See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/348645198 Trends in submersed aquatic plant communities in a large, inland lake: impacts of an invasion by starry stonewort ( Nitellopsis obtusa ) Article in Lake and Reservoir Management · January 2021 DOI: 10.1080/10402381.2020.1859025 CITATIONS READS 0 28 3 authors, including: Brian Ginn Lake Simcoe Region Conservation Authority 36 PUBLICATIONS 777 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Aquatic plant communities in Lake Simcoe View project Status of invasive Dreissenid mussels in Lake Simcoe View project All content following this page was uploaded by Brian Ginn on 22 January 2021. The user has requested enhancement of the downloaded file. LAKE AND RESERVOIR MANAGEMENT https://doi.org/10.1080/10402381.2020.1859025 Trends in submersed aquatic plant communities in a large, inland lake: impacts of an invasion by starry stonewort (Nitellopsis obtusa) Brian K. Ginn, Emma F. S. Dias and Toshia Fleischaker Lake Simcoe Region Conservation Authority, Newmarket, ON, Canada ABSTRACT KEYWORDS Ginn BK, Dias EFS, Fleischaker T. 2021. Trends in submersed aquatic plant communities in a large, Aquatic plants; Eurasian inland lake: impacts of an invasion by starry stonewort (Nitellopsis obtusa). Lake Reserv Manage. watermilfoil; invasive XX:XXX–XX. species; invasive species competition; Lake Simcoe; macrophytes; Aquatic plant and macroalgae (collectively, macrophyte) communities from Lake Simcoe starry stonewort (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 com- munity, 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 bio- mass 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) abate- ment 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 los- ing 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, macro- algae 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. Aquatic plants and macroalgae, in this article col- For these and other ecological reasons, macro- lectively called macrophytes, are an important, phytes are protected in Ontario with strict limita- yet often understudied, biological community in tions on their removal (OMNRF 2017). Despite many lakes. In addition to being some of the their importance, in nutrient-enriched systems main primary producers in freshwater ecosys- macrophytes can increase in biomass and reach tems, macrophytes have other important eco- “nuisance levels” according to lake users, interfer- logical roles, such as providing habitat and ing with shallow-water recreational activities such shelter for minnows and warmwater fish species as swimming and boating, clogging water intakes, (e.g., perch, Perca flavescens; smallmouth and and causing aesthetic problems to landowners largemouth bass, Micropterus dolomieu and when dead macrophytes wash up on shorelines Micropterus salmoides; walleye, Sander vitreus; (Chambers et al. 1999). Furthermore, excess and muskellunge, Esox masquinongy); reducing macrophyte biomass can interfere with ecological turbidity and stabilizing substrates; and slowing functioning, such as nutrient and biogeochemical wave action, thus preventing shoreline erosion cycling, dissolved oxygen concentrations, interac- (Chambers et al. 1999, Alexander et al. 2008). tions with other biological communities, and lake CONTACT Brian K. Ginn [email protected] Supplemental data for this article is available online at https://doi.org/10.1080/10402381.2020.1859025. ß 2021 North American Lake Management Society 2 B. K. GINN ET AL. productivity (Carpenter and Lodge 1986, bugensis; Evans et al. 2011, Ginn et al. 2018). In Chambers et al. 1999, Alexander et al. 2008). As addition, the invasive macroalgae starry stone- a result, in areas with high macrophyte biomass wort (Nitellopsis obtusa) has been present in the there is often a need for control and management Great Lakes Region since the 1970s (Karol and efforts to maintain macrophytes at non-nuisance Sleith 2017), has been found in nearby Ontario levels. In south-central Ontario, many lakes lakes (Midwood et al. 2013, Harrow-Lyle and became nutrient enriched as surrounding water- Kirkwood 2020), and was found in Lake Simcoe sheds were transformed from natural cover to in 2009 (LSRCA, unpubl. data). agricultural and urban land uses. In many cases, Building on this previous research, our objec- these lakes have been the source of complaints tives in this study were to (1) track changes in from watershed residents where macrophytes the aquatic macrophyte community of Lake increased in amount, and have impacted the aes- Simcoe since our initial lakewide survey in 2008 thetic qualities these residents associate with (Ginn 2011); (2) assess the ecological and recre- waterfront property. ational impacts of a new invasive species, starry Lake Simcoe has been a source of concerns stonewort, on the lake; and (3) investigate control from watershed residents over excessive macro- and management strategies for areas of high phytes and shoreline accumulations (or wash- macrophyte biomass. ups) since the 1970s (Millard and Veal 1971). In response to these concerns, studies have been undertaken to investigate changes in the macro- Methods phyte community and balance natural ecological Site description and previous studies function with human needs. Millard and Veal Lake Simcoe (44 25’N, 79 25’W) is the largest (1971) undertook the first survey on Lake Simcoe inland lake (surface area 722 km2) in south-cen- to document the locations, species composition, tral Ontario, Canada (Figure 1(a–d) and Table 1). and density of aquatic “weed” beds. For the most The lake is typically divided into 3 sections: a part, they reported a shoreline free of macro- large main basin; the fjord-like Kempenfelt Bay phytes due to wind and wave exposure, with to the west, which has the deepest depths (Zmax increased densities being limited to 6 sheltered ¼ areas. Subsequent studies in the 1980s and 2006 42 m, Figure 1(d)); and the relatively shallow ¼ ’ (Neil et al. 1985, 1991; Stantec 2007) targeted one (Zmax 18 m), meso-eutrophic Cook s Bay to the ’ south, with the highest densities of aquatic mac- of these areas (Cook s Bay) that had the highest 2 density of macrophytes, likely the result of rophytes. Land uses in the 3307 km watershed muddy substrates and high P loading from the are primarily agriculture ( 47%) and natural Holland River, which drains agricultural lands heritage cover ( 40%), but urban area (currently and collects urban runoff from the towns of 12%) has increased in recent decades (Palmer Aurora, Bradford, Newmarket, and East et al. 2011, 2013a, 2013b), likely due to the close ’ Gwillimbury. The next full lake survey, in 2008 proximity to Canada s largest city (Toronto; by Ginn (2011), confirmed the 6 areas of high metropolitan area population 6.2 million; macrophyte biomass from 1971, identified several Statistics Canada 2020). new areas, and reported increases in biomass Since European settlement in the watershed since the 1980s. Although P concentrations in (starting ca. 1796), Lake Simcoe has undergone Lake Simcoe had declined since the 1980s environmental changes consistent with eutrophi- (OMOECC 2015), macrophyte biomass has cation that have included increased P concentra- increased since then, likely from expansion of tions and phytoplankton biovolumes, very low suitable habitat space, due to increased water hypolimnetic dissolved oxygen (DO) concentra- clarity from P abatement strategies and the effects tions, and recruitment failure in recreationally of invasive dreissenid mussels that were recorded important coldwater fish species (e.g., lake trout, ca. 1991 (zebra mussels, Dreissena polymorpha) Salvelinus namaycush; lake whitefish, Coregonus and 2004 (quagga mussels, D. rostriformis clupeaformis; and cisco, Coregonus artedi; Palmer LAKE AND RESERVOIR MANAGEMENT 3 Figure 1. Maps of Lake Simcoe: (a) location of lake and catchment basin within Ontario, Canada; (b) Lake Simcoe with 215 sample sites used in the 2008 macrophyte survey; (c) Lake Simcoe with 244 sample sites used in the 2013 and 2018 macrophyte