CARRINGTON, E; University of Rhode Island: Disturbance to mussel beds on NE rocky shores: is it predictable?

On temperate rocky coasts, mussels often form dense beds in the mid-intertidal zone that dominate primary space and provide secondary habitat for other organisms. The frequency and severity of disturbance to mussel beds therefore plays an important role in structuring intertidal communities. One common form of disturbance to mussel beds is dislodgment by storm waves, which can generate hydrodynamic forces that exceed mussel attachment strength. Carrington (2002) used field measurements of wave height and mussel attachment to develop a biomechanical model to predict mussel dislodgment on Rhode Island shores. The model predicts strong dislodgment events (up to 40%) during hurricane season (Sep-Nov), a period when large waves coincide with relatively weak mussel attachment. Because mussels double their attachment strength from Oct-Mar, large storms arriving in winter or early spring are predicted to effect less dislodgment. As a field test of this biomechanical model, mussel abundance has been monitored biweekly for three years at two wave exposed sites in Rhode Island Sound. Mussel percent cover was quantified in individual photoquadrats using a 200 random dot technique. While there was considerable spatial variation in mussel abundance, dislodgment events were more frequent and severe in early fall and were rare during winter, as predicted by the biomechanical model. Interestingly, two major mortality events were observed that were not predicted. These disturbances were evidently due to exposure to extreme air temperatures (heat in summer 2001, cold in winter 2002). In comparison to disturbance due to temperature stress, disturbance by wave dislodgment not only appears to be more frequent, but also more temporally predictable.