Distribution and limnological drivers of submerged aquatic plant communities in Lake Simcoe (Ontario, Canada): Utility of macrophytes as bioindicators of lake trophic status

Species composition, distribution, and biomass of submerged aquatic plants were studied in Lake Simcoe (Ontario, Canada) as was the use of macrophytes as indicators of lake trophic status. While previous studies in this lake targeted one eutrophic embayment, this study covered the entire lake area, identified five areas of high plant biomass and recorded 16 macrophyte species; the community was dominated by Ceratophyllum demersum (39.1% of the total biomass), the invasive species Myriophyllum spicatum (27.4%), Elodea canadensis (10.7%) and Chara spp. (9.7%). Canonical correspondence analysis (CCA) identified four significant limnological variables related to plant biomass: (a) depth, likely a proxy for light levels; (b) substrate type, related to nutrient availability, stability, and wave exposure; (c) phosphorus loading from the closest tributary; and (d) size of the area drained by a tributary. Since initial (1971) macrophyte surveys on Lake Simcoe, the community has been dramatically altered by expansion of an invasive species (i.e. M. spicatum) resulting in declines of native shallow-water species (e.g. Chara spp.). The arrival of zebra mussels (Dreissenia polymorpha) ~ 1995 and reductions in phosphorus loading have increased water clarity, extending the maximum depth of plant colonization (6.0 m in 1984 to 10.5 m in 2008), and almost tripled macrophyte biomass (1.2 kg·m−2 in 1984 to 3.1 kg·m−2 in 2008). Increased plant biomass, and a loss of species diversity due to spread of M. spicatum are interpreted by macrophyte indices as a decrease in lake trophic status, which likely explains why these indices did not follow trends in reduced phosphorus loading.