Primary producer and seabird associations with AVHRR-derived sea surface temperatures and gradients in the southeastern Gulf of Alaska

Identification and characterization of critical habitat is an important component of marine conservation. In this study, we compare relative sea-surface temperature (SST) and gradients in SST as derived from advanced very high-resolution radiometry (AVHRR) with primary producer biomass, indexed by remotely sensed Chlorophyll a concentrations (from the sea-viewing wide field-of-view sensor [SeaWiFS]), and seabird observations collected during replicate surveys along Line P in the southeastern Gulf of Alaska. Because of cloudiness, we used monthly composites for both Sea-WiFS and AVHRR. Seabird data were collected three times per year (‘seasons’) starting in 1996, and summarized in 4 × 4 km grids. We divided our analyses into two regions based on oceanography; the outer ‘oceanic’ region and the ‘coastal’ region. We calculated SST anomalies as residuals from mean SST calculated per region, season, and year. We calculated maximum SST gradients between AVHRR data pixels (both 4×4 km and 9×9 km resolution) and defined areas of ‘steep’ SST gradients. Within each region we found that primary producer biomass varied with SST gradients, but not as expected (i.e. we expected sharp increases in chlorophyll a with steeper SST gradients). There was, however, strong evidence of seabird associations with SST and SST gradients, especially in the coastal region where almost all species were associated with fronts. In the oceanic region, 62% of the species were associated with steep SST gradients. This study demonstrates the importance of mesoscale oceanographic structures on seabird distributions in the southeastern Gulf of Alaska, and illuminates how these relationships can be characterized using remote-sensing of habitats at large spatial scales.