THE NUTRITIONAL ECO-PHYSIOLOGY OF CHAETOMORPHA-LINUM AND ULVA-RIGIDA IN PEEL INLET, WESTERN AUSTRALIA

The uptake rates and critical tissue concentrations of nitrogen and phosphorus were determined for Chaetomorpha linum and Ulva rigida, the dominant algae in Peel Inlet, Western Australia. Both species had rate-saturating mechanisms of phosphate uptake described by Michaelis-Menten type functions; C. linum had the faster uptake rate (667 c.f. 272-mu-g P g dwt-1 h-1) although U. rigida had a lower half-saturation value. Both species displayed linear relationships between ammonium uptake rates and substrate concentrations with C. linum having the greater slope (4.4 c.f. 1.7). Chaetomorpha linum also had a linear increase in uptake rate with increasing concentration of nitrate, but U. rigida showed rate-saturating kinetics; below 750-mu-g L-1, U. rigida had the higher rate of uptake. Ulva rigida had critical tissue nitrogen and phosphorus concentrations of 20 and 0.25 mg g dwt-1 respectively. Corresponding concentrations for C. linum were 12 and 0.5 mg g dwt-1. Ulva is frequently nitrogen limited during spring in Peel Inlet, reflecting the high nitrogen requirements of this plant compared to Chaetomorpha as well as the reduced ability of Ulva to store nutrients over winter. The growth rate of Ulva is negative in winter due to low salinity, temperature and irradience, and this results in negligible biomass at the time of highest inorganic nitrogen concentration. Chaetomorpha linum persists over winter allowing it to take advantage of elevated nutrient concentrations at the time; nutrients stored during winter support high growth rates in summer. The maximum tissue nitrogen levels recorded for Ulva were only 47% higher than the critical concentration while maximum tissue phosphorus concentration was almost 9 times the critical level. This seasonal limitation by nitrogen supports an earlier hypothesis that production in the system may be nitrogen limited in summer. It was concluded that differences in nutrient requirements and aquisition strategies afforded C. linum greater competitive potential for utilizing the seasonal and short-term increases in nutrient concentrations that occur in Peel Inlet.