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Relationsh~ps between seaweed morphology and the metabolism of ammonium and carbon were examined for 9 seaweeds from northeastern New Zealand. Maximum rates of surge ammonium uptake and ammonium assimilation, in addit~on to the maximum ammonium storage pool, were strongly positively correlated with seaweed surface area volume (SA:V) ratio when expressed per g dry weight, but were largely independent of SA:V rat10 when expressed per cm2 surface area. This suggests that ammonium metabolism may be confined largely to the outermost cell layers. Similar scaling properties were found for rates of photosynthesis and, to a lesser extent, respiration. Rates of photosynthesis were similar to those derived from the literature for northern hemisphere seaweeds, while respiration rates were slightly lower among New Zealand seaweeds. These comparatively minor geographic differences suggest that previously documented differences in rates of ammonium uptake between the 2 regions are not related to carbon metabolism. Maximum rates of ammonium assimilation were determined by measunng the internally controlled rate of ammonium uptake, which closely matched the assmilat~on rate in, the green seaweed Ulva sp., the red seaweed Osmundaria colensoi, and the brown seaweed Zonaria turneriana. 'KEY WORDS: Ammon~um assimilation. Seaweeds. Photosynthesis. Scaling. Ammonium storage. Respiration. A~nn~onlunl uptake. Surface area.volume ratio

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