A comparative study of the use of inorganic carbon resources by Chara aspera and Potamogeton pectinatus

We studied the potential role of dissolved inorganic carbon (DIC) in determining vegetation dominance of Potamogeton pectinatus L. and Chara aspera Deth. ex Willd. by monitoring the seasonal dynamics of DIC in a shallow lake and comparing the use of DIC of the two species. The HCO3- -concentration in summer dropped from 2.5 to <0.5 mM with seasonally increasing Chara biomass, whereas outside the vegetation concentrations remained at 2.5 mm. Inside Potamogeton spp. vegetation DIC decreased from 2.5 to ca. 0.75 mM HCO3 (-). A growth experiment showed ash-free biomass for R pectinatus was nearly two times as high as for C. aspera at 3 mM HCO3-, but almost two times lower at 0.5 mM than at 3.0. In a separate experiment, R pectinatus precultured at a relatively low HCO3- -level had a lower net photosynthetic rate (P-max, 0.1 mmol O-2 g(-1) DW h(-1)) than C. aspera (P-max, 0.1 mmol O-2 g(-1) DW h(-)1) over the range of HCO3--concentrations tested (P-max, 0.14 mmol O-2 g(-1) DW h(-1)). In response to CO2 no significant differences between the compensation points (R pectinatus, 28 mM; C. aspera 66 mM), were observed, but the photosynthetic rate increased faster than for C. aspera than for P. pectinatus. Under field conditions, the use Of CO2 is not important since inside vegetation CO2 -concentrations were below 10 muM, and thus, not available for photosynthesis of either species during the main part of the growth season. It is suggested that C. aspera may be a better competitor for HCO3- than P. pectinatus in conditions with a low HCO3- supply. As HCO3- is a strong limiting factor for growth inside the vegetation and probably the only carbon source available, the superior ability of C. aspera to use HCO3- - may be an important factor explaining its present dominance in Veluwemeer. (C) 2002 Elsevier Science B.V. All rights reserved.