Variability in hydrolytic ectoenzyme activity was investigated at 3 stations along the salinity gradient of the Hudson River estuary and at 1 station in western Long Island Sound between October 1996 and October 1998. Activities of aminopeptidase, β-glucosidase, chitinase, lipase and alkaline phosphatase were estimated at each station using fluorogenic substrate analogs (methylumbelliferyl [MUF]-and methylcoumarinylamide [MCA]-labeled). Potential hydrolysis rate constants for these ectohydrolases varied widely over time and among stations; from < 0.01 to 90% d -1 . Most rate constants were highest in July and August. However, temperature only weakly correlated with activities of 4 of the 5 ectohydrolases in the Hudson River (r = 0.35 -0.58, x = 0.42) and was not significantly correlated with activity of any ectohydrolase in Long Island Sound. Activities of most ectohydrolases varied independently of dissolved and particulate organic carbon, bacterial abundance, chlorophyll a and primary production. Aminopeptidase activity was strongly correlated with bacterial production (r = 0.78-0.89; p < 0.0001; n = 46) and negatively with total inorganic nitrogen concentration (r = -0.57; p < 0.0001; n = 47) among all stations. However, each station had distinct enzyme activity profiles, suggesting acclimation of bacterial communities to different nutritional regimes. For example, alkaline phosphatase and aminopeptidase activity dominated the oligohaline station, while at the marine station, activity was more evenly divided among all ectohydrolases but lipase. Activities of aminopeptidase, β-glucosidase and chitinase were highly correlated with turnover of 3 H-leucine, 14 C-glucose and 14 C-acetate, suggesting a close coupling between hydrolysis of polymeric source materials and consumption of a variety of monomeric products.

We examined variations in organic carbon (OC) pools and microplanktonic carbon fluxes at three stations in the Hudson River estuary during 12 cruises between Octobers of 1996 and 1998. Phytoplankton biomass and net primary production varied from 5 to 40 mol C L Ϫ1 and 0.3 to 318.3 mmol C m Ϫ2 d Ϫ1 , respectively. Biomass and production of bacterioplankton in the surface layer commonly exceeded those of phytoplankton, varying from 0.2 to 72 mol C L Ϫ1 and 1.4 to 70 mmol C m Ϫ2 d Ϫ1, respectively. Median planktonic respiration varied from 275 to 605 mmol CO 2 m Ϫ2 d Ϫ1 between stations along the salinity gradient. Primary production/respiration (P : R) ratios varied temporally and spatially between 0.003 and 6.60, averaging 0.22. Carbon mass balances revealed that under low river discharges (Ͻ250 m 3 s Ϫ1 ), the estuary processed 2.4-fold more carbon internally and advected 2.7-fold less carbon seaward than under higher flows. Annual OC budgets suggest that ϳ19 ϫ 10 9 mol C yr Ϫ1 of total organic carbon (TOC) entered the estuary, whereas 30 ϫ 10 9 mol TOC yr Ϫ1 was exported seaward, representing a net gain of 11 ϫ 10 9 mol TOC yr Ϫ1 within the estuary. Microplankton processed ϳ32 ϫ 10 9 mol C yr Ϫ1, of which 5.6 ϫ 10 9 mol C was attributed to photo-and chemoautotrophic (ϳ72%) and heterotrophic (ϳ28%) production; the remainder was respired to CO 2 (26.5 ϫ 10 9 mol CO 2 yr Ϫ1