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

Abstract

The nitrogen ecophysiology of 4 intertidal seaweeds (Stictosiphonia arbuscula, Apophlaea lyallii, Scytothamnus australis, Xiphophora gladiata) 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. S. arbuscula and S. australis 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 S. arbuscula had the greatest capacity for N acquisition, regardless of season.