Climatic variation alters supply-side ecology: impact of climate patterns on phytoplankton and mussel recruitment

Climate change will modify ecosystems, but a dearth of long-term data sets on critical ecological factors makes prediction of mechanistic pathways and scope for ecological changes difficult. Using 14-18 year time series of phytoplankton abundance and mussel recruitment at up to 10 sites along 320 km of coast in a rocky intertidal ecosystem, we examined the influence of climatic variation at 3-7 year (El Nino-Southern Oscillation [ENSO]), 7-10 year (North Pacific Gyre Oscillation [NPGO]), and 20+ year (Pacific Decadal Oscillation [PDO]) timescales. Phytoplankton and mussel recruitment revealed order-of-magnitude increases and seasonal shifts in the 2000s compared to 1989-1999, but seasonal shifts were in opposite directions with phytoplankton blooms occurring earlier and recruitment occurring later in the year. The NPGO had the strongest relationships with both phytoplankton and recruitment, while relationships between these factors and ENSO and PDO were weak. Increases were associated with cooler ocean temperature and intensification in the ocean gyre-scale circulation reflected in a stronger North Pacific Current (NPC). Phytoplankton abundance and mussel recruitment were positively correlated, suggesting that increases in larval food may have driven the changes in mussel recruitment. With our prior demonstration that mussel growth rate is sensitive to phytoplankton concentration, these results suggest an important effect of bottom-up forcing on the dynamics of coastal mussel populations. We conclude that central supply-side ecological processes in coastal ecosystems are sensitive to long-term ocean basin-scale oceanographic and atmospheric fluctuation. Despite these changes, intertidal community dynamics have changed only in relatively subtle ways, suggesting a role for local ecological interactions in dampening the effects of dominant modes of climate forcings in coastal ecosystems.