The green ribbon: Multiscale physical control of phytoplankton productivity and community structure over a narrow continental shelf

Abstract

Chlorophyll concentration, phytoplankton biomass, and total and nitrate‐fueled primary productivity increase toward the coast over the 12‐km‐wide continental shelf of the southern portion of the Southern California Bight. These gradients are accompanied by changes in phytoplankton community composition: the outer shelf is characterized by offshore assemblages including pelagophytes and oligotrophic Synechococcus ecotypes while the inner shelf is dominated by diatoms, coastal Synechococcus ecotypes, and the picoeukaryote Ostreococcus. Across the small horizontal scale of the shelf, large changes in the vertical distribution and flux of nitrate maintain elevated productivity, driving variability in the vertical distribution of biomass and the integrated biomass and productivity of the entire shelf. Temporal variability from hours to days in chlorophyll fluorescence as measured by an autonomous profiling vehicle demonstrates that phytoplankton respond vigorously and rapidly to physical variability. The interaction of physical processes at different temporal and spatial scales is responsible for the observed biological gradients. These dynamics include: (1) vertical shear in the alongshore currents, (2) local wind forcing, (3) the internal tide, and (4) remote, large‐scale variability. Individually, these mechanisms rarely or never explain the phytoplankton community composition and metabolic rate gradients. These results and a reanalysis of historical data suggest that monitoring thermal stratification at the shelf break and the tilt of the thermocline across the shelf will augment our ability to predict phytoplankton productivity, community composition, and biomass, thereby improving our understanding of fisheries dynamics and carbon cycling in the coastal ocean.