development of mangrove habitats along ria shorelines in north and northwestern tropical australia

Development of mangrove habitats along ria shorelines in north and northwestern tropical Australia*

V. Semeniuk** 21 Glenmere Road, Warwick, W.A. 6024, Austral ia

Keywords: Aust ra l ia , Mangal , Mangrove , Mangrove habi ta t , Ria shorel ine

Abstract

A l o n g ria shore l ines m a n g r o v e assemblages are closely re la ted to hab i ta t types and there is a recur r ing pa t t e rn in their d i s t r ibu t ion tha t is l inked to the or igin and his tory of the geomorph ic units. The coas ta l g e o m o r p h o l o g y is re la ted to ances t ra l l and fo rms deve loped pr io r to the pos t -g lac ia l t ransgress ion, as well as to t ida l levels, aspect , m o d e r n shore - fo rming processes, and types of h i n t e r l a n d / t i d a l f lat contacts . These h is tor ica l and process pa t te rns deve lop dis t inct g e o m o r p h i c units which dic ta te the d i s t r ibu t ion of hab i ta t s and their mangrove assemblages .

M a n g r o v e assemblages are classified accord ing to their habi ta t : main t idal flat, h in te r land fringe, a l luvial fan, sp i t / chen ie r , t ida l creek bank , t idal creek shoal and rocky shore. Wi th in a given hab i ta t there are var ious phys ico-chemica l grad ien ts which are ins t rumenta l in deve lop ing zona t ion within the assemblages. The var ie ty of in ternal zones within an assemblage is related to the richness of species within the regional species poo l which in turn is related to cl imate.

Introduction

Studies of mangrove ecology genera l ly have con- cen t ra ted on species d i s t r i bu t ion at reg ional and local (zona t ion) scale ( M a c N a e , 1968), succession ( C h a p m a n , 1976), and smal l -scale phys ico-chemical and b io logica l processes within the env i ronment (e.g. Scho lande r , 1968; Saenger , 1982). Othe r

* Nomenclature follows ** Acknowledgements. Preliminary data for this study were collected in 1978 while the author was researching the Mitchell Plateau area and Dampier Archipelago on behalf of Amax Exploration Pty Ltd and Woodside Offshore Petroleum Pty Ltd; quantitative data on mangrove assemblages in Port War- render were collected while investigating mangrove ecosystems for Mitchell Plateau Bauxite Company. Access and logistic aid by these companies are gratefully acknowledged. Data from Darwin were collected over several years while engaged on research work for Dames & Moore Consulting Engineers/De- partment of Lands and Housing, Northern Territory. The manu- script was critically read by P. N. Chalmer and I. LeProvost.

workers , no tab ly ear th scientists, have in tegra ted studies of sed imento logy , s t r a t ig raphy and geo- chemis t ry within mangrove envi ronments with a view to documen t ing the impor t ance of mangroves in p r o d u c i n g recognizable s t ra t ig raph ic units; these inves t igat ions general ly are not a imed at p rov id ing i n fo rma t ion for b io logica l studies. Thus few studies have descr ibed a f r a m e w o r k of the na tu ra l system for b io logica l purposes to expla in why mangroves or for tha t mat ter , o ther coasta l ha lophytes , occur where they do. An example of this a l te rna t ive ap- p roach is shown by the work of Phleger (1977) on mar ine marshes. The s tudy provided a med ium- scale f r a m e w o r k within a sett ing of coas ta l lagoons to define smal l -scale units as habi ta t s for mar ine marshes , and the ph i lo sophy of the a p p r o a c h is direct ly app l i cab le to mangroves .

W h e r e p h y s i o g r a p h y and hab i ta t s a long coasts are descr ibed , many studies to da te have t rea ted mangrove env i ronments s imply as incl ined shore-

Vegetatio 60, 3-23 (1985). © 1985. Dr W. Junk Publishers, Dordrecht. Printed in the Netherlands.

lines and provide only maps of tidal zones (levels) and mangrove zonation. There generally is no differentiation of habitat types, although various habitats can be inferred from published maps and vegetation associations. Furthermore, where descrip- tions of physiography, geomorphology, substrates, sedimentology, salinity, etc. are provided it is generally as a setting or backdrop to mangroves rather than as a basis to understanding the development and maintenance of mangrove habitats. The works of Thom (1967, 1982) serve as an exception to this. In a theoretical dissertation Thorn (1982) developed the model that hydrodynamic and sedimentologic processes interact to produce a dynamic landscape at the interface of land and sea, and viewed succession as a response to a dynamic habitat. However, his model did not proceed beyond large-scale coastal morphologic types and thus did not provide the small- to medium-scale models nor the small-scale documentation to explain the distribution and

maintenance of mangrove species and populations. This study has concentrated on mangrove as-

semblages along ria shorelines and provides a framework to understanding the distribution, composition and zonation of mangroves at large, medium and small scales for these coastal settings. This three-scale approach helps to explain the occurrence of mangroves along the coast, the occurrence of broad mangrove assemblages within habitats, and the local variability and zonation of species in response to small-scale gradients within habitats. Ria shores are common around the globe, particularly in tropical humid-subhumid regions and thus the concepts and results presented here have widespread applicability regardless of the regional species pool. The approach of course can be applied to any type of shoreline.

The study areas are located in the tropical north and northwest of Australia (Fig. I a). Aerial photo- graphs from 14 areas were used to obtain an indica-

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tion of regional variability; five accessible regions were chosen for detailed on-site study at Port Dar- win, Port Warrender, Mitchell River Estuary, Point Samson/Cossack area and Dampier Archi- pelago/Mai t land River area. At each locality, geomorphic units were mapped as a framework for detailed transect studies. Along selected transects, cores, pits and trenches were used to investigate soils, stratigraphy, groundwater hydrology and groundwater/soi lwater salinity. The structure and composit ion of mangrove vegetation was described along belt transects oriented to intersect zonation or gradients across habitats. Vegetation composition was quantified at selected sites by counting species within (5 to 20) replicate 5 m × 5 m qua- drats.

Ria shorelines in northern and northwestern Aus- tralia: their physical features

Regional setting and description of study areas

Ria shorelines are highly indented coastlines (Fig. 1 B, C) comprised of inlets and embayments formed where a dissected rocky terrain has been flooded by the post-glacial transgression (Bates & Jackson, 1980). Coasts are termed here ria regardless of scale if they exhibit these features: (1) a dissected rocky hinterland of moderate to high relief (>10 m), and (2) embayments which are marine flooded terrestrial valleys. Ria shorelines are located in three main areas along the northern and northwestern coast of Australia: the Darwin region, the Kimber- ley coast and the Pilbara coast (Fig. IA).

The Darwin region contains Port Darwin which is an embayment some 450 km 2, bordered by a dissected plateau of Precambrian rocks and Cai- nozoic laterites (Christian & Stewart, 1953). Port Darwin is dominated by embayments, narrow gulfs and tidal land-connected islands (Figs. 1C, 2A; Semeniuk, 1984). Darwin's climate is tropical hu mid (Gentilli, 1972; using Thornthwaite 's ( 1931) method) with wet summer monsoon and dry winter seasons. There are some 97 rainy days per year, with about 1 536 mm of precipitation; evaporation is 2 773 mm/yr ; mean annual temperature is 28 ° C (Bureau of Meteorology, 1976).

The Kimberley coast fringes the dissected Kim- berley Plateau composed of uplifted Precambrian

rock (Fig. l B)(Geological Survey of Western Aus- tralia, 1975). The regional coastal zone is dominated by embayments, two of which are Port War- render and Mitchell River estuary, some 160 km 2 and I l0 km 2 resp. The climate of the study sites is tropical subhumid with wet summers and dry win- ters. Some l 554 mm of rain falls annually within 95 days; evaporation is 2 900 mm/yr ; mean annual temperature is 25 o C.

The Pilbara coast borders an uplifted dissected block of Precambrian rock (Geological Survey of Western Australia, 1975). The coast is dominated by limestone barrier islands and fan-shaped deltas; however, where resistant Precambrian rocks crop out locally as at Point Samson/Cossack, Dampier Archipelago/Mait land River area (Fig. I A) there are ria shorelines with small embayments, narrow gulfs and tidal land-connected islands (Semeniuk et al., 1982). The climate of the Pilbara coast is tropical arid with 320-360 mm of rain falling annually in 20-30 raindays; evaporation is ca 3 500 mm/yr ; mean annual temperature is 26 ° C.

Since these areas span a range of climates from humid to arid there is variable input of freshwater from the hinterland (by sheet flooding, numerous small creeks, rivers and subterranean seepage). The significance of the climate gradient in terms of this freshwater input to development of mangrove habitats and mangrove diversity will be discussed later.

All the study areas are in macrotidal environments. The maximUm tidal range is 7.8 m at Port Darwin, 8.6 m at Port Warrender and 5.6-6.5 m along the Pilbara coast (Australian National Tide Tables, 1983). Land breeze/sea breeze systems and thunderstorms generate wind waves which are important as agents for sediment reworking and transport in coastal processes.

Large-scale features: coastal morphology

Ria shorelines exhibit 5 intergradational types of large-scale morphology (i.e. within frames of references ca 100 km2). These are (Fig. 2):

(1) Narrow embayments where entrenched river courses have been flooded and /o r infilled with tidal deposits; tidal flats > hinterland margins > tidal creeks > alluvial fans > rocky shores.

(2) Broad embayments where open undulating terrain has been flooded and /o r filled with tidal deposits; tidal fiats > hinterland margins > tidal

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Fig. 2. Maps showing typical configuration and elements of some ria shorelines. A. Port Darwin showing narrow embayments, broad embayments with spits and tidal land-connected islands. B. Mitchell River estuary showing long, narrow, straight embayment with subsidiary small narrow (tributary) embayments. C. Port Warrender showing long, narrow, sinuous embayment to the south and small broad embayments to the north.

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creeks > spits/cheniers > alluvial fans /> rocky shores.

(3) Islands, in clusters or in chains where hills have been isolated by marine flooding and tidal deposits; islands exposed to waves have bordering splits/cheniers and, where protected, have muddy tidal land connections to the mainland and other islands; tidal flats > spits/cheniers ~> rocky shores.

(4) Cliff/rocky shores where extensive stretches of coast are exposed to consistent waves and there is negligible sediment accumulation.

(5) Sand)' shores where extensive stretches of coast are exposed to consistent waves with accumulation of sand.

M angrove formations extensively colonize main- ly tidal lands only ofembayments , protected islands or portions of islands where there is little reworking by waves. Only these environments are considered further in this paper.

Medium-scale features: geomorphic/stratigraphic units

Ria shorelines contain a range of medium-scale geomorphic units (within frames of reference ca 1 km2); the units are (Fig. 3A): (1) supratidal/terrestrial hinterland; (2) hinterland/tidal flat margin; (3) high tidal alluvial fan; (4) tidal flat; (5) tidal creeks; (6) spits/cheniers; (7) bouldery or rocky shores.

Dependent upon aspect, geomorphic configuration and sediment infill, embayments of ria coasts are in various stages of completion in the development of these units; accordingly, in some cases not all units are present (although a majority usually are). The geomorphic units also generate, by sedimentary accumulation, their own distinct stratigraphic units (Table 1; Fig. 4). The geomorphic / - stratigraphic units are important in developing unique habitats (since each unit has its own suite of substrates), and also has an important function in tidal flat hydrology (e.g. geomorphic units function as specific recharge areas, stratigraphic units function as aquifers and /o r facilitate recharge and in- terchange of groundwaters).

The origin of geomorphic/strat igraphic units is linked to ancestral geomorphology developed prior to marine transgression and to modern terrestrial and marine geomorphic processes of sedimentation/ erosion (Fig. 3B). Many of the units are located at specific tidal levels because processes forming

them are located only, or predominantly, at these levels. The large-scale coastal morphology is due to marine flooding of a dissected terrain: valleys became embayments; spurs/ridges became headlands. After inundation the ancestral morphology contributed the following elements to the medium- scale geomorphology: (1) active streams, which dis- charged sediment down their axes; (2) colluvium or soil-covered valley slopes which contributed materials to the hinterland/tidal flat margin; (3) local, or widespread, protruding bedrock which developed islands. The marine and terrestrial processes of sedimentation/ erosion then acted upon the inundat- ed surface to develop the range of medium-scale geomorphic units (Table 2).

Small-scale features: substrates and salinity gradients

Small-scale features of tidal flats include those physico-chemical phenomena (i.e. variable mosaics of substrates and the groundwater/soilwater regimes) which occur at local sites (typically within frames of reference ca 100 m 2 up to 1 000 m2).

Substrates Substrates (= soil) play a major role in tidal

environments in determining the distribution of flora and fauna. The recharge and /o r aquifer properties of homogeneous mud, root-structured mud or burrow-structured mud are all different (Davis & De Wiest, 1966; Hillel, 1971; Daubenmire, 1974; Chapman, 1976; Bolt, 1982; Bresler et al., 1982). The internal drainage and /o r water-retention properties of deep sand, homogeneous mud, burrow-structured mud, and sand underlain by mud, also are significantly different. So too the nutrient storage and nutrient-retention capacity of various substrates (such as clay mineral mud, carbonate mud, quartz silt) are different (MacNae, 1968; Hil- lel, 1971; Daubenmire, 1974; Bolt, 1982). Therefore it has been necessary to document the full range of structure, fabric, texture and compositional types within substrates.

The components of substrates are mud, sand, shell, rock gravel and bedrock. These when accum- ulated and mixed, provide a variety of substrates. Burrowing biota, such as crabs, serve to further mix sediments and produce homogeneous and bioturbated products. The main substrates encountered

Table 1. Geomorphic/stratigraphic units and soils of ria coasts.

Geomorphic unit Description Stratigraphic unit; geometry, lithology and relationships

Distribution according to tidal level

Types of soils

Suptratidal/terrestrial moderate to high hinterland relief rocky terrain;

colluvial soil slope; valleys and alluvial ribbons

Hinterland/tidal flat narrow zone (10 50 m margin wide) of contact

between tidal fiat and hinterland, correlated with a break in slope at the edge of the hinterland

High tidal alluvial fan elongate to fan- shaped to deltoid alluvial deposit (10-250 m across) debouching from hinterland drainage

Tidal flat

S pits/cheniers

Bouldery or rocky headland shore

Tidal creeks

broad zone (100 m to >1 km), gently inclined surface; vegetated by mangroves between ca MSL and MHWS

elongate to recurved finger or bar-shaped deposit (10-50 m wide)

cliff varying to steep, bouldery shore

bifurcating and/or meandering steep- sided drainage courses

bedrock mantled by above HAT, to sheets of soils, landward of all tidal colluvium, or alluvium units

colluvium sheet/ along HAT zone ribbon; muddy gravels fringing the or muddy boulder hinterland deposit overlying bedrock sandwiched between main tidal fiat wedge and hinterland

gravel/sand fan or wedge; sandy gravels, sand, or muddy sand and gravel overlying bedrock and inter- fingers with tidal fiat deposits

muddy tidal fiat extends from HAT to wedge; muddy sediments LAT, encompasses most onlap hinterland/tidal flat margin; occupies major portion of embayment

sand or shelly sand bar; overlying tidal fiat deposits

boulder apron or ribbon; overlying bedrock

tidal creeks generally incise tidal fiat and spit/chenier units; mid-channel areas are composed of sand and mud shoals

HWS up to above HAT; limited to discrete localities along the hinterland/tidal fiat contact

of the tidal zone

depending on position, extends from HWN to EHWS as units emanating from headlands

extends from HAT to LAT, limited to discrete localities at the entrance to embayments

mouth of creeks are at low water; headwater portions of creek are at HWS; main trunk depending on size of creek is incised to levels of LWN and LWS

rock gravel sheets boulder deposits interlayered mud/ sand, homogeneous

rock gravel sheets, bioturbated and root- structured mud, mud/ sand and sand

bioturbated and root- structured mud, texture mottled mud and sand, muddy sand; and homogeneous mud

homogeneous sand, laminated sand interlayered sand/mud

rocky gravel sheets, boulder deposits

sand and mud

a Soils developed on the supratidal/terrestrial hinterland are not described since they do not support mangroves.

in th is s t u d y in o r d e r o f a b u n d a n c e inc lude: (1)

b i o t u r b a t e d a n d r o o t - s t r u c t u r e d m u d ; (2) h o m o -

g e n e o u s m u d ; (3) t e x t u r e - m o t t l e d m u d a n d s a n d ;

(4) h o m o g e n e o u s m u d d y s a n d ; (5) r o o t - s t r u c t u r e d

m u d d y s a n d ; (6) h o m o g e n e o u s s a n d ; (7) l a m i n a t e d

s a n d ; (8) r o c k g r av e l shee t s ; (9) b o u l d e r d e p o s i t s ;

a n d (10) i n t e r l a y e r e d m u d a n d sand .

Th i s w ide va r i e ty o f s u b s t r a t e s c a n be r e l a t ed to

g e o m o r p h o l o g y , t ida l levels a n d r e l a t ed p roces se s ,

s u c h t h a t t h e r e is a r e c u r r i n g p a t t e r n o f soi ls fo r a

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Fig. 4. Examples of stratigraphic units and their interrelationships along ria shorelines. These examples show the similarity of stratigraphic sequences and their interrelationships regardless of locality and climate. The figures also show the occurrence of the main mangrove species across the geomorphic/s t rat igraphic unit for each location and indicates that the stratigraphy is related to Holocene processes. A, C & F illustrate the relationship of spits, colluvial materials along the hinterland/t idal flat margin, and the main tidal muddy wedge. A is from Dampier Archipelago, C is from Port Warrender and F is from Port Darwin. B, an example from Port Darwin, illustrates the relationship of colluvial material along the hinterland/t idal flat contact and the main tidal muddy wedge. D, from Port Darwin, and E, f rom Dampier Archipelago, illustrate the relationship between the high tidal alluvial fan and the main tidal flat muddy wedge. The stippled pattern at the surface of the profiles in E is a sand sheet on the salt flat.

Table 2. Origin of geomorphic units.

II

Geomorphic unit Origin

Main processes Details of processes

Hinterland

Hinterland/t idal flat margin

High tidal alluvial fan

Main tidal flat

ancestral topography

marine reworking and terrestrial sedimentation

terrestrial sedimentation and marine reworking

marine sedimentation

Tidal creek marine erosion

Spits/cheniers marine sedimentation

Bouldery and rocky intertidal shore

marine erosion and marine reworking

current processes of terrestrial pedogenesis, erosion, transport and sedimentation

marine reworking of colluvial deposits; zone of mixing between muddy tidal flat sediments and hinterland; later, ongoing sedimentation (sheet wash off terrestrial environments) contributes to this unit

fluvial discharges of sediment from creeks onto high tidal flats; sedimentation keeping pace with tidal flat accretion develops an interdigitating stratigraphic relationship between alluvial gravel /sand and tidal flat muddy sediments; further sedimentation builds the alluvial fan up above the level of the tidal flat (salt flat)

sedimentary progradation develops a tidal flat wedge, the upper surface of which is the low-inclined high tidal surface; sediments accrete to above M H W S levels and there is development of hypersaline ground and soilwater such that mangroves die back forming extensive salt flat

marine erosion of the tidal flat by drainage incisions

wave processes acting on rocky headlands at high tidal level cause the migration and progradation of linear shell grit and sand deposits

formed by marine erosion of spurs of a rocky terrain and marine reworking of their colluvial or soil-covered slopes

given geomorphic unit (Table 1 and Fig. 4). The essence of the relationship between geomorphy and the substrate types is that: (l) dependent on sediment supply, the shores are rocky or covered with a variety of sediments; (2) dependent on wave/ tidal energy, the sediment-covered shores later are washed free of sediment or are covered by sand winnowed free of mud; (3) dependent on wave ex- posure, storm action and frequency of flooding, the shores are underlain by sand in low tidal zones and by mud in progressively high tidal zones; and (4) dependent on fluviatile input, there is a varying amount of sand and gravel along the hinterland edge or along the axis of terrestrial drainage chan- nels.

Groundwater/soilwater Salinity regimes on tidal flats are important in

regulating aspects of mangrove recruitment, survi- vorship, growth and zonation (MacNae, 1968; Chapman, 1976; Cintr6n et aL, 1978; Semeniuk, 1983). Groundwater/soilwater salinity is linked to tidal height, recharge frequency, recharge mechanisms, substrate, stratigraphy and evapotranspira- tion. Ultimately there are two main sources of water (i.e. oceanic and fresh) but aspects such as evapora-

tion, recharge, mixing and flow rates combine to provide five groundwater/soilwater bodies (Fig. 5): (I) marine-derived oceanic to hypersaline groundwater/soilwater of muddy flats; recharged daily; (2) marine-derived highly hypersaline groundwater/ soilwater of muddy flats; recharged fortnightly to monthly; (3) marine-derived oceanic to hypersaline groundwater/soilwater of spits/cheniers; recharged fortnightly to monthly; (4) freshwater flowing through alluvial fans and along colluvial sheets/ bedrock interfaces into the tidal flats producing groundwater/soilwater of mixed salinity (5) freshwater under the hinterland.

Each groundwater type resides in a distinct stratigraphic unit (= aquifer) and has its own mechanisms (dynamics) or recharge, migration, physico- chemical processes, evaporation/transpiration and salinity (Table 3). These processes within aquifers, and the disposition of aquatards/aquacludes, result in: (1) a gradient of increasing (hyper)salinity across tidal flats; (2) a zone of mixing between freshwater seepage and tidal flat (hypersaline) groundwater; and (3) mixing of seawater within spits/cheniers, and between spits/cheniers and tidal flat ground- w a t e r .

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:: :: ::::::::::::::::::::::::::::::::::::::::::::::::::::: . . . . . . . . . . . . . . . . . . . .

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B. RECHARGE MECHANISMS

HINTERLAND/TID FLAT COLLUVIUM SHEET

12

,• OCEANIC WATER: DALLY RECHARGE ' ~

I OCEANIC WATER: FORTNIGHTLY j TO MONTHLY RECHARGE

FRESHWATER:SEASONAL RECHARGE

FRESHWATER: PERENNIAL RECHARGE

Fig. 5. A. Diagram illustrating the geometry, disposition and interrelationships of the main aquifers that are host to groundwater on the tidal flat. The description of the geomorphic/strat igraphic units that form the basis of the aquifers is presented in Tables I and 3. B. Summary diagram of the main mechanisms by which oceanic and fresh water recharge and maintain the aquifers.

13

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14

Mangrove habitats and mangrove assemblages

H a b i t a t s

The combina t i on of geomorphology/s t ra t i - graphy, substrates, aquifers, physico-chemical processes and salinity develops several distinct habitats for mangroves. It is evident that geomorphic units are a key factor in unders tand ing the d is t r ibut ion of mangrove assemblages because the other environ- mental factors such as stratigraphy, substrates, etc. are l inked to, or related to medium-scale geomorphology. The mangrove habi tats are: (1) h in ter land margin; (2) high tidal alluvial fans; (3) tidal fiat; (4) tidal creek; (5) spits/cheniers; (6) rocky /bou lde ry headland shores.

There also is a gradient of physico-chemical features within each habi ta t such that mangroves are zoned. Examples of gradients impor t an t to mangrove zona t ion are: gradients of salinity and frequency-of-f looding across broad tidal fiats; salinity, f requency-of-f looding and soil texture gradients across spits; salinity and soil texture gradients across h in t e r l and / t i da l flat margins; and salinity gradient across high tidal alluvial fans.

M a n g r o v e a s s e m b l a g e s

D e f i n i t i o n

In this paper mangroves have been aggregated into assemblages defined to be informal groups of readily identif iable or diagnostic species which tend to recur (even if in varying propor t ions) under condi t ions of similar habitats. There is no implicat ion that there is species in terdependence within an assemblage. Many of the componen t species of these assemblages vary in abundance , and species most numerous locally may be less a b u n d a n t elsewhere even though the overall compos i t ion is the same. Assemblages may be zoned but individual species overlap in d is t r ibut ion which makes it difficult to assign a group of organisms to a specific or discrete zone. Some species may be used to characterize a ' zone ' but they frequent ly extend into the ad jo in ing species 'zone' .

R e g i o n a l spec i e s p o o l

Prior to describing assemblages it is necessary to consider species pool in each region. The study areas are in different climates and regionally the

coasts may be viewed as being located within a climate gradient; the trend from Port Darwin to the Pi lbara is increasing aridity. Mangrove diversity (species richness) decreases with aridity and the climate gradient is reflected in species richness for a given region: 19 mangrove species occur in Port Darwin, 13 occur in Port Warrender and 7 occur along the Pi lbara coast (Table 4).

Table 4. Occurrence of mangrove species and strandline plants a in the study areas.

Species Darwin Port Pilbara region Warrender region

region

Mangrove species A egialitis annulata X X A egiceras corniculatum X X A vicennia marina X X Bruguiera exaristata X X Bruguiera gymnorhiza X x Bruguiera parviflora X X Camptostemon schultzii x x Ceriops decandra x Ceriops tagal x x Cynometra ramiflora x Excoecaria agallocha X X Hibiscus tiliaceus X tumnitzera racemosa X X Osbornia octodonta X X Pemphis acidula X Rhizophora stylosa x X Scyphiphora hydrophylacea x Sonneratia alba x X Xylocarpus sp. b X X

Strandline plants a Diospyrosferrea var.

humilis X x Melaleuca spp. X X

a This category termed here 'strandline plants' refers to those species which occur along the strandline, growing in terrestrial as well as high tidal environments. The species may be viewed as facultative mangroves. They are included here because they make important contributions to the composition of mangrove assemblages in some regions. Xylocarpus has been identified as X. australasicus in the Port Warrender region by Semeniuk et al. (1978); however, there appears to to taxonomic problems at present with this identif- ication. There may be 2 species of ,~vlocarpus in this region (personal observations) and the species idenitifed to date may be X. mollucensis(P. G. Wilson, pers. comm.). In the Darwin region there are at least 2 species of Xylocarpus (see Saenger et al. (1977) and Wells (I 982)). For purposes of this paper, in view of the taxonomic problems, all species of Xvlocarpus are not differentiated.

15

Mangrove assemblages according to habitat

Given the range of habitats and the var ia t ion of

physico-chemical condi t ions within them, there is

therefore a wide range of mangrove assemblages

and zones that inhabi t ria shorelines. These assem-

blages are largely restricted to the medium-sca le

habi tats (Figs. 3C, 6). The assemblages are named

after their habi ta t as follows: (1) 'ma in tidal flat '

assemblage; (2) 'a l luvial fan' assemblage; (3) 'h in ter -

land fringe' assemblage; (4) ' sp i t / chen ie r fr inge' as-

semblage; (5) 'creek bank ' assemblage; (6) 'creek

shoal ' assemblage; and (7) ' rocky shore ' assem-

blage. F o r a given area each assemblage is composed of

species drawn f rom the regional species pool and

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HINTERLAND ~ SPIT/CHENIERMANGAL WITH FRINGING

I I SALT FLAT I I MANGALAND HEADWATERsALONG CREEK aANKS MANGAL ON MID-CREEK LOW TIDAL FLAT SHOALS

MANGAL FRINGING ~ UNDIFFERENTIATED MANGAL THE HINTERLAND ON TIDAL FLAT MANGAL ON HIGH TIDAL ALLUVIAL FAN

ZONED MANGAL TIDAL FLAT

Fig . 6. Maps showing distribution of mangrove assemblages within embayments of ria shorelines. These maps are drawn from numerous areas: A, B & D are from Port Darwin; C is from Port Warrender; D is from Dampier Archipelago. Within each embayment the various mangrove assemblages occurring on a specific geomorphic/habitat unit are evident. Compare these maps with Figure 3. Although there is zonation within an assemblage as expressed here by various shades of stippling, differentiation of zones into formally named units is not intended here; the composition of such internal zonation is presented in Figure 7 and Table 5.

16

HABITAT

HIGH TIDAL ALLUVIAL FAN AND HINTERLAND / T IDAL FLAT MARGIN

50 - 2 0 0 m I I

MAIN T I D A L FLAT

I 0 0 - 2 0 0 0 m I I

T IDAL CREEK : BANK AND SHOALS

I 0 0 - 5 0 0 m

I I

SPIT I CHENIER

500 m

I

ROCKY HEADLAND

50 - 5OOm

I I

COASTAL REG ION

PILBARA PORT WARRENDER

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/. * SALT FLAT~ × " ~ . ×,

~ l / 30

29 :51

~ × ×

×

DARWIN

24 25 26 27 28 × x * x × .... ..~ v x x x x x

: i!!: !:ii;

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I I 33

Fig. 7. Summary diagram showing internal zonation of mangroves within a given mangrove assemblage. The numbers designating a particular zone are referred to in Table 6 where the component species of a zone are listed and quantified.

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22

thus assemblage composition changes with climate. The occurrence of assemblages according to habitat, and the components of assemblages with re- spect to climate are summarized in Figures 6 and 7 and Table 5.

Zonation (or variation within assemblage). The mangals investigated in this survey were examined primarily according to the main habitats, but within habitats there is small-scale zonation in response to gradients of salinity, soil variability, height above MSL etc. (Fig. 7). Table 6 shows the species abundance within the zones. These data show that 'richness' or variety ofzonat ion within assemblages is dependent upon climate: assemblages at Darwin are well zoned with a multitude of species contribut- ing to the definition of zones; at the other extreme, in the Pilbara area, with fewer species, the same gradients within habitats do not result in a multitude of zones. The zonation in the Darwin region with the greatest species richness is described below to illustrate the types of mangal zonation within habitats.

Main tidal flat (proceeding from landward to sea): (1) a salt flat with scattered Ceriops; (2) zone of Ceriops (locally mixed with Avicennia); (3) zone of Bruguiera mixed with Ceriops; (4) Rhizophora zone; (5) a seaward Sonneratia zone.

Hinterland fringe (proceeding from landward to sea): (I) a landward Hibiscus zone; (2) a zone of Lumnitzera, Excoecaria, Ceriops and locally Bru- guiera; (3) a zone of seaward Ceriops; (4) a salt flat.

High tidal alluvial fans (proceeding from landward to sea); (1) a landward zone of Bruguiera, Diospyros, Scyphiphora, Xylocarpus; (2) middle zone of Bruguiera, Ceriops, Rhizophora; (3) a seaward zone of Ceriops and Bruguiera.

Spit/chenier margins (proceeding from landward to sea): (1) a landward zone of Hibiscus and Pem- phis; (2) zone of Lumnitzera, Excoecaria, Scyphi- phora; (3) a seaward zone of Ceriops and Bruguie- r f l .

Tidal creek banks: (1) a seaward zone of Rhizo- phora and Camptostemon; (2) a zone of Rhizopho- ra, Bruguiera parviflora, Ceriops tagal, Ceriops decandra.

Discussion

Apart from obvious correlations that may be made between mangrove ecology and a variety of physico-chemical factors that illustrate the close relationship between mangroves, soil and water (MacNae, 1968), several other conclusions can be drawn from this study. Firstly there is a similarity of physical features at all scales along ria coastlines regardless of their regional climate setting, and hence a similarity and recurring pattern in distribution of habitats. Secondly it is obvious there are distinct assemblages composed of species drawn from the regional species pool; these assemblages are linked to discrete habitats. This relationship underscores the need to understand habitat distribution in conjunction with vegetation mapping and phytosociological studies. The fact that distinct and varied assemblages can be related to habitats per- haps is best exemplified by contrasting humid zone and arid zone assemblages. The more humid climates highlight the differences between discrete habitats because there are additions of species from the regional species pool to the assemblages within a habitat; whereas markedly dissimilar habitats in arid zones have limited and nearly similar components within assemblages because of a limited species pool.

Thirdly it is clear that zonation within assemblages is related to the internal gradients within habitats. Zonation is developed by the forced distribution of species selected from the limited components of the assemblage that colonize the habitat. The forcing factors that induce the selection of species are conditions such as salinity, water log- ging, soils and inundation; these factors influence population dynamics of both individual species and aggregations of species. This aspect thus underscores a need to develop an understanding of physico-chemical gradients as a framework to, or in conjunction with, studies in mangrove zonation and phytosociology.

Finally, the terminology for frames of reference (i.e. large, medium and small scale) can be applied to mangrove systems to describe the various scales of detail used in their description. The extensive occurrence of mangals in embayments or along protected shorelines between MSL and HWS is a large-scale feature. Distinct mangrove assemblages within habitats are medium-scale features. Individ-

ual populations or zones within an assemblage developed in response to the variable physico- chemical gradients are small-scale features.

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Accepted 2.2.1984.

development of mangrove habitats along ria shorelines in north and northwestern tropical australia

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