Marine Biology Program
Biology Department
FIU Homepage
Graduate Student Position available

Marine Biology Links
Boat Operations
Boat Schedule
Truck Schedule

Marine Biology Program
Florida International University
3000 NE 151 Street
North Miami, FL 33181; USA
Phone: (305) 919 5882
Fax: (305) 919 5896

Office: AC-I 379
Lab: AC-II 350
Lab phone: (305) 919 4226

No office hours offered this semester

Photo- and heterotrophic pico- and nanoplankton in the Mississippi River plume: distribution and grazing activity
Frank J. Jochem

The abundance of pico- and nanophytoplankton, bacteria and heterotrophic nanoflagellates, and grazing rates on phototrophic pico- and nanoplankton and bacterioplankton were assessed along a salinity gradient (0.2–34.4) in the Mississippi River plume in May 2000. Grazing rates were established by serial dilution experiments, and analysis by flow cytometry allowed differentiation of grazing rates for different phytoplankton subpopulations (eukaryotes, Synechococcus spp., Prochlorococcus spp.). Grazing rates on phytoplankton tended to increase along the salinity gradient and often approached or exceeded 1 day–1. Phytoplankton net growth rates (growth – grazing) were mostly negative, except for positive values for eukaryotic nanoplankton in the low-salinity, high chlorophyll region. Grazing pressure on bacteria was moderate (~0.5 day–1) and bacteria gained positive net growth rates of ~0.3 day–1. Eukaryotic nanophytoplankton were the major phototrophic biomass and protozoan food source, contributing 30–80% of the total consumed carbon. Bacteria were the second most important food source at 9–48% of the total consumed carbon. Synechococcus spp. and Prochlorococcus spp. remained an insignificant portion of protozoan carbon consumption, probably due to their low contribution to the total pico- and nanoplankton biomass. Group-specific grazing losses relative to standing stocks suggest protozoan prey preference for eukaryotes over bacteria. Protozoan grazers exerted a major grazing pressure on pico- and nanophytoplankton, but less so on bacteria.

Published in: Journal of Plankton Research 25: 1201-1214; 2003

PDF LogoClick to download PDF reprint (362 kB)

Growth and grazing rates of bacteria groups with different apparent DNA content in the Gulf of Mexico
Frank J. Jochem, Peter J. Lavrentyev, Matthew R. First

Growth rates and grazing losses of bacterioplankton were assessed by serial dilution experiments in surface waters in the Mississippi River plume, the northern Gulf of Mexico, a Texas coastal lagoon (Laguna Madre), southeast Gulf of Mexico surface water, and the chlorophyll subsurface maximum layer in the southeast Gulf of Mexico. Bacteria were quantified by flow cytometry after DNA staining with SYBR Green, which allowed for discrimination of growth and grazing rates of four bacteria subpopulations distinguished by their apparent DNA content and cell size (light scatter signal). Total bacteria growth rates (0.2–0.9 day-1) were mostly balanced by grazing losses, resulting in net growth rates of -0.18 to 0.45 day-1. Growth rates of DNA subpopulations varied within experiments, sometimes substantially. In most, but not all, experiments, the largest bacteria with highest DNA content exhibited the highest growth rates, but a relationship between DNA content and growth rates or grazing losses was absent. Small bacteria with the lowest DNA content showed positive growth rates in most experiments, sometimes higher than growth rates of bacteria containing more DNA, and were grazed upon actively. Low-DNA bacteria were not inactive and were an integral part of the microbial food web.

Published in: Marine Biology 145: 1213-1225; 2004

PDF LogoClick to download PDF reprint (411 kB)