The sandy shores of beaches can be considered as a very harsh environment to live in (Ted Klenk, 1999). Survival in such a habitat requires an organism to withstand strong wave and current action, tidal rise and fall, unstable substrate, heavy predation and wide variations in salinity and temperature (The Otter Island Project). Any organism found in this type of harsh environment is specialized and highly adapted (The Otter Island Project). Fine, sandy beaches often occur in areas with light wave action (California’s Ocean Resources, 1995).
While a more coarse sand is found with heavy wave action (California’s Ocean Resources, 1995). The slope gradient of a beach also helps determine the grain size of the sediment (Bascom, 1959). The steeper a beach is, the larger the sand particles (Ted Klenk, 1999). The water retention of coarse sand is quite low allowing temperatures to rise easily at low tide (MB2050 lecture notes). Any organisms on a coarse particle beach will not only have to deal with the very unstable substrate but also the real danger of desiccation (MB2050 lecture notes).
Fine sediment areas do not have much water moving through the particles (MB2050 lecture notes). Therefore any oxygen available is used up quickly by burrowing organisms (MB2050 lecture notes). There comes a point beneath the substrate where there is insufficient oxygen for organisms to respire, this is called the anoxic layer (MB2050 lecture notes). The anoxic layer is therefore much deeper under coarse sediments then fine (MB2050 lecture notes). The fish community of a surf zone is largely controlled by three factors (Robertson & Lenanton, 1984).
The form of primary production input to the surf zone, the water movement pattern, and the geomorphology of the sandy beach (Robertson & Lenanton, 1984). Fish such as anchovies, herrings and mullets often inhabits surf zones around the world (The Otter Island Project). The aims of this experiment were to examine physical features and sediment-related characteristics of Pallarenda beach and the influence of these characteristics on the occurrence and distribution of the intertidal taxa. This experiment also aimed to describe the fauna in the surf zone region of Pallarenda beach.
Materials and Methods: The beach studied during this experiment was Pallarenda Beach just North of the city of Townsville in North Queensland. The co-ordinates of the location were 19o11. 8S, 146o46. 6E. The beach experiences a tropical climate, the experiment was carried out on the 4th of March in the year 2000 after much rainfall. The class was divided up into three different groups for the experiment to be carried out. Beach Profile: An observer sat at the top of the foreshore above the high tide mark, this position was considered station 1.
The observer lined up the horizon with the relevant measurement on the 3m measuring rod at each station down the beach so the drop off could be determined. The eye level of the observer was measured so an adjustment could be made to the drop off measurement making it accurate. The locations of the 12 stations from the observer were measured with a tape measure spanning down to the water line at low tide. Sediment analysis: At each station a sediment sample was taken, the anoxic layer and water table were searched for and notes on any surrounding organisms were recorded.
The sediment samples were taken back to the lab to be analysed. The sediment samples were analysed using a wet sieving procedure. For each station sample the percentage of fine sediment was determined as well as the resuspension category of the fine sediment to determine the amount of mud and silt present. The sediment was washed through a sieve; the sediment that went through the mess was considered fine sediment. The amount of fine sediment obtained to work out a percentage of the total sediment at each station. A small amount of the fine sediment was then used to determine the mud and silt content.
The sediment was put in a vial with water and shaken. The time taken for the sediment to settle was recorded and applied to a resuspension category. Other physical features: Penetration depth was determined by pushing a wooden rod through the sediment using a reasonable and continuous force at each station. Biological Survey: Each group carried out a trawl along the surf zone parallel to the beach for 3 to 5 minutes. The different organisms captured from each trawl were recorded. Results: Figure 1 shows the profile of Pallarenda beach including the water table and anoxic layer.
The lowest point on the profile was a stream of water, which was running perpendicular to the profile. The water line was at the last station on the profile. A projection of the anoxic layer has been included as it was often to deep to be found easily by digging. Figure 2 shows the percentage of fine sediment at each station along the beach profile. There was a large amount of fine sediment both at the start and finish of the transect. Figure 2 also shows the resuspension categories of the fine sediment. Level one being the lowest and level five being the highest. Table 1 shows the different organisms found at each station along the transect.
It also shows basic characteristics of the beach along the profile. The numbers of organisms found higher up the beach were very minimal. Table 2 shows the different organisms caught in the three trawls carried out through the surf zone. There was no exact numbers recorded but some species occurred a lot more often than others. Table 1: Terrestrial organisms found at Pallarenda beach.
StationAnimals and features 1Vegetation line 2End of vegetation line 3Shell line, Ghost crab 4Shells 5Wet sand on beach surface, Ghost crab 7 8Fresh water runoff, crab burrows, Ghost crab, Hermit crab. 10 11 12Bivalves, hermit crabs, sand dollars, polychaete worms and gastropods. Table 2: Presence and occurrence of marine organisms in the surf zone of Pallarenda beach. OrganismF, frequent species. +, Species present. Sting Ray+ Herring+ Anchovy+ Long Tom+ Gar fish+ WhitingF Pony fishF Trevally+ Mullet+ Thread fin fish+ Flat fish+ Puffer fish+ PrawnsF Mackerel+ CrabF Jew fish+ Snub nose dart+ Shovel nose shark+ Discussion: The profile of Pallarenda beach as shown in figure 1 is very typical of a North Queensland, medium energy beach.
The lack of organisms present higher up the beach was due to the harsh conditions at low tide. Ghost Crabs, Ocypode quadrata, can survive these areas as a result of their adaptations (The Otter Island Project). The ghost crab burrows during the day and feeds at night to avoid the heat of the day (The Otter Island Project). These crabs can actively take up moisture from damp sand to wet their gills (The Otter Island Project). The anoxic layer is very deep above the areas with coarse sand. This is in accordance with what was mentioned in the introduction.
The coarse grained sand has more water passing though it to increase the availability of oxygen deeper beneath the surface. The water table was fairly close to the surface in comparison with the anoxic layer. At station nine the water table was above the surface of the sand. This water was fairly fresh due to the large amount of rainfall in the area previously. This fresh water affected the abundance of surrounding organisms. Down near the water line at station twelve there was a very high abundance of marine organisms, the salinity in this area was normal compared to the relatively fresh water stream higher up the profile.
However there was evidence of the ghost crab, which is an osmoregulator, burrowing near the stream. Figure 2 shows some usual results in that there is a large amount of fine sediment between stations one and two that has a steep gradient. The rest of the profile and sediment data abide by this rule mentioned earlier. The fine sediment could be linked to the fact that the first station is the very top of the high tide mark where there would be quite low wave energy compared to the middle of the profile at stations seven and eight.
The amount of mud contained in the fine sediment was a lot higher when there was a high total percentage of fine sediment at a station. The animals found in the trawl were very typical of a surf zone. Surf zones as far away as California have very similar species found there (The Otter Island Project). The seasonal rainfall at Pallarenda severely affected the organisms on shore. The surf zone organisms however were very characteristic of this type of environment and climate.