OCEANOGRAPHIC PROCESSES OF CORAL REEFS: Physical and Biological Links in the Great Barrier Reef - Chapter 5

5 Water Circulation in Mangroves, and Its Implications for Biodiversity Eric Wolanski, Yoshihiro Mazda, Keita Furukawa, Peter Ridd, Johnson Kitheka, Simon Spagnol, and Thomas Stieglitz CONTENTS Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Tidal Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Water Flow through the Forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Mixing, Flushing, and Seed Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Waves in Mangroves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Maintenance of Biodiversity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Sedimentation and Sea-Level Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Fine Sediment as a Tracer for Mixing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Groundwater Flow and Bioturbation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Recruitment of Prawn Larvae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 INTRODUCTION There are two dominant types of mangrove swamps, the riverine type that fringes rivers and tidal creeks, and the open water type that is directly exposed by waves (Lugo & Snedaker, 1974). The former type is the most common, with a strip of man-groves typically 50 to 300 m wide fringing the tidal creek or river on either side. Such an example is the 5-km-long mangrove-fringed Merbok River estuary in Malaysia (Figure 1). The second type is generally present only in embayments protected by shallow reefs and mud or sand banks that allow wave attack only around high tide. Missionary Bay in Australia (Figure 1b) is a typical example of an extensive man-grove swamp that is protected from the prevailing tradewinds but nevertheless is occasionally attacked by waves in the monsoon season. Along coral reefs, mangroves 53 © 2001 by CRC Press LLC 54 Oceanographic Processes of Coral Reefs can also be present and are protected from excessive wave attack by waves breaking on the fringing reefs. Along muddy coasts a strip of mangroves, typically a few hun-dred meters wide, can fringe the open coast, and these are very frequently under wave attack; nevertheless, they survive. Such is the case of the Thuy Hai coast of the Gulf of Tonkin in Vietnam (Figure 1c). The vegetation consists of many species. Basically there are six prevailing pat-terns of vegetation structure near the substrate, i.e., that part of the vegetation that interferes with the water currents (Figure 2). The structure varies from simple trunks looking like vertical rods, to a buttress formation, to a complex network of roots and pneumatophores. TIDAL CIRCULATION Field studies of tidal circulation in mangrove-fringed tidal creeks show that peak velocities in the creek can exceed 1.5 m/s, while 20 m away in the mangrove forest the speed rarely reaches 0.07 m/s. Thus the system can be divided into two basic com-ponents: firstly, the tidal creek where fully non-linear, open water hydrodynamics prevail; secondly, the mangrove swamp where vegetation reduces the currents by friction (Wolanski et al., 1990; Mazda et al., 1995). In the forest the water circulation results from a balance between the frictional slope (both bottom-induced and that due to the vegetation) and the slope of the surface as water flows downhill. In the creeks, inertia and acceleration effects can be important. The water slope in the swamp is about equal to the frictional slope, Sf, which can be calculated using Manning’s for-mula. This formula relies on an empirical Manning coefficient, n. A priori, the value of n is unknown. It can be calculated for mangroves by two methods, described below. One method requires varying the value of n in the mangroves in a hydrody-namic model until the observed currents in the creek are well reproduced by the model. The second method requires calculating details of the currents through the forest. The system can be divided into a number of cells of irregular shape and size fit-ted to the topography of the creek and the swamp (Figure 1a). The swamp is only inundated by the highest tides. Typically this occurs only a few times a month for a few hours at a time. At neap tides, lasting typically 7 to 10 days, the vegetated swamp is not inundated. At the border between salt pans and mangroves the tidal inundation can be even more infrequent. The complex currents are steered by topography and vary spatially and tempo-rally (Animation 1). The spatial differences are evidenced by the significant differ-ences in the velocity in the creek and in the swamp (Figure 3). The temporal differences occur at tidal frequency in the creek, where the flow direction reverses typically every 6 h for a semi-diurnal tide (Figure 3). In the swamp, flow reversal can occur within an hour after tidal inundation begins. At the mouth of the creek, the velocity can exceed 1.5 m/s (Figure 3). There is also a large vertical shear of the cur-rents that is due to bottom friction preferentially slowing down the bottom waters. As mentioned above, an estimate of the value of the Manning friction parameter n can be found from the model by varying the value of n in the swamp, ns, until the © 2001 by CRC Press LLC Water Circulation in Mangroves, and Its Implications for Biodiversity 55 observed and predicted currents in the creek agree (Figure 3). This calibration leads to the estimate of ns 0.1 to 0.2 (Wolanski et al., 1980 and 1992), which is five to ten times higher than the value in the creek (n 0.025). This value of ns is the same as that derived semi-empirically for flow through salt marsh grass (Burke & Stolzenbach, 1983) and through crops (Petryk & Bosmajan, 1975). This demon-strates that the flow through the forest is strongly controlled by the vegetation in both mangroves and salt marshes. WATER FLOW THROUGH THE FOREST At scales of 1 to 10 cm the flow through the vegetation is highly turbulent with eddies, jets, and stagnation apparent (Figure 4). To measure this flow, Furukawa and Wolanski (1996) and Furukawa et al. (1997) introduced small, floating tracers and photographed their trajectories around the trees. The trajectories they measured are an integrated view of the flow around a tree (Figure 5). Both the mean circulation and the turbulent fluctuations due to unsteady flows can be calculated from such data. A wake is apparent behind the roots. In other regions, zones of accelerated flows are found, sometimes leading to jets between two roots. Stagnation or deceleration zones are also readily apparent. The overall result is a pattern of stripes of zones of alter-nating accelerated and decelerated flows. This pattern results from the interaction between vortices generated by each root. The resulting flow is highly turbulent, with the areas of accelerated flows having turbulent velocities typically three times larger than the mean flow. This turbulence is important in maintaining the mud in suspen-sion (discussed further on). When averaged over many trees, the flow resistance from the vegetation can be parameterised by the use of an overall drag coefficient, Cd. The use of an overall drag coefficient is common practice in engineering when calculating the flow resistance from an obstacle such as an airplane wing or the pier of a bridge. It is dependent on the Reynolds number Re U W/ where U is the undisturbed velocity, W the obstacle width, and the kinematic vis-cosity. Mazda et al. (1997b) showed that for flows through a mangrove forest the length scale W is the effective vegetation length scale (Figure 6), W (V Vm)/A where for a m2 area of the substrate, V is the volume (surface area 1 m2 the depth of water) and Vm is the total volumes of obstacles (mangrove roots, tree trunks, etc.) in the volume V, and A is the total projected area of the vegetation obstacles to the flow in the control volume V. Thus W is a measure of vegetation density and can be obtained from simple field measurements. Parameterisation of the vegetation density shows (Figure 7) that the flow through a mangrove forest, regardless of species or location, follows a physical law. The value of Cd decreases with increasing values of © 2001 by CRC Press LLC 56 Oceanographic Processes of Coral Reefs Re. For Re 50,000, corresponding to an increasing separation between trees, Cd converges toward 0.4, which is its correct magnitude in flows around a single cylin-der (Batchelor, 1967). At low values of Re (10,000), Cd reaches a value as high as 10. This indicates that in shallow waters the prop roots and pneumatophores play an important role in slowing down the flow through the vegetation. These results are thus physically pleasing as it implies a universal law which may apply for skin fric-tion in mangroves as a function of depth and vegetation density. MIXING, FLUSHING, AND SEED DISPERSION The flushing rate of a tidal creek–mangrove system is controlled primarily by lateral trapping in the mangroves and internal circulation in the creek. Lateral trapping is the process of temporary water storage in the swamp at rising tide, while swift tidal currents prevail in the tidal creek or river. On returning to the creek at ebb tide, this water mixes with “new” water. This process can be parame-terised in an empirical eddy diffusion coefficient, Kx (Wolanski & Ridd, 1986; Ridd et al., 1990). It is an accepted practice in the engineering literature to use an eddy dif-fusion coefficient to parameterise mixing in estuaries (Fischer et al., 1979). This coef-ficient is useful because it enables one to calculate the residence time, T, of water in a creek–mangrove system (Wolanski, 1992): T ~ L2 / Kx where L is the length of the creek. The trapping phenomenon is key to estimating the outwelling rate of nutrients from mangrove forests. The water circulation generates strong longitudinal gradients in the nutrient concentration (Animation 2). These gradients can be different in the mangroves and in the creek, especially at ebb tide when strong currents can prevail in the creek while the swamp water is nearly stagnant. The lateral trapping phenom-enon controls the flushing of mangrove-fringed tidal creeks. Numerical models of the flushing of 5-km-long mangrove-fringed tidal creeks suggest typical residence times of the order of 7 to 10 days (Wattayakorn et al., 1990; Ridd et al., 1990). Another method to estimate the value of Kx and hence the residence time is from the salt balance. During the dry season and in the absence of surface or groundwater freshwater inflow, most mangrove creeks become hypersaline. At steady state this excess salt is flushed out by longitudinal diffusion. This salt balance is expressed mathematically as Aa Kx dS/dx As E S where Aa is the creek cross-sectional area, As the surface area of the creek–mangrove swamp system, S the salinity, E the evapotranspiration rate, and x the distance along the tidal creek. Kx can be determined from this equation, as all the other terms can be measured in the field. In mangrove creeks about 5 km long in Australia and Thailand this technique yields estimates of the residence time of about 7 to 10 days. © 2001 by CRC Press LLC Water Circulation in Mangroves, and Its Implications for Biodiversity 57 Hence tidal creeks have the potential to retain nutrients and pollutants for long periods. A large swamp containing many mangrove creeks (Figure 1b) along a muddy coast acts as a lateral trap for coastal waters. This is because the waters moving in and out of the creeks with the tides mix little with the offshore waters (Wolanski & Ridd, 1986) but exchange readily between creeks. As a result the residence time of coastal waters can be extremely long. For instance, in the mangrove-fringed Hinchinbrook Channel, the residence time is about 50 days (Wolanski et al., 1990). The flushing of a mangrove creek can be strongly affected by internal circulation due to baroclinic effects, principally the salinity. In the dry season evaporation and evapotranspiration can remove more water than is brought in from riverine inflow, and hence mangrove creek becomes hypersaline. A salinity maximum zone results so that almost none of the riverine water reaches the sea (Wolanski, 1986). The riverine dissolved and particulate nutrients do not reach the sea either; the mangrove creek retains them all. The biological implications of this long-term trapping have not been explored yet. When the tidal currents are not large enough to destroy the stratifica-tion, the hypersaline waters sink under oceanic waters near the mouth (Figure 8a). This sinking vertically stratifies the creek and can result in stagnation of the near-bot-tom waters where anoxic conditions can exist especially at neap tides (Figure 8b). At spring tides the tidal mixing is usually sufficient to prevent anoxic conditions. The input of freshwater in mangrove-fringed creeks can cause gravitational cir-culation typical of temperate estuaries, with lighter, brackish water floating over denser oceanic water at the bottom. This induces a flow in opposite directions near the surface and near the bottom (Fischer et al., 1979). At spring tides tidal inflows and outflows of water generate strong tidal currents in the creek. Generally these currents are strong enough to thoroughly mix the mangrove creek. However, during neap tides this is not the case. At rising tide the fresher surface waters are exported laterally into the mangroves (Figure 9). At falling tide this water returns to the estuary where it has been observed to stay trapped in a narrow river plume along the banks of the estuary. This process inhibits mixing and increases the residence time. Thus mangrove creeks can stratify and destratify in salinity at weekly intervals due to the spring-neap tidal cycle. These processes may introduce a cycle with a period of 2 weeks in the productivity of the fringing mangroves. The groundwater inflow may also lead to stratification in mangrove creeks, e.g., Mida Creek in Kenya (Figure 10) studied by Kitheka (1998). The substrate is lime-stone and groundwater inflow is piped by natural cavities to a few exit points in deeper reaches of the creek. The inflow of groundwater at the bottom causes vertical salinity anomalies characterised by the presence of a layer of lower salinity water at the bottom of deep channels and relatively higher salinity at the surface. The top-to-bottom difference in salinity can reach 1.40 psu. The presence of fresher water at the bottom makes the water mass unstable, leading to an overturning of the creek water. The resultant ventilation of the bottom waters may prevent anoxic conditions. At Mida Creek this process occurs throughout the year, except during droughts. Floating matter, such as seeds and vegetation detritus, is moved primarily by sur-face currents. These are affected by cross-channel currents caused by salinity. This circulation is characterised by an axial convergence or divergence at the middle of the © 2001 by CRC Press LLC ... - tailieumienphi.vn