University of Central Florida Undergraduate Research Journal - Oyster Reef Restoration: Impacts on Infaunal Communities in a Shallow Water Estuary
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Introduction

The eastern oyster, Crassostrea virginica, provides an abundance of ecosystem services that benefit estuaries.  Oysters filter out excess nutrients and phytoplankton in the water, improving local water quality (Coen et al. 2007, Grabowski and Peterson 2007); they are also known carbon sinks (Peterson and Lipcius 2003, Chambers et al. 2018) and act as wave breaks to mitigate erosion (Meyer et al. 1997).  Acting as ecosystem engineers, oysters create reef habitats that are utilized by many commercially important fishes and crabs and threatened species of wading birds.  Many estuarine animals utilize oyster reefs for foraging, and crabs and juvenile fish also use the reefs as refuge from predators. 

Over the past century, however, 85% of shellfish reef habitats have been lost worldwide (Beck et al. 2011).  The global loss of oyster reefs is attributed to over-harvesting, exploitation, and habitat loss and degradation from anthropogenic use.  As ecosystem engineers, the loss of oyster reefs has detrimental affects on estuarine ecosystems through the loss of the ecosystem services provided by the reefs.  Therefore, oyster reef restoration is crucial to restore the ecological function of oyster reef habitats (Coen and Luckenbach 2000).

Intertidal oyster reefs in Mosquito Lagoon, Florida have experienced large losses in acreage since 1943 (Garvis et al. 2015).  The decrease of oyster reefs in this area is attributed to recreational boat wakes (Grizzle et al. 2002). Wave motion and sediment loading caused by boat wakes is correlated with an increase in oyster reef dead margins. (Wall et al. 2005; Garvis et al. 2015).  Boat wakes create waves that dislodge live oyster clusters and wash them up on the reef above the water level.  The oysters die, resulting in piles of bleached white shell.

Oyster reef restoration in Mosquito Lagoon helps restore dead reef margins to living reefs.  Oyster mats, consisting of mesh mats zip-tied with disarticulated oyster shell, are laid out on flattened dead reef margins and held down with cement weights (Garvis et al. 2015).  Oyster larvae recruit on the disarticulated shell and a new reef is able to establish.  This method of restoration prevents oyster clusters from being dislodged by boat wakes and has proven to be very effective.  Three and a half years following restoration, restored reefs had equal live oyster densities as natural reefs in Mosquito Lagoon (Birch and Walters 2012).

Oyster reefs provide habitat to infaunal organisms that hold significant positions in the estuarine food web (Meyer and Townsend, 2000).  Infaunal organisms are small, marine organisms that burrow in the sediment (e.g. worms, clams).  Many threatened and endangered wading birds and commercially important fishes and crabs depend on infauna as a main food source.  On intertidal oyster reefs in the North Inlet Estuary of South Carolina, a species of infaunal amphipods was found to make up 10% of wading birds’ diets in the area (Grant 1981).  The rest of the wading birds’ diets consisted of infaunal polychaetes and bivalves.  Juvenile fish in Alaskan estuaries were found to rely on polychaetes, bivalves, and decapods to make up 90% of their diet (Grabowski et al. 2002).  On restored mudflat oyster reefs in North Carolina, increases in juvenile fish abundances were positively correlated with the abundance of infaunal food sources and oyster habitat structure (Grabowski et al. 2005).  These studies suggest large infaunal communities are critical to supporting higher trophic level species in coastal estuaries. 

Oyster reefs function as foraging grounds for many important species, and restoration has been shown to increase the complexity of food webs in estuaries.  A literature review on shorebird diets in the Western Hemisphere suggests that management efforts to improve food sources for shorebirds should focus on the restoration and management of ecosystem processes.  Management and restoration increased the populations of naturally-occurring invertebrate and infaunal organisms, therefore providing an important food source for shorebirds in the Western Hemisphere (Skagen and Oman 1996).  In the Chesapeake Bay, three-to five-year-old restored oyster reefs increased the energy transfer to higher trophic levels in the reef community (Paynter and Rodney 2006).  Restoration increased the biomass of prey species that are a primary food source for commercially and recreationally important fish in the area.  These observations demonstrate that mature, restored reefs have the ability to support more complex trophic structures than degraded, non-restored reefs.

Infaunal organisms are strong indicators of oyster reef productivity not only because of their important role in the food web, but because they are typically the first organisms to recolonize a habitat after a disturbance.  A study done in Tampa Bay, Florida on short-term faunal recolonization demonstrated that infaunal habitats were recolonized within hours after removal of these organisms (Bell and Devlin 1983).  Within 25 hours, the infaunal species abundance had returned to the level it was before the removal occurred.  If infaunal species are the first macro-organisms to recolonize an oyster reef after the disturbance of restoration, it is likely that these early successional species may facilitate other organisms colonizing the reef soon thereafter.

Several studies have examined the impact of restoration on faunal abundance, but few have assessed the impact of habitat restoration on infaunal abundance (Meyer and Townsend 2000, Hadley et al. 2010).  To our knowledge, no studies have been conducted in Mosquito Lagoon to understand how infaunal organisms are impacted by oyster reef restoration.  We predict that if restoring dead oyster reefs allows them to function as natural, live reefs and live reefs maintain a high abundance of infauna, then infaunal abundance and composition will increase over time after restoration.

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