Production and major nutrient composition of three grass species on the Magela floodplain, Northern territory, Australia

Abstract

Seasonal changes in dry weights and nutrient concentrations in three aquatic grasses on the seasonally inundated Magela floodplain in northern Australia were investigated over an 18 month period. The dry weight of the aquatic grass species Pseudoraphis spinescens (R. Br.) Vick., Hymenachne acutigluma (Steudel) Gilliland and Oryza meridionalis Ng varied with water depth on the floodplain. Maximum dry weights (1.67±0.21 kg m −2, 1.41±0.10 kg m −2, 0.51±0.10 kg m −2, respectively) occurred at the end of the wet season when water depth was decreasing. The perennial species P. spinescens and H. acutigluma had two growth periods and an annual productivity of 1.91±0.26 kg m −2 and 2.09±0.38 kg m −2, respectively, compared with 0.51±0.10 kg m −2 for the annual O. meridionalis. Relative to other aquatic and wetland plant species, the former two grasses have high production rates. The major nutrients nitrogen, phosphorus, sulphur, chloride, magnesium, calcium, sodium and potassium were generally present in low concentrations compared with concentrations in other plant species. The annual O. meridionalis had distinct seasonal changes in nutrient concentrations, whereas for P. spinescens and H. acutigluma the changes were not as obvious. An assessment of the nutrient loads that could potentially be turned-over in the detritus originating from these grasses was made. Compared with the annual input to the floodplain of major nutrients in creekwater the grass debris/detritus contained substantially more nitrogen, phosphorus and potassium, less sodium and chloride, and similar amounts of sulphur, magnesium and calcium. Thus, the grasses are an important potential component in the chemical turnover pathways on the floodplain.