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From Long-Term Data to Understanding: Toward a Predictive Ecology
2015 LTER ASM Estes Park, CO - August 30 - September 2, 2015

Changes in zooplankton habitat, behavior, and acoustic scattering characteristics across glider-resolved fronts in the California Current Ecosystem

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Presenter/Primary Author: 
Jesse Powell
Mark D. Ohman

We report cross-frontal changes in the characteristics of plankton proxy variables measured by autonomous Spray ocean gliders operating within the California Current Ecosystem (CCE) LTER site.  Our study is based on 23,000 glider dives to 500 m depth made over a period of 6 years of continuous observations.  A comparison of conditions across the 154 positive frontal gradients (i.e., where density of the surface layer decreased in the offshore direction) showed that waters on the denser side of the fronts typically showed higher Chl-a fluorescence, shallower euphotic zones, and higher acoustic backscatter than waters on the less dense side.  Transitions between these regions were relatively abrupt.  For positive fronts the amplitude of zooplankton diel vertical migration (DVM), inferred from a 3-beam 750 kHz acoustic Doppler profiler, increased offshore of fronts and covaried with optical transparency of the water column.  Average interbeam variability in acoustic backscatter also changed across many positive fronts within 3 depth strata (0-150 m, 150-400 m, and 400-500 m), revealing a front-related change in the acoustic scattering characteristics of the assemblages.  The extent of vertical stratification of distinct scattering assemblages was also more pronounced offshore of positive fronts.  Depth-stratified zooplankton samples collected by Mocness nets corroborated the autonomous measurements, showing copepod-dominated assemblages and decreased zooplankton body sizes offshore and euphausiid-dominated assemblages with larger median body sizes inshore of major frontal features.  These results suggest that ocean frontal features in the CCE region spatially structure the pelagic ecosystem and are sites of modified plankton growth, aggregation, DVM behavior, and, we infer, regions of altered predator-prey interactions.