MICROBIAL-PRODUCTION, ENZYME-ACTIVITY, AND CARBON TURNOVER IN SURFACESEDIMENTS OF THE HUDSON RIVER ESTUARY

The detrital food web is a major nexus of energy flow in nearly all aq uatic ecosystems. Energy enters this nexus by microbial assimilation o f detrital carbon. To link microbiological variables with ecosystem pr ocess, it is necessary to understand the regulatory hierarchy that con trols the distribution of microbial biomass and activity. Toward that goal, we investigated variability in microbial abundance and activitie s within the tidal freshwater estuary of the Hudson River. Surface sed iments were collected from four contrasting sites: a mid-channel shoal , two types of wetlands, and a tributary confluence. These samples, co llected in June to August 1992, were sorted into two to four size frac tions, depending on the particle size distribution at each site. Each fraction was analyzed for bacterial biomass (by acridine orange direct counting), bacterial production (by H-3-thymidine incorporation into DNA), fungal biomass (by ergosterol extraction), fungal production (by biomass accrual), and the potential activities of seven extracellular enzymes involved in the degradation of detrital structural molecules. Decomposition rates for particulate organic carbon (POC) were estimat ed from a statistical model relating mass loss rates to endocellulase activity. Within samples, bacterial biomass and productivity were nega tively correlated with particle size: Standing stocks and rates in the <63-mu m class were roughly twofold greater than in the >4-mm class. Conversely, fungal biomass was positively correlated with particle siz e, with standing stocks in the largest size class more than 10X greate r than in the smallest. Extracellular enzyme activities also differed significantly among size classes, with high carbohydrase activities as sociated with the largest particles, while oxidative activities predom inated in the smallest size classes. Among sites, the mid-channel sedi ments had the lowest POC standing stock (2% of sediment dry mass) and longest turnover time (approximately 1.7 years), with bacterial produc tivity approximately equal to fungal (56 vs. 46 mu g C per gram POC pe r day, respectively). In the Typha wetland, POC standing stock was hig h (10%); turnover time was about 0.3 years; and 90% of the microbial p roductivity was fungal (670 vs. 84 mu g C per gram POC per day). The o ther two sites, a Trapa wetland and a tributary confluence, showed int ermediate values for microbial productivity and POC turnover Differenc es among sites were described by regression models that related the di stribution of microbial biomass (r(2) = 0.98) and productivity (r(2) = 0.81) to particle size and carbon quality. These factors also determi ned POC decomposition rates. Net microbial production efficiency (prod uction rate/decomposition rate) averaged 10.6%, suggesting that the se diments were exporting large quantities of unassimilated dissolved org anic carbon into the water column. Our results suggest that studies of carbon processing in large systems, like the Hudson River estuary, ca n be facilitated by regression models that relate microbial dynamics t o more readily measured parameters.