Empirical data that describe the metabolic balance of stream ecosystems in human-dominated watersheds are scarce. We measured ecosystem metabolism in 23 open-canopied lowland streams draining urban and agricultural areas in the Fuji River Basin, central Japan. Gross primary production (GPP) and community respiration (CR) were estimated using the diurnal dissolved oxygen (DO) change technique, with the reaeration coeffi cient (K 2 ) determined from seven empirical depth-velocity equations. Because the predicted values of K 2 showed variation among the depth-velocity equations, the estimates of stream metabolism also varied according to the equations. However, CR was almost always greater than GPP, resulting in negative net ecosystem production (NEP) and GPP/ CR ratios below unity for most of the study reaches. Highly heterotrophic streams were found in intensively farmed watersheds, suggesting that organic matter loading from agricultural lands is likely to be a source of allochthonous carbon fueling excess respiration in the study streams. In contrast, streams draining more urbanized areas were less heterotrophic. The present results suggest that lowland streams in agriculturally developed watersheds are associated strongly with terrestrial ecosystems as a source of organic carbon. The resultant strong respiration might become the dominant process in ecosystem metabolism, as reported for headwater streams, large downstream rivers, and estuaries.

To identify the nitrate source and spatial distribution of nitrogen isotope values originated from forest and agricultural lands, fifty-seven stream water samples from Fuji River Basin were analyzed for nitrate concentration and nitrate-nitrogen (δ 15 N-NO 3-) and oxygen (δ 18 O-NO 3-) isotope values. The δ 18 O-NO 3 showed that the nitrate originated through nitrification. These values were plotted against fraction of each land use category in the study catchments. The δ 15 N-NO 3 was positively correlated with the fractions of agricultural and residential areas in the catchments. The increasing of δ 15 N-NO 3 with fraction of agricultural area converged to 5.4‰ which was similar to a reported nitrate isotope value of groundwater at orchard area in Kofu-Basin. The increasing δ 15 N-NO 3 with more than 1% residential area suggested that potential source of nitrate contamination was domestic waste water. An increase of δ 15 N-NO 3 in the catchments with less than 98% forest area suggested the sources of nitrate contamination were anthropogenic. The estimated value of δ 15 N-NO 3 in forest catchment was 0.9±1.2‰.