MB2050, 2000-03-05

Sandy shore field exercise

Introduction.

It is remarkable that one of the most familiar marine habitats should be the one of the least studied. Until quite recently, sandy beaches i.e. area between high and low water marks (Oxford 1995) have been seen as marine deserts, and knowledge of their biology was far behind that of most other marine systems. Sandy beaches are among the most physically controlled marine habitats. They do not have the spatial complexity provided by plants or coral reefs as other tropical marine habitats, and the distribution and abundance of the flora and fauna, as well as the processes linking different species, are controlled primarily by the interaction between water and sediment (Robertson, 1984).

The biological productivity of a beach varies and is much less than for many other marine systems. The commercial values of beaches are mainly from recreation, tourism and mining for rare minerals (Snedaker & Getter 1985).

The aims of the study are:

1. To examine the physical features and sediment-related characteristics of Pallarenda Beach – Townsville, a tropical sandy shore habitat, and their possible influences on the occurrence and distribution of the intertidal taxa in the area

2. To describe the fauna in the surfzone region of Pallarenda Beach.

Materials and methods.

This study was conducted the 5th of March 2000 at Pallarenda Beach (19° 11.8’S, 146° 46.6’E), north of the city of Townsville.

Beach Profile

The Beach Profile was measured by marking out nine stations with eight meters between with a tent peg. By noting the height from the origin of measurement on the rod towards the horizon at every station, the profile of the beach was recorded.

The density of the sand on the beach was measured by pushing a wooden rod through the sediment using reasonable and continues force.

Sediment analysis

By using a spade, the depths to the different layers at every station were recorded.

Approximately 1000ml of sediment samples were taken from each station and were later analysed in the laboratory.

To analyse of the samples, 100ml of sediment were measured in a graduated cylinder.

This was then washed through a sieve using a wash bottle to extract the fine sediment that was measured and recorded in the graduate cylinder.

For each station a spoonful of sediment was placed into a 30ml vial, and filled up with clean water and shaken for a few seconds. Then the time required for the water in the vials to clear i.e. for suspended sediment to settle, was recorded. The time required varied depending on the grain size of the suspended sediment present.

Resuspension categories were defined according to the time required to clear as follows:

1= <10secs, 2 = 10-30secs, 3 = 31-60secs, 4 = 61-120secs, 5 = >120secs

Biological survey (intertidal and surf-zone)

The intertidal zone was surveyed by taking gentle steps around to observe and record the living animals in the zone.

By dragging a net to bottom of the surf-zone, evaluation of what kind of animals that live in the surf-zone could be done. Stinger suits and wetsuits were used for protection against jellyfishes and other harmful animals. The catch was identified, recorded and then released.

Results.

The results are presented in figures 1 and 2 and tables 1, 2 and 3.

With a gradient of 0.04m/m indicate that Pallarenda beach has a very gentle slope. (Fig. 1, see separate paper)

From the surfzone to station 6 more than 85% of the sediment was considered fine, around station 6 the fine sediment dropped to less than 20% for at station 1 again raise to levels above 80%. (Fig. 2, see separate paper)

Table 1. Plants and animals encountered at each station.

Station 1. Ipomea pes-caprae (present)

Station 2. Mangrove debris, shells visible (1-3cm)

Station 3.

Station 4. 25mm crab-holes (no sightings)

Station 5. Ghost crabs Ocypode sp.

Station 6. Polycheate tubes (Papery), more crab tunnels.

Station 7. Polycheate tubes -Diapatra sp. (Shells, detritus, rubbery)

Station 8. Sand dollars -lots -Arachnoides placenta. Hermit crabs, bivalves -Tellina sp.

Gastropods Polynices sp. (Moon snail). Ghost crabs Ocypode sp.

Polycheate tubes -many. Balanoglossus sp. Debris -enteropneust worms.

Station 9. Sand dollar trails under the sand, also bivalves Tellina sp. Polycheate tentacle trails.

Table 2. The beach’s physical features.

Date: 3/8/00

Station	Penetration Depth (m)	Anoxic Layer	Water table	Resuspension Category
1	0.032	Not	none	1
2	0.023	visible	none	1
3	0.003	from	0.4m	1
4	0.003	digging	0.3m	1
5	0.002		0.6m	2
6	0.002	0.06m	surface	2
7	0.002	0.05m	surface	2
8	0.001	0.07m	surface	2
9	0.001	all	surface	3

The penetration depths indicate that the first two stations had softer sediment than the rest.

The anoxic layer did not show until station 6 – 9 i.e. the area between the high and low tidal marks and at the last station the anoxic layer was on the surface.

The resuspension category for the first four stations was 1, category 2 was found on stations five to eight and on the last station closest to the sea, had a category 3.

Table 3. Biological survey over different species caught in the surfzone.

Whiting Sillaginidiae

Ponyfish Leiognathidea

Trevally Carangidae

Mullet Mugilidae

Flatfish Cynoglossidae

Pufferfish Tetraodontidae

Batfish

Discussion

Pallarenda Beach is a low energy and a low gradient (0.04m/m) tropical sandy beach (Fig. 1).

The fine sediment is distributed so the last four stations i.e. station six to station nine have above 85% of fine sediment. This tends to indicate that the sediment in the intertidal area is muddier than further up the beach. Station one had also over 85% fine sediment and that tend to show that the waves and/or the wind have transported the finest sediment to the upper dunes where it has accumulated (Fig. 2).

The animals (longer then 1-2mm, generally less than 10cm) on tropical sandy beaches are typically made up of species of molluscs, crustaceans and polychaetes (McLachlan 1983). The distribution of animals at the stations was, not surprisingly, concentrated in the intertidal area where the Ghost crab Ocypode sp., was the animal living the furthest away from the low tide mark. This show that the crabs’ exoskeleton helps prevent desiccation and makes it possible for the species to live high up towards the high tide mark at low tide (Table 1).

The polycheate tubes encountered show the ability of other animals to survive without an exoskeleton through building a tube which the worm is capable of shutting, so when the tide goes out, the worm simply closes it’s tube and wait until the tide comes in again.

Further down the beach, at station eight, large numbers of Sand dollars, Arachnoides placenta, were discovered and they made very obvious trails in the sand. The sand was saturated with water and had small shallow pools present that enable A. Placenta to live above the low tide mark.

The highest number of species (7) and abundance of organisms around this station probably also reflects the proximity of water’s edge and also the saturation of the substrate.

It is possible to compare different Australian and some international sandy beaches but care must be taken though different sampling methods are used (Robertson, 1984).

The animal life at a tropical sandy beach like Pallarenda Beach would not be found on a rocky shore, though the animals shown in Table 1 need the soft sediment to borrow and bury them self in.

The fish found (Table 3) between the high and low tide marks lives in the surf area during low tide.

References:

McLachlan, A. 1983, Sandy beach ecology, in Sandy Beaches as Ecosystems, McLachlan, A. & Erasmus, T. (eds), Dr W. Junk, The Hague, pp 321-80

Robertson, A.I. & Lenanton, R.C.J. 1984, Fish community structure and food chain dynamics in the surf-zone of sandy beaches: the role of detached macrophyte detritus. J. Exp. Marine Biology Ecology, vol. 84, pp. 265-83.

Snedaker, S.C. & Getter, C.D. 1985, Coastal Resources Management Guidelines, Renewable Resources Information Series, Coastal Management Publication no. 2, National Parks Service, US Department of the Interior and US Agency for International Development, Washington DC.

Thompson, D. 1995, The Concise Oxford Dictionary, 9th Ed. Oxford University Press, Oxford