Patterns of phytoplankton diversity and mortality due to grazing across trophic gradients in the Southern California Current

The dilution method is a widely used technique to determine rates of community phytoplankton growth and mortality due to grazing in pelagic communities. Here, we present results of microscopy and metagenomic analysis of 16S and 18S rRNA genes from dilution experiments conducted as part of the California Current Ecosystem Long Term Ecological Research (CCE LTER) program. Samples from experiments conducted in the mixed layer and deep chlorophyll maxima were analyzed from coastal vs. offshore sites, an ephemeral frontal site, and during the anomalously warm conditions of summer 2014. Distinct patterns of community biomass (µg C L^-1) and diversity (Shannon index) with respect to sampling depth (surface versus chlorophyll max) and distance to shore (coastal versus offshore) emerged. Eukaryotic diversity was negatively correlated with chlorophyll a concentration, likely due to the dominance of a few diatom species, and was highest in spring 2007 during declining diatom bloom conditions. Relative phytoplankton growth and mortality rates estimated from metagenomic data indicate unique patterns of growth and mortality among diatoms, dinoflagellates, prymnesiophytes, and prasinophytes, which diverge from overall community rates determined through pigment analysis. Trends in the diversity, biomass, and mortality rates of plankton taxa across trophic gradients highlight the complex trophic interactions that would otherwise be hidden in overall mean rates of community growth and grazing.