The geomorphological and zonational development of mangrove swamps in the Townsville area, North Queensland

Spenceley, Anthony Phillip (1980) The geomorphological and zonational development of mangrove swamps in the Townsville area, North Queensland. PhD thesis, James Cook University.

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Abstract

Research was undertaken in three mangals in the Townsville area. Two were located on an open accreting coastline and the third on an estuarine coast. Mangals on the lee-side of Magnetic Island and Orpheus Island were examples of an open accreting coastline whilst the mangal at Saunders Beach, on the right bank of Althaus Creek, was an example of an estuarine coast. Because of ease of access, the first and last sites were studied intensively. The aims of the project were:

(i) to investigate factors that influence mangrove zonation;

(ii) to investigate the role mangroves play in the sedimentary processes operating on the intertidal slope; and

(iii) to investigate the evolution of the swamps.

The soil chemical characteristics of an open accreting coastline were found to be different from those of an estuarine coastline. The chemical data from Magnetic Island displayed a marked spatial trend but, on the whole, tacked a temporal variation. At Saunders Beach, the soil chemical data displayed significant variations both in time and space.

Statistical analysis of the data using factor analysis revealed a basic similarity in the underlying structure of the data sets. Over 50% of the covariance was explained in terms of a salt factor and a pH factor. Grouping the zones together according to their soil chemical characteristics produced a contrary picture. Stations from within the Rhizophora zone on Magnetic Island were not grouped together, nor were the stations within the Ceriops zone. However, stations within a particular zone at Saunders Beach were grouped together. The reverse situation occurred on the bare or sparsely covered areas on the upper intertidal slope. Zones on Magnetic Island were grouped together but those at Saunders Beach were not. Consequently the zonation of mangroves in the Townsville region cannot be adequately explained in terms of the soil chemical variables that were analyzed.

Two types of salt flats were identified, relict and contemporary. Relict salt flats were caused by a relative fall in sea level whilst the contemporary salt flats were caused by a break in the vegetation canopy. Consequently they need not be part of the usual vegetational sequence in seasonally dry climates.

It was found that maximum bed load was transported in the seaward or creekward section of the swamps declining landward. However, above approximately mean high water spring tides (MHWST) amounts of bed load increased. A similar trend was observed in the pattern of accretion. Greatest variations in erosion or deposition were seen in the seaward or creekward margins of the swamps declining landwards to approximately MHWST, beyond which the variations again increased. The increases on the upper part of the intertidal slope were due in part to particle size differences. Other controlling factors that may be important are the initial rippling wavelet on the flood and ebb tides; overland flow from the terrestrial environment; intercepted rain water streaming down tree trunks and being concentrated between buttress roots; intercepted rain water dripping off the tree trunks, concentrating its energy on a small area.

Series of grids were used to simulate pneumatophores. It was found that the grid spacing directly influenced the rate of accretion. The experiment reinforced the idea that mangroves stabitize the intertidal slope sediments, the fine rootlets binding the soil particles. Electrolytic and chemical factors may also be associated with the increase in cohesion of the sediment.

The evolutionary history of the swamps was described. Each swamp had a different sequence of development. Coupled with a different set of species being present, each mangal has developed a unique vegetational pattern in response to the continually changing environmental conditions. Interactions that operate within mangals of the Townsville area have been incorporated into a model of mangal development. Mangal development and the distribution of species within mangals are seen as a reflection of the species pool available for colonization, geomorphological history of the area, soil type and the changing environmental conditions which affect the normal interspecific competition.

Item ID: 28249
Item Type: Thesis (PhD)
Keywords: mangroves; Magnetic Island; Orpheus Island; Saunders Beach; soils; Rhizophora; Ceriops; zonation; salt concentration; sedimentation
Date Deposited: 08 Aug 2013 04:48
FoR Codes: 06 BIOLOGICAL SCIENCES > 0607 Plant Biology > 060799 Plant Biology not elsewhere classified @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960802 Coastal and Estuarine Flora, Fauna and Biodiversity @ 50%
97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
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