EFFECT OF BRYOZOAN COLONIZATION ON INORGANIC NITROGEN ACQUISITION BY THE KELPS AGARUM-FIMBRIATUM AND MACROCYSTIS-INTEGRIFOLIA

The effect of bryozoan colonization on inorganic nitrogen acquisition by Agarum fimbriatum Harv. and Macrocystis integrifolia Bory., collect ed from the west coast of Vancouver Island, British Columbia, Canada, was examined in laboratory experiments during June and July 1992. Piec es of kelp blades that were completely covered on one side by the bryo zoans Lichenopora novae-zelandiae Busk or Membranipora membranacea, L. , or uncolonized (clean treatment), were used to estimate the rate at which nitrate and ammonium were removed from the surrounding seawater. In addition, the rate of ammonium excretion by bryozoans isolated fro m their associated kelp was measured and also estimated from the resul ts of the uptake experiments. Values obtained were used to estimate th e contribution of ammonium excreted by bryozoans to the total amount o f inorganic nitrogen available to the associated kelp. Both bryozoan s pecies reduced the ability of the associated kelp to remove nitrate an d ammonium from seawater but provided a source of ammonium to the kelp through excretion. The nitrogen status of colonized and clean kelp di sks was determined from the ratio of total particulate carbon to total particulate nitrogen (C:N ratio). The C:N ratios for A. fimbriatum co lonized with either L. novae-zelandiae or M. membranacea were similar (C:N=12 to 14), and differences between colonized and clean treatments were not significant. For A. fimbriatum, therefore, the C:N ratio ind icates that this species was not nitrogen limited at the time of the p resent study. In contrast, both colonized and clean disks of M. integr ifolia were nitrogen limited, but colonized disks (C:N=19) were signif icantly less limited by nitrogen than clean disks (C:N=29). Results ar e discussed in relation to the different environments inhabited by bot h kelp species and are consistent with the hypothesis that ammonium ex creted by bryozoans was an important source of inorganic nitrogen to M . integrifolia, but not to A. fimbriatum, at the time of the study.