SCALING OF AMMONIUM UPTAKE BY SEAWEEDS TO SURFACE AREA VOLUME RATIO -GEOGRAPHICAL VARIATION AND THE ROLE OF UPTAKE BY PASSIVE DIFFUSION

Rates of ammonium uptake per g dry weight for seaweeds from the Mokohi nau Islands, northeastern New Zealand were much lower than published v alues for northern hemisphere (east coast of North America and Baltic Sea) seaweeds. For the New Zealand seaweeds, the rate of ammonium upta ke expressed per cm(2) surface area was relatively constant (23.9 +/- 3.4 nmol cm(-2) h(-1)), irrespective of seaweed surface area:volume (S A:V) ratio. Moreover, there was a linear relationship between rates of ammonium uptake per g dry weight and ammonium concentration for 2 of the species used, Xiphophora chondrophylla and Ulva sp., which had low and high SA:V ratios, respectively. These results are consistent with most or all of ammonium uptake being due to passive diffusion of NH3. In addition, of 3 other species investigated, Pterocladia capillacea, Porphyra sp. and Enteromorpha sp., only P. capillacea exhibited satur ation kinetics.