Effects of aquatic plant management on stream metabolism and oxygen balance in streams

1. In unshaded, nutrient-rich streams, prolific growth of stream macrophyte s often results in flows that over-top the banks and in high primary produc tion and respiration that may result in extreme diel variations in dissolve d oxygen. Consequently, water protection authorities commonly remove macrop hytes periodically. 2. We investigated the effect of plant removal on stream metabolism and oxy gen balance in two Swiss streams with a high macrophyte biomass. We monitor ed the concentration of dissolved oxygen before and after macrophytes were removed by cutting and dredging, and calculated rates of gross primary prod uction and ecosystem respiration by means of diel oxygen curves. 3. The removal of plants, which had reached a dry biomass of 320-420 g m(-2 ) immediately before plant removal, had a different impact on stream metabo lism in the two streams. In the first (plants removed in May), neither prim ary production nor ecosystem respiration were significantly affected. in th e second (plants removed in late July), gross primary production and ecosys tem respiration were reduced by about 70%. In this latter stream gross prim ary production increased in the first 2 weeks after plant removal but never recovered to pre-disturbance levels. . The removal of plants coincided with only a moderate increase in nocturna l oxygen concentration (+1 mg L-1). This, and the rapid partial recovery of stream metabolism in the second stream, suggests that an increase in the o xygen concentration after plant cutting is transient in unshaded, nutrient- rich streams.