COS 255-1 - An Energy Crunch Hypothesis to explain the collapse of mussel beds (Mytilus californianus) in the Southern California Bight

Abstract: Prior analyses of sea surface temperatures (SST) indicate that the Southern California Bight (SCB) showed little influence of seasonal upwelling, but a clear signal of the wider ocean warming that began in the late-1970s. To investigate possible effects of rising SST on a shore community, we compiled historical photographs of mussel (Mytilus californianus) beds extending back to 1973-1983 at 3 locations. We used image analysis and GIS to access the changes. Over 4-5 decades, all beds showed deep declines in coverage, some disappearing. Two beds showed partial recovery coinciding with a recent La Nina event. Prior studies elsewhere on the Pacific Coast showed that mussel bed losses occurred when the upper boundaries descended towards the lower boundaries. The descent was attributed to increasing exposure to lethal air temperatures during low tides. However, in the SCB, beds diminished not because the upper boundaries descended, but rather, because the fluctuating lower boundaries ascended, eventually converging on the comparatively stationary upper boundaries. We also note that (1) the shore level of the lower boundary is known to shift with changes in the relative rates of predation and mussel productivity (recruitment and growth), (2) that some lower boundary movements appeared to respond to events influencing ocean productivity, and (3) that longitudinal studies of mussel growth on the of the locations showed greater growth and survival during a La Nina event than the subsequent El Nino. Therefore, we propose that in the SCB rising SST has reduced ration and elevated mussel metabolic costs, diminishing the mussels’ ability to recoup losses to predation, itself possibly driven higher by rising SST. Thus, ocean warming, by altering predator-prey energetics, may produce gradual but pervasive effects that ramify through some marine benthic communities.