Like many Eastern US salt marshes, East Harbor salt marsh lagoon on Cape Cod was isolated from tidal flow in the 1800s, resulting in near-freshwater conditions and loss of native salt marsh species. After partial restoration in 2002, salt marsh species recolonized East Harbor, and soft shell clam (Mya arenaria) recolonization was prolific, with peak densities reaching 3,200 individuals m -2 in 2005. However, severe macroalgal blooms in 2006-2007 resulted in anoxia and mortality of benthic organisms (including clams), and macroalgae still dominate primary productivity at the site. The causes of algal blooms at East Harbor are not understood. We conducted a mesocosm study (2009)(2010)(2011) to evaluate whether soft shell clams contributed to macroalgal blooms by altering nutrient cycling and increasing water clarity. In 2011, we added periwinkles (Littorina littorea) to evaluate whether grazers reduced algal productivity. In all three years of our study, macroalgal biomass and percent cover, benthic and water column nutrients, and water clarity were significantly higher in treatments with clams than controls. In 2011, periwinkles significantly reduced algal growth even when clams were present. These findings suggest that soft shell clam recolonization of East Harbor played an important role in algal blooms by increasing nutrient availability and water clarity, and that the absence of gastropod grazers allowed macroalgae to proliferate. Full tidal flushing may mitigate macroalgal blooms at East Harbor and in other, similar systems by reducing nutrient loading that increases algal productivity, and by maintaining water temperatures within a range that allows algal grazers to establish.