PREDICTING THE EFFECT OF SEA-LEVEL RISE ON MANGROVES IN NORTHWESTERN AUSTRALIA

The patterns of mangrove distribution in tropical northwestern Austral ia are related to coastal dynamics, habitats and salinity. They also r espond to the sedimentology of the tidal Data that back them, to coast al (sheet) erosion, and to the effects of some industrial impacts. The se patterns provide information useful in predicting the variable effe cts of sea-level rise on mangroves. For instance, fundamental changes to soil regimes and salinity can be expected as tidal Bat surfaces and groundwaters sedimentologically and hydrologically adjust to new leve ls of wave base and frequency of inundation as sea-level rises. Since mangrove assemblages and their zones are closely related to shore prof ile, soils, habitat statigraphy and salinity fields, any change in the se can lead to alteration of the structure and composition of mangrove systems. The mangrove response to a rising sea level will depend on t he environmental setting of the mangrove system This includes the rela tive geomorphic and sedimentologic homogeneity of the ccaat, its tidal range, its stability, and the history of Holocene sea levels in regar d to development of coastal gradients and the climatic Betting which d etermines the variety of species that will respond to this Holocene se alevel rise and the type of reproduction the mangroves will utilise to keep pace with encroaching seas. A dichotomous key is presented which suggests that the response of mangrove coasts to a rising sea level w ill be suite varied from coastal sector to sector and even from site t o site within a single coastal sector and climate setting. Some case s tudies illustrate the probable effect of rising sea level on mangrove systems in Western Australia The macrotidal shores of King Sound, a re latively simple coast in terms of habitat and stratigraphy, is eroding naturally by creek and cliff erosion and by sheet erosion progressing at 1-3 cm/yr. This erosion specifically simulates the effects of a ri sing sea. With coastal retreat, the mangroves are migrating landwards, generally keeping pace with the retreat. Mangroves colonise by seedli ng recruitment on the new substrates that become available through the processes at erosion, inundation, and dilution of hypersaline groundw ater of the salt Bats. As erosion and progressive dilution of hypersal inity proceeds, each zone within the mangrove belt displaces the adjoi ning one. Thus, sccl level rise in a system like King Sound would most likely result in the migration of mangroves, with similar composition and structure, into habitats made available by increased inundation I n arid zones, however, where mangrove population is maintained by vege tative reproduction, sheet erosion of tidal Bats also causes landward migration of zones, but the individual zones keeps pars with a relativ e rising sea level by vegetative processes. Elsewhere in NV Australia, various mangroves assemblages with different composition, structure a nd population maintenance have developed slang highly indented (ria) s hores, in a heterogeneous suite of habitats that have evolved over the late Holocene. These habitats are defined by their geomorphic setting , sedimentologic processes, stratigraphic evolution, and ground water dynamics, and each is related to a specific height in relationship to sea level. A sea-level rise would inundate the various geomorphic/habi tat systems, dislocating their suite in relation to the formative sea level. It is likely that these mangroves would not adjust as rapidly a s the more homogeneous systems, and hence be disrupted.