The seagrass communities of Moreton Bay, Queensland

Aquatic Botany, 1 (1975) 191--202 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

THE SEAGRASS COMMUNITIES OF MORETON BAY, QUEENSLAND

P.C. YOUNG* and H. KIRKMAN**

*CSIRO Division of Fisheries and Oceanography, Cronulla, N.S.W. (Australia) **Fisheries Research Station, CSIRO, Deception Bay, QM. (Australia)

(Received June 2nd, 1974)

ABSTRACT

Young, P.C. and Kirkman, H., 1975. The seagrass communities of Moreton Bay, Queensland. Aquat. Bot., 1: 191--202.

In this paper the composition of the seagrass communities of Moreton Bay, Queensland, is described. Six species were found: Zostera capricorni Aschers.,Halodule uninervis (Forsk.) Aschers., Halophila ovalis (R.Br.) Hook.f., Halophila spinulosa (R.Br.) Aschers., Cymodocea serrulata (R.Br.) Aschers. and Magnus and Syringodium isoetifoliurn (Aschers.) Dandy. These were found in five different distinct phanerogamic communities: two monospecific communities, one multispecific community, a community characterised by a single dominant species, and a community in which two species were of equal importance.

The zonation of these was determined by depth, salinity, turbidity and substrate characteristics. It is postulated that the absence of Posidonia has enabled various species to co-exist and remain intact instead of being part of a series leading to a Posidonia meadow.

INTRODUCTION

Intertidal vegetated areas are generally considered to be important habitats for juvenile fish and commercial prawn species (Penaeidae). Thus Allen and Inglis (1958) described juvenile prawns inhabiting vegetations of saltcord grass, Spartina alternifolia, and Strawn (1954) described them from beds of Thalassia testudinum. Because of this importance it was decided that as part of a programme of research into juvenile penaeid prawns, the seagrass communities of south-east Queensland should be investigated. In this area only two regions exist where very extensive seagrass beds occur -- Hervey Bay and Moreton B a y - and the latter was investigated because of its association with an extensive prawn fishery.

Moreton Bay is a large wedge-shaped depression deepening towards the north- west, approx. 100 km long and ranging in width from 1 km in the south to a maximum of 31 km near its northern limit (Fig.l). It extends from the mouth of the Nerang River at a latitude of 27°56'S to a free opening between Bribie and Moreton Islands at a latitude of 27 ° 2'S. Except for this northern opening, which is 14 km wide, and the three narrow openings at South Passage,

:% LEGEND ~.~i p.:~ ~--

1 Bribie Island ~ i ; ~ 2 Toorbul Point ~ ~ f ~ ¢ 3 Cook's Rock

Ca0oo,ture Rivor 5. Deception Boy 6 Moreton Boy 7 Tangalooma 8 Mud Is land r/f 9 South Passage

10 Ami t y 11 WangaWallen Banks 12 Peel Island .~ 13. Dunwich 14 Cleveland Point 15. Coochiemudlo Island 16 Pelican Banks 17 North Stradbroke Island 18. Macleay Island 19 Po in tTa lburp in 20. Jumpinpin 21. South Stradbroke Island

,22. Aldershots Banks 23 Southport Bar 24. Nerang River

~ Sand bars and banks etc.

~ 2 7 ° 6 '

27o56 ,

Fig.1 (a) Moreton Bay, Queensland showing banks of dow to 2 m below mean low water in depth. (b) Details o f Moreton Bay to show the distribution of the seagrass meadows.

Jumpinpin and the Southport Bar, all of which are less than 2 km wide, its 1 300 km 2 of water and low islands are enclosed by the mainland and thelarge, high, offshore islands of Moreton, North Stradbroke and South Stradbroke.

The tidal influx within the Bay occurs along a north--south axis at right

Community type 4

Community type 3

KEY TO SEAGRASS COMMUNITIES

Community type 2

Community type 5

I Community type 1

angles to the east--west inflow. This occurs along the deep eastern side whilst the rivers f low onto a shallow western and southern fringe. Shear forces pro- duced by the tidal currents produce lateral mixing of fresh water eastwards and the water exchange on each tidal cycle is only 5--6% (Newell, 1971).

Maxwell (1970) divided the sedimentary framework of Moreton Bay into three facies as follows. Firstly, a clean sand in which less than 1% mud is present, best developed in the eastern and extreme northern and southern parts of the Bay where it forms a fairly narrow fringe 1--3 km wide along the edge of the barrier islands, expanding considerably at the South Passage tidal delta to more than 13 km where it extends almost to the western shore near Cleveland Point. It also crosses the nor th of the Bay as the northern ridge system. The second facies is muddy sand, between 1 and 50% mud, occupying the western part of the Bay except where the South Passage tidal delta has en- croached. The third facies is mud and sandy mud, containing more than 50% mud and occurs in the deep northern section beyond the 12-m line and in the protected channels near the south-western shore and in the lower regions of all the inflowing streams.

At present ancient dune deposits on the barrier islands provide most of the sandy sediments of the eastern part of the Bay. The main contribution from the western land mass is mud, restricted largely to the western and southern parts of the Bay. Relict coral reefs and coral communities contribute to a restricted carbonate facies in an L-shaped bank from Mud Island south to Coochiemudlo Island then east to Peel Island and Macleay Island.

Despite the increasing interest in the ecology of seagrasses, very little study has been made of those found in Australia. Ferguson-Wood (1959} described some east Australian seagrass communities and Den Hartog included Australian seagrasses in his Monograph (1970), but apart from these two sources, published reports on major works on the ecology of seagrasses of Australia are non-existent.

METHODS

Following a preliminary survey, a comprehensive s tudy of Moreton Bay was undertaken in which all the littoral areas were visited by shallow draft boats and the composition of their seagrass communities determined by eye. Their extent was then corroborated by aerial survey (Fig.2).

Selected sites were established in various of the communities and the biomass of each seagrass species was estimated. Here a 0.063 m 2 quadrat was thrown. The plants within the quadrat were removed entirely (including roots) and taken to the laboratory. Here the plant material was removed from the soil, sorted into species, dried for 24 h at 105°C and weighed. Ten or 20 samples were taken from quadrats thrown at random. The number taken was decided according to the diversity of the community . This sampling was re- peated every 2 months for a year.

At the beginning of the programme, two samples were taken of the substrate at each site. This was done by driving a corer 3.4 cm in diameter 36 cm into the substrate. The corer with sediment in situ was deep frozen with CO2 and removed to the laboratory where it was held in deep freeze until sub- sequently processed.

Fig. 2. Aerial photograph of Wanga Wallen Banks, showing seagrass communities. 1 : community type 2 ; 2 : community type 3 ; 3 : community type 4 ; 4: sand; 5 : mangrove (Avicennia marina) fringe to North Stradbroke Island.

The hor izons were separa ted by eye and a mechanica l analysis was made o f each. The fo l lowing c o m p o n e n t s were measured -- clay, silt, f ine sand, and coarse sand. The m e t h o d s used for the mechanica l analysis were those of Piper (1950) .

T h r o u g h o u t the year measu remen t s were t aken of the salinity and tempera- tu re of the wate r over the seagrass beds at fo r tn igh t ly intervals.

RESULTS

There are six species of seagrass growing in M o r e t o n Bay: Zostera capricorni Aschers., Halodule uninervis (Forsk . ) Aschers. , Halophila ovalis (R.Br.) Hook. f . , Halophila spinulosa (R.Br.) Aschers. , Cymodocea serrulata (R.Br.) Aschers. and Magnus and Syringodium isoetifolium (Aschers) Dandy. The specific

identi ty of these was ascertained by reference to the works of Ferguson-Wood {1959) and Den Hartog (1970). H. ovalis, Z. capricorni, and S. isoetifolium were identified on floral characteristics and H. spinulosa and H. uninervis on leaf structure. C. serrulata was identified by leaf scar arrangements as flowers, although present, had no t been previously described. These six species belong in five different distinct phanerogamic communit ies (Table I).

(1) Zostera capricorni and Halophila ovalis. (2) Cymodocea serrulata. (3) Syringodium isoetifolium. (4) Zostera capricorni, Halodule uninervis and Halophila ovalis. (5) Halophila ovalis and Halophila spinulosa. Within each of these communit ies the proport ion of one species to another

may vary widely.

(1) Z. capricorni and H. ovalis

This communi ty inhabits muddy to sandy mud localities, sheltered mud flats or sand banks (see Table I). Although the roots of Z. capricorni are quite numerous and well at tached to the substrate this plant does not grow in strong currents in Moreton Bay. H. ovalis is of only secondary importance in this communi ty and may be absent altogether where dense stands of Z. capricorni predominate. H. ovalis appears to require the presence of Z. capricorni to pro tec t it against tide and wave action. This communi ty may grow in a sublittoral or littoral position, the depth to which it grows being limited by light and by exposure to air. Light penetration in the muddy areas is no t great. However the longer the immersion time for Z. capricorni the denser the stand and the larger the plant and morphological differences may be seen in Z. capricorni and H. ovalis related to their position and the depth of the communi ty (Fig.2 and 3). The small plants came from mud flats near Cleveland Point which are ou t of the water for more than 2 h twice a day. The larger plants are from a muddy channel near the Aldershots Banks which is never out of water.

Flowering of Z. capricorni occurs between late September and March each year. No flowering of H. ovalis has been observed in this community .

In one area of the Deception Bay foreshores a massive mortal i ty of Z. capricorni occurred. This may have been primarily due to a disease, followed by extensive grazing. This disappearance was rapid, and followed by the loss of H. ovalis as the roots of this plant appeared unable to withstand the tide and wave action on their own.

This communi ty can tolerate wide ranges of salinity: salinites as low as 3 0 o were recorded during the s tudy in some localities. Epiphytic algae were uncommon; however a b loom of Sargonaema phyloforme (Sonder) Kylin appeared on Z. capricorni from the Aldershots Banks in May.

(2) C. serrulata

This monospecific communi ty occurs extensively only in one area. This occurs from the northern end of North Stradbroke Island for abou t 3 km south along the Wanga Wallen Bank. It occurs to a depth of about 3 m where it grows on a sandy b o t t o m in the sublittoral zone. This location has only transient changes in salinity, lying as it does close to the open ocean. C. serrulata has thick densely matted roots completely covering the substrate and tolerates a fairly strong current (up to 1.3 m/sec). Male flowers were found in January but little is known of flowering habits or flowering time. Older C. serrulata leaves were covered with algal epiphytes. There is no noticeable seasonal change in plant biomass.

(3) S. isoetifolium

This plant grows in a monospecific meadow just below neap low tide and is always immersed in water. The littoral edges of this meadow may associate with the Z. capricorni, H. uninervis and H. ovalis communi ty at the Wanga Wallen Bank off North Stradbroke Island and at Cooks Rock off Toorbul Point but there is a strong delimitation on the seaward side where it abuts the C. serrulata meadows at the Wanga Wallen Bank. Flowering occurs between January and February and at various times of the year epiphytic algae such as Polysiphonia sp., Ectocarpus sp., Hydroclathrus clathratus (Berg.) Howe and various diatoms form epiphytic blooms on the leaves. This is most noticeable during the winter months. Udotea argentea Zanard. at Cooks Rock grows as single plants sparsely separated in the communi ty . A slight decrease of biomass of S. isoetifolium occurs in spring, i.e. September to December. This species is only found of f Cooks Rock and at the Wanga Wallen Bank on sandy substrate where water entering from outside Moreton Bay tends to maintain oceanic salinity.

(4) Z. capricorni, H. uninervis and H. ovalis

This communi ty occurs only on sandy bo t toms in littoral areas where it grows from the region of mangrove pneumatophores to below the low tide mark. There it is replaced by S. isoetifolium. In shallower areas the plants of each species may be very much smaller than those in deeper water. This can also be seen where larger plants grow in hollows or lagoons (Figs.3 and 4). Salinity and turbidi ty ranges taken over a year (Table I) indicate that water turbidities are lower than for communi ty type 1 and although salinities may be similar in range, the mean salinity over a year is usually higher where species belonging to communi ty type 4 occur. This communi ty occurs at Cooks Rock, the Wanga Wallen Bank, and in patches around the Aldershots Banks. In these areas this communi ty is gradually replaced by communi ty type 1 as the mud content of the substrate increases. This at North Stradbroke

® jJ"

f.J i /

i "/ / I'fl \'~ ....

Fig.3. Zostera capricorni: (a) larger morphological form; (b) smaller morphological form; (c) leaf tip venation.

Island stretches as far south as Dunwich by which time H. uninervis is no longer present and H. ovalis is somewhat reduced. At Toorbul Point it grades into a communi ty type 1 towards Beachmere and the Caboolture River. The foreshores of Deception Bay south of the Caboolture River were covered originally by communi ty type 1. Since 1972 seagrasses have disappeared from this area and there are now indications that Z. capricorni is dying from the communi ty type 4 meadows at Cooks Rock. The calcareous green alga U. argentea grows in the sand amongst the seagrasses, and during winter (June- September} blooms of the brown alga H. clathratus occur. Other algae

Fig.4. Halophila ovalis: (a) larger morphological form; (b) smaller morphological form.

epiphytic on these seagrasses are Dictyota intricata (Ag.) Grev. and Laurencia heteroclada Harvey.

(5) H. ovalis and H. spinulosa

The western side of Moreton Island is bordered by clean sandy beaches with parallel sand bars and deeper channels. The water here tends to be oceanic (see Table I). Flanking the beach, near Tangalooma, a lens-shaped bed of this communi ty grows. This communi ty does no t afford complete ground cover bu t is characterised by a lattice type growth with runners of both Halophila species crossing the sand in straight lines. The H. ovalis here shows the larger morphological form (Fig.4). Light penetration is high and the communi ty grows to a depth of abou t 5 m.

These species have also been found in association at the Aldershots Banks. Here turbidi ty is greater, lower ranges of salinity are experienced and there is a much higher percentage of silt and clay. This small area in the sublittoral zone between Z. capricorni and a deep channel had, in 1972, a partial cover of H. spinulosa with small amounts of H. ovalis. During that year the species changed and the area became dominated by H. ovalis with only a few sprigs of H. spinulosa remaining. Flowers of H. spinulosa were no t found during this study.

These two different habitat areas give some indication of the wide range that these species may occupy. H. spinulosa is also found along the edges of many channels sometimes associated with Caulerpa mexicana Sonder ex Kiitzing. H. spinulosa may grow at a greater depth than others because it needs less light. However it is quickly replaced by other species. Thus a patch o fH. spinulosa which was observed in the Welsby Light area on an otherwise bure patch of sand has gradually been overrun by H. uninervis and H. ovalis. So H. spinulosa is usually found in areas too deep or turbid for other sea-

gr~sses whilst H. ovalis although found throughout Moreton Bay has morphological variability according to the location in which it is found.

DISCUSSION

Most investigations into the ecology of seagrasses have determined that usually a succession series occurs leading to a climax; Den Hartog (1973) has grouped these series into four types as follows: (1) a series leading to a Posidonia meadow; (2) a series leading to a Thalassia meadow due to absence of Posidonia; (3) a series in which seagrasses are a stage in the succession towards a higher organized communi ty and (4) the series in which the seagrass is the initial as well as the terminal stage of development, e.g. a Zostera meadow.

Den Hartog's study, and those of others (e.g. Aleem, 1955; Phillips, 1960) have generally been concerned with monospecific communities, and there is a tendency to assume that a climax communi ty is usually monospecific.

In the present s tudy we found two monospecific communities, one multispecific communi ty , a communi ty characterised by a single dominant species, and a communi ty in which two species were of equal importance.

These five major communit ies have been found no t only in Moreton Bay, but also further north in Tin Can 'Bayand the Great Sandy Strait. In these areas, however, both Thalassia and Posidonia are absent and we suggest that these communities may be stable because of the absence of Posidonia. Dense monospecific stands of Posidonia australis Hook. f. occur further south, along the coast of New South Wales where it obviously forms a climax situation. We have found Thalassia hemprichii (Ehrenb.) Aschers. occurring further north at Thursday Island on the tip of the Cape York Peninsula. Here this species appears to replace Z. capricorni as it was found in association with H. ovalis and H. uninervis in similar areas to those in Moreton Bay in which Z. capricorni, H. ovalis, and H. uninervis were found. However, we did not find it in monospecific meadows. In this same geographical area Cymodocea serrulata, which formed monospecific meadows in Moreton Bay and Tin Can Bay, was found in association with T. hemprichii, Enhalus acoroides (L.f) Royle, H. uninervis and H. ovalis and was no t found in monospecific meadows.

It would appear then, that Moreton Bay is remarkably well endowed with seagrasses and the zonation of the communities which they comprise is determined by depth, salinity, turbidity, and substrate characteristics. The absence of Posidonia has enabled various species to co-exist and for communities to remain intact instead of being part of the series leading to a Posidonia meadow.

Work on seasonal growth, biomass and species composit ion will be reported in another paper.

ACKNOWLEDGEMENTS

We should like to thank Dr C. den Hartog of the Katholieke Universiteit, Nijmegen for confirmation of our identifications of the specific identities of

t h ~ M o r e t o n B a y seag ras ses a n d a l so Miss J. O ' R e a g a n w h o p r e p a r e d t h e i l l u s t r a t i o n s f o r t h i s p a p e r .

REFERENCES

Aleem, A.A., 1955. Structure and evolution of the sea-grass communities Posidonia and Cyrnodocea in the southeastern Mediterranean. In: Essays in the Natural Sciences in Honour of Captain Allan Hancock. Univ. of S. Calif. Press, Los Angeles, Calif., pp.279--298.

Allen, D.M. and Inglis, A:, 1958. A pushnet for quantitative sampling of shrimps in shallow estuaries. Limnol. Oceanogr., 3(2): 239--241.

Den Hartog, C., 1970. The sea-grasses of the world. Verh. K. Ned. Akad. Wet. Afd. Natuurk., Ser. II., 59(1): 1--275.

Den Hartog, C., 1971. The dynamic aspect in the ecology of sea-grass communities. Thalassia Jugosl., 7(1): 101--112.

Ferguson-Wood, E.J., 1959. Some East Australian sea-grass communities. Proc. Linn. Soc. N.S.W., 34(2): 218--226.

Maxwell, W.G.H., 1970. Sedimentary framework of Moreton Bay, Queensland. Aust. J. Mar. Freshwater Res., 21: 71--89.

Newell, B.S., 1971. The hydrological environment of Moreton Bay, Queensland 1967--68. CSIRO Div. Fish. Oceanogr. Tech. Pap., 30: 1--35.

Phillips, R.C., 1960. Observations on the ecology and distribution of the Florida sea-grasses. Prof. Pap. Ser. Mar. Lab. Fla., 2: 1--72.

Piper, C.F., 1950. Soil and Plant Analysis. Univ. Adelaide Press, Adelaide, W.A., 368 pp. Strawn, K., 1954. The pushnet, a one-man net for collecting in attached vegetation. Copeia,

3: 195--197.

The seagrass communities of Moreton Bay, Queensland

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