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

Abstract

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.KEY WORDS: Nonindigenous invasive species · Larval abundance and dispersal · Physical oceanographic processes · Coastal embayment · Bryozoan · Membranipora membranacea · Electra pilosa Resale or republication not permitted without written consent of the publisher

Delivery of zooplankton to the surf zone during strong internal tidal forcing and onshore winds in Baja California

“…For bryozoans (cyphonautes), tidal height was the factor that best explained their abundance patterns. Saunders & Metaxas (2010) suggested that onshore transport of cyphonautes occurs during wind-driven downwelling events; however, in our study we did not have any such events. Differences found in the delivery mechanism between studies may be related to regional variability in the vertical distribution of larvae.…”

“…Differences found in the delivery mechanism between studies may be related to regional variability in the vertical distribution of larvae. For example, Saunders & Metaxas (2010) found higher abundances of cyphonautes closer to the surface in Nova Scotia, whereas in California, near our study site, other authors (Bernstein & Jung 1979, Yoshioka 1982, Pineda 1999 found that they occurred below the thermocline in strongly stratified Table 2. Models representing hypotheses of onshore larval transport at San Miguel beach.…”

The association of coloniality with parental care of embryos

“…Also, the obligate dispersal of feeding cyphonautes larvae before metamorphic competence might enlarge the area that progeny occupy, in contrast to the distribution of a brooder, Bugula neritina , in an eelgrass bed, which was limited by a short distance of larval dispersal (Keough & Chernoff, 1987). However, for two nonbrooders, Membranipora and Electra , the dispersal during feeding and growth of the cyphonautes larvae exceeded what was necessary for occupying a local habitat (Saunders & Metaxas, 2010).…”

Bay-scale patterns in the distribution, aggregation and spatial variability of larvae of benthic invertebrates

“…The highest concentrations of bryozoans occurred at 18 m, and showed a positive relationship with fluorescence (Lloyd et al 2012a). In contrast, bryozoan abundance in St. Margarets Bay (also in Nova Scotia) was higher at 4 m than at 12 m (Saunders & Metaxas 2010). However, the salinity and temperature at 4 m depth in St. Margarets Bay was more similar to that at 12 m in St. George's Bay.…”