Allometric evidence for the dominant role of surface cells in ammonium metabolism and photosynthesis in northeastern New Zealand seaweeds

Relationships 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 addition to t he maximum ammonium storage pool, were strongly positively correlated with seaweed surface area:volume (SA:V) ratio when expressed per g dry weight, b ut were largely independent of SA:V ratio when expressed per cm(2) 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 photosynthesi s were similar to those derived from the literature for northern hemisphere seaweeds, while respiration rates were slightly lower among New Zealand se aweeds. These comparatively minor geographic differences suggest that previ ously documented differences in rates of ammonium uptake between the 2 regi ons are not related to carbon metabolism. Maximum rates of ammonium assimil ation were determined by measuring the internally controlled rate of ammoni um uptake, which closely matched the assimilation rate in the green seaweed Ulva sp., the red seaweed Osmundaria colensoi, and the brown seaweed Zonar ia turneriana.