JOHNSON, A.S.*; PRATT, M.C.: Safety in numbers: Do algal canopies reduce dislodgment?

Persistence in the wave-exposed rocky intertidal implies resistance to dislodgment by waves. The ratio of breaking force to drag indicates safety from breakage, with values > 1 indicating safety and values < 1 indicating danger. For Chondrus crispus we estimated drag on thalli at site-relevant flow speeds to calculate the Environmental Stress Factor (ESF), which is a safety factor calculated for a specific time period (season or life-history stage). Estimation of ESF for Chondrus in the autumn revealed (1) that ESF was equal between exposures even though the mass-specific force ratio was greater at the protected site, and (2) a paradoxical result that all Chondrus thalli studied ought to have dislodged (average ESF = 0.63), but had not. This paradox may be resolved by consideration of the protection conferred by canopies of Chondrus: a canopy may effectively raise a thalli?s force ratio. When the site-relevant drag was reduced by the force reduction quantified in canopies (83%), the average ESF for these populations shifted to a value greater than one (mean canopy-corrected ESF = 3.7, with only 1 thallus having an ESF <1). In the New England rocky intertidal, a co-dominant, canopy-forming species, Mastocarpus stellatus, coexists with Chondrus. At a given site, the largest thalli of Chondrus are always larger than the largest thalli of Mastocarpus and would be, absent a canopy effect, more likely to dislodge. Perhaps differences in protection conferred by different canopies explain why larger Chondrus persist with Mastocarpus. Thus, differential canopy effects in different species are likely to influence both dislodgment and the outcome of competitive interactions.