Response of a community dominant to climate patterns in rocky intertidal ecosystems

Using 8-15 year-long datasets, we tested the effects of El Niño-La Niña and Pacific Decadal Oscillation phases on growth of the rocky intertidal mussel Mytilus californianus, the dominant competitor in wave-exposed rocky intertidal communities on the west coast of North America.  We find strong associations between both climatic patterns and mussel growth, with faster growth during warm water events (El Niño and warm phase PDO).  Further, growth also varied in space, being faster at sites on one cape compared to sites on another cape 70 km to the north.  Analysis of parallel datasets on chlorophyll-a concentration, a proxy for food availability, and on sea water temperature quantified in the intertidal indicates that growth rates of mussels are an integrated response to both food and temperature fluctuations.  Consideration of either factor alone, temperature or food, yields minimal explanatory power.  The growth responses showed dramatic shifts between the 1990’s when they differed between capes and the 2000’s, when slower growth rates at one cape caught up to faster growth rates at the other cape.  These results imply that variation in the dynamics of mussel-dominated intertidal systems are driven in part by both bottom-up factors and thermal environments, and that these dynamics are changing, with contrasting consequences under two alternative models of the effects of climate change on coastal upwelling ecosystems.