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Photosynthesis of Amphibious and Obligately Submerged Plants in CO₂-Rich Lowland Streams

Kaj Sand-Jensen and Henning Frost-Christensen
Oecologia
Vol. 117, No. 1/2 (1998), pp. 31-39
Published by: Springer in cooperation with International Association for Ecology
Stable URL: http://www.jstor.org/stable/4222130
Page Count: 9
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Photosynthesis of Amphibious and Obligately Submerged Plants in CO₂-Rich Lowland Streams
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Abstract

Small unshaded streams in lowland regions receive drainage water with high concentrations of free CO₂, and they support an abundant growth of amphibious and obligately submerged plants. Our first objective was to measure the CO₂ regime during summer in a wide range of small alkaline Danish streams subject to wide variation in temperature, O₂ and CO₂ during the day. The second objective was to determine the effect of these variations on daily changes in light-saturated photosynthesis in water of a homophyllous and a heterophyllous amphibious species that only used CO₂, and an obligately submerged species capable of using both HCO3 - and CO₂. We found that the median CO₂ concentrations of the streams were 11 and 6 times above air saturation in the morning and the afternoon, respectively, but stream sites with dense plant growth had CO₂ concentrations approaching air saturation in the afternoon. In contrast, outlets from lakes had low CO₂ concentrations close to, or below, air saturation. The amphibious species showed a reduction of photosynthesis in water from morning to afternoon along with the decline in CO₂ concentrations, while increasing temperature and O₂ had little effect on photosynthesis. Photosynthesis of the obligately submerged species varied little with the change of CO₂ because of HCO3 --use, and variations were mostly due to changes in O₂ concentration. Independent measurements showed that changes in temperature, O₂ and CO₂ could account for the daily variability of photosynthesis of all three species in water. The results imply that CO₂ supersaturation in small lowland streams is important for the rich representation of amphibious species and their contribution to system photosynthesis.

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