Disturbance to mangroves in tropical-arid Western Australia: hypersalinity and restricted tidal exchange as factors leading to mortality

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Disturbances to mangroves in the arid Dampier–Cossack region of Western Australia are described. Of the 70 km2 of mangals surveyed, 12 km2 are now dead, over 95% of this following permanent ponding of seawater within a salt evaporator. Other examples of death and morbidity are related to altered tidal exchange from road construction and asphyxiation from dumping of dredge-spoil. Changes in soil properties under variable tidal wetting at landward, hypersaline sites show that tidal recharge has most influence on the upper layer of the soil profile where groundwater salinity is diluted temporarily during inundation. Two factors are implicated in mass mortality of mangroves associated with roads: intermittent ponding of seawater which submerges pneumatophore roots for days rather than hours and diminished tidal recharge which increases soil salinity beyond the upper concentration (90‰) associated with living mangroves in this region.

Introduction

The mangroves of the arid Pilbara coast in north-west Australia experience low rainfall (< 300 mm annually), high summer temperatures and evaporation rates nearly an order of magnitude greater than rainfall (Gentilli, 1972; Fig. 1). Consequently the ratio of rainfall: evaporation as a measure of coastal aridity (Hamilton & Snedakar, 1984) is very low (0.8–0.21) and the natural pressures faced by mangroves here are likely to be considerable particularly in an environment where tidal recharge produces fluctuating (often high) groundwater salinity and limited rainfall reduces the opportunity for freshwater recharge. These natural stresses will be aggravated by industrial and urban expansion in a region where the water supply servicing coastal towns is met largely from groundwater (Brown, Harrison et al., 1983). Development has already impinged to varying degrees on the mangals, of which the most visible examples are the large salt evaporator close to Dampier, shore facilities for offshore natural gas production as part of the North-West Shelf Gas Project and sites for stockpile of iron ore linked by rail to ore deposits in the interior (Nicholson, 1980; Chittleborough, 1983). In conjunction with industrial development has been increasing urban growth, and, with it, the need for improved access to the coast, which has opened further mangrove communities to a variety of disturbances associated with road and site construction.

The north-west region is the only section of coastline in Australia supporting mangroves in a tropical-arid climate. Despite this and the increasing pressure now placed on these communities by development, there have been no studies undertaken to measure the dynamics of these communities nor to examine how the trees cope with their harsh environment. This paper is the first of several reporting results of field studies conducted between 1983 and 1985 to examine disturbances associated with mangroves in the region, to quantify ranges of edaphic and atmospheric factors operating on the trees and to measure the physiological responses to changes in some of these factors. A preliminary survey of mangrove communities in the region in 1983 suggested a link between the appearance of moribund and dead trees and excessive salt concentrations in the groundwater (Gordon, 1983). Though the salinity ranges of groundwater and soils in mangrove communities of this region have been reported previously, particularly the spatial variability in relation to zonation (Semeniuk 1983, 1985; Semeniuk & Wurm, 1987), there has been little information yet gathered on small-scale changes in soil properties in response to variable tidal recharge or how these are affected by man-induced perturbations. The extent of changes in several soil properties were measured under variable tidal recharge at landward locations in the mangals, including a site where road construction has disturbed the natural exchange of seawater and there is widespread death of mangroves landward of the road. Differences between soil properties in relation to tidal exchange on each side of the road have been used to gauge the impact of this disturbance on the water supply to the trees. These data provide background information for interpreting the water relations and salt exchange of the mangroves at these sites (Gordon, in preparation).

Section snippets

Study area

The study area is located 1300 km (directly) north of Perth and extends from Cape Preston to Cape Lambert (Fig. 2). The geomorphology and mangrove floristics of the Dampier Archipelago region have been described recently (Semeniuk, Chalmer et al., 1982; Semeniuk & Wurm, 1987). This coastline consists of Precambrian igneous bedrock sometimes topped with Pleistocene limestone, and this overlain by a veneer or gravel, sand or mud of variable depth derived from biogenic breakdown and fluvial

Areas of mangals

Areas were calculated using maps of mangal distribution prepared for the Environmental Protection Authority, Western Australia from ground truthing of colour and false-colour infra-red aerial photographs (Department of Lands Administration, Western Australia) and an electronic digitized planimeter (Summagraphics Corp. Fairfield, Connecticut, USA). The geographic limits of the 19 maps are shown in Fig. 2.

Study sites

Sites visited to assess disturbance are located close to the main industrial and urban

Areas of mangals

Areas of living mangals within the limits of the 19 maps surveyed are given in Table 1 and are designated as zones of ‘Avicennia’ and ‘Rhizophora’ based on the clear colour separation apparent between these mangroves in false-colour infra-red aerial photographs. Other members of the Rhizophoraceae family in the region, Ceriops tagal and Bruguiera exaristata, have similar leaf colour to Rhizophora stylosa and it is not possible to differentiate between them at the elevation of the survey

Discussion

Mangroves surveyed here make up less than 3% of the 2400 km2 occurring in Western Australia (Galloway, 1982), and about 12% of Australia's tropical-arid mangals, confined to the north-west coast between North-West Cape and Cape Keraudren. Over and above their aesthetic value, introducing attractive green forest to an otherwise desolate coastline, they provide refuges for birds and fish (Semeniuk, Kenneally et al., 1978; Kenneally, 1982) and physical barriers to storm surge and erosion on a

Conclusions

The study illustrates how mangrove communities in this arid region are susceptible to man-induced disturbance, particularly where this interferes with tidal inundation. Restricted tidal exchange introduced by roads tends to increase soil salinity beyond the survival threshold of the mangroves and alter the duration over which their specialized root systems are submerged. For the future survival of mangroves under threat from disturbance on this coastline, consideration should be given to

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