Effects of CO2 concentration on growth of filamentous algae and Littorella uniflora in a Danish softwater lake

An experiment was conducted from May to November in Lake Hampen, Denmark, to study the effect of higher CO2 concentration on the biomass of filamentous algae. Three enclosures (1.5 m diameter) were enriched with free CO2 to ∼10 times atmospheric equilibrium (∼170 μM) and three enclosures were kept at atmospheric equilibrium (∼17 μM). The isoetid Littorella uniflora dominated the vegetation in the enclosures. Low concentrations of nitrate and phosphate in the water were observed, especially in the summer months. During the summer, a high biomass of filamentous algae (dominated by Zygnema sp.) developed in both types of enclosures (18–58 g dry wt. m−2 in July and August), but the biomass of algae was significantly higher (1.9–38 times) in the CO2 enriched enclosures than in enclosures with low CO2 concentration. L. uniflora biomass, especially leaf biomass, also showed a significant positive response to increased CO2 concentration (75.0 ± 10.4 and 133.3 ± 42.5 g dry wt. m−2 at low and high CO2 concentrations, respectively) even though the massive filamentous algal growth decreased the light intensity. Both filamentous algae (in August) and L. uniflora showed lower tissue concentrations of N and P at high CO2 concentration.