Interactive effects of flow speed and particle concentration on growth rates of an active suspension feeder

Suspension-feeding organisms dominate subtidal habitats in the Gulf of Maine (GOM) at depths of 25-40 m. Parasmittina jeffrersi is an encrusting cheilostome bryozoan found in this zone throughout the GOM. Regional variation between coastal and offshore locations in both flow speed and particle concentration makes the GOM an ideal location to study the effect of these factors on the growth of P. jeffrersi. Particle flux measurements at two offshore and two coastal sites indicated that flow speeds were >3-fold greater offshore than at coastal sites. Chlorophyll a concentrations and particulate quantity and quality were greater at offshore sites, but sedimentation rates were greater in the coastal region. The growth rate of P. jeffreysi was measured at each of the sites using unobstructed colonies free from spatial competition and colonies that remained within their natural community. Both unobstructed colonies and those in quadrats within the natural community displayed no regional difference in growth rates, despite higher particle flux in the offshore region. The interactive effects of flow speed and particle concentration are acting upon growth rates in opposite directions, resulting in the net effect of no regional differences in growth. Growth of unobstructed colonies at one of the coastal sites (Halfway Rock) was less than that at any of the other three sites and may have resulted from the combination of high sedimentation and extremely low flow at this site. A comparison between unobstructed colonies and those in natural quadrats suggested that growth was limited by spatial competition in the natural community. A significant site interaction effect indicates that this response did not occur at all four sites and was absent at Halfway Rock because of poor growth of unobstructed colonies. Any differences in growth due strictly to differential food acquisition (e.g., unobstructed colonies) appear to be dampened by competitive interactions in the natural community.