Bacterioplankton dynamics in the Patuxent River: Response to nitrogen abatement

Principal Investigator:

Cynthia C. Gilmour

Start/End Year:

1994 - 1999


Academy of Natural Sciences Estuarine Research Center

Co-Principal Investigator:

Douglas G. Capone, Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science


To look for changes in heterotrophic bacterioplankton production, biomass and C cycling in the Patuxent River as point source N loadings to the river decline. To examine the limitations on pelagic bacterial production. To use the information collected from this research project in conjunction with other studies of C and nutrient cycling the Patuxent to work toward a C budget for the Patuxent. We will extend our baseline studies of N & P cycling in the river through the current period of sharp reduction of N inputs. In addition to determining changes in bulk water characteristics, we will specifically examine the relative importance of planktonic heterotrophs, relative to autotrophs, in the uptake and regeneration of N & P as a function of season and site in the river. Our efforts will be conducted directly in parallel with determinations of bacterial biomass, bacterial secondary production and heterotrophic metabolism in the river by Gilmour and Capone. Finally, we will make preliminary estimates of planktonic N2 fixation and be prepared to expand this effort should a sudden shift to diazotrophic cyanobacteria take place in the plankton populations. Upcoming upgrades to sewage treatment plants are projected to reduce N loading to the Patuxent River by about 45% over the next few years. This situation offers a unique opportunity to examine changes in the structure and function of the microbial food web in a natural, eutrophic system as a function of N loading. Our objectives are to study the changes in the absolute magnitude of bacterial production and biomass, and the magnitude relative to primary production, as DIN loading declines, and to examine the factors that specifically control (or limit) bacterioplankton growth, nutrient uptake and regeneration and how these factors change as a function of N loading. The addition of biological N removal (BNR) to sewage treatment plants on the Patuxent River in the early 1990s has provided a natural laboratory in which to study the effects of reduced N loading in a eutrophic, primarily N-limited estuary. Funded by Sea Grant since 1994, we have examined the response in bacterioplankton dynamics as N loads and N:P ratios decline in the Patuxent. Intensive measurements have been made concurrently with water quality, primary production (PP), and zooplankton measurements by Maryland DNR. Preliminary analysis supports the hypothesis that eutrophication shifts the balance of estuarine production away from PP toward bacterial production (BP). The reduction in point source N loading to the Patuxent has been reflected in about a 20% reduction in TN in the mesohaline, but no significant decline in algal growth. Unusually high spring flows in 1993, 1994 and 1996 have made trend analysis difficult.

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