Ecosystem metabolism in piedmont streams: Reach geomorphology modulates the influence of riparian vegetation

We measured the impact of riparian zone vegetation on ecosystem metabolism in paired forested and meadow reaches on 13 streams in southeastern Pennsylvania and Maryland, USA. Metabolism estimates were based on open-system measurements of dissolved oxygen changes, with reaeration determined from propane evasion. Daily gross primary productivity (GPP) in meadow and forested reaches averaged 2.85 and 0.86 g O-2 m(-2) d(-1), respectively, at water temperatures of 12 degrees C or greater when the forest canopy was developed and 1.74 and 1.09 g O-2 m(-2) d(-1), respectively, at temperatures below 12 degrees C when the canopy was bare. Community respiration (CR24) also was greater in meadow reaches than in forested reaches, averaging 5.58 and 3.57 g O-2 m(-2) d(-1), respectively, in the warm season and 4.87 and 2.88 g O-2 m(-2) d(-1), respectively, during the cold season. Thus, both meadow and forested reaches were heterotrophic. Forested reaches were always wider and nearly always shallower than companion meadow reaches. When ecosystem function was assessed per unit of stream length, the difference in average GPP between meadow and forested reaches was reduced from three-fold to 1.9-fold in the warm season, and mean GPP was greater in the forested reaches during the cold season. Mean CR24 per meter stream length was greater in forested reaches during both seasons. Even though riparian shading reduced primary productivity per unit area of streambed, the greater stream width of the forested reaches counteracted that reduction in part. Thus, when rates of ecosystem function were expressed per length of stream, differences between reaches were always smaller than when expressed per area, and activity per unit stream length was sometimes greater in forested reaches than in meadow reaches.