The U.S. Long Term Ecological Research (LTER) network expanded in 2017 to encompass four marine sites with a pelagic focus, complementing existing time-series maintained by NSF, NOAA, and multiple international funding agencies and enhancing our ability to document multidecadal ecosystem responses to climate variability and change across a global gradient. The four LTER sites span a range in terms of physical environment, geography, bathymetry, and terrestrial influence. Palmer LTER, on the western Antarctic Peninsula and extending 200 km offshore, is dominated by seasonal changes in sea ice cover and light availability. The California Current Ecosystem LTER is an eastern boundary upwelling regime and primarily an open ocean site with minor terrestrial inputs. The Northeast U.S. Shelf and Northern Gulf of Alaska LTER sites extend from the continental shelf into the open ocean, with significant terrestrial influences. All of the sites are associated with productive fisheries, and all are experiencing warming trends in surface air temperatures with associated impacts on mixed layer temperature and depth, and/or sea ice cover. This session will showcase ongoing studies of ecological responses at the LTER sites in terms of primary production, community composition and structure, carbon and nutrient cycling, and disturbance. We strongly encourage submissions related to these topics from other (non-LTER) national and international pelagic time-series sampling programs. Recent cross-LTER site synthesis efforts will also be highlighted.

Lead Organizer: Katherine Barbeau, Scripps Institution of Oceanography, UC San Diego (kbarbeau@ucsd.edu)

Co-organizers:

Russ Hopcroft, University of Alaska Fairbanks (rrhopcroft@alaska.edu)

Heidi Sosik, Woods Hole Oceanographic Institution (hsosik@whoi.edu)

Oscar Schofield, Rutgers University (oscar@marine.rutgers.edu)

Presentations

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06:00 PM

MEASUREMENT OF BACTERIAL PRODUCTION AND BACTERIVORY WITHIN A PLANKTONIC FOOD WEB IN A PRODUCTIVE TEMPERATE CONTINENTAL SHELF ECOSYSTEM: METHOD VALIDATION AND ECOLOGICAL IMPACT (9695)

Primary Presenter: Alexandra Sinno, Univeristy of Rhode Island Graduate School of Oceanography (alexandra_sinno@uri.edu)

Marine heterotrophic bacteria are the most abundant, diverse, and metabolically active organisms in the ocean and while abundances are routinely measured in oceanographic surveys, measurements on growth and bacterivory rates by protists are sparse. The Northeast Shelf Long-Term Ecological Research (NES-LTER) project routinely measures the growth of marine phytoplankton and grazing rate by protists and recently added bacterial growth and bacterivory measurements. To achieve this, we 1) identified the appropriate Flow Cytometer cell concentration range; 2) validated the method using dilution levels and microscopy to cross-validate; and 3) determined a reliable analysis duration with SYBR-Green DNA Stain. We found that using a bacterial concentration of < 500 cells µL^-1 avoids instrument over-saturated and subsequent underestimates of bacterial concentrations. Despite these steps, we observed a 33% underestimation of bacteria using the flow cytometer when compared to microscope counts (R²=0.999). Nonetheless, flow cytometry was more precise across replicates than microscopy (%CV Microscope=9%; %CV Guava=1%). Finally, we determined that SYBR-Green is viable for analysis up to 70 minutes after staining. Following method validation, we successfully quantified bacterial growth and bacterivory rates along the NES-LTER transect for the first time. This places these rates within the larger context of the planktonic food web and allows them to be included as routine measurements within the NES-LTER project, building on the knowledge that the NES-LTER project has cultivated.

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