New approach for using remotely sensed chlorophyll a to identify seabird hotspots

Satellite imagery of chlorophyll a concentration (chl a) provides the only holistic perspective of productivity in marine ecosystems worldwide, yet its utility for understanding and predicting the distribution of upper trophic-level predators remains equivocal. We tested the hypothesis that remotely sensed chl a measurements spanning a decade can predict marine bird movements and distributions and provide insight into persistent seabird 'hotspots'. To test this hypothesis, we developed a temporally and spatially explicit chl a variance metric that explained up to twice the variation (90%) in seabird distributions than mean chl a values in our model system, the California Current. Our chl a variance metric-the frequency of chlorophyll peaks index (FCPI) - quantifies the frequency of chl a anomalies that exceed 1 standard deviation from the mean, relative to a spatial model for the ecosystem. Using the FCPI, we identified locations of high chl a anomaly persistence amidst considerable habitat heterogeneity from British Columbia, Canada, to Baja California, Mexico. Calculating variance and persistence metrics such as FCPI over large temporal scales may be preferable to analyzing mean or median chl a values over shorter time scales if understanding or predicting predator distributions is a goal of the research. As a static habitat layer, we suggest that the FCPI integrates time lags, physical forcings, and food web processes that can thwart attempts to link primary productivity and predator distributions in pelagic ecosystems. Additionally, identifying important ecological areas reflecting persistently elevated productivity and predator abundance is critical to marine spatial management and conservation.