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 ma croalgae and an exotic macroalga from Port Phillip Bay. All species exhibit ed rate-saturated mechanisms of uptake described by Michaelis-Menten uptake kinetics. At the highest concentration examined (28.6 mu mol NH4-N) Hincks ia 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 decipie ns (53 mu mol NH4-N g dry wt(-1) h(-1)). Maximum surge uptake rate was high est for H. sordida and lowest for P. decipiens (802 and 57 mu mol NH4-N g d ry wt(-1) h(-1) respectively). The introduced phaeophyte Undaria pinnatifid a had an intermediate capacity for ammonium uptake which was dependent on b lade maturity. Differences in the ratio of maximum uptake rate to half-satu ration rate between surge and assimilation uptake phases suggest a propensi ty for some species to take up ammonium at low concentrations. The relation ships between nutrient uptake and growth among species would afford mature U. pinnatifida, H. sordida and Ulva sp. a competitive advantage for ammoniu m uptake in winter during high N availability, whereas P. decipiens would b e able to exploit low N concentrations in spring and summer.