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 ( Q(P)) and seasonal mean nearshore light attenuation ( Kd(PAR)) 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 3.5- and 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 0.96 degrees C decade(-1) 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.