Report – MB2050 – Marine Flora and Fauna

MB 2050 – 2000-05-02

Marine Flora and Fauna Of Picnic Bay Fringing Reef.


Coral reefs is located in the tropics and sub-tropics between approximately 30° N and 30° S, where the mean seawater temperature never falls under 18° C (Done et al. 1996).

In 1997, over 1.6 million visitors visited the most extensive coral reef system in the world, the Great Barrier Reef. The popularity of the G.B.R. among tourists is not that hard to understand, as it is one of the most diverse ecosystems on earth. It is the home of about 1500 spp. of fishes, around 400 spp. of hard and soft coral, about 4000 kinds of molluscs and thousands of kinds of sponges, worm, crustaceans, echinoderms and other odd creatures (GBRMPA,1998).

There is mainly four different kinds of reefs and these are; barrier, patch/shoal, cay, fringing and atoll reefs. In Australia all kinds of reefs can be encountered except atoll reefs which develops on the top of submerged volcanoes (Done et al. 1996). Where reef forms it includes both coastal and island fringing reefs which are assemblages of corals which develops in the shallow waters surrounding the edge of a continent or a island (Done et al. 1996).

It has been suggested by De Wreede (1976)that several environmental aspects influence the seasonal abundance of benthic algae in the tropics. Light and water temperature and water movement especially in the warmer months, are the key factors in determining the seasonal changes in algal communities in the tropics (Vuki et al. 1994).

The aims is to examine the marine flora and fauna on the intertidal fringing reef of Picnic Bay, Magnetic Island.

Also to describe the flora and fauna in relation to the substratum, its abundance and distribution from the spring line.

Materials and methods.

Site description

This study was conducted at Picnic Bay is a shallow gradient beach, located (19° 10.8’S, 146° 50.6’E), on Magnetic Island, 5km north offshore from the city of Townsville.

Magnetic Island is located in the dry tropics. I.e. relatively dry winters, with monsoonal rain during the summer months which can result in very low salinities in Picnic Bay (Walker, 1981).

The study was conducted the 15th of April 2000 between approximately 1pm – 4pm. Low tide (0.71m) occurred at 1.50pm (Brodie, pers. comm., 2000).

Data collection

A transect was established perpendicular to the dune system and extended towards the water’s edge for approximately 150m.

A 1 m2 quadrat was placed at random, at a major change in substrate composition or every 10 m interval, along the transect to record the dominant species present and the percentage of the particular substratum, and amount of invertebrates animals within each quadrat.


Figure 1 indicates that the sediment over the transect follows a predicted pattern with the majority of the sediment, in the first 60 meters, being mud with values up to 100%.

From about 60 to 110 meters, an area with fine sand and a mix of coral rubble constitutes the sediment (Fig. 1).

The fine sand transforms to courser sand and more coral rubble at about 110 meters to the end of the transect (Fig. 1).

Not many animals were found except for hermit crabs that were present with about 2-5% all over transect. Some occasional bivalve, chiton, pearl oyster and a juvenile scorpion fish were found (Table 1).

Sargassum spp. were abundant from about 45 meters to the end of transect (Table 1).

Between 11 and about 40 meters Halophila spp. were present (Table 1).

The presence of Halodule spp. was divided into two patches, one from about 10-45 meters and another from 110-145 meters (Table 1).

Hallimeda spp. were present from the beginning up to about 84 meters where Cymodocea spp. took over for the remaining of the transect (Table 1).


The known seasonal changes in populations like Sargassum spp.

(Vuki et al. 1994) may have resulted in a misleading conception of the biological flora and fauna of the study. Also a tropical cyclone passed the area three weeks before the study which might have affected the outcome of the study. Further more it has been shown by Vuki et al. (1994) that the minimum density of Sargassum spp. occurs at the time of the study.

The sediments of the fringing reef system in Picnic Bay show seaward gradients of increasing mean grain size and carbonate content.

When comparing the data from Figure 1 and Table 1 tends to indicate that Sargassum spp. occurred where mud didn’t.

Halophila and Halimeda spp. are found from the beginning of the transect up to about 80 meters, when Cymodacea spp. becomes more abundant for the rest of the transect. This can be due to that Halophila and Halimeda spp. favours fine grained sediment while Cymodacea spp. favours course grained sediment.

The data also tend to indicate that hermit crabs are not dependent on a particular type of substratum.

The study was biased in the way that estimates of percentages of flora and fauna were conducted and it was made harder due to the water went turbid very easily.The last third of the transect was not conducted in a satisfactory way due to this fact.

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