Influence of bryozoan colonization on the physiology of the kelp Macrocystis integrifolia (Laminariales, Phaeophyta) from nitrogen-rich and -poor sites in Barkley Sound, British Columbia, Canada

The encrusting bryozoan Membranipora membranacea is the dominant epibiont o n the giant kelp Macrocystis integrifolia. Previous studies suggest that br yozoans influence the nitrogen (N) and photosynthetic physiology of the und erlying kelp. We predicted that for N-limited M. integrifolia, if bryozoans supply the underlying tissue with excreted ammonium, (1) the kelp tissue w ould utilize this N in chlorophyll synthesis. thereby compensating for redu ced light levels that result from bryozoan colonization, and (2) levels of carbon-based secondary metabolites (polyphenolics) would be reduced. For N- replete M. integrifolia. we expected bryozoans to have little or no influen ce on kelp physiological parameters. Soluble tissue nitrate. tissue percent C and percent N. polyphenolic and chlorophyll a. c and fucoxanthin content , and ammonium uptake were determined for M. integrifolia tissue disks that (1) had no bryozoans (clean treatment): (2) were covered by bryozoans on o ne side of the disk (50% treatment): or (3) were completely covered with br yozoans (100% treatment). The bryozoan excretion rates were estimated as 2. 25 x 10(-4) mu moles g(-1) wet mass bryozoan s(-1). However, contrary to ou r predictions, there was little effect of bryozoan colonization on pigment content at the N-poor site, but colonization significantly affected pigment content at the N-rich site, where concentrations of chlorophyll a and fuco xanthin were lower with bryozoan colonization. Bryozoan colonization also h ad little effect on tissue percent N, percent C, or polyphenolic content at either site. The presence of bryozoans significantly decreased ammonium up take rates of algal tissues. Tissue percent N and pigment content were high er at the N-rich site compared to the N-poor site for all bryozoan cover tr eatments. Polyphenolic content was three to four times higher at the N-poor compared to the N-rich site for all bryozoan treatments. Results presented here contrast with those of an earlier study in which bryozoans apparently provided kelp with N and the implications of this difference are discussed .