Suspension feeding organisms dominate subtidal habitats in the Gulf of Maine (GOM) at depths of 25-40 meters. PARASMITTINA JEFFREYSI is an encrusting cheilostome bryozoan found in this zone throughout the GOM. Space is generally a limiting resource in these communities, and initial observations suggested that P. JEFFREYSI was able to rapidly overgrow neighboring organisms. Regional variation in the GOM between coastal and offshore locations for both flow speed and particle concentration make it an ideal location to study the effect of these factors on the growth of P. JEFFREYSI. A secondary goal was to determine if differences in growth rates between sites would lead to differences in either the demography or competitive ability of this organism. Experiments were conducted over a 5 year period at 2 offshore and 2 coastal sites.

Particle flux measurements indicated that offshore flow speeds were over 3-fold greater than flow speeds at the coastal sites. Chlorophyll a concentrations, as well as particulate quantity and quality were generally greater at offshore sites, while sedimentation rates were greater in the coastal region. Both transplanted colonies and those within natural quadrats displayed no regional difference in growth, suggesting that the interactive effects of flow speed and particle concentration are acting upon feeding success and growth in opposing directions at each location, resulting in the net effect of no difference.

The amount of surface area, or percent cover, occupied by a sessile colonial organism depends on both density and individual colony sizes, while rates of recruitment and mortality provide a more complete understanding of the factors determining spatial cover. An examination of the spatial variation in each of these population parameters for P. JEFFREYSI indicated differences among sites for all parameters except percent cover. This suggests that: (1) percent cover alone may not be a sufficient indicator of differences in population dynamics among sites; (2) although there were no differences among sites in the spatial cover of P. JEFFREYSI, this relationship was maintained through different processes at each site. Demographic comparisons using log-linear analysis of size-classified transition matrices indicated that the subsequent fate of P. JEFFREYSI was conditional on original colony size and independent of both spatial and temporal effects. In relative terms, temporal variation was found to exert a greater effect than spatial variation on the demography of this bryozoan.

There was no overall difference in the competitive ability of P. JEFFREYSI at coastal and offshore sites. The gain or loss of spatial cover for P. JEFFREYSI was primarily due to disturbance effects at the offshore sites, while interactions with a hydroid complex (hydroids and detritus bound by amphipod tubes) were of greatest importance at the coastal sites. In the few time intervals where the strength of competitive interactions appeared to be significant, it was due to interactions with a single abundant species. Competitive interactions between P. JEFFREYSI and most species were indeterminate, with either species overgrowing the other in certain cases.