PRELIMINARY REPORT ON THE BENTHIC INVERTEBRATE/SEDIMENT SURVEY OF KING SOUND
G Pearson, M Lavaleye, E Oldmeadow, M Pepping
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.
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
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
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.
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 total number of species we found in the samples were 19 out of the
following taxonomic groups:
The most abundant species are:
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)
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.
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.
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.
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