Physical forcing of distributions of bryozoan cyphonautes larvae in a coastal embayment

In shallow rocky subtidal ecosystems in the western North Atlantic, outbreaks of the nonindigenous encrusting epiphytic bryozoan Membranipora membranacea cause defoliation of laminarian algae, facilitating a phase transition from native kelp beds to meadows of invasive algae. We quantified spatial (m [depth] to km [sites]) and temporal (hourly to monthly) patterns in the abundance of larvae of M. membranacea, and a morphologically similar native species, Electra pilosa, in relation to physical structure (temperature, salinity, density) of the water column in St. Margarets Bay, Nova Scotia, Canada, on 8 dates from September to November 2007. During the study period, the water column ranged from strongly stratified to well mixed, and cross-shore movements of water masses (wind-driven upwelling and downwelling) were indicated by shoaling of the isoclines. When a strong pycnocline was located between the 2 sampled depths (4 and 12 m), larvae of both species were more abundant in the shallower, warmer, fresher layer. The linear relationship between strength of stratification and the relative abundance of larvae between depths was significant over 2 temporal (hourly, weekly) and 1 spatial (km) scale examined for M. membranacea, but not for E. pilosa. Highest abundance of larvae of both species in the warm fresh surface layer suggests onshore transport during wind-driven downwelling events. Dissimilar patterns in size-frequency distributions between the indigenous and nonindigenous bryozoan larvae suggest that species-specific characteristics of larvae of M. membranacea may be a contributing factor to its success as an invader in the western North Atlantic.