The bioprotective properties of the blue mussel (Mytilus edulis) on intertidal rocky shore platforms.

Biogeomorphological processes are an important component in the geomorphic evolution of rocky coasts. Sessile organisms, including those which form dense biological covers, enhance and/or retard weathering and erosion. The common blue mussel (Mytilus edulis) is found on rocky shores in many parts of the world, typically forming extensive beds several centimetres thick. Yet, unlike other canopy forming marine organisms (e.g., seaweeds), the influences of mussels on processes of rock breakdown lack quantitative analysis. This study assesses the potential of M. edulis to act as a biogeomorphic agent through weathering experiments and field trials on a mudstone shore platform in Wales, UK. Monitoring on the shore platform showed that daily near-surface temperature maxima, range and short-term variability were lower on surfaces colonised by M. edulis compared to those that were bare. This was supported by laboratory simulations which showed that (artificial) mussel beds consistently reduced the internal (subsurface) temperature of rock samples during 'low-tide' periods. As mechanical rock weathering processes are influenced by surface and subsurface temperature regimes, it is inferred that these stabilising effects may translate to a reduction in the efficiency of particular rock breakdown processes. Rock hardness (Equotip Piccolo) measurements collected from the shore platform support this assertion as mussel-colonised surfaces were found to be significantly harder than bare surfaces, indicating they had experienced less breakdown during exposure to intertidal weathering and erosion. A supplementary field experiment based on the disintegration of gypsum 'domes' as an indirect measure of water/wave motion both within and outside of mussel beds further indicates that mussels may act as bioprotective agents by reducing turbulence at the rock surface. Overall, this study presents the first empirical evidence of mechanisms by which mussels may protect intertidal rocks from weathering and erosional processes through the moderation of near-surface microclimate regimes and water motion in temperate environments. Importantly, compared to other canopy forming species, such as seaweeds, the observed dampening effects of M. edulis are likely only small in magnitude. However, as mussels can dominate the surfaces of rocky shores at certain tidal heights, they may act as locally important geomorphic agents that warrant further consideration as one component of the collective effect of bioprotective and bioerosive species in these environments. Further work is now needed to examine the geomorphic roles of mussel beds on a variety of other rock types, and the extent to which their impacts vary in time and space in relation to other biological, chemical and physical agents of change. • The influences of mussels on rock breakdown processes are poorly understood. • The weathering roles of mussels were examined via field and laboratory experiments. • Rock surface and subsurface thermal regimes were dampened under mussels. • Rock hardness and material loss during wave inundation were also reduced. • Mussels can act as locally important biogeomorphic agents on rocky shores. [ABSTRACT FROM AUTHOR]