Urban influences on Cladophora blooms in Lake Ontario

Cladophora biomass and bloom occurrences were highly variable across Lake Ontario during 2008. Cladophora growth rates were strongly phosphorus (P) limited, and P loading from local watersheds appeared to be the underlying driver for the spatial variability in Cladophora biomass. Cladophora growth rates are likely more sensitive to P loading than prior to dreissenid invasion, since these mussels can transform particulate P into soluble forms with increased bioavailability. While increased P bioavailability due to dreissenid mussel excretion was likely important in many areas, there was little evidence that P from metabolic waste products of dreissenid mussels was sufficient to produce severe blooms in absence of localized P enrichment. Our results indicated that the effective management of Cladophora blooms in Lake Ontario should occur through managing P loading at local scales while ensuring lake-wide P concentrations do not increase. When monitoring and managing these blooms in Lake Ontario it will be important to consider that ambient concentrations of soluble reactive phosphorus (SRP) are likely under biological control in areas with extensive Cladophora blooms, that Cladophora may obtain SRP from underlying dreissenid beds, and that SRP concentrations in overlaying waters may not reflect the sum of P available for growth. As dreissenids can transform particulate P into bio-available P, management of P from localized sources should focus on reducing both total P and soluble P loading to nearshore waters.

► Cladophora biomass and nutrient status examined at seven sites in Lake Ontario.
► Cladophora growth was strongly phosphorus limited at all sites.
► Cladophora biomass was highly correlated with conservative tracers of human activity.
► Dreissenids were incapable of causing blooms in absence of local nutrient loading.
► Cladophora management should focus on localized P reductions.