Phytoplankton and microbial abundance and bloom dynamics in the upwelling shadow of Monterey Bay, California, from 2006 to 2013

Wind-driven upwelling variability and local topography cause an upwelling shadow in the northern region of Monterey Bay, California, to persist seasonally. The present study applied partial least squares regression to a 7-yr time series collected within this retentive feature for the purpose of evaluating the environmental controls on total autotrophic phytoplankton (as chlorophyll a) and picoplankton (Synechococcus spp., picoeukaryotes, and heterotrophic bacteria) abundance. A bloom threshold was defined and applied to all biological groups to evaluate seasonal and inter-annual abundance patterns. Microbial and phytoplankton abundances in the upwelling shadow were positively associated with warmer, nutrient-depleted water. Consistent with these results, two-thirds of phytoplankton blooms occurred in October-November, when surface temperatures were warm and ammonium concentrations were greatest. These blooms were predominantly composed of dinoflagellates, 64% of which were known toxin-producing species. Although the overall relationship of phytoplankton to river discharge rates was negative, phytoplankton blooms in 2006, 2007, 2010, and 2012 followed early rainfall events, which flush nitrogen from the surrounding farms into the bay. Despite the fact that the regional measure of upwelling, the Bakun upwelling index, is seasonally low in the autumn, pulses of cold, nutrientreplete water were advected into the upwelling shadow, additionally supporting late-year blooms. Physical and chemical processes occurring over multiple time scales controlled bloom dynamics in the upwelling shadow of Monterey Bay.