Climate-driven collapse of mussel beds (Mytilus californianus) in the Southern California Bight and the twilight of a keystone interaction.

In this study historical photographs of sites in the southern reaches of the Southern California Bight are compared with similar views taken in 2015-18. Using photographic models in GIS, mussel beds showing extensive areal covers of very large individuals in the 1960s and 1970s were greatly diminished or completely absent in modern times. Several lines of evidence indicate the collapse is driven by rising sea surface temperatures and falling ocean production. The mean size of matrix mussels diminished, suggesting reduced ration. The vertical range on the shore contracted for each bed, as predicted by models with reduced mussel recruitment and growth. A 33-year time series of panoramas at Bird Rock, Catalina Island were modeled within GIS and show a stepping down of areal coverage, entailing steep declines coinciding with El Nino events and stasis in the intervening years. Similar declines in mussel beds are seen elsewhere along the mainland of Southern California during this same time period. The El Nino/La Nina cycles were embedded in a long-term trend with the late 1970s marking the end of a 30+ year period of relatively cool SSTs and the beginning of the El Nino events. The observed decline in mussel beds is potentially altering a longstanding keystone interaction between mussels and the California Spiny Lobster (Panulirus interruptus) which, to date, has had positive ecological and economic impacts in California. Recent stable isotope data reflect a shift in the diet of spiny lobsters from mussels to less calorie rich prey items such as limpets and shore crabs. This climate driven shift in lobster diet may have long term ecological and economic consequences for the Southern California Bight ecosystem.