Light and the annual variation of oxygen‐ and carbon‐based measurements of productivity in a macrophyte‐dominated river1

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Abstract
Daily integrals of primary productivity were measured for 369 days in the macrophyte‐dominated Gryde River (NW Jutland, Denmark) between July 1979 and September 1980. All seasons were distinct, with high primary productivity during summer, a gradual transition in autumn, low primary productivity in winter, and a rapid transition to summer conditions during April. Those seasons corresponded with the condition of the macrophytes, with high biomass and surface area during summer, a gradual decline in autumn, senescence during winter, and development of new shoots during April and May. The remarkably constant relationship between daily integrals of primary productivity and light (in comparison with that in rivers without macrophytes) could be described during both summer and winter by a hyperbolic equation, with more shade adaptation but less maximum photosynthesis during winter than during summer. The hyperbolic relationship held for both oxygen‐ and carbon‐based measurements. The mean value for the photosynthetic quotient (P.Q.) was close to one, suggesting little transport of oxygen and inorganic carbon to or from roots. We could not discern effects on primary productivity of other variables such as temperature, which seemed unimportant in comparison to light.