Warming shows differential effects on late-season growth and competitive capacity of Elodea canadensis and Potamogeton crispus in shallow lakes

Abstract

Submerged macrophytes are likely to be affected by climate changes through changes in water temperatures and length of growing season. We conducted a lab experiment to examine the influence of a late-season temperature increase on growth, biomass allocation, and acclimation of 2 submerged macrophyte species, Elodea canadensis and Potamogeton crispus. We also ran competitive interaction experiments between the 2 species with mono- and mixed-species cultures in pots placed in outdoor heated mesocosms (5 years at ambient temperature and a higher temperature following the IPCC A2 scenario downscaled to local conditions but enhanced by 50%). In the lab, macrophytes collected in the 2 types of mesocosms were grown at ambient temperatures (12 °C in September and 8 °C in October) and 4 °C higher. Warming had an overall stronger effect on E. candensis than P. crispus, particularly within the low temperature range studied. Hence, the relative growth rate (RGR) of E. canadensis acclimated to ambient mesocosm conditions increased 6-fold from low (8 °C) to high (16 °C) temperature, whereas the RGR of P. crispus increased <2-fold. In the competitive interaction experiment, warming increased the biomass and RGR of E. canadensis in the monoculture. In addition, warming increased shoot elongation of the plant in both the monoculture and mixed culture. P. crispus was generally unaffected by warming when grown in both monoculture and mixed culture, but competition decreased the elongation of shoots pre-adapted to ambient conditions and grown in the warmer mesocosms. The decomposition rate of E. canadensis accelerated with warming but was unaffected in P. crispus. We conclude that E. canadensis is a stronger competitor than P. crispus under warmer late-season conditions; however, it may have a higher demand for oxygen due to the increased decomposition rates at higher temperatures, particularly in the peripheral growing season, with potential profound effects on lake ecosystems. Although acclimatisation was evident, we suggested that temperature changes will affect the growth pattern of the 2 plant species and thereby perhaps induce a switch in macrophyte species dominance.