Light and Temperature Controls of Aquatic Plant Photosynthesis Downstream of a Hydropower Plant and the Effect of Plant Removal

Abstract

Many rivers around the globe are now regulated, and the altered hydrological regime can lead to mass development of aquatic plants. The invasion by plants is often seen as a nuisance for human activities leading to costly remedial actions without clear knowledge of the consequences for aquatic biodiversity and ecosystem functioning. Mechanical harvesting is often used to remove aquatic plants and knowledge of plant growth rate could improve management decisions. Here we used a simple light-temperature theoretical model to make a priori prediction of aquatic plant photosynthesis tested against an open-channel diel change in O2 mass balance approach. A Michaelis-Menten type model was fitted to observed gross primary production (GPP) standardised at 10°C using a temperature dependence derived from a thermodynamic theory of enzyme kinetics. The model could explain 87% of the variability in GPP of a submerged aquatic plant (Juncus bulbosus L.) throughout an annual cycle, in the River Otra, Norway. The annual net plant production was about 2.6 (1.0±4.2) times the standing biomass of J. bulbosus suggesting high continuous mass loss through hydraulic stress and natural mechanical breakage of stems, since the biomass of J. bulbosus was relatively constant throughout the year. J. bulbosus was predicted to be resilient to mechanical harvesting with photosynthetic capacity recovered within two years following 50-85% plant removal. This was confirmed with an independent field experiment. We emphasise the value of using a theoretical approach, such as metabolic theory, over statistical models, in which a posteriori results are not always easy to interpret.

Keywords: photosynthesis, macrophyte removal, regulated river, metabolic theory, light, temperature