Simon Spagnol

Dynamics of the turbidity maximum in King Sound, tropical Western Australia

King Sound is a 100-km-long embayment located in tropical northwestern Australia with a spring tidal range of 11 m. This is the second largest tide in the world after the Bay of Fundy in Canada. Intertidal areas cover about 800 km2. The upper reaches of the sound are turbid with fine suspended sediment concentration reaching 3 kg m−3. Field studies of the dynamics of water and fine sediment were carried out in the dry seasons of 1997 and 1998. The tide was a propagating wave, shoaling and dissipating by friction as it entered the sound. This mode of propagation generated an asymmetric tidal current with a stronger current at flood than at ebb. An evaporation-driven salinity maximum zone was found in the upper reaches of the sound, and this was also where the turbidity maximum occurred. Tidal pumping by the tidal asymmetry and, possibly, the biological filter formed by muddy marine snow, trapped the fine sediment in the upper regions of King Sound. Wind-driven waves contributed significantly to entrainment of bottom fine sediment, possibly through wave pumping of the sediment and not wave-induced orbital velocities. Field data suggest that erosion of bottom fine sediment was proportional to the sixth power of the tidal current and the third power of the wave height.

Fluxes of nutrients and dissolved and particulate organic carbon in two mangrove creeks in northeastern Australia

In Coral and Conn Creek, northeastern Australia, the variations in concentrations of nitrate, phosphate, silicate, dissolved organic carbon (DOC) and particulate organic carbon (POC) were measured over tidal cycles on five occasions and along each creek on four occasions. The fluxes of these five properties were then estimated using two methods. The first method is the so‐called Eulerian method, whereby water flow and material concentration are measured at a fixed station near the creek mouth and the net flux is calculated by adding up flux increments over a tidal cycle. The second method first derives the longitudinal eddy diffusion coefficient from the salt mass balance equation and then calculates material fluxes from their observed gradients along the creek. The use of the latter method is permitted only in the absence of freshwater inputs.The Eulerian method was not sensitive enough to examine whether there was any statistically significant difference in fluxes of nutrients, DOC and POC between ebb and flood periods. This casts some doubt over the meaning of individual flux estimates. It is, however, worth mentioning that 17 out of 25 flux estimates were positive (= import) in Coral Creek, whereas only eight positive flux estimates occurred in Conn Creek. In Coral Creek, the average flux values for nitrate, phosphate and DOC were positive, but negative for silicate and POC. In contrast, the average flux values for all properties were negative in Conn Creek. This may be due to the difference in amount of freshwater input between Coral and Conn Creek.The presence of freshwater inputs from upstream sources restricted the use of the salt mass balance equation to the Coral Creek data collected in September, 1996. However, the study of the variability of nutrient, DOC and POC concentrations along the creek could provide valuable insight into their behavior in Coral and Conn Creek. For example, the concentrations of silicate and DOC were consistently higher upstream than downstream and the distance–concentration relationship was statistically significant in seven out of eight measurements. The concentrations of nitrate and POC also decreased from upstream to downstream, but the trend was statistically significant in only 2–3 measurements. The concentration of phosphate was higher downstream than upstream in four measurements and in two of these four measurements, the trend was statistically significant. These results suggest that in Coral and Conn Creek, silicate and DOC are usually exported to adjacent coastal waters, whereas the import and export of nitrate, phosphate and POC are often finely balanced.

Inorganic sediment budget in the mangrove-fringed Fly River Delta, Papua New Guinea

Six oceanographic moorings were maintained for 8 weeks across the mouth of the mangrove-fringed Fly River estuary from April to June 1995 in the southeast trade wind season. A further 4 moorings were deployed for 8 weeks along the estuary channel in 1992, also in the southeast trade wind season. These data were used to estimate net exchange of suspended sediment between the estuary and the Gulf of Papua. A net inflow of fine sediment into the estuary from the coastal ocean was found to be considerable, about 40 tonnes s-1 or about 10 times the riverine inflow rate, resulting in a calculated, spatially averaged vertical accretion rate of 2 mm year-1. Mangroves may account for trapping 6% of the riverine sediment inflow or about 1/4 of the riverine clay inflow. If this sediment was distributed only over the observed accumulation zones near islands the local accumulation rates in these zones would reach 4 cm year-1. Estimates of soft sediment mass accumulation rates (1–10 kg m-2 year-1) in the channel from Pb-210 and C-14 measurements from cores of deltaic mangrove mud cannot account for this accumulation rate on a 100–1000 year time scale. The fate of the remaining sediment is unknown, it may be exported from the estuary in the monsoon season.

Field and model studies of the fate of particulate carbon in mangrove-fringed Hinchinbrook Channel, Australia

A field and model study was undertaken in 1996/1997 of the dynamics of water, fine sediment and particulate carbon in the northern region of the mangrove‐fringed Hinchinbrook Channel, Australia. The currents were primarily tidal and modulated by the wind. Biological detritus acted as a coagulant for the fine cohesive sediment in suspension in the mangrove‐fringed, muddy coastal waters. Plankton and bacteria were the major aggregating agents at neap tides, and mangrove detritus at spring tides. The micro‐aggregates were typically several hundreds of micrometer in diameter and enhanced the settling rate. The fate of fine sediment and particulate carbon was controlled by the dynamics of the coastal boundary layer, a turbid shallow coastal water zone along the mangrove‐fringed coast. A tidally‐modulated, turbidity maximum zone was found in this layer. Wind stirring increased the turbidity by a factor of five.The channel behaves as a sink trapping fine sediment and particulate carbon. However, the sink was ‘leaky’ because the dynamics of the coastal boundary layer generated a net outflow of fine sediment out of the channel along the western coast. The biologically enhanced settling of cohesive sediment limited the offshore extent of the muddy suspension to within a few hundreds of meters from the coast.At spring flood tides, some of this particulate carbon was advected into the mangrove forest where it would remain trapped. On a yearly basis about six times as much particulate carbon was exported out of Hinchinbrook Channel through the coastal boundary layer than was trapped in the fringing mangroves.

HYDRODYNAMICS OF DARWIN HARBOUR

Although macrotidal, Darwin Harbour is poorly flushed, especially in the dry season when the residence time in the upper reaches is of the order of 20 days. Much of the riverine fine sediment remains trapped in mud flats and mangroves with little escaping to the sea. The complex bathymetry of headlands and embayments generate complex currents comprising jets, eddies, and stagnation zones that can trap pollutants inshore. The tidally averaged circulation may control the location of the sand banks, indicating a feedback between the bathymetry and the water circulation. The environment in Darwin Harbour has the potential to degrade and the water circulation in the harbour must be considered when planning developments.

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

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

Patchiness in the Fly River plume in Torres Strait

Abstract Oceanographic studies were carried out from August 1994 to March 1995 on the intrusion of the Fly River plume in Torres Strait. Measurements at offshore coral reefs revealed an event of decreased salinity (≈24) while salinity of the water over the reefs fluctuated between 30–34 the rest of the time. Modelling suggests that this event resulted from the reversal of longshore currents advecting old river plume water back past the river mouth. There the new river water mixed with the old river plume water generating a patch of low-salinity water. While such events may be infrequent, they have the potential to leave a terrestrial signature on offshore coral reefs, in terms of (1) an input of terrigenous sediment and (2) the possible incorporation of riverine particulate metal into the food chain. The impact during an intrusion event may be significant. In the long term the riverine material is diluted in calcareous sediment produced throughout the year by bio-erosion of coral reefs.

The impact of damming the Ord river on the fine sediment budget in Cambridge gulf, northwestern Australia

Abstract Cambridge Gulf is a turbid embayment about 50 km long, with a mean water depth of about 12 m. The spring tidal range is 8 m. At its apex, the gulf divides into two estuaries, namely the West Arm that drains the mostly undisturbed Durack and Pentecost rivers, and the East Arm that drains the Ord River. As a result of a dam constructed in 1970 on the Ord River, the river flood regime has been greatly affected within the East Arm, having silted by an average of 3 m. By contrast the West Arm bathymetry has remained practically unchanged for the last 100 years. Oceanographic studies suggest that the West Arm exports fine sediment at a rate of about 40,000 ton d−1, and that most of that sediment is now imported into the East Arm and does not reach Cambridge Gulf. Negligible net sediment flow was measured at the mouth of the gulf. This suggests that the pre-dam Cambridge Gulf received a total sediment inflow of about 100,000 ton d−1, and about zero at present. Sediment is currently being redistributed within the gulf, the west coast may be accreting and the east coast receding.

Chapter 15 Merging scales in models of water circulation: perspectives from the great barrier reef

Publisher Summary This chapter describes the technique chosen to include the effect of the oceanic circulation in a 2-D model of the large-scale circulation on the continental shelf. This circulation is found to be modulated at an intermediate spatial scale by the interaction of the tidal circulation with individual reefs, and this process is modeled by merging large-scale and reef-scale 2-D circulation models. The oceanography of the Great Barrier Reef (GBR) is made particularly complex by the extraordinary complex bathymetry. In the GBR, the obstruction of the flow by the presence of reefs steers and modifies, even at these large scales, the oceanic inflow and the longshore currents. Obstruction by large reefs or a reef matrix steers prevailing currents toward areas of low reef density. This provides the modeler the challenge to merge the large-scale oceanic circulation with the shelf-scale general distribution of reefs over the shelf. The resulting currents through a reef matrix, and the deflection of the prevailing currents around a reef matrix, are modulated by the tides.

Fine sediment dynamics in the mangrove-fringed, muddy coastal zone

[Extract] Studies of the fate of fine sediment at the mouth of muddy rivers have revealed that the sediment generally falls rapidly out of suspension and does not follow the brackish water in the river plume. The Amazon River plume is a well-documented example (Geyer et a1. 1991; Kineke et al 1996). This observation has been attributed to the formation in coastal waters of large flocs, also called micro-aggregates, that accelerate the settling of the fine sediment. Such flocs have also been found in the Dutch coastal zone (Eisma et a1. 1990), the North Carolina inner shelf (Wells 1989) and the Amazon shelf (Berhane et al. 1997). These floes can exceed many hundreds, sometimes thousands, of µm in diameter. Their settling velocity is thus much higher than that of the floes usual1y found in very turbid estuaries, these flocs are seldom much larger than 100 µm in diameter (Gibbs 1985; Gibbs et at 1989; Wolanski and Gibbs 1995). The formation of micro-aggregates is thus important in controlling the fate of riverine sediment The processes generating the micro-aggregates have been little studied, though it has been speculated they were biological in nature (Cloern et al. 1983; Nelsen and Trefry 1986; Monaco et al. 1990). Recently Zimmerman and Kausch (1996) and Ayukai and Wolanski (1997) documented the planktonic origin of these micro-aggregates in respectively the Elbe River estuary and the Fly River plume in Torres Strait, Australia. These micro-aggregates essentially formed a mud-affected “marine snow “. The marine snow was previously documented only for the open ocean where there was negligible sediment in suspension (Alldredge and Silver 1988; Alldredge et al. 1993), In the Fly River, plume microphotographs of the undisturbed flocs (Ayukai and Wolanski 1997) revealed that the fine sediment particles that have escaped trapping in the estuary attach themselves to the sticky surface of the marine snow. The resulting micro-aggregates settle out of suspension near the mouth of the Fly River while the plume extends tens of kilometers further offshore. In the case of the Fly River the extremely high turbidity in the estuary prevents significant plankton growth in the estuary itself where the flocs are small (typically <100 µm in diameter; Wolanski and Gibbs 1995). Thus the Fly River estuary behaves like a physical filter while the coastal zone behaves as a biological filter. Presumably in less turbid systems the plankton growth can be significant in the estuary which behaves both as a physical filter and a biological filter; this indeed seems to be the case in the Elbe River estuary (Zimmermann and Kausch 1996). In all these systems the fine sediment settling zone extends onto the inner shelf several kilometers to several tens of kilometers away from the river mouth.