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Ocean fronts in the Southern California Current System and their role in structuring zooplankton distributions, diel vertical migration, and size composition


Ocean fronts are dynamic gradients that divide waters with differing hydrographic properties. Fronts also play important ecological roles in structuring plankton distributions, modulating primary and secondary production, and delineating predator foraging areas. Here, I utilize autonomous "Spray" ocean gliders to describe the spatial and seasonal distribution of deep-water fronts and their impact on the plankton within the Southern California Current System (SCCS). To test the suitability of the Spray's 750 kHz acoustic doppler profilers (ADP) for mapping zooplankton distributions, I first present results from a seatruthing study in which I mounted an ADP on a Mocness plankton net. I show that the relative mean volume backscatter (rMVBS) measured by the ADP is correlated with the summed zooplankton cross-sectional area (a proxy for biomass). I also find that the relationship between rMVBS and zooplankton biomass is strongest for zooplankters with an estimated spherical diameter greater than 1.6 mm. Observed rMVBS was best explained by euphausiid and copepod biomass. From Spray cross-shore sections between October, 2006 and December, 2011, I identified 154 distinct surface layer density fronts. The strongest and most numerous fronts occurred in spring, and were located closer to the coast, whereas summer and fall fronts were found further offshore. Fronts were weakest and least numerous in winter. Across all seasons, fronts structured plankton distributions. Horizontal gradients in physical variables (e.g., surface density, temperature, and salinity) co-varied with horizontal gradients in MVBS and Chl-a fluorescence, and the magnitude of biological gradients was higher at frontal areas compared with non- frontal areas. Frontal areas were also clearly associated with elevated Chl-a and zooplankton acoustic backscatter. Fronts divided distinct plankton habitats and associated assemblages. Chlorophyll a fluorescence maxima and euphotic zones were deeper offshore of fronts. The amplitude of diel vertical migration by zooplankton also increased offshore, in association with increased optical transparency in the upper ocean. Additionally, ADP data indicate that zooplankton assemblages change across fronts. I employ an inverse acoustic model to estimate zooplankton size spectra, inferring that larger-bodied zooplankters contributed a greater proportion to overall biomass inshore of these fronts. Vertically-stratified zooplankton samples from both inshore and offshore regions corroborated the model results

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