Oyster reef connectivity : ecological benefits and associated vulnerabilities

Global oyster abundance has declined ~85 % over the past 200 years, primarily because of overharvesting (Beck, Brumbaugh, and Airoldi 2011; Kirby 2004). Healthy oyster reef systems benefit the environment in many ways, including water-quality improvement, shoreline protection, increased biological and habitat diversity, and carbon sequestration. To maintain these environmental benefits, reef-restoration efforts that produce healthy, sustainable oyster reefs are essential. To this end, the US Army Corps of Engineers (USACE) has been involved in reef-restoration projects in many locations, including extensive efforts in the Chesapeake Bay (Virginia, Maryland), coastal regions of New York and New Jersey, and the Gulf of Mexico. There are many benefits to creating and maintaining oyster reef systems that are well connected, for both oysters and other organisms within the reef and surrounding habitats. This technical note presents the current knowledge of benefits and costs to restore oyster-reef connectivity along the East and Gulf Coasts of North America. Connectivity of oyster reefs can refer to the physical location of reefs with respect to one another as well as to the dynamics of the genetic links within a metapopulation or to the extent to which larval transport and recruitment unite reef communities. For the purposes of this technical note, connectivity is defined as the spatial aggregation of reefs, though we address impacts of genetic and larval flow as well. Reef connectivity positively affects many ecosystem services and dynamics but can also have unintended consequences (that is, negative externalities). This technical note reviews the benefits and costs of increasing connectivity and presents a brief example of how trade-offs may occur between these potentially opposing ecological objectives. Here, we focus on the eastern oyster, Crassostrea virginica, which inhabits the East and Gulf Coasts of North America, though many of the concepts and principles discussed may apply to other oyster species as well.