Prior to the establishment of the exotic dreissenid mussels in the Great Lakes, benthic filter feeding communities, common in the marine, were a minor contributor to nutrient regeneration and retention. Mussels efficiently recycle both allochthonous and autochthonous particulate nutrients and have clarified coastal waters. This reengineering of the coastal benthic regime has favored increased growth of benthic plants and resulted in the reemergence of the nuisance alga Cladophora which also leads to increased benthic retention of nutrients seasonally. We have estimated the P recycling by dreissenids using in situ incubation chambers with suitable controls, measured and modeled Cladophora growth and measured tissue stoichiometery to estimate P retention to maximum biomass.  We have used hydroacoustic methods to estimate areal coverage and biomass of Cladophora along urbanized, agricultural and low impact (parkland) shorelines on Lake Ontario. 

Results/Conclusions

Coastal and offshore nutrient monitoring data show that nutrient concentrations in Lake Ontario are meeting targets under the Great Lakes Water Quality Agreement and are substantially lower than in the 1970’s and early 1980’s when Cladophora formerly grew to nuisance proportions.  Cladophora has reemerged as an aesthetic and fouling issue under the current low P external loading regime. Dreissenids are now the most important proximate source of available P even along urbanized coastal shorelines in Lake Ontario. Abundances of dreissenids and Cladophora are similarly high along urban, agricultural and low impact park shorelines.  Direct land catchment contributions of total P can account for <10% of P sequestered by Cladophora during its growing season. Dreissenids harvest continuously advected  pelagic phytoplankton and recyle sufficient P to sustain Cladophora demand up to maximum biomass. Maximum biomass is still strongly P deficient and is lower than historic biomasses prior to mandated P control at point sources on Lake Ontario.  Because of increased water clarity post dreissenids, the potential for Cladophora growth is greater than even the historic experience.  Control of Cladophora growth at current abundance requires maintaining current limitations on point source loadings and reduction would likely require much greater attention and reduction to non-point phosphorus loading.