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.