Thin phytoplankton layer formation at eddies, filaments, and fronts in a coastal upwelling zone

On two cruises in August and September 2003 (hereafter cruises 2 and 3) during wind relaxations and transitions to upwelling conditions, thin layers of phytoplankton were observed in or a few meters below the stratified transition layer at the mixed layer base and in regions of sheared flow on the flanks of eddies, filaments, and fronts near Monterey Bay, California. On an earlier cruise in August (cruise 1), no thin layers were found after a prolonged wind relaxation. Chlorophyll concentrations and shear were both an order of magnitude less than on cruises 2 and 3. Our vertical profiles were made using a fluorometer mounted on a conductivity–temperature–depth package, which was lowered from the ship as slowly as 0.25ms-1 every 10 km on five ∼80-km∼80-km cross-shore transects. Remotely sensed sea-surface temperature, chlorophyll, and currents are required to understand the temporal and spatial evolution of the circulation and to interpret the quasi-synoptic in situ   data. Decorrelation scales are ∼20km from lagged temperature and salinity covariances. Objectively mapped sections of the in situ   data indicate the waters containing thin layers were recently upwelled at either the Point Sur or Point Año Nuevo upwelling centers. Spatially limited distributions of phytoplankton at the coastal upwelling centers (∼40km alongshore, 20 km cross-shore, and 30 m thick) were transformed into thin layers by current shear and isolated from wind-driven vertical mixing by the stratification maximum of the transition layer. Vertically sheared horizontal currents on the flanks of the eddies, filaments, and fronts horizontally stretched and vertically thinned phytoplankton distributions. These thin, elongated structures were then observed as thin layers of phytoplankton in vertical fluorescence profiles at four stations on cruise 2 and eight stations on cruise 3. Light winds during relaxations did not mix away these thin layers. On cruise 2, thin layers were found at eddies at the inshore and offshore ends of a 100-km-long filament, while broader subsurface chlorophyll maxima were found along the filament. This result suggests that higher-resolution sampling along and across a filament may find thin layers forming and dissipating along its length. On cruise 3, thin layers were found at three adjacent stations across an upwelling front and may have extended continuously for >20km, but neither species composition nor bio-optical data are available to confirm this conjecture. The thin layers were 1–5 m thick in the vertical at full width half maximum and had peak fluorescence values from 7–30mgm-3. (Bottle chlorophyll samples showed fluorometer chlorophyll readings may have been 1.3–1.5×too1.3–1.5×too large, but the scatter in this relation was considerable especially at the larger fluorescence values detected in thin layers.) While sheared currents thinned an initially thick subsurface chlorophyll maximum into thin layers, the peak values in these thin layers exceeded concentrations in the upwelled source waters and were unexplained by our data.