and Brian King ... In the northern and southern regions of the lagoon, the mean depth is only 5 to 10 m ..... Lee, T. N. & Mayer, D. A. 1977 Low-frequency currents ...
Estuarine, Coastal and Shelf Science
(1990)
Flushing Barrier
Reef Lagoon,
Eric
of Bowden Reef
Wolanski
and Brian
Australian Institute of Marine Queensland 4810, Australia
31,789-804
Great
King
Science, P.M.B.
No. 3, Townsville M.C.,
Received 1 August 1989 and in revisedform 25June 1990
Keywords: lagoon flushing; numerical models;inter-reefal hydrodynamics; tidal eddies;Great Barrier Reef Field and numerical studieswere undertakenin 1986and 1987of the water circulationaroundandover BowdenReef,a 5-km long kidney-shapedcoral reef lagoonsystemin the Great Barrier Reef.In windy conditions,theflushingof the lagoonwasprimarily due to the intrusion into the lagoonof topographically inducedtidal eddiesgeneratedoffshore. In calm weather,sucheddiesdid not prevail and lagoonflushing wasmuch slower.The observedcurrentsat sitesa few kilometresapart in inter-reefal waters,have a significanthorizontal shear apparentlydue to the complexcirculation in the reef matrix. Under suchconditions, sensitivity testsdemonstratethe importanceof including this shearin the specificationof openboundaryconditionsof numericalmodelsof the hydrodynamicsaround reefs. Contrary to establishedpractice, the water circulation arounda coral reef shouldnot bemodelledby assumingreefsarehydrodynamically isolatedfrom surroundingones.Little improvementappearslikely in the reliability of reef-scalenumerical modelsuntil the inter-reefal shearcan be reliably incorporatedin suchmodels. Introduction In a review of reef lagoon circulation, Pickard et al. (1990) and Hamner and Wolanski (1988) have shown that the flushing of coral reef lagoons is controlled primarily by three processes: firstly, the tidal flushing, similar to that in an estuary; secondly, the unidirectional wind- and/or wave-driven current over the reef flat; and thirdly, by the presenceof a strong prevailing current in inter-reefal waters which may drive a unidirectional current through the lagoon with this offshore water intruding into the lagoon through one passageand leaving through another. These processes,and the resulting flushing properties of reef lagoon systems,are amenable to numerical modelling. Two-dimensional models have recently been proposed where generally only one reef, or one reef passage,is included in the model domain, assuming there are no other reefs in the vicinity (Black & Gay, 1987; Sammarco & Andrews, 1988; Wolanski et al., 1988, 1989). Wolanski and Hamner (1988) have shown that these two-dimensional models cannot be used to predict the fate of buoyant particles such ascoral eggs,becauseof the strong three-dimensional water circulation which aggregates buoyant particles along topographically controlled fronts. One aggregating process 0272-7714/90/120789+
16 $03,00/O
@ 1990 Academic
Press Limited
790
E. Wolanski
t.3 B. King
Bowden
/
0
A *Top
FG ‘*
HI ”
JC
D
Reef
Left
Right
5 km
Model
Figure 1. (a) and (b) Location maps and mooring Broadhurst Reef is the large reef located north-west
Bottom boundaries
sites, (c) location of current of Bowden Reef.
meters.
is the three-dimensional circulation in an eddy in shallow water, and this mechanism was recently verified numerically with the use of a three-dimensional barotropic model (Deleersnijder et al., 1989). Most aggregating processes occur offshore from the reef but in its vicinity (Wolanski & Hamner, 1988), and in this case the flushing of a reef lagoon is still an important process as it determines the residence time in reef waters of non-buoyant particles including dissolved nutrients, fish eggs, or some pollutants. The Coral Spawning Experiment (CORSPEX), a multidisciplinary study of the fate of coral eggs around a coral reef, involving a field study of the water circulation around Bowden Reef, was undertaken during the mass spawning of the corals in 1986 and 1987. Bowden Reef is a 5-km long kidney-shaped coral reef with a lagoon, embedded in the reef matrix on the mid-shelf of the central region of the Great Barrier Reef (Figure 1). The waters around Bowden Reef are 40 to 50 m deep with a smooth bottom. Bowden Reef has an uninterrupted reef flat to the east, north and south. The reef flat just emerges at slack low water spring tide. In its central region, the lagoon is typically 10 to 20 m deep [Figure l(c)]. In the northern and southern regions of the lagoon, the mean depth is only 5 to 10 m and there are numerous coral outcrops. During the 1986 coral spawning season, light wind generated negligible net currents over the reef flat. The lagoon was flushed primarily by the tides through the wide western opening, and by a net southward current, generated by the southward East Australian Current, flowing through the deeper western side of the lagoon. Lagoon water was trapped in the shallow eastern side of the lagoon next to the reef flat (Wolanski et al., 1989). The results of the 1987 coral spawning season study show quite different results, attributed to different wind conditions and are described below.
Flushing of Bowden Reef lagoon
Methods In December 1987, timed to coincide with masscoral spawning, current meters were deployed for 10 days at sites, A, F, G, H, I, J, C and D, on a transect acrossBowden Reef lagoon [Figure l(b) and (c)] and at site E, half-way between Bowden and Broadhurst Reefs.At the deep-water sites, A, F, G, C, D and E, current meters were deployed at middepth. The meter was either an Inter-Ocean model S4 meter or an Aanderaa RCM4-S meter. Additionally, S4 meters were also deployed at 1.5 m depth under a 2-m long rod held at the surface by buoys, at sites A, F, and G. Deployment of more current meters far away from the reef was judged too dangerous in view of extensive trawling. In the lagoon, S4 current meters were deployed 2 m below the surface at sites H and I throughout the experiment, and at site K, in the southern region of the lagoon in a maze of coral outcrops, for 1 day only. At site J, over the reef flat, two S4 meters were held one above the other in a bottom-mounted aluminium frame, sampling 1 min out of phaseto avoid either meter interfering electronically with the other. The meters were located at 0.4 and 1.3 m above the reef flat. The top meter surfaced during low spring tides. On occasions, vertical profiles of currents were measuredby anchoring a small boat in the lagoon at various sites, and lowering by stepsan S4 current meter averaging current data over 1 min, sampled at 0.5-s intervals. Currents were alsomeasured by radar tracking drogues with 2 x 2 m sails. The depth of the sails was either 2 or 10 m below the surface, according to whether the drogues were deployed in the lagoon or in deep water. Aanderaa model WL5 tide gauges were deployed, attached to rigid weights on the bottom, at sitesA and J. A meteorological buoy, that included wind speedand direction sensors25 m above sea level, was moored in the lagoon half-way between sitesH and I. All the in situ meteorological and oceanographic instruments provided data at lo-min intervals. CTD data were collected along the current meter transect. Time is expressed as day number in December 1987. For instance, time 6.5 days is 6 December 1987, 12.00 h local time. The oceanographic convention is used throughout both for wind and water currents, i.e. the direction given is that towards which the water or the air flows. Results Analysis
of field measurements
A time seriesplot of sealevel is shown in Figure 2(c). The tide was semi-diurnal with a strong diurnal inequality, with 0.5-m neap tides prevailing near days 14 and 15, and 2.5-m spring tides near days 3 and 4. The differences in tidal elevations between site J and site A was always lessthan 0.02 m [Figure 2(d)] so that in the lagoon there was no large frictionally driven trapping asin a shallow lagoon or estuary. Time seriesplots of the northward and westward wind velocity components are shown in Figure 2(a) and (b). The wind was consistently westward to north-westward, with typical speedsof 5 to 15 m s-‘. Wind-driven waves broke on the reef flat on the eastsideof Bowden Reef. Within our measurement accuracy (0.01 ppt), the ambient shelf waters were well mixed in salinity. The waters were only slightly stratified in temperature with top to bottom temperature difference not exceeding 0.5 “C (Figure 3), a shallow dirunal heated layer
792
E. Wolanski
& B. King
(a) (b) Wind
velocity
90”
Sea level
(d)
I
(i) -0.51 4
1 6
8
10
14
16
Time (days)
Figure 2. (a) and (b) Time series of northward and westward wind velocity, (c) sea level, (d) difference in sea level between sites A and J, (e), (f) and (g) stick plots of currents at sites E, A and D, and (h) and (i) long-shore and cross-shelf currents at sites A and E.
for half that temperature difference. Buoyancy-driven currents are then expected to be small. On occasions, a 2-m wide front was visible in the north-west region of Bowden Reef, separating blue shelf water from greenish lagoon water. When the front extended east-west across the northern part of the lagoon, CTD profiles across it [Figure accounting
-
Flushing
of Bowden
A
F
Temperature
Figure
793
Reef lagoon
G
H
I
D
JC
(“C)
3. Typical
cross-reef
temperature
distribution
(“C).
Front
(a)
Norfh 4 0..
Distance 10
5
44
(m) 15I
1
4
South 25 7
20
2E
27 47-27.48
c 4‘z 0” 6-
“C
Temperature
aN Transect
i b)
shown
above
t WoQC?
Jellyfish
row
Tr~chodesmwm/corol -
oocc:ioo‘
C~OOOc30 .’ ..
JellyfIsh
eggs
row
Figure 4. (a) Temperature distribution across the front in Bowden Reef lagoon, arrows indicate the location of CTD profiles, and(b) a sketch of visual observations at the from showing aggregation of jellyfish, Trichodesmium algae and coral eggs.
4(a)] showed negligible temperature differences, implying that the front was barotropic. Figure 4(b) is a sketch of readily visible features of the front, with jellyfish aligned side by side in two rows on either side of the 2-m wide front. The jellyfish formed a nearly continuous stream, being so closeto one another that they often touched. In between these two rows of jellyfish, there were large quantities of Trichodesmium algae and coral eggs. The front rotated anti-clockwise with the tides assketched in Figure 5. During neap tides,
794
E. Wolanski
0600
&B.
h
King
0800h
lOOOh
1200h
~~~~~~~~~:
Figure
5. Sketch
of the location
of the front
on 7 December
1987.
the front did not intrude into the lagoon but was swept southward by the flood tidal currents along the western side of the reef. Kingsford (pers. comm.) observed that pre-settlement reef fishes (i.e. passive swimmers) were aggregated at the front. Typical drogue trajectories are shown in Figure 6. Prevailing low-frequency currents in offshore waters were much weaker than the reversing tidal current. The flood current flowed southward around Bowden Reef, and the ebb current flowed northward. The tidal ellipses, distorted by the reef, had dimensions comparable to the reef. There was a significant horizontal shear of the currents outside the lagoon (Figure 10). In the lagoon, the currents at site I never reversed direction, but instead varied from westward to northward. Stick plots of current meter data at sites E, A and D are shown in Figure 2(e), (f) and (g). In the far field, the currents were rotating anti-clockwise as a tidal ellipse, with a weak ( < 0.05 m s-l) southward current prevailing. There was a considerable difference between the current meter data at sites A and E [see Figure 2(e) and (f)]. At times the occurrence of slack currents differed by 2 h at these two sites. At other times, the longshore current at site E did not reverse sign with the tides, contrary to the situation at site A. This point is further illustrated in Figure 2(h) and (i) showing time series plots of longshore (along 145” clockwise from north) and cross-shelf (along 55”) currents at sites A and E. This was surprising as it was expected that since both these meters were in the far field they would yield similar data. In the absence of more data, one can only attribute this large horizontal shear in inter-reefal water to the complexity of the circulation in a reef matrix. Indeed, King and Wolanski (in press) have applied a depth-averaged non-linear, barotropic, twodimensional hydrodynamic numerical model to a 188-km long region of the central Great Barrier Reef continental shelf. The region includes Bowden Reef as well as 42 other reefs, and extends from the continental slope to the shore. The model has a mesh size of 2.0 km and is forced by a direct wind stress and as its open boundaries by tides and the long-shore sea level slope due to the East Australian current. The model predicts, for the prevailing wind during this field study, considerable horizontal shear in inter-reefal waters, of comparable magnitude to the observed one. The shear is found to be patchy and intermittent, and results from the presence of eddies, jets, blocking effects and free shear layers. In the model this shear is due to disturbance to the large-scale continental shelf circulation by the ensemble of reefs. There was also considerable vertical shear in the currents (see Figures 7 and 8 discussed later), suggesting the existence of strong three-dimensional currents. The cross-reef (west-east) current over the reef flat was always less than 0.2 m s-l and was fairly uniform from top to bottom [Figure 9(b)]. This current was modulated by the
Flushing of Bowden Reef lagoon
795
4 I
7 Dec. 1987
Figure 6. Typical trajectories of drogues released at sites A and G outside the lagoon, and all the trajectories of drogues released at site I in the lagoon. The latter always ended up stranded on coral outcrops after typically 1 to 3 h.
tides but there were high-frequency oscillations of the currents of typical amplitude 0.02 m s-l. In addition, a net low-frequency westward current over the reef flat of magnitude 0’ 1 m s-l, prevailed during the study and, asits fluctuations correlated with the wind fluctuations, this current was presumably wind-driven. This inflow in the lagoon would drive a westward current in the lagoon (where the depth is 10 times larger) of the order of 0.01 m s- ‘. For unknown reasons,the along-reef (north-south) current over the reef flat at site J showed little coherence through the water column [Figure 9(a)].
796
E. Wolanski c3 B. King
(b
t I
t Y -0
H
2 m s-1
9 December1987
1530-1605
Drogue
h
Figure 7. (a) Synoptic distribution of observed currents at ebb tide on the afternoon of 9 December 1987, as measured by the moored current meters (top and bottom meters are indicated), the current meters suspended from a small boat (depth below surface indicated in m), and by radar tracked drogues (thick arrows). (b) Synoptic distribution of the depth-averaged predicted currents (for Run no. 4) at the same time.
(0)
(b
D Bottom / 1ents a, I m 1
A Bonom \
-0.2
m
4 s-1 N
lODecember
Figure
8. Same as Figure
0910-1030h
7 for a flood tide.
The data from the moored current meters, the drogues and the occasionalmeasurements of currents from a smallboat, enabled the measurement of the synoptic three-dimensional water circulation in the lagoon [Figures 7(a) and 8(a)]. A convergence of currents between sites G and H is evident in both Figures. There were negligible cross-reef currents over
797
Flushing of Bowden Reef lagoon
6
a
I
I
10
12
Time (days) Figure9.Timeseriesofobserved(a)along-reefand(b)cross-reefcurrentsatsite reef flat for both top and bottom current meters.
Jover the
the reef during northward currents offshore, with inflow of water in the lagoon at the southern end and outflow at the northern end [Figure 7(a)]. During southward currents offshore [Figure 8(a)], the prevailing westward current at sites I and H could not be accounted for by the small cross-reef flow at site J, suggestingthat water leaving the lagoon at the northern region entered the lagoon at the southern region. Hydrodynamic
model verification
The two-dimensional (depth-averaged) numerical model described by Falconer et al. (1986) and generalized to accept forcing at the four open boundaries following Wolanski et al. (1989), was used to simulate the water circulation. The model is centred, implicit and includes in the momentum equations the terms representing acceleration, inertia, water surface slope, Coriolis effects, wind stress,non-linear bottom friction (using a Manning friction parameter, n), and lateral Reynolds stresses.The model boundaries, with location of current meters, are shownin Figure 1(b). The rectangular model domain and bathymetry employed is the sameas that which was successfully used for the 1986 simulations by Wolanski et al. (1989). The reef is located at the centre of the domain, comprising 60 (in the long-shore direction) by 43 grid points. The grid interval size was386 m sothat the reef was represented by 12 points in the long-shore direction and by sevenpoints in the cross-shelf direction. No other reefs were assumedto influence the domain, and the time step was 2 min. The Manning’s n value was set to 0.03 for deep water (depth > 20 m). Over the reef flat, runs were done with n=0.03, 0.15 and 0.35, to estimate the influence of the high roughnessof a coral surface. There are four open boundaries, ideally characterizing the undisturbed far field currents. The specification of these boundaries is an ill-posed problem because of the lack of knowledge of the horizontal variability of the far field currents. Presumably, this variability is due to the complex circulation of the reef matrix in which Bowden Reef is embedded. Shelf-scale analytical and numerical models of the circulation in inter-reefal waters (Andrews &Bode, 1988; Dight et al., 1988)are not helpful becausethey neglect the reef-induced circulation and predict tidal phasedifferences of a few minutes for sitesa few kilometres apart while observations at sitesA and E reveal differences of up to 2 h [Figure 10(a)]. The phase lags appeared to vary with tidal range, being the largest at neap tides [Figure 10(a)] and the smallest at spring tides [Figure 10(b)].
798
E. Wolanski & B. King
-0.3 L
I TO
I 64
0.3
-0.3 L
I
I
9.3
95
9.7
I 7-2 T\me(days)
99
I 74
I 76
I 7.8
I 10-l
/ IO.3
10.5
Time(days)
,B
@07
-
0.02
-
E 5 0
/ se-------.
__ -0.03
---_--,
1'
.-.
Q-1
-0.08 J 4
I 6
I a
I 10
I 12
14
Tlme(days)
Figure 10. (a) and (b) Band-passed tidal and (c) subtidal along-shelf currents for sites A, E and D showing the lateral shear in inter-reefal waters. (a) Gives spring tidal signals showing small phase lags while (b) gives neap tidal signals showing larger phase lags. Similar behaviour was observed in the across-shelf direction but is not shown, -, site D. Site A; - - -, site E; and -,
Previous investigators (Black & Gay, 1988; Sammarco & Andrews, 1988; Wolanski et aE., 1988) used for the open boundaries of small-scalemodels of similar dimensions as
those shown in Figure 1(b), either the tidal height predictions of shelf-scaletidal modelsor the data from one or two current meters deployed in inter-reefal waters. There have been no attempts sofar to assessthe importance of the lateral variability of the open boundary conditions in such models, on the near-reef water circulation. To addressthis issue, runs were undertaken with several sets of open boundary conditions. The results of four of them, listed below, will be discussed. Run no. 1. Top, left and right boundaries: usecurrent meter data from site E asforcing; bottom boundary: use the observed tidal data asforcing. Run no. 2. Sameasrun no. 1 but use current meter data from site A instead of E. Run no. 3. Left boundary: use current meter data from site A; right boundary: use current meter data from site D; top boundary: use current meter data from site E; bottom boundary: useobserved tidal data.
799
Flushing of Bowden Reef lagoon
I
--A-___-
0
24
12 Time
i h)
7 December
1987
Figure 11. (a) Observed and (b), (c), (d) and (e) predicted currents at site A for the four different model simulations (marked as Al to A4 for Runs no. 1 to 4).
Run no. 4. Right and left boundaries: usecurrent meter data from site A; top boundary: use site E current meter data; bottom boundary: use observed tidal data.
In all these simulations, the model was also forced by the wind as measuredat lo-min intervals. In addition, the model was run for various values of the Manning’s coefficient n. This wasfound to have no effect on the lagoon and deep water circulation, but measurably affects the circulation over the reef flat. A value of Manning’s n of 0.35 on the reef flat gave the best results (seelater). Values lower than 0.15 over the reef flat gave rise to unrealistically large high-frequency current fluctuations over the reef flat. The results are summarized in Figures 11 to 13. The model results are analysed below by visually comparing observed and predicted currents west (site A), inside (site H) and east (site D) of Bowden Reef. Run no. 1 did not satisfactorily reproduce the current observations at site A which was believed to be far enough away to be unaffected by Bowden Reef. Indeed, there were differences of up to 2 h lag between the predicted and observed timing of slack currents (seelines T, and T, in Figure 11). This suggeststhat the left open boundary condition was not well specified by assumingthe current meter data at site E were representative of the left open boundary. As a result, in all the other runs, the left open boundary was specified asthe observed current at site A. This improved, for Run no. 2, the fit between observed and predicted currents at site A (Figure 11). However, there is a large variation of the current between the left boundary and site A, suggestingan upstream influence of the reef on the current of at least 5 km. Also, the predictions of the currents at site H were still poor, with an error in the timing of slack currents of about 3 h (Figure 12). This suggeststhat one should use the data from the current meters closest to each open boundary as open boundaries forcing; hence the justification for Run no. 3. This considerably improved the predictions at site H (Figure 12), but led to a poor reproduction of the timing of slack currents at site A in the afternoon of 7 December (Figure 11). The fit at site D (Figure 13)
800
E. Wolanski
& B. King
1
12
0 Time(h)
Figure
12. Same as Figure
7 December
24 1987
11 for site H.
Id)
Time
Figure
13. Same as Figure
(hi
7 December
1987
11 for site D.
was still only marginal and this suggests Reef to be representative of the prevailing views (see later) of the predicted currents Run no. 4, where only the current meters
that site D may have been too close to Bowden current on the right boundary, though synoptic failed to support this hypothesis. This justified from sites A and E were used for open boundary
Flushing of Bowden Reef lagoon
1 12
801
24 6 December
1987
7 December
1987
Time(h)
Figure 14. Time series of observed and predicted hourly averaged cross-reef currents over the reef flat at site J. The error bars show the range of the high-frequency oscillations, both observed and predicted for various values of Manning’s n coefficient. -, Top metre; -, bottom metre; and -- -, predicted case 4.
current forcings. This yielded better results at sites A and H, but the timing of the predicted slack currents at site D was still poor (Figure 13). Experiments with various other open boundary forcings (not shown) failed to improve the overall model fit. For instance, by setting the current forcing on the top and right open boundaries to be the current meter data from site E, and the current forcing on the left open boundary to be the current meter at site A, the fit at site D was improved but this led to unrealistic current predictions at other sites. Therefore Run no. 4 was selectedasthe ‘ final ’ simulation. The results emphasize the difficulties in specifying the open boundary forcing in a reef model with the presence of large subtidal lateral shear [Figure 10(c)] and unexplained phase lags in the tidal signal [Figure 10(a) and (b)]. Clearly, under such difficulties, numerical models may only be able to simulate the main features of the reef-induced circulation, and the discrepancies between observed and predicted currents call for considerable caution when using small-scalemodels for predictive purposes. The comparison between the observed and predicted cross-reef current at site J, over the reef flat, shows that the major features are reproduced (Figure 14). This also shows that the unidirectional strong westward current at sitesH and I, where the depth wasabout 20 m, was too large to be due to the cross-reef flow over the reef flat. Indeed, the latter averaged 0.1 m s - ’ for a total depth of 1to 3 m and furthermore, reversed sign with the tides. Instead, the persistent current at sites I and H was due to an inflow into the lagoon in the south-western region. Note that in Figure 14 the currents have been smoothed using a moving average filter spanning 1 h. There existed at site J high-frequency oscillations of both the observed and predicted currents; amplitude is shown aserror bars in Figure 14. These may be due to lagoon oscillations (e.g. Buchwald & Miles, 1981). The magnitude of these oscillations increaseswith decreasing values of the Manning’s coefficient n (seethe predictions in Figure 14 for n = 0.15 and 0.3) and best reproduced the magnitude of the observation for n = 0.3. However, the predicted hourly averaged current over the reef flat was only weakly dependent on the value of n. A comparison of the observed and predicted synoptic current fields (Figures 7 and 8) is also quite encouraging. Lagoon flushing
Figure 15 shows, at 2-h intervals, synoptic views of the predicted, depth-averaged circulation around Bowden Reef on 7 December 1987. The results are typical of the rest of the study period. Note that the areashown in these Figures is not the full domain of the model,
E. Wolanski & B. King
802
/
I
,
,.-I
-,/.-
,r.,
‘:\
I
-,r
,
.,
.
r.
.
i
,
~
,
,;
_ .\\\
I
i‘
.
!“*. I_.
.
: ,-
,-
-
-.
71 fl
/
,
,
,’
, /
//// 1
/
/
/
I,
‘/////
I--‘////,‘,’
---x--////i/
0200 h
0600 h
lOOOh 7 December
1987
Figure 15. Synoptic December 1987.
dVelocily
views
0.5 m s-1
h
of the predicted
L
depth-averaged
J lkm
water
circulation
on 7
just a subsetof it near the centre. An examination of Figure 15 suggeststhat an eddy was formed at tidal periods offshore on the north-western side of Bowden Reef. A zone of convergence was also formed at the edge of the eddy, separating lagoon from offshore waters, and this is where a frontal eddy can be expected, and indeed wasobserved, to form. The eddy generated at the north-western tip of Bowden Reef, grew in size while moving southward then eastward intruding into the lagoon, pushing the front anti-clockwise into the lagoon, as indeed was observed (Figure 5). The front ultimately extended east-west acrossthe lagoon. It was swept out of the lagoon at the end of the tidal cycle. When the eddy had intruded in the lagoon, it injected offshore water in the southern region and removed lagoon water from the northern region. Because the lagoon waters are shallow enough for friction-driven three-dimensional barotropic water circulation processesto be generated (Wolanski & Hamner, 1988), this eddy accumulated floating particles along the front, explaining the observations sketched in Figure 4. The intrusion of the offshore-generated eddy into the lagoon also explains the prevailing northward flow at sites I and H (Figure 6) and the prevailing inflow into the lagoon at the southern side. The eddy occupied up to half of the lagoon, and, from the numerical
Flushing of Bowden Reef lagoon
803
model, one can estimate that, at most, 30% of the water ejected from the lagoon returned. Thus, one expects that the lagoon was well flushed in a few tidal periods. Qualitatively, this lagoon flushing mechanism is similar to the flushing of Florida continental shelf waters by frontal eddies spun off from the Gulf Stream located further offshore (e.g. Lee & Mayer, 1977). The presence of this eddy was neither observed, nor predicted from the model, in the field study undertaken in 1986, when the wind was light. Model results suggestthat the reason for this difference is the wind conditions. In light wind, such as in the 1986 coral spawning period, a strong net southward current prevails on the shelf and this current inhibits the generation of eddies on the northern side of the reef. When trade winds prevail, such as in the 1987 coral spawning season,net currents are weak, tidal currents reverse direction and reef-induced eddiesare generated. This implies that the wind controls the flushing of Bowden Reef lagoon, by modulating the prevailing shelf currents in the far field of the reef. The wind is alsoimportant asit drives the current over the reef flat.
Results and discussion An important coral reef lagoon flushing mechanismwas discovered, namely the intrusion in the lagoon of topographically controlled eddies generated in offshore waters at tidal frequencies at spring tides. These eddies inject oceanic water into the lagoon on one side and remove lagoon water on the other side. This flushing mechanismwas very efficient in the 1987 study period when the lagoon of Bowden Reef was flushed in a few tidal cycles. The south-easterly trade winds played a dominant role in the formation of such eddies, primarily by modifying the large-scale circulation around the reef. The flushing of the lagoon was also enhanced by the westward wind-generated currents over the reef flat which were successfully reproduced by the numerical model. In 1986, when the wind was light, a stronger net southward current prevailed and prevented the formation of such eddies. This resulted in a much longer lagoon flushing time. The sensitivity tests on the influence of the open boundary conditions suggestthat until the open boundary conditions can be accurately specified in a reef hydrodynamic model, the results of any model must be viewed with great caution, especially if the forcing at the open boundaries is taken from shelf-scale models that neglect the presenceof reefs. Little improvement appearslikely in the reliability of the reef-scale numerical modelsuntil field data are collected to enable a ‘ parameterization ’ of the shearof the currents in inter-reefal waters, in such models. This shear is induced by complex flow patterns in the Great Barrier Reef matrix. This implies that, in a reef matrix system such as the Great Barrier Reef, individual reefs are not isolated hydrodynamically from surrounding reefs, and should not be modelled alone. To include this effect in numerical models of the water circulation around a particular reef, the next generation of models may still need to usea small mesh size aswe did but will need to resolve the water circulation over a much larger domain of the Great Barrier Reef, encompassing several reefs both upstream and downstream of the study reef. This may necessitatethe useof a super computer. Available field data on the shear in inter-reefal waters are sparseand insufficient for predictive or model verification purposes, and the collection of such data may be a matter of priority for reef research. The intricate relationships between tides, wind and shelf currents, and their effects on patchiness and flushing of individual reefs, calls for further research.
804
E. Wolanski
&B.
King
Acknowledgements It is a pleasure to acknowledge the assistance of Drs I?. Ridd and D. Burrage and Mr R. McAllister, discussions with Drs R. Babcock and M. Kingsford, and the support of Dr J. T. Baker. Dr D. Burrage and two anonymous reviewers criticized and improved the manuscript. References Andrews, J. C. & Bode, L. 1988 The tides of the central Great Barrier Reef. Continental Shelf Research 8, 1057-1085. Black, K. P. & Gay, S. L. 1987 Hydrodynamic control of the dispersal of crown-of-thorn starfish larvae. 1. Small scale hydrodynamics on and around schematized and actual reefs. Victorian Institute of Marine Sciences, Technical Report No. 8,67pp. Buchwald. V. T. & Miles. 1. W. 1981 On resonance of off-shore channels bounded bv a reef. Australian Journal of Marine andFreshwater Research 32,833-831. Deleersnijder, E., Wolanski, E. & Norro, A. 1989 Numerical simulation of the three-dimensional tidal circulation in an island’s wake. Proceedings of the 4th International Conference on Computational Methods and Experimental Measurements, Capri, Italy, 23-26 May 1989. Dight, I. J., James, M. K. & Bode, L. 1988 Models of larval dispersal within the central Great Barrier Reef: patterns of connectivity and their implications for species distributions. Proceedings of the 6th International Coral Reef Symposium, Townsville, Australia, 8-12 August 1988. Falconer, R. A., Wolanski, E. & Mardapitta-Hadjipandeli, 1986 Modeling tidal circulation in an island’s wake.Journal of Waterway, Port, Coastal and Ocean Engineering, American Society of Civil Engineers 112, 234-254. Hamner, W. H. & Wolanski, E. 1988 Hydrodynamic forcing functions and biological processes on coral reefs: a status review. Proceedings of the 6th International Coral Reef Symposium, Townsville, Australia, 8-12 August 1988. Lee, T. N. & Mayer, D. A. 1977 Low-frequency currents variability and spin-off eddies along the shelf off southeast Florida. Deep Sea Research 35,193-220. Pickard, G. L., Andrews, J. C. & Wolanski, E. 1990 A review of the physical oceanography of the Great Barrier Reef 1976-1986. Australian Institute of Marine Science, Monograph Series. Sammarco, P. W. & Andrews, J. C. 1988 Localized dispersal and recruitment in Great Barrier Reef corals: the Helix Reek experiment. Science 239,1422-1424. Wolanski, E. & Hamner, W. M. 1988 Topographically controlled fronts in the ocean and their biological influence. Science 241, 177-181. Wolanski, E., Drew, E., Abel, K. M. &O’Brien, J. 1988 Tidal jets, nutrient upwelling and their influence on the productivity of the alga Halimeda in the Ribbon Reefs, Great Barrier Reef. Estuarine, Coastal and Shelf Science 26,169-201. Wolanski, E., Burrage, D. & King, B. 1989 Trapping and dispersion of coral eggs around Bowden Reef, Great Barrier Reef, following mass coral spawning. Continental ShelfResearch 9,479-496.
