Modeling the growth response of Cladophora in a Laurentian Great Lake to the exotic invader Dreissena and to lake warming

A Cladophora growth model (CGM) is calibrated and validated here to simulate attached and sloughed Cladophora biomass in daily time-steps in an urbanized location of Lake Ontario, using two years of collected input data and independent measurements of Cladophora biomass. The CGM is used to hindcast Cladophora growth using multiplicative factors of seasonal minimal tissue phosphorus concentrations (QP) and seasonal mean nearshore light attenuation (KdPAR) of the early 1970s and 1980s relative to modern data. The possible effects of climate on growth are also forecast using additive temperature increases. Cladophora QP in Lake Ontario has declined in parallel with decreasing pelagic P concentrations, resulting in reduced Cladophora biomass at all depths in the euphotic zone. KdPAR has also declined, most strongly since the mid–1990s, following Dreissena mussel invasion, driving an increase in biomass between 3.5and 10-m depth. Combining these effects, the CGM predicts that biomass along shorelines today is lower in Lake Ontario than in the 1980s. However, any increases in QP in this post–dreissenid-mussel period will result in greater Cladophora proliferation than in previous decades due to increased nearshore water clarity. Cladophora QP, while still currently lower than in the early 1980s, may be rising due to P supply to the littoral zone by the invasive mussels. The surface water temperature of Lake Ontario indicates warming of 0.96uC decade21 from 1980 to 2006. With increasing surface water temperatures, the CGM predicts an earlier spring growth but only a marginal increase in peak Cladophora biomass. Eutrophication continues to be among the most critical and pervasive issues facing coastal waters—marine and freshwater alike (Rabalais and Nixon 2002; Smith et al. 2006). Blooms of annual or ephemeral attached macroalgae are symptomatic of nutrient enrichment in littoral zones underlain by hard substrata and are typically indicative of degraded ecosystem health (Raven and Taylor 2003; Smith et al. 2005; Worm and Lotze 2006). High biomass of the globally distributed filamentous green alga, Cladophora, in particular, is regarded as a sentinel of eutrophication in alkaline systems (Dodds and Gudder 1992; Lembi 2003). Nuisance densities of Cladophora have been reported from estuaries (Gordon and McComb 1989; Valiela et al. 1997), inland seas (Kiirikki 1996; Curiel et al. 2004), and hardwater rivers and lakes with moderate energy (Power 1992; Parker and Maberly 2000). In the Laurentian Great Lakes, Cladophora is a nuisance throughout Lakes Ontario, Erie, and Michigan, where hard substrate is available, and in isolated areas near nutrient point sources in Lake Huron (Herbst 1969). There is a widespread perception that since the establishment of invasive dreissenid mussels (i.e., the zebra mussel [Dreissena polymorpha] and the quagga mussel [D. bugensis]) over the past 15 yr, Cladophora biomass has resurged in the lower Great Lakes (Mills et al. 2003; Hecky et al. 2004; Bootsma et al. 2005). The degree to which this perception is accurate, however, is unclear. Due to their high biomass in these lakes (e.g., up to 150 g shell-free DM m22; Fleisher et al. 2001), dreissenid mussels have been credited with reengineering nutrient distributions by removing suspended particulate matter through filterfeeding, then excreting dissolved regenerated nutrients as well as generating organic waste as feces and pseudofeces in the benthos (Hecky et al. 2004). This process, recently termed ‘‘benthification’’ (Zhu et al. 2006), has been implicated in shifting pools of matter with attendant nutrients from the pelagia to the benthos and increasing the solar radiant flux to the littoral benthos. Mussels can be an important source of nutrients to overlying flora (Kahlert and Pettersson 2002) and, furthermore, may remove competitive demands for nutrients via filtration of plankton (Holland et al. 1995). In the Great Lakes, long-term pelagic total P concentrations have been declining since the 1970s due in part to nutrient abatement strategies (e.g., Nicholls 2001; Millard et al. 2003) while currently, dreissenid mussels may be increasing supply of P to benthic flora (Hecky et al. 2004). The decadal-scale response of Cladophora tissue P quota (QP) to these counteracting pressures in the Great Lakes is not known. The lower Great Lakes have additionally experienced a significant increase in light penetration since dreissenid mussel establishment (Howell et al. 1996) in response to decreased seston 2 Current address: Department of Biology and Large Lakes Observatory, University of Minnesota Duluth, 2205 5th St., Duluth, Minnesota, 55812. Acknowledgments We are grateful for the field assistance of Guillaume Cuillard, Anna Desellas, Sara Ross, Jim Zettel, Tedy Ozersky, Scott Higgins, and Dave Depew; we thank Bill Schertzer for providing historical temperature records, Scott Higgins for providing critical support in applying the Cladophora growth model, and for the helpful comments by 3 anonymous reviewers. Funding for this research was provided by the Ontario Clean Water Agency and Sairah Malkin was supported by a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship and an Ontario Graduate Scholarship. 1 Corresponding author ([email protected]). Limnol. Oceanogr., 53(3), 2008, 1111–1124 E 2008, by the American Society of Limnology and Oceanography, Inc.