Controls on temporal patterns in phytoplankton community structure in the Santa Barbara Channel, California

Abstract

Characterizing phytoplankton succession in the context of physical and chemical processes is important for understanding the mechanisms driving phytoplankton species composition and succession. An understanding of these processes ultimately influences the ability to predict the contribution of phytoplankton to carbon cycling, the initiation and persistence of harmful algal blooms, and the ability to use satellites for the remote sensing of specific phytoplankton taxa important for biogeochemistry. A statistical analysis of 5 years (1998−2003) of phytoplankton pigment concentrations from the Santa Barbara Channel using empirical orthogonal functions reveals four dominant modes of variability that explain 80% of the variance in the pigment data set. The annual cycle is characterized by a switching from a mixed‐phytoplankton assemblage mode to modes dominated by either diatoms, dinoflagellates, or a combination of nano‐ and pico‐phytoplankton. The dominant two modes correspond to a prebloom condition that precedes upwelling conditions, with all identified phytoplankton groups present in low abundance and a diatom‐dominated upwelling state that develops following spring upwelling. In 2001, the EOF analysis indicated a transition toward more intense diatom blooms in spring and summer and fewer, large dinoflagellate blooms. This trend was corroborated by analyses of diagnostic pigments and CHEMTAX analysis and may be linked to an increase in local upwelling intensity between 2001 and 2003. Both spring diatom blooms occurring after 2001 were dominated by toxicPseudo‐nitzschiaspecies and led to significant marine mammal deaths in the channel in 2003.