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Nearshore Chlorophyll-a Events and Wave-Driven Transport

Continuous records of temperature and chlorophyll- a [chl- a] fluorescence were used to characterize phytoplankton variability at two shallow intertidal stations on the northern California coast. Chl- a records from spring and summer 2007 and 2008 were characterized by distinct peaks persisting for 1.5 to five days. These peaks represent bloom events, which often coincided between the two sites even though they are separated by ∼150 km. Blooms did not appear to be directly forced by individual upwelling episodes. While some events were associated with reversals of upwelling-favorable winds, there was a stronger relationship between chl- a peaks and peaks in surface swell height. This relationship was dominant in spring and early summer, but no longer evident by July. We suggest that these nearshore chl- a peaks are an accumulation of phytoplankton caused by convergence of onshore wave transport against the impermeable coastal boundary. Scaling arguments show this mechanism to be consistent with observed chl- a increase rates. This mechanism has not been previously considered as a forcing for blooms at rocky coasts, and it may have significant implications for understanding coastal productivity, larval dispersal, and nearshore water quality.

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