Flow regulation by reservoirs modifies the natural hydrologic regime and can alter fish and invertebrate assemblages, but less is known about the effects of flow regulation on stream ecosystem processes. This study examined the effects of nutrients, light, and flow regulation by upstream dams on periphyton chlorophyll a, net primary productivity (NPP), and respiration. Nutrient diffusing substrates were installed in five blocks within three streams with upstream reservoirs and three unregulated forested streams in central and western Massachusetts. Glass (inorganic substrate) or cellulose (organic substrate) disks were deployed in four nutrient treatments, nitrogen (as nitrate), phosphorus, nitrogen and phosphorus, and un-enriched controls. Nutrient response ratios were calculated for chlorophyll a, NPP, and respiration on disks as the ratio of mean nitrogen and phosphorus treatment divided by the mean control treatment for each stream. I predicted that stream metabolism and periphyton chlorophyll a would be nutrient and light-limited, and that regulation by upstream dams would both alter stream metabolism and the effect of nutrient enrichment on stream metabolism.
Results/Conclusions
Respiration on cellulose disks was limited by nutrients, two streams were phosphorus limited and one was co-limited by nitrogen and phosphorus (p < 0.05). There was no evidence of nutrient limitation on respiration or NPP on glass disks. In contrast, NPP on glass disks was positively related to light (p = 0.01). The nutrient response ratio did not differ between regulated and unregulated streams for NPP (p = 0.81) or respiration (p = 0.14) on glass disks or respiration (p = 0.78) on cellulose disks. Nutrient enrichment increased respiration on cellulose disks to a similar extent in regulated (1.8x) and unregulated (1.7x) streams. Overall, light and nutrients had stronger influences on stream ecosystem metabolism in this study that the presence of upstream dams.