1. Bottom-dwelling charophytes have been observed to replace canopy-forming pondweeds within a few years in de-eutrophied shallow lakes. Competition for bicarbonate (HCO3−) may explain this shift in vegetation dominance but inhibition of pondweeds by Chara spp. through direct competition has not been shown experimentally.
2. We tested whether charophytes inhibited growth of fennel pondweed (Potamogeton pectinatus) in the absence of belowground competition by growing plants in pots in mesocosms following a replacement series experimental design. To further understand the role of bicarbonate, we studied main and interactive effects of Chara, light and bicarbonate on P. pectinatus growth in a laboratory study.
3. Early in the mesocosm experiment, high charophyte densities had a negative effect on P. pectinatus cover at a time when bicarbonate levels were low. However, bicarbonate levels eventually converged to low levels in all treatments. At final harvest, both species exhibited lower biomasses at higher densities of conspecific pots, indicating that ultimately intraspecific competition was limiting. In a laboratory study, Chara inhibited P. pectinatus the most under a combination of high light and high bicarbonate concentrations, suggesting that Chara may negatively affect P. pectinatus by acting as a general nutrient sink.
4. Our results suggest that Chara growth can reduce bicarbonate levels, delaying but not preventing a P. pectinatus growth pulse. Given the recorded inhibition under ample bicarbonate supply, Chara’s ability to act as nutrient sink may contribute to the decline of P. pectinatus under Chara recovery in shallow lakes.