Soft Shell Clams (Mya arenaria) Contribute to Macroalgal Blooms in a Partially-restored Gulf of Maine Back-barrier Lagoon

Abstract

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.

Molluscan Community Recovery in a New England Back‐Barrier Salt Marsh Lagoon 10 Years after Partial Restoration

“…littorea , have been largely absent from the East Harbor Main Lagoon (where algal blooms are most extensive) owing to high summer water temperatures (Smith et al ). Thiet et al () also observed that the presence of L . littorea significantly reduced macroalgal growth in experimental mesocosms.…”

“…A recent study at East Harbor (Thiet et al ) suggests that rapid and prolific recolonization of East Harbor by mollusks following tidal restoration directly contributed to the algal blooms of 2005–2006, resulting in a negative feedback on molluscan richness and density. In that study, M .…”

“…In that study, M . arenaria significantly increased macroalgal biomass by increasing benthic and water column N and P levels and clarifying the water column (Thiet et al ). Similarly, zebra mussel invasion has increased macroalgal productivity in northern U.S. lakes via nutrient deposition and clarification of the water column (Lowe & Pillsbury ; Mayer et al ).…”

“…By 2009, water chemistry and nekton diversity were close to ranges that would occur naturally in the system (Smith et al ). Nonetheless, mollusk species richness and density have declined since the first mollusk survey in 2005 (Portnoy et al ; Smith et al ; Thelen & Thiet ); this has been attributed to high water temperatures and low dissolved oxygen levels associated with macroalgal blooms (Portnoy et al ; Thelen & Thiet ; Thiet et al ). Clearly, despite several positive restoration outcomes at this site, optimal functioning has not yet been achieved and long‐term monitoring is necessary to understand restoration outcomes.…”