PRELIMINARY REPORT ON THE BENTHIC INVERTEBRATE/SEDIMENT SURVEY OF KING SOUND

G Pearson, M Lavaleye, E Oldmeadow, M Pepping
14/8/98

Shorebird studies in the North-west of Western Australia have primarily been concerned with Roebuck Bay and a portion of 80 Mile Beach between Cape Missiessy and Mandora Station. An aerial survey conducted in the mid 1980s of the coastline from Wyndham to Broome failed to identify any significant shorebird roosts or concentrations of shorebirds. However Walcott Inlet and King Sound were identified as likely to contain suitable habitat. A bird survey of Doctors Creek, 20 Kms north of Derby, was carried out in November 1997 by the Broome Bird Observatory's Chris Hassell. This was on behalf of the proponents of a tidal power station and revealed shorebird numbers in moderately high density around the Creek.
The potential and extent that King Sound may support large populations of shorebirds remained, however, unsubstantiated.

In the event of shorebirds being displaced from their traditional habitats at Roebuck Bay and 80 Mile Beach it has been suggested that other sites - such as King Sound - may provide suitable alternative sites for feeding and roosting.

In 1997 an intensive survey of the northern and part of the eastern shores of Roebuck Bay was carried out by a collaborative group from CALM, Curtin, NIOZ and the Broome Bird Observatory. This survey enabled a close examination of why Roebuck Bay is so important to shorebirds. Over a period of 14 days, 800 stations were visited. 550 of these were sampled and analysed quantitatively for macrobenthic animals. Sediment samples were analysed for colour and texture in the field and retained for further laboratory analysis. A total of 17000 individual animals were collected during the process from more than 200 different taxa yielding probably more than 500 intertidal invertebrate species. This biodiversity places Roebuck Bay among the richest known intertidal areas in the world.

"The rapid expansion of Broome, WA's premier tourist location, and increasing commercial exploitation of the region threaten the nearby wetland habitats of Roebuck Bay" ( Hickey et al 1997).

In July a smaller pilot study comprising the same team of researchers from Curtin, CALM, NIOZ, and Broome Bird Observatory took place in King Sound. The WA Museum also participated with assistance with taxonomy from Shirley Slack-Smith.
59 stations were visited and core samples for benthic animals and sediments were taken. Long cores to 3 m. were also taken at specific sites. Shorebird numbers were noted when possible. Preliminary results of these are discussed in the sections on biology and geology.

Participants for the survey were based at the old Derby Leprosarium, now named Bungerun. This excellent facility provided a quality of accommodation and laboratory space quite unexpected.

Access to the edge of King Sound was, generally, not difficult but the logistics of traversing the large distances across the Sound was daunting. Several methods were tried, including
Small BBO Hovercraft - generally not successful due to the condition of the skirt and engine and the high sand ridges formed on the bed of the Sound that created substantial obstacles for the small craft.

6 wheel drive Argo amphibious craft. - unsuccessful in the thick, deep or loose substrates. It has considerable use on the harder sand flats on the Sound but even with tracks required lengthy detours to avoid boggy areas of deep silt.

Wallis Hovercraft - an excellent, if expensive way to cover large areas of soft mud, high sand ridges and open water. Provided a safe platform from which to sample and carried substantial loads of 6 passengers plus sediment samples, spare fuel and personal gear. Un-troubled by tidal movement except during launching and retrieval.

Foot traverse - the most reliable method of covering short distances between tides. Required consideration for protection from crocodiles and the unknown aspects of tidal movements and tidal bores.

Local Derby resident participation level was low but important. The local knowledge provided by local interest groups was important.

Biology
M Pepping

Species diversity and overall abundance of macrozoobenthic species in King Sound are very low, thus indicating a very hostile environment especially for sessile animals. 59 stations were sampled for biological samples with 3 cores down to 30 cm of depth taken at every sample site, covering 1\40 of a square meter per station.
The following areas were sampled (see map):
- Doctors Creek (15 stations)
- Mary Island (6 stations)
- Colac Shoals to Ask Creek (17 stations)
- Alligator Creek (10 stations)
- Pelican Pool (11 stations)

The total number of species we found in the samples were 19 out of the following taxonomic groups:
- Crustaceans (5)
- Gastropods (3)
- Bivalves (1)
- Polychaetes (4)
- Isopods (1)
- Amphipods (1)
- Nemertines (1)
- Actiniaria (1)
- Echinoderm (1)
- Insect larvae (1)

The most abundant species are:
- Scopimera (Sand bubbler crab) with 29 individuals in 14 out of 59 sites
- Corbula spec. with 66 individuals in 10 out of 59 sites

Locally high numbers of individuals were found only in one or two sites, indicating habitats with a more suitable environment do exist. Samples from one location at Alligator Creek contained 23 Hydrobiidae, 13 Pyramidellidae, 8 Corbuliidae and one Insect larvae, making it the richest site that was sampled. (For density per square meter multiply the figures by 40)
Apparently these locations are exceptions, representing one of the few areas where current velocities are lower and animals are not swept away by the tidal currents.
31 out of 59 stations didn't contain any living macrozoobenthic species, the poorest area being sampled was Pelican Pool, where only 3 out of 11 sites showed living animals in small numbers. This is probably due to strong currents and the height of the sandbanks which are well above MWS (Mean Water Spring).

The low diversity and biomass can be explained by a combination of some of the following features of King Sound:

a) High tidal current velocities of up to 1.5 - 2 m/s in open waters and 3 m/s and greater in deep, narrow tidal creeks during spring tides (Semeniuk, 1981). The currents rework the upper layers of the sediment constantly, making it impossible for sessile animals like most of the bivalves and polychaetes to survive. The finer sediments are washed out into the open ocean, leaving only clear washed, medium to coarse grained sands, which are an unsuitable environment for most species. This is because coarse sands have a low water capacity and therefore dry out quickly, have little pore space and little organic content. Current velocities are highest in tidal creek systems and in the southern parts of the embayment due to its funnel shaped bathymetry. Coarse sediments are therefore deposited near the mouth of the creeks and in the southern parts of the bay, building up high, often megarippled sandbanks, which form an especially unsuitable habitat for most species.
b) High sediment load in the water column. The waters in King Sound are very murky having a high load of small particles (clay/silt) that may clog the gills of filter feeding animals. The fine sediment load in the water column can be explained by erosion processes along the coastline of King Sound and the neighbouring tidal flats and, to a lesser extent by sediment input from the Fitzroy River during the wet season. Sheet, cliff and tidal creek erosion are the dominant processes that are shaping the geomorphology of King Sound for the last 5000 years. Older stratigraphic units (2 Holocene, 4 Pleistocene), comprising sand, interlayered sand and mud deposits, muddy sands and mud, are readily eroded and the current velocities lead to differential settlement of the particles. Gravel and coarse sands stay in the bay, while finer sediments are washed into the open sea. Only during slack tide is there a deposition of finer particles (ephemeral sediment veneer), which are stripped off again by spring tides and storms.
c) Changing levels of salinity. During the dry season the salinity of King Sound is basically that of the open ocean (32-35 psu). During the wet season salinity changes drastically because of fresh water input from the Fitzroy River and groundwater aquifers. Salinities as low as 6 psu have been recorded in the southern part of King Sound and Doctors Creek. This might be another reason for the low diversity of sessile animals, since very few species have adapted to drastic changes in salinity.
d) Macrotidal range and slope gradients. For a macrotidal environment with a tidal range of up to 11.5 m during EHWS (equinoctial high water spring) surprisingly little surface area is exposed during normal tides. This is due to the steep gradient of the inclined slope, which is typical for the geomorphology of King Sound. Within the lower lying tidal flats only sandbanks and shoals, consisting of coarse grained sands and gravel, are exposed regularly. Vast areas in between the sandbars are exposed only during spring tides. This is another mayor difference between King Sound and Roebuck Bay, the slope gradients of the latter being a lot shallower, thus exposing more surface area at a given change of the water level and offering a bigger feeding habitat for wader birds.

The animals best adapted to this harsh environment are very mobile, deep burrowing crabs like Uca spec. (fiddler crab) and Scopemira (sand bubbler crab) of the family Ocypodidae. Especially the sand bubblers rework vast amounts of sediment, feeding on the little organic matter the sandflats have to offer. This explains the dominance of this species in King Sound. Out of the class of bivalves only the Corbuliidae seem to be sufficiently adapted. This may be due to their shell shape, which is very rounded, thus offering little resistance to currents, and there small size.

Literature:
V. Semeniuk (1981): Long-term erosion of the tidal flats King Sound, North Western Australia. Marine Geology, 43 (1981) 49-64.

Geology
E. Oldmeadow
Methodology

14 cores, ranging in depth from <1 m to greater than 2.5 m, were taken at 4 localities (Alligator Creek, Doctors Creek, Two Core Creek and Mary Island) in a range of settings around the King Sound area, Derby, W.A.

The aim was to assess near surface sedimentology with the intention of understanding the processes important to the recent history of the Fitzroy River.

Alligator Creek, a medium sized (300m wide mouth) tidal dominated system was the most intensively sampled area with a total of 8 cores. The creek displays features regarded as typical of the King Sound area, in particular;

Sandy river bed composed of ferric rich quartz dominated sediment, ranging in size from silts to course grained sands.

Steep levees composed dominantly of laminated muds, which grade laterally into supratidal flats.

Supratidal flats, composed of similar material to, which exhibit severe mud cracks and a high proportion of carbonate.

The Alligator Creek system illustrates the recent (Holocene / Upper Pleistocene) history of the Fitzroy.

Other geologic features of importance were:

Erosion / deposition, seen best at the Mary Island locality, where the west side displayed erosional processes and the east side, some 2km away, was characterised by a depositional environmental.

Tidal ripple marks, which ranged in wavelength from <10cm in areas dominated by low flow velocity to >3m in areas dominated by larger velocities. There is an apparent proportional relationship between grain size and tidal velocity, the effects of which can be compared from each locality visited.

SUMMARY
It is clear that the substrates in the parts of King Sound that we sampled are significantly different to those at Roebuck Bay. King Sound is dominated by the high tidal current velocities, the periodic flooding of the Fitzroy River and the high sediment loads in the water column. These factors combine to restrict suitable habitat for benthic invertebrates and consequently, shorebirds.
Further work is justified to properly quantify the shorebird use of the Sound during the summer migration period and extend the sediment sampling to the whole of the Sound. An integrated survey of Stokes By and Walcott Inlet to the north should also be carried out to complete the picture of potential shorebird sites north of Roebuck Bay.

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