Competition between three submerged macrophytes, Elodea canadensis Michx, Elodea nuttallii (Planch.) St John and Lagarosiphon major (Ridl.) moss

During photosynthesis, submersed aquatic plant stands create 'envelopes' of raised pH and dissolved O-2 concentrations and depleted dissolved CO2 conc entrations. These may extend beyond the plant stands, suppressing the photo synthetic performance and increasing the photorespiratory stress of adjacen t species. In competition between three aquatic macrophytes, Elodea canaden sis, Elodea nuttallii and Lagarosiphon major, we hypothesised that a compet itive advantage is achieved by those species that have the best stress gene ration/toleration mechanisms. pH and free CO2 conditions were measured unde r laboratory conditions at two plant densities and compared with measuremen ts of photosynthesis and respiration at various pH and free CO2 concentrati ons. L. major raised pH faster and photosynthesised at a greater rate under the experimental conditions and may gain competitive advantage at high pH. No clear differences in photosynthetic behaviour were observed between the Elodea spp. and it is concluded that a wider range of factors than those s tudied here are probably involved in competition between the three species.