The importance of mangrove flocs in sheltering seagrass in turbid coastal waters

E. Wolanski(1), S. Spagnol(1) and E. B. Lim(2)

e.wolanski@aims.gov.au

Australian Institute of Marine Science, PMB No. 3, Townsville M.C., Qld. 4812, Australia.
Department of Mathematics, Universiti Sains Malaysia, Penang, Malaysia.

Abstract

The seagrass beds in the mangrove-fringed shallow coastal waters of Hinchinbrook Channel, Australia, survive in shallow coastal waters. They are sheltered from excessive sedimentation and turbidity by the plankton and vegetative detritus generating a marine snow that accelerates the settling of fine mud out of suspension.

Introduction

Mangrove-fringed coastal waters in a number of embayments in tropical Australia are often very turbid, with visibility reduced to a few cm in inter-tidal waters. Below the mean sea level, sometimes only tens of meters from the coast, extensive seagrass beds are found. Such is the case in Hinchinbrook Channel near Cardwell, Cleveland Bay near Townsville and Trinity Bay near Cairns. At some sites the seagrass is not found closer inshore for a number of reasons including occasional desiccation, excessive turbidity and minimal light, and storm-induced liquefaction of the bottom. High sedimentation in other locations has damaged or destroyed seagrass beds (Onuf, 1994; Schoellhamer, 1996). The seagrass thus survives in a potentially very hostile environment. The aim of this paper is to identify the physical processes that help the seagrass to survive.

Methods

The field study site was the mangrove-fringed Hinchinbrook Channel near Cardwell (Fig. 1). An oceanographic mooring was deployed at site 1 for 5 weeks in July-August 1996 and January-February 1997. The mooring included an Inter-Ocean model S4 current meter and 4 nephelometers spread between 0.2 to 1.8 m above the bottom. The nephelometers were calibrated in-situ to yield the suspended sediment concentration (SSC). Vertical profiles of temperature, salinity and SSC were obtained at stations using a SeaBird CTD equipped with a nephelometer that was also calibrated in-situ. Microphotographs of the sediment in suspension were obtained using the technique described by Wolanski and Gibbs (1995).

Results

The shallow coastal waters, especially the inter-tidal areas, were extremely turbid, both in the dry and the wet season but only at spring tides. The mooring data show the near-bottom suspended sediment concentration (SSC) reaching 1 kg m^-3 (1000 ppm; Fig. 2), but only during peak tidal currents at spring tides. At such times visibility was less than 2 cm. The rest of the time during spring tides, as well as throughout the neap tides, the SSC values were small and visibility at times exceeded 1 m.

The high turbidity zone, when it was found, was restricted to a narrow band along the coast (Fig. 1). This turbid coastal water was named the coastal boundary layer by Wolanski and Ridd (1990). This water, shared between the mangroves and the ocean, mixes very slowly with offshore waters and thus provides a buffer between the mangroves and offshore waters (Wolanski et al., 1990). The formation of a coastal boundary layer is due to the effect of bottom friction in shallow coastal waters.

The coastal boundary layer was about 800 m wide and was identified by its high turbidity with suspended sediment concentration commonly exceeding 0.04 kg m^-3 (Fig. 1) and peaking at 1 kg m^-3 (Fig. 2). This layer was parallel to the mangrove-fringed coast without diffusing offshore. In this layer the settling zone was made readily visible by a turbidity front separating clear water offshore from turbid water inshore (Fig. 3), this front was found about 100 m offshore and then only at spring, ebb tides.

The state of the fine sediment in suspension varied greatly from spring to neap tides. During neap tides, there was practically no sediment in suspension at that time. Oceanic water was found throughout the channel. The small amount of fine sediment was transported in suspension mainly as marine snow, the bacteria, plankton detritus and mucus acting as a coagulant for the fine sediment (Ayukai and Wolanski, 1997). In contrast, during spring tides, mangrove detritus was exported into the coastal boundary layer; there this organic nucleus aggregated the sediment particles and the small inorganic flocs into huge flocs (Fig. 4). These we call "mangrove flocs" and were only found in the coastal boundary layer.

Discussion

The mangrove-fringed channel is shallow. Seagrass is common at least on the western side and can be found at some sites just below the low tide elevation. Our findings demonstrate that the terrigenous fine sediment in this location is transported in suspension in any quantity only during spring tides. However this material does not extend far offshore under non-storm conditions. In coastal waters the mean diameter of the suspended sediment flocs is several hundreds of microns (Fig. 4) as opposed to a few tens of microns elsewhere. As a result of the increased floc size, the sediment rapidly settles out (Gibbs, 1985). This rapid settling of the sediment is apparent from the time series data of SSC at the mooring site, the sediment clearly settling out in 4 m of water in 1 hour after peak spring tidal currents. This corresponds to a settling velocity of about 0.1 cm s^-1. This velocity is 100-1000 times faster than that of the unflocculated mud particles, and 10 times faster than that of the small flocs found elsewhere. Near the coast in the settling zone, the turbidity is the largest. This shelters the waters further offshore from high sedimentation and excessive turbidity.

There are thus two competing gradients of environmental stress for the seagrass. Firstly, the suspended sediment concentration decreases with distance offshore - this would favour seagrass. And secondly, the depth increases with distance offshore, hence light is reduced at the bottom, and this is unfavourable to the seagrass. The location for optimum growth of seagrass can then vary seasonally and from site to site according to the bathymetry and the sediment dynamics.

At Hinchinbrook, this mechanism presumably is the process enabling the existence of seagrass beds immediately offshore from the extremely muddy coast.

The formation of macro-aggregates is important because it accelerates settling. When the macro-aggregates are formed by oceanic plankton, the terrigenous sediment is trapped in a coastal plume which can be several km wide (the Amazon river plume, Berhane et al., 1997; the Fly River plume, Ayukai and Wolanski, 1997; the Dutch coastal zone, Eisma et al., 1990; the North Carolina coastal zone, Wells, 1989). Our study demonstrates that the mangroves accelerate the formation of macro-aggregates. Along a mangrove-fringed coast the width of the fine sediment trapping zone is greatly reduced, occurring over scales of tens to hundreds of meters. Much of the fine, terrigenous sediment does not even reach the 5 m depth contour and is trapped in shallower waters of the settling zone. The mangrove flocs may later be recirculated back into mangroves where they settle (Furukawa and Wolanski, 1996).

Seagrass beds cannot survive in areas of high turbidity (Onuf, 1994; Schoellhamer, 1996). In mangrove-fringed coastal waters the coastal boundary layer acts as a sponge layer for fine terrigenous sediment, the mangrove flocs accelerating the settling of terrigenous sediment. Inshore, high turbidity, desiccation and bottom liquefaction prevent or inhibit seagrass. Offshore, seagrass is found due to the waters being clear or, when turbid, the high turbidity occurs only for a short period (Fig. 5). The width of the sediment settling zone where seagrass is inhibited can vary from location to location as it is likely to depend on a number of parameters such as the mean water depth, the stability of the bed and the stress due to desiccation and/or excessive sedimentation.

In the shallow coastal waters of Hinchinbrook channel the existence of seagrass beds is apparently due to the accelerated settling of fine sediment. Presumably a similar process protects the seagrass in other mangrove-fringed coastal embayments along the tropical coast of Australia, including Cleveland Bay near Townsville and Trinity Bay near Cairns.

The coastal boundary layer in Hinchinbrook Channel is shore-parallel because the coastline is mostly straight without headlands. As a result the prevailing currents are also shore-parallel. These currents will be deflected by engineering structures such as seawalls or dredged channels. This will bring terrigenous sediment -and decreased light- further offshore over the offshore seagrass beds where the light would be limited because of greater depth. This added sediment will further decrease light and the offshore seagrass beds may thus become affected. The inshore seagrass may be influenced by increased settling in the presence of seawalls and increased turbulence in the presence of a dredged channel.

Acknowledgments

This research was supported by the Australian Institute of Marine Science, the Kansai Electric Production Corporation - Kansai Environmental Engineering Company, and the IBM International Foundation.

References

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Berhane, I., Sternberg, R.W., Kineke, G.C., Milligan & T.G., Kranck, K. (1997). The variability of suspended aggregates on the Amazon continental shelf. Continental Shelf Research, 17, 267-285.

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Furukawa, K. & E. Wolanski (1996). Sedimentation in mangrove forests. Mangroves and Salt Marshes, 1: 3-10.

Gibbs, R.J. (1985). Estuarine flocs: Their size, settling velocity and density. Journal of Geophysical Research, 90: 3249-3251.

Onuf, C.P. (1994). Seagrasses, dredging and light in Laguna Madre, Texas, U.S.A. Estuarine, Coastal and Shelf Science, 39, 75-92.

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Figures

Fig. 1. Hinchinbrook Channel, location map showing the oceanographic mooring site 1, the cross-channel transect line, and the suspended sediment concentration along this transect during (a) the dry season and (b) the wet season at ebb tide when water was exported from the mangroves to flow past the transect line. The high turbidity zone, with suspended sediment concentration exceeding 0.04 kg m^-3, is called the coastal boundary layer.

Fig. 2. Sample of time series data from the mooring site at spring tide: sea level and suspended sediment concentration at three elevations above the sea floor. At neap tides there was by comparison negligible sediment in suspension.

Fig. 3. Photograph from the mooring site showing the high turbidity front marking the offshore extent of the settling zone (also marked by arrows). This front runs parallel to the mangrove-fringed coast.

Fig. 4. Microphotograph of flocs in the coastal boundary layer of Hinchinbrook Channel during spring tides in the dry season. The view spans about 500 microns.

Fig. 5. Sketch of the coastal boundary layer along the mangrove-fringed coastline of Hinchinbrook Channel. The coastal boundary layer covers the turbid zone shown in Fig. 1. It includes a settling zone inshore and a friction-dominated zone offshore. The arrows indicate the direction and strength of the tidal currents, the currents offshore are larger than those inshore. Mangrove flocs and other marine snow in the coastal boundary layer accelerate the settling of the terrigenous sediment. This settling is intense in the settling zone and this protects the seagrass found further offshore in the coastal boundary layer. The offshore edge of the settling zone is marked by a turbidity front shown in Fig. 3.