Bacterial abundance and production and community respiration were measured at several mainstem and tributary stations

Bacterial abundance and production (thymidine and leucine incorporation) were measured along a salinity gradient from the Mississippi Rlver (0 %o) to the open waters of the Gulf of Mexico (36 %a) during July-August 1990 and February 1991. Bactenal production in surface waters was maximal at intermediate salinities (15 to 30 %o). Nutrlent enrichment experiments suggested that bacterial growth near the outflow of the river was C limited whereas bacteria in plume waters of intermediate salinities were P and N limited. Rates of plankton community oxygen demand measured during winter were also maximal at intermediate salinities indicating an area of increased heterotrophic activity. The oxygen demand associated with heterotrophic bacterioplankton activity during summer was an important factor leading to hypoxic conditions in bottom waters of the Louisiana continental shelf. In summer, bacterial abundance and production ranged from 0.25 to 3.34 X log cells 1.' and from 4 to 90 pg C 1-' d-l, respectively. In winter, the corresponding ranges were 0.36 to 1.09 X log cells I-' and 3 to 20 pg C 1-' d-' Depth-integrated bacterial production on the Louisiana shelf decreased from 443 k 144 mg C m-2 d-' in summer to 226 f 124 mg C m-2 d-' in winter. Using empirically-derived bacterial growth efficiency values of 19 and 29 %, we estimated that bacterial production in summer could be supported by 10 to 58 % of phytoplankton production. In winter, the amount of carbon needed to support bacterial production exceeded phytoplankton production suggesting that bacterial growth during this season was heavily dependent on riverine sources of organic matter.

We tested whether natural assemblages of marine bactenoplankton undergo periods when rates of macromolecular syntheses are uncoupled (unbalanced growth). In seawater cultures of bacteria, rates of DNA and protein syntheses (thymidine and leucine incorporation) and changes in the DNA amount and cell size were compared to fluctuations in growth rate. Rates of DNA and protein syntheses became uncoupled when the bacterial assemblage shifted between growth rates. During these periods of unbalanced growth, rates of bacterial protein synthesis changed faster than rates of DNA synthesis. Variations in the DNA concentration and size of cells paralleled changes in rates of DNA and protein syntheses. The C : DNA ratio of bacteria varied 2-fold depending on the growth state with an average value of 4.6 -t 0.48. The delay between unbalanced growth and shifts in the growth rate was on the order of hours and was always shorter than the generation time. Information about unbalanced growth may reveal the delay between fluctuations in the environment and the corresponding bacterial response. In addition, the unbalanced growth model may explain why rates of thymidine and leucine incorporation occasionally do not covary in pelagic ecosystems.

We examined the simultaneous incorporation of [3H]thymidine and ['4C]leucine to obtain two independent indices of bacterial production (DNA and protein syntheses) in a single incubation. Incorporation rates of leucine estimated by the dual-label method were generally higher than those obtained by the single-label method, but the differences were small (dual/single = 1.1 + 0.2 [mean ± standard deviation]) and were probably due to the presence of labeled leucyl-tRNA in the cold trichloroacetic acid-insoluble fraction. There were no significant differences in thymidine incorporation between dual-and single-label incubations (dual/ single = 1.03 ± 0.13). Addition of the two substrates in relatively large amounts (25 nM) did not apparently increase bacterial activity during short incubations (<5 h). With the dual-label method we found that thymidine and leucine incorporation rates covaried over depth profiles of the Chesapeake Bay. Estimates of bacterial production based on thymidine and leucine differed by less than 25%. Although the need for appropriate conversion factors has not been eliminated, the dual-label approach can be used to examine the variation in bacterial production while ensuring that the observed variation in incorporation rates is due to real changes in bacterial production rather than changes in conversion factors or introduction of other artifacts.

The significance of bacterioplankton in the flow of carbon and energy and in trophic dynamics of the upper Laguna Madre, Texas (USA), was estimated by measuring bacterioplankton abundance and production over an 18 mo period and over several diel cycles. Bacterioplankton production was estimated from incorporation rates of thymidine (DNA synthesis) and leucine (protein synthesis). These independent inhces of bacterial growth were generally in agreement and yielded nearly identical annual estimates of bacterial production (25.24 g C m-2 yr-' based on thymidine and 25.12 g C m-' yr-I based on leucine). Assuming a 30 % growth efficiency, the annual bacterioplankton growth could be supported by 15 % of the total primary production (seagrasses and phytoplankton), 17 % of the above-ground production of the dominant seagrass, Halodule wrigh~, or 103 % of the phytoplankton production. Bactenal abundance was high throughout the year, often exceeding 1 X 10'' cells I-'. Bacterioplankton production varied seasonally and over the diel cycle, with maximal values during warmer months and dunng daytime. Although changes in water temperature could account for some of this variation, shifts in the quantity and quality of the organic substrates supporting bacterial growth appeared to be the major factors regulating the variations in bacterioplankton production. Bacterioplankton in the Laguna Madre are a large and rapidly growing source of biomass potentially available for higher trophic levels. If this biomass is efficiently used by grazers, bacteria may be a major 'link' between seagrass production and secondary producers in the Laguna Madre ecosystem.

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Identifying the source of polychlorinated biphenyls (PCBs) is the first step to eliminating contamination within anadromous salmonids and other aquatic species. We evaluated PCB concentrations in the muscle tissue of 4-year old Puget Sound and coastal hatchery Chinook salmon in an effort to identify potential locations where contamination occurs. Ten muscle samples were taken from each of two Puget Sound hatcheries and two coastal hatcheries in Washington State to determine PCB concentrations. Two technical mixtures of PCBs, Aroclor 1254 and Aroclor 1260, were measured in the tissue samples collected. Aroclor 1254 was detected in all samples, while Aroclor 1260 was detected in 16 of 40 samples. Generalized linear modeling (GLM) was used to evaluate the influence of region (Puget Sound and coast), hatchery, sex, fish length, and percent lipids on PCB concentrations. Twenty different GLMs representing multiple null hypotheses were ranked using Akaike Information Criterion (AIC). Six of the 20 models evaluated had substantial support for being the best model to describe PCB concentrations in Chinook salmon. The six models with substantial support were region and lipids; region; hatchery and lipid; region and sex; region, sex, and lipids; and region and length. Region appears to be the most influential variable explaining the variation in PCB concentrations in Chinook salmon in the Pacific Northwest. PCB concentrations in Chinook salmon muscle tissue from Puget Sound hatcheries were significantly greater (mean 49.26 μg/kg wet weight and standard deviation 40.55 μg/kg) than those from coastal hatcheries (mean 17.41 μg/kg wet weight and standard deviation 6.8 μg/kg). Our results suggest that some contamination likely occurs in the Pacific Ocean. However, a larger source likely exists within Puget Sound or along the migratory route of Puget Sound Chinook salmon. The observation of PCBs in Chinook salmon also has important implications for human health and salmon carcass supplementation.