Trends in submersed aquatic plant communities in a large, inland lake: impacts of an invasion by starry stonewort (Nitellopsis obtusa).
Aquatic plant and macroalgae (collectively, macrophyte) communities from Lake Simcoe (Ontario, Canada) were studied in lakewide, >200 site surveys in 2008, 2013, and 2018. Over this period, mean macrophyte biomass increased 5-fold, from 29.9 g (dry)/m2 in 2008 to 153.9 g (dry)/m2 in 2018, due to the arrival and expansion of invasive starry stonewort (Nitellopsis obtusa). First recorded in Lake Simcoe in 2009, starry stonewort has greatly altered the macrophyte community, particularly in shallow (<3 m) water where it outcompeted invasive Eurasian watermilfoil (Myriophyllum spicatum). By 2018, starry stonewort comprised 67.6% of the total macrophyte biomass in Lake Simcoe. In shallow, mesotrophic Cook's Bay, comparison to studies from the 1980s shows an increased plant biomass due to increased water clarity, from phosphorus (P) abatement and invasive dreissenid mussels, with further increases after 2011 due to starry stonewort. Starry stonewort may continue to impact nearshore ecology, with shallow-water fish species losing habitat and refugia as the "forest-like" structure of the plant community is replaced by large, dense aggregations of starry stonewort. Recreational uses will also be impaired and landowner complaints of macrophyte wash-ups will increase, with municipalities and lake-based businesses bearing the cost of mitigation and control strategies. Future research should consider the impacts of starry stonewort to P cycling as, unlike aquatic plants that uptake sediment P, macroalgae use dissolved P as a nutrient source. A lack of communication and reporting on starry stonewort has enabled its spread through south-central Ontario and the Great Lakes Region. Moving forward, we need a better understanding of starry stonewort biology and need to develop effective control and management strategies.