Tropics Of North-eastern Australia Research Articles

Abundances of benthic microfauna in relation to outwelling of mangrove detritus in a tropical coastal region

Mangrove litter outwells from tldal forests and deposits onto the adjacent coastal seafloor within the central Great Bamer Reef lagoon in tropical northeastern Australia. Previous studies in this region indicate that this detrital loading greatly influences edaphic characteristics (e.g. C/N ratios and POC-soluble tannin concentrations) and supports highly abundant and productive, sedimentary bacterial communities. Mean ciliate and flagellate densities (> 20 pm) within surface (0 to 1 cm) sediments in this region ranged from 23 to 511 cells cm-3 and from 40 to 806 cells cm-3, respectively. Densities of nanoprotozoans (5 to 20 pm) ranged from below detection limits to 260.5 X 103 cells cm-3. Seasonal fluctuations of all microfaunal groups were relatively minor. Total fauna1 densities generally declined with sediment depth, but low densities of sarcodinids, amoeboflagellates, and yeasts and yeast-like cells were discovered below the sediment surface at all of the stations sampled. Experimental additions of aged mangrove litter to mixed laboratory populations of Euplotes spp., hymenostomid and hypostomatid ciliates, and zooflagellates resulted in either no or poor growth, suggesting that outwelled litter is not a nutritious food. Densities of microfauna in these sediments are low compared to densities in temperate coastal habitats, and appear to be influenced more by preferences in sediment type than by detrital outwelling or by proximity to the mangrove forests. Poor food quality may partially explain the low densities and lack of a discernible effect of outwelling on the microfauna. Several factors such as low dissolved nutrient levels, high C/N ratios and low microalgal abundances are also cited to account for the low densities, but low water content and poor nutritional quality of litter are considered to be the major factors regulating the benthic microfauna in this region.

The influence of crabs on litter processing in high intertidal mangrove forests in tropical Australia.

Measurements of litter fall and litter removal by crabs, in conjunction with estimates of litter decay by microbes and tidal export of litter from three high-intertidal mangrove forests were made during a year-long study in tropical northeastern Australia. In forests dominated by Ceriops tagal and Bruguiera exaristata, litter standing stocks remained low on the forest floor (mean 6 g·m-2), although litter fall was high; 822 and 1022 g·m-2·y-1, respectively. Sesarmid crabs removed 580 (Ceriops) and 803 (Bruguiera) g·m-2·y-1, or 71 and 79%, of the total annual litter fall from the forest floor. Relative to the rate of litter removal by crabs, microbial turnover of whole, unshredded litter was insignificant, accounting for <1% of annual litter fall. Export of litter by tides was estimated to remove 194 (Ceriops) and 252 (Bruguiera) g·m-2·y-1 or 24 and 25% of annual litter fall. In a forest dominated by Avicenniamarina, in which an ocypodid crab was more abundant than sesarmids, litter standing stocks were higher (mean 84 g·m-2) and crabs removed less litter; 173 g·m-2·y-1 or 33% of the annual litter fall of 519 g·m-2·y-1. Microbial turnover of intact litter was more important in the Avicennia forest (168 g·m-2·y-1 or 32% of annual litter fall), and tides exported 107 g·m-2·y-1 or 21% of litter production. In areas where sesarmid crabs were absent or rare in Ceriops forests, there were significantly higher standing stocks of litter and slower rates of leaf removal. Taking into account the probable assimilation efficiencies of sesarmid crabs feeding on mangrove leaves, we estimate that in Ceriops and Bruguiera forests leaf processing by crabs turns litter over at >75 times the rate of microbial decay alone, thus facilitating the high sediment bacterial productivity in these forests. The importance of litter processing by crabs increases with height in the intertidal in tropical Australia, in contrast to New World mangrove forests, where the reverse is true.

The Nature of Herbaceous Vegetation Associated with Termitaria in North- Eastern Australia

(1) The distributions of herbaceous species on and around the termitaria of grassand litter-feeding termites were examined at four locations in tropical north-eastern Australia. (2) Plant growth on the upper mound surfaces is largely confined to extended periods of high humidity during the summer wet season but occurs most extensively on the bases of the mounds. (3) Species composition changes markedly with increasing distance from the mound margin. A higher proportion of rapidly growing annual grasses and functionally-annual herbs occurs on the mounds and close to the mound margins. Slower growing perennial grasses and sedges are predominant further out. (4) The altered species composition is largely attributed to the higher nutrient status of the materials eroded from the mound surfaces, diffusion from the mounds, modified drainage conditions and, possibly, differential grazing by vertebrates and invertebrates. (5) At one site, the standing crop was shown to diminish with increasing distance from the mound margin. (6) Some ecological consequences of these vegetation patterns are discussed.

Erosion of termite mounds under natural rainfall in semiarid tropical Northeastern Australia

The early stages of erosion of six unoccupied termite mounds at a site in northeastern Australia with a semiarid tropical climate have been studied in order to determine overall erosion rates. An attempt has been made to assess which rainfal parameters are best related to the measured sediment losses from the mounds. The conical mounds, built by Amitermes vitiosus Hill, varied in basal diameter from 0.36 m to 0.47 m and in mass from 35.5 kg to 61.6 kg. Over the two summer wet seasons studied, they lost on an individual mound basis between 4.3% and 10.2% of their masses (average 7.1%). At this rate the mounds would be completely destroyed after the order of 30 years of exposure to natural rainfall, although the actual erosion rates over such a time-span may be non-linear. Kinetic energy and momentum estimates, rainfall erosion indices (EI 5-EI 60), and the maximum amounts of rain falling continuously in 5-minute intervals up to 1 hour long were the most reliable predictors of sediment yields from the mounds. Total rainfall during each sediment sampling period is almost as good a predictor and is much more readily determined from daily rainfall data. The occurrence of high intensity rain (equivalent hourly intensities of 90 mm hr −1 or greater) of short duration during the summer wet season was found to be critically important in the termite mound erosion process. The main effect of such rain would seem to lie in the ability of the large drops to break down the hard-setting, dense, organically-cemented surfaces of the mounds to initiate sediment removal. The finer drops of the less intense rains have a much reduced ability to transport sediment from the mounds.

Some marine diatoms from the Australian region, especiallly from Port Phillip Bay and Tropical North-eastern Australia. 2. Survey of the genus Palmeria and of the family Lithodesmiaceae including the new genus Lithodesmioides.

Based on plankton samples collected in 1970 and on a survey of the literature, the Australian members of the genus Palmeria and of the family Lithodesmiaceae are critically listed or described. Both species of Palmeria are new for the Australian region, as are one species each of Lithodesmium and of Ditylum, two species of the genus Lithodesmioides, one species plus one variety of Bellerochea, as well as one species and one form of Neostreptotheca. Lithodesmium duckerae, Lithodesmioides polymorphum and L. minutum, Ditylum buchananii, Bellerochea horologicalis var. recta and Neostreptotheca torta and its fo. triangularis are described for the first time. Palmeria ostenfeldii, a species associated with an epiphytic ciliate, is sep- arated from P. hardmaniana. In Ditylum brightwellii, the proportion of thecae with the slotted versus the fimbriate form of the marginal ridge is shown to be probably influenced by the ambient water conditions. In the same species, the ontogenies of the resting spores (including germi- nation) as well as of the gametes are described. Lithodesmioides presents new instances of fissipariety (split wall character) in both of its species. 'Simple pores', which have recently been demonstrated by Li and Volcani to be distinct in origin from areolae, are common in all described species of Lithodesmium, Lithodesmioides and Ditylum in two forms as either 'branching point pores' or 'intracostal pores'.

Application and appraisal of a visual estimation technique for composition and yield sampling of grass-legume pastures in the wet tropics of north-eastern Australia

A visual estimation technique (SVET) was applied to assess the yield and botanical composition of the rapidly growing, highly seasonal and morphologically diverse pastures of the wet tropics. The technique uses the comparative yield method for total dry matter yield, and the framework of the dry-weight-ranking procedure for botanical composition. Widespread use of cumulative ranking and divided ranking gave a close numerical description of each quadrat. Methods of visual estimation based on dry-weight ranking were evaluated for pastures dominated by sown grass and including a sown legume as a minor but important component. Although the usual dry weight ranking method showed large departures from the known composition, SVET was able to assess the pastures with small overall departures from results obtained by hand-sorting. The full theoretical potential of SVET could not be attained in the field, where ranks had to be estimated. Nevertheless, with sufficient quadrats and trained operators, SVET can assess composition and yield of these tropical pastures with acceptable precision. The relationship between total yield and visual rating was linear but the regression equation for data quadrats departed from its corresponding calibration equation in either slope or coincidence in 11 of 34 cases.