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AME 16:109-118 (1998)
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Abstract
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Influence of the benthos on growth of planktonic estuarine bacteria
Charles S. Hopkinson Jr1,*, Anne E. Giblin1, Robert H. Garritt1, Jane Tucker1, Meredith A. J. Hullar2
1The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA
2Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, USA
*E-mail: chopkins@lupine.mbl.edu
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ABSTRACT: Estuarine planktonic bacteria were incubated (bacterial bioassays) in water previously held in close contact with benthic sediments to assess the potential importance of benthic dissolved organic matter for planktonic bacterial production. The
study was conducted using water and sediments collected from along a fresh to salt water gradient in the Parker River and Plum Island Sound Estuary in northern Massachusetts, USA. Initial concentrations of dissolved inorganic nutrients were elevated
relative to ambient estuarine water while initial concentrations of dissolved organic carbon were higher during a June bioassay and lower during a December bioassay relative to ambient water. Specific growth rates of planktonic bacteria were calculated
from changes in cell numbers during the exponential growth phase. Growth rates were very high, ranging from about 3.6 to 6.5 d-1. Rates at individual sites were generally higher in June than in December and higher in low salinity waters than in
high salinity waters. These rates are substantially higher than bacterial growth rates typically found in the water column of Plum Island Sound (1.5 to 65x, depending on method) but had the same spatial pattern as is commonly found in the estuary. Growth
rates were comparable to those observed during experimental additions of marsh macrophyte leachate to incubations. Growth rates were linearly related to initial dissolved organic carbon (DOC) concentration in the bioassays, suggesting that substrate may
limit bacterial growth. Stable isotopic compositions (d13C and d15N) of microbes grown in the bioassay indicated utilization of benthic derived DOC even during times when the net
flux of DOC was from overlying water into sediments. Further, the d15N of bioassay microbes suggested that microbes utilized benthic derived inorganic N to supplement an organic diet deficient in organic nitrogen. It
appears that growth rates were elevated as a result of dissolved organic matter exchange with bottom sediments. These results suggest that there were gross fluxes of organic matter across the sediment-water interface that were not apparent from net
changes in DOC concentration in the overlying water pool. This study indicates that benthic systems must be seen not only as sites of inorganic nutrient remineralization important in support of planktonic primary producers, but also as sites of dissolved
organic matter generation and nutrient remineralization important in support of bacterioplankton production.
KEY WORDS: Bacteria · Bacterial growth · Dissolved organic matter · DOC · DON · Estuarine sediments
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Published in AME Vol.
16, No. 2
(1998) on November 27
ISSN: 0948-3055.
Copyright © Inter-Research, Oldendorf/Luhe, 1998
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