Abstract
Sessile epifauna that excrete ammonium as a waste product are often closely associated with surfaces of macroalgae that are able to absorb and use ammonium for growth. This close association means that nitrogen provisioning from epifauna to host macroalgae is probable, but barriers to uptake, formed by epifaunal colonies, and the physiological status of colonised macroalgal tissue may limit the amount of ammonium absorbed by colonised macroalgae. Evidence for the movement of nitrogen from sessile epifauna into and around the giant kelp Macrocystis pyrifera was gathered using a N-15-enriched stable isotope tracer. Experiments in which N-15-labelled phytoplankton was fed to the epifaunal bryozoan Membranipora membranacea did not to find conclusive evidence for the flow of nitrogen released by bryozoan colonies into colonised M. pyrifera tissue. The low degree of transmission of N-15 observed may be due to the barrier to nitrogen uptake that is formed by M. membranacea tissue and the low affinity for ammonium uptake by M. pyrifera blades during winter. M. membranacea is unlikely to provide significant benefits to host macroalgae via nitrogen provision in many situations, due to its sheet-forming habit and its absence during summer low-nitrogen periods. In situ incubations of M. pyrifera blades in N-15-enriched ammonium were used to determine if ammonium provided by epifaunal colonies could be taken up by mature blades tissue and exported to the stipe for long-distance transport to actively growing meristems. Uptake and export of labelled nitrogen from basal blades into the stipe was measured, and the potential for long-distance transport of nitrogen from older blades was confirmed. Active transport of nitrogen from older macro algal tissue colonised by epifauna such as hydroids could be an important mechanism allowing kelp with specialised long-distance solute transport systems to tolerate nitrogen limitation.