Dynamics of heterotrophic bactena and phytoplankton m the Delaware Estuary (USA) were studied over 3 yr along a sampling transect that enconlpassed the enbre sahnity gradient (0 to 32%) Bactenal abundance and chlorophyll a concentration were only weakly correlated (r = 0 17) Area1 bactenal production (from 3H-thymidme and '4C-leucine incorporalon rates) covaned w t h phytoplankton production in all geographic regions of the estuary (r = 0 ?0), although phytoplankton production explained less of the variance in bactenal production upstream (12"C) Over the entire estuary, the ratio of annual bactenal production to phytoplankton production was 0 4 In contrast, in the upper estuary bactenal production exceeded phytoplankton production by 1 4-fold, suggesting that allochthonous sources of organic matter are more important than phytoplankton in supporting bactenal growth in t h s region Dunng 5 yr of study, the summer average (May to September) of bactenal abundance and production vaned 2-fold, whlch could largely be explained by the summer phytoplankton production Even though bacteria and phytoplankton are uncoupled during the change of seasons, year-to-year vanation in bactena is apparently controlled by pnmary production and the fraction of primary production processed by heterotrophlc bactena is relatively constant

The thymidine and leucine methods were examined and used to estimate bacterial production in the Delaware Estuary. During growth experiments that minimized grazing on bacteria, conversion factors for both thymidine and leucine were initially high and then rapidly decreased to values lower than commonly-used factors (2.0 X 10" cells mol-' for thymidine). The low thymidine conversion factors may have been due to l3~]thymidine incorporation into protein, which was 72 % of total incorporation in untreated samples from the estuary. Addition of glucose reduced the leucine conversion factor from 7.4 to 3.6 X 1016 cells mol-' and the thymidine conversion factor from 1.53 to 0.68 X 10'' cells mol-'. Ammonium additions had no effect. The thymidine and leucine approaches gave similar estimates of bacterial production in the Delaware Bay (20 to 70 X 106 cells I-' h-'). Both methods confirmed that bacterial production was highest at about 40 to 50 km upstream of the mouth, which coincides with the peak in primary production. Bacterial production was about 30 % of primary production in most regions of the estuary, but increased to over 100 O/ O in the turbidity maximum where primary production was low. Bacterial production in the Delaware Estuary is apparently controlled by phytoplankton production in spite of large allochthonous sources of dissolved organic matter.

Bacterial n~acromolecules have often been used successfully as estimates of bacterial biomass in environmental samples. Less is known about the relationship between macromolecular content and rates of growth and activity. The ratio of RNA to DNA in bacteria has long been thought to be proportional to levels of metabolic activity and growth. We have used the nucleic acid fluorochrome thiazole orange combined with specific nuclease digestions to determine RNA:DNA ratios and compared these data to more standard measures of bacterial growth and activity. More than 100 samples from a wide variety of estuarine and marine environments were examined and the average RNA, DNA, and protein content per bacterial cell was found to be 9.44 + 6.25 fg, 5.75 * 2.35 fg, and 29.29 i 11.57 fg, respectively. Initial experiments demonstrated a strong correlation of RNA:DNA ratios with growth rates for a laboratory organism. We then compared RNA:DNA ratios to cell specific rates of 3H-thymidine and '"C-leucine incorporation as estimates of growth rates for naturally occurring bacterioplankton communities in whole water and the <0.8 pm size fraction in estuarine and coastal waters in the Northern Gulf of Mexico. Nucleic acid ratios were generally very low in the natural communities, usually below those determined for late stationary phase pure cultures. No significant or consistent relationship was observed between RNA:DNA ratios and rates of 3H-thymidine and I4C-leucine Incorporation in naturally occurring bacterioplankton comn~unities. No relationship could be ascertained whether compared against location or water temperature. Thymidine incorporation rates were found to be completely independent of cellular protein content w h~l e leucine incorporation rates were more related to protein content. Our data suggest that while RNA, DNA, and protein content are strongly correlated with bacterial numbers in our environmental samples, the low metabolic activity and the heterogeneous composition of bacterioplankton communities may preclude the use of these parameters as biochemical indicators of activity in the environment.

Variation in the distribution of stable nitrogen isotopes among different size classes of particulate organic nitrogen (PON) in aquatic environments may be partly explained by isotope effects associated with regeneration of NH,* by the microbial food web. Protists fed the marine bacterium Vibno natl-ieg~ns were grown in batch and continuous culture to define the isotope discrimination between NH,', microbial biornass, and dissolved organic nitrogen (DON) for culture systems closed or open to exogenous nutrient inputs. In batch cultures of a flagellate Pseudobodo sp. and a scut~cocil~ate Uronema sp., the nitrogen isotope discriminat~on between b~omass and NII.,' was 3 to 5 "6, during exponential growth of protists corresponding to the highest rates of b~omass-specific NH,' release The 6"N of D O N , calculated by mass balance, was very depleted in 15N relative to NIi,' and biomass dunng exponential and early stationary growth when D O N and dissol\ied free amino acid (DFAA) concentrations increased. In contrast to batch culture results, for continuous culture of the flagellate the Isotope discrimination between biomass and NH,' and the change in the calculated 6I5N of DON were small. 1 to 2 X,. Based on rates of thymidine incorporation and turnover of DFAA, protist excretion and bacterial uptake of DON were less coupled for exponentially growing protists fed starved bacteria in batch cultures than for the flagellate fed active bacteria growing in a 2-stage continuous culture. Coupling between release and utilization of DON, as well as isotope effects associated with protist metabolism, are proposed to constrain the nitrogen isotope dynamics of NH,', suspended PON, and DON within marine ecosystems, especially those environments dominated by regenerated primary production.

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