Seasonal variation in the attachment strength of blue mussels: Causes and consequences

The midintertidal zone of temperate rocky coasts is often dominated by mussels that form dense beds and exclude other primary space holders. The rate at which storms remove mussels from their substrate consequently has a controlling influence on intertidal community structure. The rate of disturbance by waves can be predicted mechanistically by comparing the attachment strength of an organism to the force it encounters. One simplifying assumption often implemented with this approach is that organismal strength is temporally constant. This study demonstrates that such an assumption is not valid for Mytilus edulis in Rhode Island, where byssal attachment strength (tenacity) increases twofold in winter compared to summer. Time series analysis of monthly samples (1998– 2000) indicates tenacity generally tracks seasonal fluctuations in wave height, which suggests that mussels sense and respond to changes in their flow environment. However, the ability of mussels to respond to wave climate is not precise; during September and October (hurricane season), mussels remain weakly attached and are prone to major mortalities. Mussel reproductive condition also displays seasonal cycles, peaking 3–4 months after attachment strength. This suggests a possible energetic constraint on the ability of mussels to respond to their flow environment, as mussels trade byssal thread production for gonad development. This study demonstrates that accurate predictions of disturbance events must consider not only a storm’s magnitude, but its timing relative to the cycle in attachment strength. Given the recent shift to increased hurricane activity in the North Atlantic, it is likely that mussels will begin to suffer more frequent and more severe disturbances compared to those that occurred during 1971–1994. Investigations of the rocky intertidal zone have made important contributions to our understanding of how disturbance and patch dynamics influence nonequilibrium communities (e.g., Menge 1976; Lubchenco and Menge 1978; Paine and Levin 1981; Sousa 1985). Disturbance in the rocky intertidal zone is often dominated by a single physical factor, the hydrodynamic forces generated by breaking waves. In recent years, a number of studies have detailed the mechanisms by which moving water interacts with intertidal plants and animals (e.g., Denny et al. 1985; Carrington 1990; Gaylord et al. 1994; Denny 1995), contributing to the development of a mechanistic approach to the prediction of disturbance in marine communities by Denny (1995). Denny’s six-step approach couples wave statistics and hydrodynamics with measurements of organismal attachment strength to predict disturbance as a function of time and the waviness of the sea offshore. Preliminary tests of Denny’s approach are promising; it correctly predicts the survivorship of a variety of organisms along an exposure gradient, the size of wave-swept algae, and the percentage of mussels dislodged from beds during winter storms (Gaylord et al. 1994; 1Corresponding author ([email protected]). Formerly E.C. Bell.