Seabird affinities for Gulf Stream frontal eddies: Responses of mobile marine consumers to episodic upwelling

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Abstract
Thus study combined ship-board counts of seabirds with satellite hydrography to determine whether Gulf Stream frontal eddies influences spatial and seasonal patterns of apex consumers on the southeastern United States continental shelf and slope. Stratified sampling indicated seabird densities at eddies were 7-15 .times. higher than in resident shelf and Gulf Stream water, ranging from 2.31-10.91 birds km-2 compared to 0.15-1.51 birds km-2 in moneddy regions. At any given time eddies covered 3-17% of the outer shelf and upper slope, yet 28-59% of total seabird numbers in this domain aggregated within areas influenced by eddies. Gulf stream eddies consist of an upwelled, nutrient- and phytoplankton-rich cold-core and an oligotrophic warm filament entrained from the Gulf Stream. Seabird densities within eddy cold-cores were 4-12 .times. higher, ranging from 2.91-16.35 birds km-2 compared to 0.16-3.70 birds km-2 in the warm filament. Seabird abundances withineddy cold-cores were higher (a) near the frontal boundaries between water masses, and (b) within older upwelled (upstream) cold-core water as opposed to recently upwelled (downstream) cold-core water. Eddy size accounted for 99%, 94%, and 98%, respectively, of the between-eddy variation in seabird abundance, biomass, and minimum daily food requirements per unit area. Between-eddy differences in seabird density, biomass, and food requirements showed no relationship to eddy age. The frequency and the extent of satellite-detected, near-surface upwelling originating from eddies on the outer shelf and upper slope decreased during late summer and early fall. Total seabird density (pooled species'' abundances) showed no relationship to this seasonality. However, taxa that are particularly dependent on upwelling (Family Hydrobatidae: Oceanites and Oceanodroma) exhibited declines in abundance that correlated with seasonal decreases in near-surface upwelling. Seabird patchiness within the outer shelf and upper slope was greatest during months of frequent eddy upwelling, indicating that small-scale seabird distribution within domains may be affected by upwelling seasonality. Eddies influence seabird distribution and abundance at space scales of 10-50 and 50-150 km, and at time scales of 2-14 and 30-180 days. Seabird responses to environmental heterogeneity caused by eddies give strong circumstantial evidence for energy transfer beyond primary producers at these episodic upwelling features.