## LSU Historical Dissertations and Theses

1991

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Oceanography and Coastal Sciences

R. Eugene Turner

#### Abstract

Observations and experiments were conducted to evaluate methodologies for measuring aquatic bacterial population dynamics and grazing activity in ecologically dissimilar ecosystems. Samples were taken from Auke Bay, AK, the Mississippi River at Plaquemine, LA, the Terrebonne Bay estuary, LA, and the Mississippi River plume. Four methods were compared for measuring bacterioplankton growth and grazing mortality rates: dilution, filtration, antibiotic, and the Servais et al. (1985) procedures. The dilution method appeared to be the best among those compared. Among all sites, bacterial density ranged from 0.02 to 10$\sp6$ ml$\sp{-1}$, and bacterial growth and grazing mortality rates, estimated using the dilution method, ranged from $-$0.016 to 0.11 h$\sp{-1}$ and 0.012 h$\sp{-1}$ to 0.134 h$\sp{-1}$, respectively. The number of bacteria grown and grazed in all study locations were nearly in balance, implying that grazers can increase in magnitude equivalent to bacterial growth. Grazing is the major factor controlling bacterial density in all study areas, although at particular sites temperature and food availability also have an important role. Seasonal changes in bacterial abundance and growth rate were not a function of temperature in Auke Bay and Terrebonne Bay estuary, but were in the Mississippi River. Phytoplankton are important as a source of carbon for bacteria in Auke Bay, but perhaps not in the Lower Mississippi River. There were 8% per hour turnover of bacterial biomass (ranged of 1 to 20% per hour) among all sites. In Terrebonne Bay, each flagellate grazed between 13 to 86 (average 23) bacteria per hour. There were virtually no flagellates present when bacterioplankton densities were less than 0.5 $\times$ 10$\sp6$ ml$\sp{-1}$, suggesting that there are at least two qualitatively different bacterial population dynamics in aquatic environments: one with, and one without significant grazing by flagellates. When there is an absence of grazing, bacteria are a "sink". In the presence of grazing, bacteria may be a "sink" or a "source" ("link") of organic matter for the higher trophic levels. The value of comparisons in widely dissimilar aquatic ecosystems has been demonstrated.

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