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

Abstract The encrusting bryozoan Membranipora membranacea is the dominant epibiont on the giant kelp Macrocystis integrifolia. Previous studies suggest that bryozoans influence the nitrogen (N) and photosynthetic physiology of the underlying kelp. We predicted that for N-limited M. integrifolia, if bryozoans supply the underlying tissue with excreted ammonium, (1) the kelp tissue would utilize this N in chlorophyll synthesis, thereby compensating for reduced 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 influence 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 one side of the disk (50% treatment); or (3) were completely covered with bryozoans (100% treatment). The bryozoan excretion rates were estimated as 2.25 × 10−4 μmoles g−1 wet mass bryozoans−1. However, contrary to our 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 fucoxanthin were lower with bryozoan colonization. Bryozoan colonization also had little effect on tissue percent N, percent C, or polyphenolic content at either site. The presence of bryozoans significantly decreased ammonium uptake rates of algal tissues. Tissue percent N and pigment content were higher at the N-rich site compared to the N-poor site for all bryozoan cover treatments. 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.