Zooplankton aggregation at glider-detected ocean fronts in the California Current Ecosystem

Here we report the results from six years of front observations from 2006 through 2011 along two transect lines within the Southern California Current System (SCCS) using autonomous gliders. Over the course of the study composed of 124 completed transects (22,980 profiles), our front detection algorithm identified 180 surface density fronts along the two sampling lines. The incidence of surface density fronts showed distinct seasonality, with significantly fewer fronts occurring during winter months followed by the onset of more numerous and stronger fronts in the nearshore during Spring. Fronts occurring during summer and fall were also more numerous than winter fronts, and located further offshore than spring fronts. During the study, horizontal density gradients in the surface layer (0-50 m) were significantly correlated with horizontal gradients in surface layer chlorophyll-a fluorescence as well as with acoustic backscatter (ABS) recorded by a 750 kHz acoustic Doppler profiler, suggesting that density fronts on average were zones of rapidly changing phytoplankton and zooplankton biomass concentrations. Surface density fronts were also more likely to be zones of enhanced average ABS compared to regions flanking the fronts, suggesting that fronts were often zones of enhanced zooplankton biomass, not merely regions of changing biomass. Gradients in other hydrographic variables such as temperature, salinity, and in some cases spice and horizontal shear, were significantly correlated with ABS and chl-a fluorescence gradients, though the presence of strong density gradients remained the single best predictor of strong ABS and fluorescence gradients. We examine implications of the observed patterns in front occurrence for prey foraging by highly mobile predators such as birds, marine mammals and larger fish, and discuss the implications of the results in the context of long term trends in the SCCS.