Mangroves in the Great Barrier Reef World Heritage Area: current status, long-term trends, management implications and research. NC Duke. 22-24 Victoria ...
-
Mangroves in the Great Barrier Reef World Heritage Area: current status, long-term trends, management implications and research NC Duke 22-24 Victoria
Street, Townsville
Q/d 4810
Abstract Mangroves are a coastal marine environment, characteristically biomass-dominated by trees. They support a high biodiversity of marine and terrestrial biota, as well as providifig a haven for estuarine fauna, and a nursery ground for other fauna ranging from flying foxes and seabirds, to offshore fish and crustaceans. The uses and benefits of mangroves equate to our direct use of some of these biota but it also includes other indirect benefits such as protection of coastal foreshores and estuarine margins from erosion. Mangrove environments in, and adjacent to, the Great Barrier Reef World Heritage Area are in relatively good condition, although there are clear indications that pressures on them are increasing rapidly. Localised impacts are accumulating to a point where large areas, once thought to be able to withstand change, are now threatened. And, detrimental changes appear to exceed societies’ current responses to protect mangrove environments and to reduce the overall impact of the growing number of smaller impacts. Human activities affect the establishment, growth, survival and biodiversity of mangrove plants, and their impacts range from: direct removal and damage of mangrove plants; conversion of mangrove lands to other uses; construction of breakwaters and. other alterations to water courses and local hydrology affecting depositional planes and sediment levels; changes to air and water quality as increased dust, turbidity, temperature and the addition of chemicals; catastrophic events of pollution bringing long-term impacts like large oil spills; and the introduction of exotic pests and pathogens from land and sea sources. Pressures on mangrove environments are real, and there is an increasing obligation on environmental management authorities to clearly describe coastal and estuarine areas according to the best scientific advice. Based on these descriptions, the next step would be to apply protection status, and in particular, designating specific areas for total protection with surrounding areas as buffers. There has never been such a profound urgency to have coastal management plans in place if we wish to preserve rare natural stands, especially adjacent to more populated areas in the region. The obligation on management authorities extends to their taking a leading role in advising Governments on the uniqueness, fragility, vulnerability and ecological tolerance of mangrove ecosystems, as well as on their benefits. And, once management authorities and all interest groups have made decisions about which areas are to be preserved, future development proposals cannot be a matter of compromise between special action groups and developers since it is the environment we wish to preserve which ultimately must determine where the limits of change are set. In appreciation of the urgency, it is also recommended that we continue to fill gaps in our knowledge and understanding of mangrove forests by further supporting long-term monitoring programs investigating, in particular: ecological processes; loss of mangrove area; and the restoration of damaged mangrove stands. Importance
and value of mangrove ecosystems
Mangrove forests form a unique ecosystem bordering coastal margins, linking land-based biota with those in the sea. These coastal plants are highly valued and regarded internationally, based on their abilities to thrive in saline conditions of daily tidal inundation, and in their support of a wide range of animals. The canopy of these halophytic, tidal swamp plants is frequented by terrestrial fauna, while other animals walk across the forest floor at low tide. During low tide, 288
Mangroves
in the Great Barrier
Reef World Heritage
Area
the plants exchange gases with the air, but after the tide returns, a dramatic change takes place. The flooded forests become the domain of estuarine and marine animals. These tidal waters also provide daily nutrition for the trees but they also facilitate the dispersal and propagation of their progeny. Just as corals form the structural basis for coral reef ecosystems, mangrove plants make up the structural base for this dominant intertidal ecosystem. The complex of roots, stems, branches and foliage support many organisms which would not exist there otherwise. Some animals are full-time residents, while others are transient users, living in mangroves seasonally while still other users of mangroves never live in mangroves but feed offshore on mangrove-reared baitfish and plankton (Cappo 1995a, b). Mangroves are a major and often primary source of carbon and nutrients exchanged offshore into the Great Barrier Reef lagoon (Robertson et al. 1992). The comparable structural role of mangrove plants and coral colonies, is based on their respective long-lived organisms of each system. Both mangroves and corals mark their growth with seasonal growth rings, although the records for mangroves are expected to be much shorter than those of some corals, in older trees this reaches up to 100 years. A comparison of respective dendrochronologies for mangroves and corals, particularly for associated nearshore communities, might provide further characterisation of longer term trends in coastal rainfall and runoff, past environmental history, and forest demography. Specifically, mangrove ecosystems are recognised for their abilities: in stabilising coastal foreshore areas, in providing high levels of primary production and atmospheric carbon fixation, and in their role of sheltering and feeding juvenile fishes and crustaceans during seasonal migratory cycles (Roberston and Alongi 1992). A specific account of the values of mangrove wetlands is given by Lugo and Brinson (1978), and the chief points are listed briefly in Table 1. In this paper, I briefly review the current status of our knowledge of mangrove habitats in and adjacent to the Great Barrier Reef World Heritage Area (GBRWHA). Also included, are recommendations for their management and the direction of future research on mangroves. For convenience, mangroves are described both within the GBRWHA and those bordering its coastal low water boundary since the relationship between mangroves and coastal waters is intimate. For this reason, it is suggested that mapping coastal boundaries using the seaward fringe of the mangrove canopy is inappropriate in defining coastal terrestrial margins since this line matches mean sea level, while for coastal areas without mangroves, the boundary chiefly coincides with highest astronomical tides. The extent of mangroves
in the Great Barrier
Reef World
Heritage
Area
Mangrove forests and saltmarsh vegetation occupies approximately 4000 km* of the coastline bordering north-east Queensland and within the GBRWHA. The distribution of forested stands is analogous to the distribution of reefs in this region, where isolated reefs and coral islands of the Great Barrier Reef form an archipelagous string extending north-south. Mangroves also form an impressive string of stands, although they chiefly hug the coast, filling the mouths of coastal estuaries and bordering embayment enclaves along coastal margins, as well as nearshore islands. The extent of intertidal vegetation, notably mangrove and saltmarsh, in sections of the GBRWHA was estimated partially by a number of authors (Table 2) although their estimates differ. It seems likely that these values reflect differences in interpretation of remote sensing images, or in the relative accuracy of measurements, but it is not believed they indicate any real
289
State of the Great Barrier
Reef World Heritage
Area Workshop
changes in vegetation coverage. Galloway (1982) provided a complete series of estimates for mangrove vegetation in seven coastal regions in the GBRWHA, totalling around 2069 km2; being approximately 18% of the total area of mangroves in Australia from the same study. Estimates of mangrove area by Dowling and McDonald (1982) appear to under-estimate stands in both Princess Charlotte Bay (Region 3j, Hinchinbrook (Region 4) and south of Lucinda (Region 5), and to over-estimate those in the southern region (Region 6); total areas are similar however. Estimates for particular regions by Danaher (1995) and Ebert (1995) support the specific regional estimates for mangroves scored by Galloway (1982). The Danaher (1995) values also demonstrate the importance of making clear distinctions between saltpan and mangrove areas. Ideally, both areas need to be estimated for each region since they together occupy the intertidal area. Furthermore, their relative extent is influenced by climatic factors, and a relationship between mangroves and saltpan area is inversely correlated, such that in areas of low rainfall, the area of saltpans are proportionally larger (Fosberg 1961). Table 1. A qualitative
list of values of saltwater wetlands (Lug0 and Brinson 1978)
Water
- store flood waters
Organic productivity
- conserve water during drought periods - desalinate salty water - high primary productivity
- high secondary productivity (e.g. commercial and sport fisheries) - high export of organic foods to other ecosystems
Biogeochemical
- high wood production in mangroves - high capacity to recycle nutrients - high storage of organic matter and CO, sink - net oxygen production - many biogeochemical cycles are closed by reducing N, C, S, Fe, etc., in anaerobic muds - heavy metals, radioactive isotopes, and other poisonous chemicals are
sequestered in anaerobic muds Geomorphological
- high potential for erosion control
- protection of coastlines against storms, tides and winds - high potential to build land
Biotic
- serve as fisheries nurseries, bird rookeries, and refuges for terrestrial animals - gene banks for haline and euryhaline plant and animal species
Other values
- natural laboratories for teaching and research
- location for recreation and relaxation - rich organic soils used in agriculture, aquaculture, or as fuels - location for solid waste disposal or construction activities - importance as natural heritage, particularly when they become scarce
- representative of personal intangible values
Mangrove floristics in the Great Barrier Reef World Heritage Area cl Mangroves are essentially a marine habitat, albeit a specialised one, but those all important intertidal plants which characterise mangroves have all evolved from terrestrial ancestors. The plants are important, since without them, there would be no mangrove ecosystem. As noted, they provide structure for the habitat, as well as a significant portion of the short and longer term net primary production on which the trophodynamics of the ecosystem are based. Mangrove plants, furthermore, do not come from a single genetic source, and Avicenniaceae and Sonneratiaceae are the only plant families which are comprised exclusively of mangrove taxa. For the world, the total number of mangrove plants is 70 taxa of 21 families, ranging from a ground fern to a range of angiosperms, notably a palm, shrubs and trees (Duke 1992). In the GBRWHA, there are 37 taxa of 19 families (Table 3), representing a significant portion of the
290 I -
Mangroves
in the Great Barrier
Reef World Heritage
Area
worlds genetic variation in mangrove plants. Some species, like Avicennia marina, Rhizophora stylosa and Bruguiera gymnorrhiza, are widespread in the Indo West Pacific biogeographic region, while others, like Ceriops australis, Bruguiera exaristata, Diospyros littoralis, are more restricted to the Australasian region, and only Lumnitzera X rosea is more-or-less restricted to the GBRWHA. It is also of interest that Avicennia marina var. eucalyptifofia merges more with a south-eastern Australian variety, var. australasica, towards the southern boundary, south of the Tropic of Capricorn. Table 2. Estimates of areas of mangrove vegetation in the Great Barrier Reef World Heritage Area Area of mangrove in km* (plus saltpan area) Dowling and Galloway Danaher Ebert (1982) McDonald (1995) (1195)
Coastal Regions
1 Cane York to Evanson Point 2 Evanson Point to Bathurst Head (Princess Charlotte Bay) , 3 Bathurst Head to Cardwell 4 Cardwell to Lucinda (Hinchinbrook Island and Channel) 5 Lucinda to Clairview Bluff 6 Clairview Bluff to Bustard Head 7 Islands of the GBRWHA Factors influencing
the distribution
354
(1982) 395
93 243
230
10
216
120
645 498 20
365 935
-
240(261)
-
99(420) >94(>149)
-
-
>214 -
of mangroves
The number of mangrove taxa in the GBRWHA generally decreases with increasing latitude south (Fig. 1) such that in some northern locations, like the Olive River, there are around 27 species, while in the south, a floristically diverse site at Port Clinton has only 13 species. This indicates the importance of temperature through the region, but species diversity is also correlated with a range of variables, including rainfall, river catchment size, estuary length and geological history (Duke 1992). In Fig. 1, the relationship between rainfall and species numbers is shown by the numbers of species reaching their southern distributional limits in the south of the three wetter regions (marked by the 1400 mm mean annual rainfall isohytes). In general, highest species diversity is found in sites where rainfall is higher, and in riverine estuaries with larger catchment areas. Species occurrence is also characterised by their distribution upriver which may be described in terms of either, or both, downstream and upstream limits. These limits are essentially correlated with salinity, and as such, they are comparable between river systems, such as for example, Alligator Creek (a dry climate, smaller catchment system; Fig. 2), the Mulgrave River (a wet climate, larger catchment system; Fig. 3), and the Claudie River (a wet climate, medium catchment system; Fig. 4). Thus species might be described as upstream or downstream species, with overlapping ranges within an estuary. For the Claudie River (Fig. 4), the line through the centre of the figure serves not only to notionally divide upstream and downstream species, but it also marks the upstream limit of mangroves defined by Danaher (1995), based on satellite imagery. The Danaher study therefore missed four key upstream species, including Nypa the mangrove palm (found in only six other river systems in the GBRWHA), and Sonneratia lanceolata (found in only one other river system in the GBRWHA). Other upstream species missed in this report, in other river systems, include Dolichandrone (found in only one river system in Australia and in the GBRWHA) and S. caseolaris (found in eight river systems only in the GBRWHA). Satellite remote sensing is considered to be unsuitable for mapping riparian estuarine vegetation. Thus, although the mapping study by Danaher (1995) has considerable value, it is also important to understand its limitations and to make appropriate
291
-
State of the Great Barrier
Reef World Heritage
Area Workshop
adjustments when applying these results in the management of estuarine mangrove ecosystems, describing fish habitats, and so on, since these maps do not account for key mangrove species which characterise particular drainage systems. It is the current view that mangrove species distributions in the GBRWHA are essentially relict (Duke 1992), and they comprise distinct and genetically isolated populations, further emphasising their fragility and vulnerability to environmental change. Table 3. Mangrove plant taxa in the Great Barrier Reef World Heritage Area Family Acanthaceae Arecaceae Avicenniaceae Bignoniaceae Bombacaceae Caesalpiniaceae Combretaceae
Ebenaceae Euphorbiaceae Lythraceae Meliaceae Myrsinaceae Myrtaceae Plumbaginaceae Pteridaceae Sterculiaceae Sonneratiaceae
Rhizophoraceae
Rubiaceae
Taxa Acanthus ebracteatus Acanthus ilicifolius Nypa fruticans (Avicennia marina var. australasica) Avicennia marina var. eucalyptifolia Dolichandrone spathacea Camptostemon schultzii Cynometra iripa Lumnitzera littorea Lumnitzera racemosa Lumnitzera X rosea Diospyros littoralis Excoecaria agallocha Pemphis acidula Xylocarpus granatum Xylocarpus mekongensis Aegiceras corniculatum Osbornia octodonta Aegialitis annulata Acrostichum speciosum Heritiera littoralis Sonneratia alba Sonneratia caseolaris Sonneratia X gulngai Sonneratia lanceolata Bruguiera~cylindrica Bruguiera exaristata Bruguiera gymnorrhiza Bruguiera parviflora Bruguiera sexangula Ceriops australis Ceriops decandra Ceriops tagal Rhizophora apiculata Rhizophora X lamarckii Rhizophora mucronata Rhizophora stylosa Scyphiphora hydrophyllacea
292 -
Mangroves
in the Great Barrier
Reef World Heritage
Area
Species/Taxa
WEDNESDAY
IS.
IARNIA CK. .JACKY JACKY CK
!z-
HAGGERSTONE OLlV R. - dNGARO0
LOCKHART
IS. R
R.
MclVOR
R.
ENDEAVC’JR ANNAN A. COOPERS
R.
CK.
Speciedaxa 1. 2. 3. Z:
;: :: 10. 11. 2: 14. 15. 16. 17. 16. 19. 20. 21. 22. 23. 24. 25. 26. 2 29:
E: 32. 33. 34. 35. 36. 37.
Acanthus ebracteatus Dolichandrone spathacea Sonneratia lanceolata Camptostemon schu!tzii Sruguiera cylindrica Diospyros littoralis Sonneratia X gulngai Sonneratia caseolaris Lumnitzera X rosea Rhizophora mucronata Lumnrtzera littorea Ceriops decandra Elruguiera sexangula Nypa fruticans Sruguiera parviflora Cynometra iripa Ceriops tagal Heritiera littoralis Scyphiphora hydrophyllac Sonneratia alba Rhizophora X lamarckii ~~~,s~~$y,pd?~~~ Sruguiera exaristata Xylocarpus granatum Osborma octodonta Xylocarpus mekongensis Pemphis acidula Lumnitzera racemosa Aegialitis annulata Ceriops australis Rhizophora stylosa Bruguiera gymnorrhiza Acrostichum speciosum Excoecaria agallocha Aegiceras corniculatum Avicennia marina
1. Distribution of mangroves in the GBRWHA. The six regions described in Table 2, are marked by filled circles on the coastline, and numbers in circles.
Figure
293
State of the Great Barrier Reef World Heritage Area Workshop U$%ver
- Alligator ‘18’S
190 1.100 mm 139 sqm km 2.78 15
sDecies Species/Taxa Acanthls ebracteatus ~.mptos,emon schulhii Aegialilis anndata Osb&nia atcdonla Rhizophora apiculala Rhizophora slyioSa %““emia alba Scyphiihwa hydrophyllama Lumnihera racemosa Lumnilzem littorea ceriaps lagal &uguiera cyiindrii Rhizophora X lamarckii Ekuguiera exaristata Chops decwdra Avicennia marina salneralia x gulngai Xylaarpls mekwgensis mugliera gymnonhiza Ehgtiera pbmra Rhizophcxa rrmcrcnata Xykxarpls granatum Acrostichum SpecioMun Heritiere littoralis ExcoecaM agallocha Aegioeras comicdatum Cynomeka iripa Acanlhus ilicifdius Bruguiera sexangula sonneratia -laris sonneralia la”ceolala
7’ +
Figure 2. Upriver checklist-of mangrove species in Alligator Creek UpRiver - Mulgrave 170 14’S 2,000 mm 813 sq km 2.20 m 20 species
Distance
Upriver
from
Mouth
SpecieslTaxa Acanmus ebrasteatus Camptostemon schultzii Aegialilis anrwhta Osbcmia octodonta Rhizophora apiculala Rhirc@ora styiosa sonnerar alba Scyphiphaa hy&o@yilacea Lu”l”it?era wxmosa Lwmit?.?ra liicfea calicQs tagal Eruariera cviindrica
Rhizopiwra
. . .
.
.
x limarddi
Bmgdera exaristata Ceriops decandra Avioennia marina Sonnemlia X gulnaai Xylocarpls mek&ge&is Bmguism gym”orrhiza Brugriera patinora Rhizophxa nwcronata Xylocarplsg~trrm Acrostichum specicsum Heritiera lilloralis Ex-da agakch.3 Aegiceras comiculalum Cynomeba iripa Acantius ilicifdius
.
.----*
.-.-----------t--* l
---.-. .-.-. . .-W .
.
-
- -
_ _ _ _ _ _.
-
*-
-.
-
. -
l
- -
_ .-.
.-9 .-
-.
*--CC..---.-.
Nypa fruticans Ddichardronespathac Diospyfm lilloralis
Figure 3. Upriver checklist of mangrove species in the Mulgrave River 294
Mangroves
in the Great Barrier
Reef World Heritage
Area
U$River - Claudie 120 50’S 1.500 mm 4bO sq km 2.23 m 28 species Species/Tan Acanthus ebradeatus Camptostemon schulhii Aegialitis annulata Osbomia ccbdonta Rhizophora apiculata Rhizophora stylosa sonneralfa alb Scyphipbxa hydrophyllacea LumniQera racemosa LumniQera littorea GkQs tagal Brugufera cyiindrica Rhiiophora X hmarckfi Bwgtiwa examtala Cericps decandra Avicennia marina SMlneratia X wlnwi Xybcarpls mek&ge&fs Bruguiera gymrwrrhiza Bruguiera paniflora Rhizophora mcralata Xyiocarpus granahlm Anmtichun speciosum Heritiera littoralis ExcoecaM agallocha A&eras comiculatum Cynometra iripa Acanthus ilicifdius Brugtiera sexangula sonneratia caseo1aris sonneratia lancedata Nypa buticans . Dolictandrone spathacea Diospyms fittorafis
Figure
4. Upriver
Long-term
checklist
trends affecting
.
.
.
-
_
-
_
_
-”
.*-----”
.
.
_
_
_
_
_
.
_
_
_
.--
_
.
--.-,
..---*
..---
--”
---..-,
.-,
.---.-* --*--,
.0.---.-q
.
.
_.
_
_
-“-
_
_
-.
_____
-.
_____
we1
-. .
.------.*---.-(
--*--*
.------
*
.
..-----
.
l
l
.---.-.
.
_
_
_
*
._____
_(
_
_
_
_
-t
.
_
_
_
_________.__
-.
).....( _ _ _ -.c
of mangrove
sm.
.-*---e
“----..
-----.
-*---.
*
.-*--, _ _ _ .-
(
l
--._ -*
--*
..---.-.
--.
--.--.---
).
_________
).
.---*
.------C).
______ l
*---l--.
-*--*
______
*.
.---*-.
species in the Claudie
River
the extent of mangroves
Long-term change to the extent of mangrove forests was assessed recently in two locations north Queensland, namely the Johnstone River estuary (Russell and Hales 1994) and the Hinchinbrook Channel islands and Missionary Bay (Ebert 1995).
in
The Johnstone River is a large river and estuarine system situated within the wet tropics region. Large parts of the catchment area are used in farming and agriculture, and few areas remain undisturbed from pre-settlement condition. Russell and Hales (1994) assessed vegetation cover of both freshwater and mangrove wetlands in this catchment, comparing aerial photographs from 1992 with those taken in 195 1 (Table 4). They identified a very high loss of freshwater wetlands, around 65% being approximately 18 km’. By contrast, there was a net increase in mangrove area, around 15%, being approximately 0.3 km2. The gain in mangroves was chiefly observed in the lower estuary as expansion into tributaries of the estuary. An explanation for these changes may be that losses to terrestrial vegetation upstream have led to erosion of topsoil, and mangroves have colonised the resulting sediment deposition banks downstream. Table 4. Changes in areas of vegetation cover for the Johnstone River, 195 I to 1992 (Russell and Hales 1994) Wetland Mangrove Freshwater
Ground
Cover
Area in 1951 (ha) 176 2677
Area in 1992 (ha) 202 925
295
Net Change @a) + 26 - 1752
% Change + 14.8% - 65.4%
State of the Great Barrier
Reef World Heritage
Area Workshop
The Hinchinbrook Channel, with a mangrove area of around 164 km*, contains a group of mangrove islands with a combined total area of around 37 km’. The mangroves on these islands are mostly quite tall (around IO m), and it is generally estimated that a large number of trees are older than 50 years. Based on accurately positioned aerial photographs (notably on data gathered by AUSLIG), Ebert (1995) compared vegetation cover in 1991 with that in 1943 (Table 5). He concluded that there was no appreciable net change in the total area of mangrove and saltpans (in fact around 1% gain, being 0.5 km’). However, there was a marked net change in the relative proportions of intertidal vegetation where saltpan area decreased by 78%, essentially replaced by tall mangrove forest which also replaced some short mangrove. In this context, it is of interest to compare the correlation between the ratio of mangrove to saltpan area with wet and dry climatic regions (Fosberg 1961), noting that saltpan area may be reduced to zero in wetter regions. An explanation for this occurrence therefore, may be that there was an increase in annual rainfall reducing ground water salinities over the period. However, the increased biomass of intertidal vegetation of the Hinchinbrook Channel islands may also be related to increased nutrient supply from the Herbert River outflow, and subtle changes in sediment deposition. A further assessment of the range of influencing factors is required. Table 5. Changes in areas of vegetation cover for the Hinchinbrook 1995) Ground
Cover
- tall - short
Mangrove
- all trees
- saltpan All Mangrove Terrestrial
Area in 1943 (ha) 3543.9 38.0 358 1.9 207.0 3788.9 22.0
Area in 1991 (ha) 3779.0
11.0 3790.0 46.0 3836.0 21.9
Channel islands, I943 to, I99 I (Ebert
Net Change (ha) + 235.1 - 27.0 + 208.1 - 161.0 + 47.1 - 0.1
% Change
+ 5.8% - 77.8% + 1.2% - 0.1%
In an assessment of the geological history of the Hinchinbrook region (Ebert 1995), the mangrove islands were described as being of similar age, and composed of sediments derived from the Holocene post glacial marine transgression. Mangroves may have colonised these .sediments as the sea level dropped after the Holocene period. The creeks draining the mangrove islands then evolved in response to tidal flushing to produce the current patterns. There is also some erosion on their northern edges, and it is suggested that mangrove growth may only occur after a large input of allochthonous sediments which has not occurred over the last fifty years. Current accumulation rates are insufficient to promote expansion of the mangrove islands. This relative stability of mangroves was also observed in Missionary Bay (a mangrove area of 50 km2) on Hinchinbrook Island (Ebert 1995). And, the pattern where saltpans were replaced by mangrove forests was repeated over the same period from 1943 to 199 I, while there was little or no expansion into surrounding waterways. In view of these examples, it seems likely that many mangrove areas in the GBRWHA are quite old, and were changing slowly in the absence of human disturbance, responding to events over very long time-scales, except where they occupy the mouths of larger rivers in wetter regions. Human
effects
It is unfortunate for mangroves that they are often mostly prevalent in sites preferred for coastal cities and industrial development. In tropical latitudes, estuaries are a major focus for commercial and recreational activities. For these reasons, urban developments, ports and
296
Mangroves
in the Great Barrier
Reef World Heritage
Area
foreshore structures surround these estuarine habitats, often replacing mangroves, or large portions of them. It is apparent that the greatest long-term effect on mangroves in this region is in their removal and disturbance by people. This takes place in often small incidents, but each one adds to a total, larger accumulative impact. By comparison, natural trends are virtually unnoticed in human time scales, noting the examples described above. Therefore, there is an urgent need to quantify the loss of mangroves caused by humans in coastal areas of Queensland, since this information is needed most in any assessment of the impact and longer term trends in mangrove forests of the GBRWHA. The chief reason for the urgency is not only because of the significance of disturbance at any one site, but more importantly, to allow an assessment of accumulative damage throughout the region. Only after this information is available will it be possible to confidently manage mangrove ecosystems to preserve them and utilize them in a sustainable manner. Over the last 30 years, there has been a rapid growth in public and scientific interest in mangroves prompted both by our increasing awareness of the fragility of similar natural environments, and by the greater demand for coastal land for development in tropical regions. This is chiefly due to population increases which have doubled in many north Queensland centres over this period. But, attitudes have changed also, and the language used by people to describe mangroves seems to have become less derogatory, marked by fewer references to ‘scrub, swamps and bogs’, to more about ‘trees, forests and mudflats’. With such subtle changes in attitude, there is hope that there is a growing respect for these natural environments as places not only to be directly exploited and used, but as places which are important in preserving the well-being of our society, long into the future. Management There are three major challenges for the conservation and management of mangrove areas in the GBRWHA. Firstly, to develop better linkages between responsible government departments, coastal research institutions and interested people, through jointly sponsored research projects, workshops and conferences. Secondly, to increase education on mangrove environments, describing mangroves, and identifying essential links with marine ecosystems and the continuum between terrestrial catchments and the sea. In this context, it would also be important to re-iterate the concept of cause and effect, such as, for example, that what happens in catchments upstream affects habitats dowrrstream, including mangroves, and ultimately affecting coral communities along the Great Barrier Reef. Thirdly, to learn more about mangrove ecosystems and how they function, better defining environmental and ecological constraints, and in particular, focusing on management-orientated research, including their restoration. For day-to-day management, it would also be useful to compile information on existing mangrove interpretation centres around Australia, providing a base from which to improve public access and educational benefits for mangroves in the GBRWHA. Wetland management strategies and objectives have been assessed by Bennett and Goulter (1989), and they described 12 specific goals which may be applied to mangroves in the GBRWHA: 1. maintain water quality; 2. reduce erosion; 3. protect from floods; 4. provide a natural system to process airborne pollutants; 5. provide a buffer between urban residential and industrial segments to ameliorate climate and physical impact, such as noise; 6. maintain a gene pool of wetland plants and provide examples of complete natural communities; 7. provide aesthetic and psychological support for humans - recreation
297
--1
State of the Great Barrier
8. 9. 10. 11. 12.
produce control provide produce expedite
Reef World Heritage
Area Workshop
wildlife; insect populations; habitats for fish spawning and other food organisms; timber, food, fiber, and fodder; scientific enquiry.
Research Recommendations Mangrove research in this GBRWHA region had until recently concentrated on spatial variation and major trophodynamic processes. For example, botanical systematic studies by the Australian Institute of Marine Science from 1974-86 resulted in a progressive and rapid increase in the number of recognised mangrove taxa from 19 in 1968, to 28 in 1977, to 32 in 1982, to the 37 mangroves recognised today. That’s three additional species every four years for the period up to 1992. I believe the number has now stabilised, and we now have an excellent understanding of mangrove floristics and distributions in this region. But, we still have some way to go before we know how the system functions and the links with terrestrial and nearshore marine systems. We also have little information on the range of management options and strategies available; a point of great concern as the pressure to remove or alter individual mangrove stands increases in the region. To address the chief concerns, I identify a number of specific longer-term research projects on mangroves required in the GBRWHA; noting that some of these projects are already underway: l map the current and past extent of mangrove and saltpan vegetation in all coastal and island regions; l large-scale and long-term monitoring of forest plots along the coast - with an emphasis on changes in fringe areas, forest dynamics, demography, tree growth and gap restoration (this could be linked with long-term monitoring of estuarine water quality): l dendrochronological assessment of comparable growth rings in mangrove trees and nearshore corals; l genetic studies showing dispersal and distribution patterns of mangrove plants; l ecological processes within mangrove forests, noting imports and exports; l links between mangroves and nearshore fisheries/ecosystems (including coral reefs); l dependence of fish and crustaceans on mangroves - food and/or shelter; l long-term hydrodynamic and geomorphological processes in coastal areas; l effects of human-induced disturbance and pollution on mangrove ecosystems; l use of mangroves in water purification and as neutralisers of biotic effluents; l restoration of disturbed mangrove areas.
References Bennett, J. and I. Goulter 1989. The use of multiobjective analysis for comparing and evaluating environmental and economic goals in wetlands management. Geojournal (2): 213-220. Cappo, M. 1995a. Bays, bait and Bowling Australia 1 (3): 13-16.
Green - pilchards and sardines, Part 1. Sportfish
Cappo, M. 1995b. Bays, bait and Bowling Sportfish Australia 1 (4): 14- 19.
Green - inshore herrings and sardines, Part 2.
18
Danaher, K.F. 1995. Marine vegetation of Cape York Peninsula. Cape York Peninsula Land Use Strategy. Office of the Co-ordinator General of Queensland, Brisbane. Department of the Environment, Sport and Territories, Canberra and Queensland Department of Primary Industries. 64 pp. plus appendices.
298
Mangroves
in the Great Barrier
Reef World Heritage
Area
Duke, N.C. 1992. Mangrove floristics and biogeography, pp. 63-100. in A.I. Robertson and D.M. Alongi (eds). Tropical Mangrove Ecosystems. Coastal and Estuarine Studies 4 1. American Geophysical Union, Washington, DC. Ebert, S.P. 1995. The geomorphological response to sediment discharge from the Herbert River, north Queensland, 1943- 1991. Honours Thesis. Department of Geology, James Cook University of North Queensland. 78 pp. plus appendices. Fosberg, F.R. 1961. Vegetation-free Papers. 424(D). pp. 216-218.
zone on dry mangrove
coastline.
U.S. Geol. Sot. Prof,
Galloway, R.W. 1982. Distribution and physiographic patterns of Australian mangroves, pp. 3 l-54. In B.F. Clough (ed.). Mangrove ecosystems in Australia - structure, function and management. Australian National University Press, Canberra. Lugo, A.E. and M.M. Brinson 1978. Calculations of the value of salt water wetlands. In P.E. Greeson, J.R. Clark and J.E. Clark (eds). Wetland functions and values: the state of our understanding. Minnesota, American Water Resources Association. Robertson, A.I., D.M. Alongi and K.G. Boto 1992. Food chains and carbon fluxes, pp. 293-326. In A.I. Robertson and D.M. Alongi (eds). Tropical Mangrove Ecosystems. Coastal and Estuarine Studies 41. American Geophysical Union, Washington, DC. Robertson, A.I. and D.M. Alongi (eds) 1992. Tropical Mangrove Ecosystems. Estuarine Studies 41. American Geophysical Union, Washington, DC.
Coastal and
Russell, D.J. and P.W. Hales 1994. Stream habitat and fisheries resources of the Johnstone River catchment. Integrated Catchment Management Report. Northern Fisheries Cent&, Queensland Department of Primary Industries. 60 pp.
299
