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Hurricanes, larval dispersal, and oyster restoration in Pensacola Bay

A research proposal: 2007-2009

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Objective: This research program will produce a model of larval exchange among oyster reefs both within and among subsections (Blackwater Bay, East Bay, Escambia Bay) of Pensacola Bay.  The resulting model will be used to assess the strategy and success of oyster reef restoration projects in Pensacola Bay, where hurricanes have severely damaged the resource and where reef rebuilding projects are proposed as a substantial component of hurricane disaster relief efforts.  We propose to use the output from a hydrodynamic model, coupled with published information on larval biology, to predict the trajectory of larvae spawned from various reefs within each system.  The model predictions will be validated by field sampling to map the actual distribution of oyster larvae and by monitoring recruitment to reconstructed oyster reefs within and between each of the three bay subsystems.  Results will be used by the Florida Department of Agriculture and Consumer Services (DOACS) to advise the placement and effective monitoring of oyster reef restoration projects in Pensacola Bay and other areas of Florida.

Introduction: The eastern oyster (Crassostrea virginica) supports important commercial and recreational fisheries throughout the eastern and Gulf of Mexico coasts of the United States and also provides valued ecosystem services including habitat, nutrient recycling, sediment stabilization, and filtration capacity (Kennedy et al., 1996; MacKenzie et al., 1997).  Despite the direct and indirect economic value of the eastern oyster, this species is either imperiled (Hargis and Haven, 1999) or suffers periodic declines (Berrigan, 1990) in many parts of its range.  In Florida, declines can be attributed to a variety of factors that are location or event driven, but in the panhandle region hurricanes are a common driving force of destruction (Berrigan, 1988).

The most economically valuable and renowned population of oysters in the Florida panhandle is located in Apalachicola Bay.  The oysters occupying the Apalachicola Bay estuary have supported a commercial fishery since at least the 1800s (Swift, 1897; Danglade, 1917) and have been a target for research into the factors regulating oyster distribution and ecology for almost as long (e.g. Livingston et al. 2000).  However, oysters are common in most of the bays and estuaries that comprise the southern shore of the Florida panhandle and those oysters also support commercial fisheries whose production may equal or exceed that of Apalachicola Bay during some years (Arnold and Berrigan, 2002).  The oyster resources in those systems are less studied and less understood than are the oyster resources of Apalachicola Bay despite their value to the economy and ecosystem of Florida.

The oyster resources of Apalachicola Bay and the other Florida panhandle estuaries are not isolated entities, but instead function as an integrated metapopulation (sensu Hanski and Gilpin, 1991) , or a group composed of spatially, or related to specific study areas, isolated local populations that are interconnected via the larval life form.  For approximately two weeks following spawning, a developing oyster year class passes through a pelagic larval phase that provides the opportunity to not only replenish the source population but also to export propagules to spatially separated (and sometimes distant) locations that may or may not at that moment support a healthy oyster population.  The larval life phase thus represents a critical event in the life history of the oyster because it provides the most fundamental and natural opportunity for oysters to populate new habitats and to repopulate extant but depauperate or impoverished reefs.  Unfortunately, the dynamics of oyster larval dispersal are poorly understood with regard to retention, dispersal, and the maintenance of the oyster metapopulation.

As noted above, the larval phase provides the conduit for genetic communication among various local populations of oysters.  This larval conduit operates at many scales, from reef-to-reef to estuary-to-estuary.  Some reefs, and possibly some reef areas and estuaries, function as source populations because they contribute larvae not only for their own maintenance but also for the maintenance of other reefs or reef systems.  In contrast, some reefs function as sinks because they are dependent for population maintenance upon larval inputs from neighboring or distant reefs.  This arrangement is dynamic: a reef that acts as a source at one time may function as a sink at another time.  The transition from source to sink results from the timing of spawning relative to production and transport factors such as standing stock, egg quality, tidal stage, weather, coastal and open ocean hydrodynamics, etc.  Those factors will determine if, and to what degree, larvae are locally retained, transported to nearby reefs, or successfully exported to other estuarine systems.  This exchange process not only influences the population abundance and structure of oyster reefs throughout the panhandle but also the genetic composition of the metapopulation (Buroker, 1983).  Understanding the mechanisms of larval exchange is therefore fundamental to understanding the process of recovery from natural disturbance and to effectively managing the process of oyster reef restoration.

Job I: Construct an oyster larval dispersal model for the Pensacola Bay system.

Objective: Collect information on water level, tides, meteorological conditions, current patterns, larval duration, vertical distribution, etc necessary to construct and calibrate a three-dimensional larval dispersal model for Pensacola Bay.

Procedures: Employ a physical oceanographer from a recognized institution to develop a basic 3-D hydrodynamic model of Pensacola Bay.  Utilize biologist employed by the Florida Fish and Wildlife Conservation Commission’s (FWC) Fish and Wildlife Research Institute (FWRI) and paid by this grant to gather all available information on oyster larval dynamics.

Job II: Identify oyster reef locations.

Objective: Obtain specific coordinates for each oyster reef presently identified in the Escambia, East, Blackwater Bay estuarine system, and prepare a Geographic Information System map layer representing those locations.

Procedures: Oyster reef locations will be obtained through close cooperation between FWC/FWRI and DOACS staff.  Areas where the location of oyster reefs is not known will be surveyed by aerial surveys and on-the-ground sampling to define the geographic location and general extent of each reef.  Note that specific reef boundaries are not required for this project because modeling will be a point-to-point exercise.

Job III: Simulation of oyster larval dispersal.

Objective: Conduct simulations of larval dispersal under various hydrodynamic and biological conditions and produce condition specific prediction maps from those simulations.

Procedures: Using the results from Job II to select points of origin and the results of Job I to provide predictions of larval trajectories from those points of origin, simulate larval dispersal to various sites within each bay system, between bays, and from each bay to the shelf (a loss term for Pensacola Bay but not necessarily for the panhandle oyster metapopulation).

Job IV: Map actual oyster larval distribution.

Objective: Map the actual 3-D location of oyster larvae under several different hydrographic regimes and statistically compare to predictions from the coupled physical-biological model.

Procedures: Conduct surface and bottom plankton sampling at various sites throughout Pensacola Bay, the exact timing and location of which will be determined based upon model results.  Plankton samples will be analyzed for oyster larvae using genetic techniques similar to those employed for hard clam studies in the Indian River Lagoon (Arnold et al., 2005).  Comparisons between modeled and actual oyster larvae distribution patterns will be analyzed using spatial statistics, or statistics based on a specific area of study.

Job V: Monitor recruitment to restored oyster reefs.

Objective: Determine the pattern and rate of recruitment to restored oyster reefs relative to patterns and rates predicted from the coupled model.

Procedures: Sample restored oyster reefs in the targeted sub-bays of Pensacola Bay to determine recruitment rates of oyster larvae to each reef, including vertical patterns of recruitment.  Map recruitment patterns and compare with predictions from coupled model.  Identify areas of good and bad agreement between modeled and measured results.  Assess model validity based upon results, including identification of weaknesses in the model and the cost and means for improvement.