Phalaropes feeding at a coastal front in Santa Monica Bay, California

Abstract

The spinning behavior often exhibited by phalaropes when feeding at freshwater sites is rarely observed at sea. Instead, phalaropes are typically observed slowly swimming forward while foraging on marine neuston concentrated in surface convergence zones. Small-scale coastal ocean fronts, eddies and internal waves capable of generating such convergences are extremely common, albeit ephemeral, features in the Southern California Bight. This region is marked by a complex flow regime, resultant in part from its variable coastal morphology. We used satellite data (AVHRR) and in situ measurements (CTD, surface drifters) to describe and track a coastal front in Santa Monica Bay, California, centrally located in the Southern California Bight. A high number of Red-necked Phalaropes ( Phalaropus lobatus) were associated with this feature over the course of several days. Neuston tows and gut content analyses revealed these phalaropes were primarily feeding on fish eggs and assorted debris that were abundant at the sea surface in this front. No phalaropes were observed spinning anywhere in the vicinity. Previously unpublished metabolic activity rates for phalaropes indicate that spinning is much more energetically expensive than is swimming at a comparable speed. Convergences associated with fronts (or eddies, internal waves, etc.) in the Southern California Bight apparently provide phalaropes with a rich, easily accessible and steady supply of food without having to resort to the energetically costly behavior of spinning.