Kinetics of nitrate, ammonium, and urea uptake by four intertidal seaweeds from New Zealand

The competitive ability for N uptake by four intertidal seaweeds, <i>Stictosiphonia arbuscula</i> (Harvey) King et Puttock, <i>Apophlaea lyallii</i> Hook. f. et Harvey, <i>Scytothamnus australis</i> Hook. f. et Harvey, and <i>Xiphophora gladiata</i> (Labillardière) Montagne ex Harvey, from New Zealand is described by the uptake kinetics for NO₃⁻, NH₄⁺, and urea. This is the first study to report uptake kinetics for N uptake by a range of southern hemisphere intertidal seaweeds in relation to season and zonation. Species growing at the highest shore positions had higher NO₃⁻ and urea uptake at both high and low concentrations and had unsaturable NH₄⁺ uptake in both summer and winter. Although there was evidence of some feedback inhibition of V_max for NO₃⁻ uptake by <i>Stictosiphonia arbuscula</i> growing at the lower vertical limits of its range, rates were high compared with species growing lower on the shore. Our results highlight the superior competitive ability for N uptake of certain high intertidal seaweeds, and consistent with our previous findings we can conclude that intertidal seaweeds in southeast New Zealand are adapted to maximizing N acquisition in a potentially N-limiting environment.