Water Temperature as a Limiting Factor in the Colonization of a Partially-Restored Coastal Lagoon: Case Study of a Gastropod Herbivore and Control of Macroalgae

Abstract

East Harbor (Truro, Massachusetts, U.S.A.) is a tidally-restricted salt marsh lagoon that has undergone partial restoration since 2002. After reintroducing seawater to the system following nearly 140 years of impoundment, remarkable transformations in plant and animal communities have occurred. But while a host of marine fish, crustaceans, and benthic invertebrates have become established throughout the system, an important herbivore, common periwinkle (Littorina littorea) has not. This gastropod is absent throughout the open lagoon where in the past several years macroalgae (mainly Enteromorpha intestinalis) has proliferated to nuisance levels. Grazing experiments with common periwinkles suggest that this species could significantly reduce the extent of macroalgae biomass there. The inability of this organism to colonize the lagoon may be related to high water temperatures. Laboratory experiments suggest that 4 days of constant water temperatures ≥ 28°C or 6+ daily hours of 30°C for several days result in high mortality. Data from in situ temperature loggers show that such conditions occur in the lagoon during July-August. Increased tidal flushing could lower water temperatures throughout East Harbor and allow this species to expand its range. This, in turn, could limit the development of macroalgae through herbivory. This study highlights the role of keystone species and the importance of restoring suitable temperature regimes for the functional recovery of hydrologically-impaired systems.

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

“…The 2006-2007 macroalgal bloom at East Harbor caused very high clam mortality, and today clam densities remain much lower than they were in 2005 (Thiet et al in press); thus, the cycle of heavy clam colonization, subsequent macroalgal blooms, and resultant clam dieback may signify this system's process of equilibrating to greater stability in population densities and physical properties following restoration. Nonetheless, although severe macroalgal blooms have not occurred in East Harbor since 2007, macroalgae continue to dominate the aquatic plant community at the site and threaten the productivity of widgeon grass, eelgrass, and benthic fauna (Smith 2011). Seeding the system with certain gastropod grazers may keep algal growth in check, but some gastropods, such as periwinkles, will likely only survive if summer water temperatures at the site are lower (Smith et al 2011).…”

“…Nonetheless, although severe macroalgal blooms have not occurred in East Harbor since 2007, macroalgae continue to dominate the aquatic plant community at the site and threaten the productivity of widgeon grass, eelgrass, and benthic fauna (Smith 2011). Seeding the system with certain gastropod grazers may keep algal growth in check, but some gastropods, such as periwinkles, will likely only survive if summer water temperatures at the site are lower (Smith et al 2011). This can be achieved by increasing the daily tide range in the main lagoon and northwest cove, thereby reducing water temperatures and creating a more hospitable environment for periwinkles.…”

“…The high water residency time also causes elevated summer water temperatures (up to 35°C) (Portnoy et al 2007, Smith et al 2011, which reduce the survival of some gastropod grazers like periwinkles that typically regulate macroalgal growth in functioning salt marshes (Smith et al 2011). The 2006-2007 macroalgal bloom at East Harbor caused very high clam mortality, and today clam densities remain much lower than they were in 2005 (Thiet et al in press); thus, the cycle of heavy clam colonization, subsequent macroalgal blooms, and resultant clam dieback may signify this system's process of equilibrating to greater stability in population densities and physical properties following restoration.…”

“…occurred in the lagoon in 2006-2007, resulting in severe anoxia and fish and shellfish mortality (Portnoy et al 2007). Today, macroalgae remain the dominant form of plant life in the lagoon and continue to threaten the establishment of widgeon grass (Ruppia maritima), eelgrass (Zostera marina), and various benthic fauna (Smith et al 2011). External inputs of organic and inorganic N into the system are minimal, and organic N export from the system • 47 is high (Portnoy et al 2007); thus, nutrient loading from external sources is not implicated in algal blooms at the site.…”

“…Periwinkles were observed in Moon Pond (not the lagoon) in 2007 but not in 2008 and were absent from all areas of East Harbor in 2011 (Thiet et al in press). Smith et al (2011) recently showed that periwinkle survival in experimental mesocosms dropped precipitously at water temperatures between 28-30°C. This suggests that periwinkle colonization of East Harbor is, in part, limited by high water temperatures in the lagoon, which regularly reach 28°C and can be 30-35°C during summer months (Portnoy et al 2007, Smith et al 2011.…”