The effect of alkalinity on photosynthesis-light curves and inorganic carbon extraction capacity of freshwater macrophytes

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The influence of variable bicarbonate (HCO 3- ) alkalinity was assessed on the photosynthetic performance of four submerged macrophyte species of contrasting growth form and distribution but all reportedly capable of HCO 3- use: Hydrilla verticillata, Egeria densa, Potamogeton schweinfurthii and Potamogeton lucens. Whole-shoot photosynthesis-irradiance curves and pH-drift capacity were measured at two alkalinities, a low and a high concentration (0.5 and 2.0 meq 1 -1 HCO 3- ). E. densa, H. verticillata and P. lucens had higher gross maximum photosynthesis (P max ) and respiration rates at the higher HCO 3- availability. The species also differed significantly in P max and respiration for both alkalinities, with P max estimates ranging between 3 and 22 mg O 2 g AFDW -1 h -1 . Half-saturation constant estimates (K 1/2 ) did not differ between species or treatments, but were comparatively low (<10 μmol PAR m -2 s -1 ). Incorporation of a light inhibition term did not improve the explained variation of the fitted curves significantly. Overall, respiration rates were high (2-13 mg O 2 g AFDW -1 h -1 ), largely due to the high incubation temperature employed (25°C). In the pH-drift experiment H. verticillata reached a pH above 9.5 in both alkalinities after 48 h, which is higher than all the other species tested. In the higher alkalinity pH-drift experiment, only E. densa failed to raise the pH. All species raised the pH more rapidly in the low alkalinity medium probably due to its weaker buffering capacity. Final (96 h) maximum pHs were also higher in low alkalinity. Across species, however, no correlations existed between any of the P-l curve parameters including net photosynthesis at 200 μmol PAR m -2 s -1 and carbon extraction capacity expressed as drifted pH after 48 h. In parallel growth experiments, H. verticillata maintained growth even at 3 meq 1 -1 (RGR = 0.06 per day), whereas in E. densa RGR declined rapidly beyond 1 meq 1 -1 , which is consistent with the pH-drift experiments. © 2002 Elsevier Science B.V. All rights reserved.




Kahara, S. N., & Vermaat, J. E. (2003). The effect of alkalinity on photosynthesis-light curves and inorganic carbon extraction capacity of freshwater macrophytes. Aquatic Botany, 75(3), 217–227.

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