Phytoplankton thin layers caused by shear in frontal zones of a coastal upwelling system

Using multidisciplinary observations from regional- to small-scale, we examined the development of thin phytoplankton layers in water mass frontal zones of a coastal upwelling system. Two fronts developed successively in the same region of Monterey Bay, California, USA, during August and September 2003: (1) when warm, fresh offshore waters flowed into the bay following relaxation and reversal of upwelling favorable winds, and (2) when a cold upwelling filament flowed into the bay after upwelling favorable winds resumed. Thin phytoplankton layers were observed during the presence of both fronts. The layers exhibited peaks in chlorophyll fluorescence and optical backscattering, indicating biomass maxima in the layers. Maximum chlorophyll concentrations in the layers ranged from 11 to 37 μg l–1and were 4 to 55 times greater than background levels. Layer vertical thickness ranged from 1 to 5 m, averaging 2.3 m. All thin layers were in the thermocline, near the 12.5°C isotherm, and the depth of the layers varied between 12 and 33 m, in parallel with variations in thermocline depth. Synoptic mapping of the first frontal zone shortly before the thin layers developed showed strong phytoplankton patchiness. The role of vertical shear in thin layer formation from phytoplankton patches is supported by multiple results: (1) most thin layers (92%) were associated with sharp changes in the direction of horizontal currents; (2) layer thickness was significantly (p < 0.03) negatively correlated with shear; (3) the median shear profile, computed from all thin layer velocity profiles, peaked sharply at the center of thin layers.