaction between wave-induced, tidal and peak-to-trough) than at neap tides (1 m tides; low-frequency currents. Oceanographers call this Wolanski et al., 1997). effect tidal friction (Pedlosky, 1982) and engineers a The Great Barrier Reef (Figure 1) is characterized stress (Massei, 1989). Radiation stresses can by a juxtaposition of regions of low reef density control water circulation over a barred (Slinn (where the reef block only 10% of the length along et al., 1998). There are many reported instances the shelf) and high reef density (where the reefs block where the net currents are significantly reduced by the about 90% of the length; Pickard et al., 1977). Each presence of strong reversing currents driven by waves of these regions is a few hundred km in length. or tides. Among the first reported cases was a shallow Previous studies of reef have largely gulf in South Australia (Provis & Lennon, 1983). neglected to consider this large-scale variability. A Waves enhance the bottom friction for -driven large spring-neap cycle exists on the Great Barrier currents, reducing the flushing of shelf waters and Reef. Therefore, one expects on physical grounds shallow bays such as the Irish Sea and Boston Harbor that regions of high reef density may be less per­ (Signell et al., 1990; Davies & Lawrence, 1995; meable to low-frequency currents at spring tides Signell & List, 1997). Tides also enhance the bottom than at neap tides. Wolanski (1994) coined the friction for wind-driven currents. As a result the term ‘ sticky water ’ to explain this likely effect. His low-frequency currents in and the Gulf supporting data were however sparse, consisting of Carpentaria are measurably reduced, by a factor of only of drogue trajectories over five days and of two to three, by the strong tidal currents (Wolanski, occasional satellite pictures of chlorophyll distribution 1993; Wolanski et al., 1988). suggesting trapping at spring tides in regions of high This effect introduces low-frequency variability in reef density. regions with large differences between spring and In this study, current meter data are used to neap tides. For instance in the Hudson River , unambiguously demonstrate the ‘ sticky water ’ the bottom drag coefficient increases by about 30% phenomenon in the Great Barrier Reef. A mathemati­ Full sized figures, tables and animations are stored on the CD- cal model is used to show that this effect is due to not ROM accompanying this article. Use a Web browser to access only bottom friction but also to energy dissipation in the start page ‘ default.htm ’ and follow the links. The help file ‘ help.htm ’ provides answers for some common problems. secondary flows behind reefs. It is shown that the ‘'E-mail: [email protected] combined effects of tidally-modulated friction and

0272-7714/00/010027 + 06 $35.00/0 (C) 2000 Academic Press 28 E. Wolanski and S. Spagnol

T a b le 1 . Current meter sites, January-March 1994 this region. The trajectories of water-born tracers were predicted from these data using the Lagrangian Water depth Elevation (m) advection- model described by Oliver et al. Site (m) of current meters (1992) for which the eddy-diffusion coefficient was set to 3-0 m 2 s~ \ A 37 10 and 18 Tidally-predicted currents were calculated from B 55 10 and 30 field data using tidal harmonic analysis. The tidally- C 65 20 D 114 38 predicted currents include the mean current over the E 7 5 whole period of observations. The residual currents were calculated as the difference between the ob­ served and tidally-predicted currents. The wind- driven currents were calculated as the linear fit energy dissipation in secondary flows result in a higher between wind and residual currents. residence time of water in a region of high reef density The results from the field and the model were at spring tides than at neap tides. This effect also visualised using IBM’s Data Explorer (Gallowayet al., diminishes connectivity between regions of high and 1995). low reef density at spring tides. Possible implications for the dispersion and recruitment of coral fish larvae are discussed. Results

The waters were vertically well-mixed in M ethods and salinity. The tides were semi-diurnal with a strong The field study was carried out along a cross-shelf spring-neap (c. 3 m spring tides peak-to- transect on the outer shelf of the central Great Barrier trough, c. 1 m neap tides). In calm , a strong Reef of Australia (Figure 1). The transect passes longshore southward current, about 0-2 m s-1, was between Bowden Reef and Darnley Reef. North of observed on the outer shelf (site D) and this net Bowden Reef, reef density is low, i.e. the reefs block longshore current was also apparent on the shelf at site about 10% of the distance along the shelf. South of A (Animations 1 and 2). The tidal currents were Bowden Reef the reef density is high, i.e. the reefs steered very differently by the topography both at block about 90% of the length along the shelf. neap tides (Animation 1) and at spring tides (Ani­ Offshore, in the adjoining , the net mation 2). At site A the currents were longshore, with is southwards with the the water flowing southward at rising tide. At sites B (Wolanski, 1994), and in calm weather this southward and C they rotated anti-clockwise with the main axis current also prevails through the Great Barrier Reef. oriented cross-shelf. At site D, on the shelf slope, the Hence the transect line is located just upstream of a currents also rotated with the tides, the main axis region of high reef density. being cross-shelf. These tidal currents modulated a Vector-averaging Aanderaa and InterOcean S4 longshore southward net current. At site E on the reef current meters were deployed along a cross-shelf crest the currents reversed 180° with the tides with no transect at sites A to D (Figure 1) from January to rotation because of the topography of the channel, and March 1994. The distance between B and D is 46 km. they were the largest at spring tides for all sites, A fifth mooring was located at site E in the only gully peaking at 1-3 m s - \ 9 m deep and 54 m wide through the reef crest at During the two days shown in Animations 1 and 2 Darnley Reef. A tide gage was also bottom-mounted for respectively neap and spring tides, there was a net in shallow waters at Old Reef. Wind data from nearby southward current of about 0-15-0-2 m s-1 at both Davies Reef were also obtained. CTD data were inshore and offshore ends of the region of high reef obtained at each mooring site at moorings’ deploy­ density (sites A and D). During that time calm ment and recovery. Table 1 summarizes the water weather prevailed and the wind-driven currents were depth and immersion depths of the meters. All current negligible. These two animations illustrate what meters and the tide gage recorded 30 min averaged happens when in calm weather a net current meets a currents. The water depth on the shelf varies between region of high reef density. At neap tides (Animation 40 and 100 m. In this region only the crest of the reefs 1) the currents at site B pointed for several hours come out of water at low spring tides. towards the passage between Old and Darnley Reef. The depth-averaged 2-D model of King and Hence, the current was able to filter through the reef Wolanski (1996) was used to calculate the currents in matrix. However, at spring tides (Animation 2) the Sticky waters in the Great Barrier Reef 29 currents were deflected offshore or inshore and largely the matrix of reefs (Animation 6). The situation is flowed around, instead of through, the reef matrix. quite different at spring tides when 75% of the tracers Clearly thus at neap tides the net southward current move around the model barrier reef and only 25% found its way through the passage, while at spring move through the model barrier reef (Animation 4). tides this current was deflected sideways around Hence only half as much tracers filter through the reef the reef matrix. Hence, the matrix of reefs of Old and matrix at spring tides than at neap tides. Darnley reefs was porous to the prevailing south­ ward longshore current at neap tides, and impervious C onclusion at spring tides. Hence the waters appear ‘ sticky ’ at spring tides and not at neap tides. The Great Barrier Reef is characterized by a juxta­ position of regions of low reef density (where the reefs block only 10% of the length along the shelf) and D iscussion regions of high reef density (where the reefs block Strong tidal currents prevail in this region as shown by about 90% of the length; Pickard et al., 1977). Each of the animations for the mooring sites. The model of these regions is a few hundreds of km in length. Beside King and Wolanski (1996) reproduces this well our study site near Bowden Reef, other high reef (Animation 3). This animation shows large tidal density regions include those near 9-5°S, 12-5°S, currents in reef passages at spring tides, peaking at 14-5°S and 21°S. There is also a marked spring-neap 1-3 m s_1. These currents generate eddies and tide cycle. Together these two factors introduce a stagnation zones behind reefs, these form and dis­ spatial and temporal variability in the water circu­ appear at tidal frequencies. These eddies extract lation through the Great Barrier Reef that previous considerable energy which is ultimately dissipated studies have neglected. The results of our field and by bottom friction (Wolanski, 1994; Denniss & model studies suggest that this variability controls the Middleton, 1993; Furukawa & Wolanski, 1996) large-scale circulation in the Great Barrier Reef. In because these eddies disappear in a few hours when particular a region of dense reef matrix is more the tidal currents reverse. As a result there is consid­ impervious (i.e. its water more ‘ sticky ’) at spring erably more dissipation of energy in a region of high tides than at neap tides. This finding also implies that reef density than elsewhere. Thus on physical grounds tidal harmonic analysis leads to erroneous results it would appear likely that at spring tides a net current because the direction of the principal tidal axes is would be deflected away from a region of high reef different at spring and neap tides. density. The sticky water effect appears qualitatively similar To more clearly illustrate this effect the 2-D model in coral reefs (this study) and forests of King and Wolanski (1996) was run for an ideal, (Furukawa et al., 1997). Eddies are generated behind model barrier reef (Figure 2 and Animation 4). The every mangrove root and branch, in the same manner model barrier reef forms a region of high reef density that eddies also form behind coral reefs. In with slate-like forming a matrix in the middle the flow also tends to be steered away from regions of a channel 42 km wide and 61 km long. The channel of high vegetation density. In mangroves this leads is open at both the northern (top of Figure 2) and to enhanced siltation and enhanced trapping of southern (bottom) ends. The sea floor is assumed to mangrove seeds in regions of high vegetation density. be flat at 40 m. A net southward current is forced by If the analogy between mangroves and coral reefs imposing a sea-level gradient along channel, while a holds, the sticky water effect may also enhance larvae semi-diurnal tide is forced at both the southern and recruitment and trapping in a region of high reef northern open boundaries. These two tides are lagged density. in order to generate a reversing tidal current. At spring Visualization appears essential to explore such com­ tides the tidal currents peak at 1-3 m s ~1 in reef plex data sets of currents and tracer plumes. The passages and generate tidal eddies in the lee of islands tracers can represent fish and coral propagules coming (Animation 5). Water-born tracers are released from a region of low reef density located upstream. upstream of the matrix of reefs. The model then demonstrates that of the propagules At neap tides much of the water enters the originating from a low density region, twice as many model barrier reef and escapes sideways to join the are recruited in a downstream high reef density at southward current outside and along the model neap tides than at spring tides (Figure 3). Also of barrier reef (Figure 2). About 50% of the water-born propagules originating from a high reef density region, tracers initially located upstream from the model twice as many are retained in that region at spring barrier reef find their way, at a reduced speed, through tides than at neap tides. 30 E. Wolanski and S. Spagnol

These processes may affect the fisheries References of the Great Barrier Reef. Indeed they imply that Davies, A. M. & Lawrence, J. 1995 Modeling the effect of the recruitment of coral fish larvae which have a wave-current interaction on the three-dimensional wind-driven free-drifting phase (Leis, 1991) varies both spatially circulation in the eastern Irish Sea. Journal of Physical and temporally. Temporal variation would be due to Oceanography 25, 29 15. Denniss, T. & Middleton, J. H. 1994 The effect of viscosity and the spring-neap tidal cycle; and spatial variation would bottom friction on recirculating flows. Journal of Geophysical be dependent on reef density. These two effects 99,10183-10192. operating together would inhibit fish larvae from a Furukawa, K. & Wolanski, E. 1996 Shallow-water frictional effects in wakes. Estuarine, Coastal and Shelf Science46, 599-608. region of low reef density seeding a region of high reef Furukawa, K., Wolanski, E. & Mueller, FI. 1997 Currents and density downstream during spring tides. In addition, a transport in mangrove forests. Estuarine, Coastal and region of high reef density tends to retain its fish Shelf Science 44, 301-310. Galloway, D., Collins, P., Wolanski, E., King, B. & Doherty, P. larvae, especially at spring tides. Since the fish larvae 1995 Visualisation of oceanographic and fisheries data for are able to swim horizontally to influence their distri­ scientists and managers. IBM Communique 3, 1-3. bution (Leis et al., 1996; Wolanski et al., 1997), it Fhighes, T. P., Baird, A. Fl., Dinsdale, E. A., Moltschanlwskyj, N. A., Tanner, J. E. & Willis, B. L. 1999 Patterns of recruitment appears likely that in a region of high reef density the and abundance along the Great Barrier Reef. 397,59-63. majority of the fish larvae may be retained at spring King, B. & Wolanski, E. 1996 Tidal current variability in the central tides. In so doing they may escape partially or com­ Great Barrier Reef. Journal of Marine Systems 9, 187-202. pletely oceanic dispersal outside of the region of high Fxis, J. 1991 The pelagic phase of coral reef fishes: larval biology of coral ref fishes. In The of Fishes on Coral (Sale,Reefs P., reef density. At neap tides however the larvae are more ed.). Academic Press, San Diego, pp. 183-230. readily exported downstream to a region of low reef Fxis, J. M ., Sweatman, FI. P. A. & Reader, S. E. 1996 W hat the density. Conversely, the sticky water effect tends to pelagic stages of coral reef fishes are doing out in blue water: Daytime observations of larval behavioural capabilities. Marine break up the Great Barrier Reef into subdivisions with Freshwater Research 47, 401-411. probably little demographic connectivity at spring Massel, S. R. 1989 Hydrodynamics of Coastal Zones.Elsevier tides. Scientific Publishing Co., 336 pp. Nihoul, J. C. J., Deleersnijder, E. & Djenidi, S. 1989 Modelling the Present field studies of coral and fish recruitment on general circulation of shelf by 3-D k-s models. -Science the 2500 km long Great Barrier Reef rely on sampling Reviews 26,163-189. a few reefs chosen only by their latitude and distance Oliver, J., King, B., Willis, B., Babcock, R. & Wolanski, E. 1992 Dispersal of coral larvae from a coral reef. Comparison between from the to sample fairly uniformly along the model predictions and observed . length of the system (Hughes et al., 1999). Our study Research 12, 873-891. suggests that this sampling strategy may be inappro­ Pedlosky, J. 1982 Geophysical Fluid Dynamics. Springer-Verlag, priate and there is a need for a new sampling strategy Berlin, 624 pp. Pickard, G. L., Donguy, J. R., Flenin, C. & Rougerie, F. 1977 A focusing on differentiating between regions of high review of the of the Great Barrier Reef and low reef density seen as individual meso-reefs. and western Coral Sea.Australian Institute of Marine Science, Our study demonstrates that in a zone of high reef Monograph Series Vol. 2, 134 pp. Provis, D. G. & Lennon, G. W. 1983 Eddy viscosity and tidal cycles density the sticky water effect results in the net cur­ in a shallow sea. Estuarine, Coastal and Shelf Science16,351-361. rents varying hugely with the spring-neap tidal cycle Signell, R., Beardsley, R. C., Gräber, FI. C. & Capotondi, A. 1990 for a constant forcing by the East Australian Current. Effect of wave-current interaction on wind-driven circulation in narrow, shallow embayments. Journal of Geophysical Research 95, However to remove the tides from instantaneous 9671-9678. satellite altimetry measurements of , averaging Signell, R. & List, J. FI. 1997 Effect of wave-enhanced bottom is required over typically two weeks of data. Satellite friction on storm-driven circulation in Massachusetts Bay. ASCE Journal of Waterway, Port, Coastal and Engineering 123, altimetry thus averages over the spring-neap tide 233-239. cycle. Since during that cycle the mean flow varies Slinn, D., Allen, J. S., Newberg, P. A. & Flolman, R. A. 1998 enormously, satellite altimetry yields a satellite- Nonlinear shear instabilities of alongshore currents over barred . Journal of Geophysical Research 103,18357-18379. average flow which has little meaning at least at Ullman, D. S. & Wilson, R. E. 1998 Model parameters estimation our study site. Our results thus suggest that satellite using data assimilation modelling: temporal and spatial variability altimetry may lead to unreliable estimates of the net of the bottom drag coefficient. Journal of Geophysical Research 103 longshore currents through the Great Barrier Reef. 5531-5549. Wolanski, E. 1993 Water circulation in the Gulf of Carpentaria. Journal of Marine Systems 4, 401-420. Wolanski, E. 1994 Physical Oceanographic Processes of the Great Acknowledgements Barrier Reef. CRC Press, Boca Raton, , 194 pp. Wolanski, E., Ridd, P. & Inoue, M. 1988 Currents through Torres This research was supported by the Australian Strait. Journal of Physical Oceanography 18, 1535-1545. Wolanski, E., Doherty, P. & Carleton, J. 1997 Directional swim­ Institute of Marine Science and the IBM International ming of fish larvae determines connectivity of fish populations on Foundation. the Great Barrier Reef. Naturwissenschaften 84, 262-268. Sticky waters in the Great Barrier Reef 31

F i g u r e 1 . Three-dimensional view of the central region of A n i m a t io n 1. Three-dimensional visualization of the the Great Barrier Reef with the mooring sites. The view is measured currents at the mooring sites during neap tides from the North looking south. Australia is to the right and and calm weather. The red arrows indicate the tidally- the Coral Sea to the left. The view is vertically distorted, predicted currents and the blue arrows the wind-driven mean depth around the reefs is 4 0 - 6 0 m, the width of the currents (the latter are negligible). Local time is indicated at outer shelf where reefs are scattered is about 50 km. the bottom. Australia is to the right and the Coral Sea to the left. The view is vertically distorted, mean depth around the reefs is 4 0 - 6 0 m, the width of the outer shelf where reefs are scattered is about 5 0 km.

F i g u r e 2 . Synoptic distribution of the depth-averaged cur­ rents in the model barrier reef at neap tides. Australia is to the left and the Coral Sea to the right. The view is vertically distorted, depth around the reefs is 4 0 m, the length of the channel is 61 km and the width 4 2 km. A n i m a t io n 2 . Visualization of the measured currents dur­ ing spring tides and calm weather. The red arrows indicate the tidally-predicted currents and the blue arrows the wind- driven currents (the latter are negligible). Local time is indicated on the bottom. Australia is to the right and the Coral Sea to the left. The view is vertically distorted, mean depth around the reefs is 4 0 - 6 0 m, the width of the outer shelf where reefs are scattered is about 5 0 km.

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F i g u r e 3 . Comparison between the predicted distribution of water-born tracers at neap tides (case A) and spring tides A n i m a t io n 3 . Visualization of the predicted depth- (case B), no wind. Australia is to the left and the Coral Sea averaged tidal currents at spring tides following the model of to the right. King and Wolanski ( 1 9 9 6 ) . 32 E. Wolanski and S. Spagnol

R u n B

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A n i m a t io n 4 . Visualization of the plume of water-born tracers released upstream of a model barrier reef at spring tides, no wind (case B). The view is vertically distorted, depth around the reefs is 4 0 m, the length of the channel is 61 km and the w idth 4 2 km.

A n i m a t io n 5 . Close-up of the visualized, depth-averaged currents at spring tides near a reef passage in the model barrier reef at spring tides.

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A n i m a t io n 6 . Visualization of the plume of water-born tracers released upstream of a model barrier reef at neap tides, no wind (case A). The view is vertically distorted, depth around the reefs is 4 0 m, the length of the channel is 61 km and the w idth 4 2 km.