Hydrogeochemical controls on the organic matter and bacterial ecology of a small freshwater wetland in the New Jersey Pine Barrens

This study investigated the effects of variable ground-water/surface-water exchange and photoinduced processes on longitudinal patterns in dissolved organic matter (DOM) and bacterial communities in a small first-order stream in the New Jersey Pine Barrens. DOM concentration, along with DOM weight average molecular weight (Mw) and absorptivity ( 280, an estimator of aromaticity), and bacterial cell counts all decreased from the stream and hyporheic zone into the shallow aquifer in a ground-water recharge zone. Further downstream, influx of ground water into the stream resulted in a lower Mw DOM pool and was accompanied by decreased cell counts. The observed effect of this ground-water discharge on bacterial numbers may be direct, if discharge temporarily dilutes cell counts, or indirect, if changes in DOM concentration and properties control the bacterial community. In either case, this study suggested the importance of considering ground-water–surface-water exchange in studies of longitudinal changes in the bacterial communities of streams.