Award: OCE-1260134

Intellectual Merit: Phytoplankton, traditionally viewed as primary producers at the base of aquatic food webs, provide an energy source for higher trophic levels. However, some phytoplankton species function as both primary producers and heterotrophic secondary consumers. Unfortunately, we currently have few estimates of the proportion of the phytoplankton community that function as "facultative" secondary consumers, their rate of secondary production, or their temporal variation in abundance. Current paradigms about trophodynamics in marine systems do not consider this potentially important "alternative" pathway of energy flow for phytoplankton. The implication is that we may be missing a significant, fundamental process that affects carbon cycling and trophodynamics in estuarine systems. Furthermore, changes in the DOC composition due to anthropogenic alterations may result in changes in phytoplankton community structure and possibly promote the proliferation of harmful algal bloom species. In terms of ecosystem function, even moderate rates of photomixotrophy could potentially alter our current understanding of phytoplankton productivity, overall C turnover, competitive interactions, and energy transfer in estuarine environments. This research used a novel approach to provide quantitative measures of the in situ rates and magnitudes of "facultative heterotrophy" in natural, estuarine phytoplankton communities over seasonal time scales in a representative estuarine ecosystem. Our results indicate that addition of labile carbon (glucose) to natural communities results in heterotrophic growth by phytoplankton, but also a reduction in the rate of chl a synthesis. Cell numbers appear to increase with a corresponding reduction in the amount of chl a per cell. Broader Impacts: This project has provided support for a Ph.D. graduate student, 2 M.Sci. graduate students, summer support for 4 undergraduate assistants, and involved upper level undergraduates as lab interns. Undergraduate interns, recruited from the Marine Science Program at USC, participated in sample analyses and learned to operate laboratory instrumentation. Data are archived in the BCO-DMO Database at http://www.bco-dmo.org/project/553534. Last Modified: 04/04/2017 Submitted by: James L Pinckney