Abiotic controls of potentially harmful algal blooms in Santa Monica Bay, California

Despite the increasing occurrence of harmful phytoplankton blooms along the North American west coast, records of phytoplankton populations and related environmental conditions are uncommon. In this study, twice monthly measurements in the upper 50m are used to assess physico-chemical conditions contributing to the growth of potentially harmful bloom taxa over two annual cycles (2004-2005) in the Santa Monica Bay, California. Results were compared to the predictions of the Intaglio model [Smayda, T.J., Reynolds, C.S., 2001. Community assembly in marine phytoplankton: application of recent models to harmful dinoflagellate blooms. journal of Plankton Research 23, 447-461.] of phytoplankton community assembly. Potentially harmful taxa were present in every surface sample and were numerically dominant during the largest observed blooms, contributing up to 92% of the total phytoplankton abundance >5 mu m. Large interannual variation was observed in the dominant taxa and bloom seasonality; Pseudo-nitzschia sp. dominated blooms in early 2004 (February and April), whereas Prorocentrum micans and Lingulodinium polyedrum blooms occurred in May and September of 2005, respectively. The Pseudo-nitzschia sp. blooms were associated with elevated nitrate, dissolved silicon and phosphate concentrations throughout the euphotic zone; the first bloom followed a strong upwelling and the second occurred during the onset of seasonal stratification. In contrast, the blooms of P. micans were associated with highly stratified, low nutrient waters. Multivariate analysis supports the roles of temperature, mixed-layer depth and nutrient concentrations as primary controls of bloom growth, following the conceptual Intaglio model. The strong presence of potentially harmful bloom species in the Santa Monica Bay during this study appears unusual in comparison to limited studies over the last several decades. (C) 2008 Elsevier Ltd. All rights reserved.