Nitrogen ecophysiology of intertidal seaweeds from New Zealand: N uptake, storage and utilisation in relation to shore position and season

The nitrogen ecophysiology of 4 intertidal seaweeds (<i>Stictosiphonia arbuscula, Apophlaea lyallii, Scytothamnus australis, Xiphophora gladiata</i>) from southeastern New Zealand is described in terms of N status, N uptake rates and N utilisation. The species growing in the highest shore position had large internal NO₃^- and NH₄⁺ pools. For all species, tissue NH₄⁺ pools were greater than tissue NO₃^- pools. Total tissue N was directly related to shore position with high intertidal species having highest tissue N, while the opposite trend was observed for C:N ratios. The ability to take up inorganic (NO₃^-, NH₄⁺) and organic (urea) N when one or all N forms were present in the culture medium was measured using time-course uptake experiments at initial concentrations of 5 and 30 µM. Nitrate uptake did not vary over time for any of the species. <i>S. arbuscula</i> and <i>S. australis</i> exhibited a surge phase of NH₄⁺ uptake at both concentrations. Urea uptake at 5 µM was generally low and consistent over time; uptake at 30 µM was highly variable. All species were capable of simultaneous uptake of all N forms. The relative importance of each N form to overall N nutrition indicated that NH₄⁺ was an important N source in winter for all species. Urea was an important N source in summer, contributing 27 to 33% to the total N acquisition for most species. A relative preference index indicated that in winter N sources were utilised in the order NH₄⁺ > NO₃^- > urea, while in summer the order was NH₄⁺ = NO₃^- > urea. Estimates of the amount of N that each species could acquire during a tidal cycle indicated that the high intertidal <i>S. arbuscula</i> had the greatest capacity for N acquisition, regardless of season.