Uptake of ammonium by four species of macroalgae in Port Phillip Bay, Victoria, Australia

The uptake rates of ammonium were determined for three species of native macroalgae and an exotic macroalga from Port Phillip Bay. All species exhibited rate-saturated mechanisms of uptake described by Michaelis-Menten uptake kinetics. At the highest concentration examined (28.6 mu mol NH4-N) Hincksia sordida had a higher rate of uptake (435 mmol NH4-N g dry wt(-1) h(-1)) than Ulva sp. (108 mu mol NH4-N g dry wt(-1) h(-1)) or Polysiphonia decipiens (53 mu mol NH4-N g dry wt(-1) h(-1)). Maximum surge uptake rate was highest for H. sordida and lowest for P. decipiens (802 and 57 mu mol NH4-N g dry wt(-1) h(-1) respectively). The introduced phaeophyte Undaria pinnatifida had an intermediate capacity for ammonium uptake which was dependent on blade maturity. Differences in the ratio of maximum uptake rate to half-saturation rate between surge and assimilation uptake phases suggest a propensity for some species to take up ammonium at low concentrations. The relationships between nutrient uptake and growth among species would afford mature U. pinnatifida, H. sordida and Ulva sp. a competitive advantage for ammonium uptake in winter during high N availability, whereas P. decipiens would be able to exploit low N concentrations in spring and summer.