Who is chlorophyll a? Phytoplankton community structure in Humboldt Bay, California

Phytoplankton community descriptions are becoming an increasingly important component in studies of oceanographic and nearshore marine ecosystems. These primary producers play a major role in the global carbon cycle and are the foundation of nutritional resources that are transferred up the food chain. The lack of a phytoplankton community description from Humboldt Bay, CA (USA) limits the understanding of primary producers within the bay and such a study would be useful to mariculture operations so that they may have a better idea of what their animals eat, including species known to form harmful algal blooms (HABs). This study seeks to describe the phytoplankton community from a long term monitoring site within Humboldt Bay along seasonal and tidal gradients and to document the occurrences of species known to produce toxins. Samples were collected from April 2007 through October 2008 nearly once per week at high and low tide. A total of 71 taxa were found in 108 samples with diatoms composing 91% of the total cell numbers (86% of total biovolume) whereas dinoflagellates made up 2% of cell numbers (17% of the biovolume). Thalassiosira, Odontella and Noctiluca composed 70% of the total biovolume for the entire sampling period and the top 15 contributors accounted for over 85 % of total biovolume. More than half of the taxa accounted for less than 1% of the total biovolume. Community structure varied the most by seasonal gradients with different communities occurring in the upwelling season of 2007, the winter of 07-08 and the upwelling season of 2008. The winter had lower biovolume with a higher tychoplankton component than the upwelling season. Upwelling 2007 had lower biovolume and had a stronger dinoflagellate signal than 2008 corresponding with a transition from El Niño to La Niña conditions. The 2008 upwelling season had the highest biovolume and was primarily composed of bloom taxa such as Thalassiosira, Chaetoceros and Odontella. Community differences between high and low tide, although not as strong as seasonal differences, were driven by transport of nearshore oceanic taxa on the incoming tide and resuspension of benthic and epiphytic diatoms on the outgoing tide. Although major HAB events did not occur locally during the sampling period, Alexandrium and Pseudo-nitzschia abundance paralleled their respective toxin concentrations from mussel samples within the bay.