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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 (methyl- umbelliferyl (MUF)- and methylcoumarinylamide (MCA)-labeled). Potential hydrolysis rate con- stants 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 abun- dance, 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.

Ectohydrolase activity in surface waters of the Hudson River and western Long Island Sound estuaries