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Assessment of Nourishment Impacts to Beach Habitat Indicator Species

A research proposal: 2005 - 2008

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Introduction

Coastal erosion is an issue of primary importance along the coasts of Florida, especially in the wake of a particularly active 2004 hurricane season in which over 600 miles of coastline were estimated to be impacted (FDEP 2004).  Loss of beaches produces a significant economic concern due to the potential loss of property and lost revenues from decreased tourism, as well as decreasing nesting habitat for endangered sea turtles.  Beach nourishment is intended and necessary to mediate loss of property, enhance tourism, improve storm protection, and replace eroded nesting habitat for sea turtles. 

 

In addition to endangered sea turtles, however, there are other more common beach fauna that inhabit the upper extent and wave-swept swash zone of the ocean beach.  These organisms include the ghost crab (Ocypode quadrata), the mole crab (Emerita talpoida), and the coquina clam (Donax spp.).   (There are two species of Donax that occur in Florida, Donax parvula and Donax variabilis.  Due to the difficulty in identifying them compared to species at small size, we will refer to them collectively as Donax spp.) All three are considered indicator species for beach habitat in Florida’s Comprehensive Wildlife Conservation Strategy.   These animals provide important ecological functions in coastal environments including cycling of organic matter and trophic, or nutrition,  transfer of both primary and secondary production to surf zone fishes and shore birds. 

 

Beach organisms are adapted to living in a wave-swept environment with heavy loads of sediment and a high degree of sand movement from normal tides and storm events. What limited information we have about sandy beach inhabitants suggests minimal impacts to them associated with beach nourishment, but few quantitative studies have been conducted to confirm this general assumption (Nelson 1993, NRC 1995, Peterson and Manning 2001, CETAC 2003).  Coastal zone managers are currently making decisions regulating quality of sand, and the timing and location of sand placement based on biological considerations primarily for worm rock (Phragmatapoma spp.) and sea turtles.  Effects on other common, but ecologically important, fauna may have cascading effects on surf zone fish and shore bird populations.  Data gaps in the general ecology and spatial, or relating to a specific study area, and temporal ( seasonal, daily, and tidal) distributions of swash-zone-inhabiting mole crabs and coquina clams and upper-beach-dwelling ghost crabs on Florida beaches, however, precludes any conclusive assessments of the short and/or long term effects of beach nourishment projects on common beach fauna. 

 

Objectives

The objectives of the proposed study are to quantify temporal and spatial patterns in abundance and size of the target species at nourished and unnourished beaches along Florida’s east and west coasts.  The geographic comparison is included because the east coast experiences a mixed semi-diurnal tide (two high and two low tides of uneven height occur within a 24-hour period) while the west coast experiences diurnal tides of lower amplitude. 

 

Methods

Study Sites:  The Florida State University (FSU) Beaches and Shores Resource Center (BSRC 2004), and the Florida Department of Environmental Protection (FDEP) Office of Beaches and Coastal Systems Strategic Beach Management Plan (FDEP 2000)  provide locations and project information for past beach nourishment activities throughout the State of Florida.  In addition, project locations for upcoming nourishment activities are identified in FDEP, Division of Water Resource Management, Bureau of Beaches and Coastal Systems 2004 Hurricane Recovery Plan (FDEP 2004).  Using these resources, we will identify three unnourished and three soon-to-be nourished beaches from the Central Atlantic Coast Region and from the Southwest Gulf Coast Region.  At all locations a stretch of beach 300 meters long will be designated as the study site.

 

Quantification of Environmental Variables:  There are several physical factors that can influence the spatial distribution and abundance of sandy beach organisms including cross-shore (e.g. beach slope, presence and height of  berm, etc.) and long-shore (cusps, headlands, jetties, etc.) beach morphology (i.e. the structure of the beach), tide conditions (diurnal vs. semidiurnal, high tide vs. low tide, neap tide vs. spring tide, etc.), sediment grain size, organic content and sorting, and temperature (see McLachlan 1983 for review). To control for tidal phase and tidal stage we will restrict sampling to 2-3 days on either side of spring low tides.  Other environmental variables will be measured at the time of sampling.

At each site, on each sampling date, a 300-meter measuring tape will run at the base of, and parallel to, the dune line.  Given that the shape of the beach face can influence faunal distributions, five points will be randomly selected along the 300-meter tape for locations at which shore-normal elevation profiles will be made to quantify width and slope of the upper beach (from the toe of the dune to the edge of the berm, or high tide line if no discernable berm is present), width and slope of the lower beach (from the base of the berm, or high tide line if no discernable berm is present, to the mid-swash zone at low tide), and presence and height of the berm.  Standard surveying techniques will be employed using a rotating laser level and stadia rod. 

Variation in sand grain size and organic content among beaches whether they are nourished or not may influence faunal distribution.  Changes in abundance and recruitment of mole crabs and coquina clams have been documented in association with nourishment projects in which grain size and organic content have been altered (Reilly and Bellis 1983, Peterson and Manning 2001).  To quantify sediment characteristics at both unnourished and nourished beaches, sediment cores (5 cm in diameter by 10 cm in depth) will be collected from the middle of the upper beach, mid-lower beach (midway between high water line or berm and top of swash zone), and from the middle of the swash zone along the shore-normal elevation profile transects.  In the lab, samples will be dried at 65^oC (149^oF) to constant weight and dry sieved to determine sediment grain size distribution, mean and median grain size, and sorting coefficient.  A portion of each sample (removed from the homogenized sample prior to drying and sieving) will be analyzed for organic content by loss on ignition (i.e. amount of material lost during combustion in a muffle furnace).  Air temperature and water temperature of the swash zone will also be recorded where the five shore-normal elevation transects, or study areas, meet the waters’ edge. 

 

Determination of Abundance and Size of Emerita talpoida and Donax spp.:  Ten points along the 300-meter measuring tape will be randomly selected to identify locations for core sampling in the swash zone to quantify Emerita and Donax abundance.  This initial sample size was chosen based on Nelson’s (1993) recommendations.  Sample size may be reduced or increased following preliminary analysis of precision of estimated mean abundances (Andrew and Mapstone 1987).  Because faunal distribution may be influenced by the presence of cusps, or waves in the sand of the lower beach (McLachlan and Hesp 1984), samples within the 300-meter wide study area will be proportionally stratified, or arranged, between the cusp and inter-cusp locations, and whether or not samples were from the cusp or inter-cusp locations will be noted.  Since the swash zone migrates and Donax and Emerita may migrate with the tide (Turner and Belding 1957, Edgren 1959, Mikkelsen 1981) we will use a non-fixed point sampling method to ensure that we are sampling within the swash zone each time.  Studies on Donax populations in Florida, however, suggest that west coast populations do not migrate with the tides, but instead remained at the same elevation regardless of tidal stage (Mikkelsen 1981).  To account for the possibility that populations do not behave similarly among sites, we will sample the upper, middle, and lower swash zone for Donax and Emerita at the 10 randomly selected locations along the 300-meter transect tape at each site. 

 

Hand held cores (7.6 cm in diameter as recommended by Nelson 1993 by 10 cm deep) will be used to sample the swash zone.  Samples will be sieved in the surf through a 1-millimeter mesh.  This mesh size will retain newly recruited juveniles of both Donax and Emerita. Samples will be transferred to plastic containers with 10 percent buffered formalin. In the lab, samples will be rinsed, and Emerita and Donax will be separated from the remaining sediment, counted and measured.  After processing, organisms will be stored in 70 percent alcohol. 

 

Determination of Ocypode quadrata abundance:  The relative abundance and size of ghost crabs will be determined by counting burrows and measuring the diameter of the burrow openings (Wolcott 1978).  The 300-meter long study site will be divided into 20, 15-meter long sections.  Five of the sections will be randomly selected and the number of crab burrows in the upper beach (high tide line to toe of dune) will be counted.  Since the width of the upper beach will vary among locations the average area of the 15-meter long sample area will be determined (width of upper beach by 15 meters), and density estimates will then be standardized to a 10 meter-squared area.

 

Assessment of Beach Nourishment Effects:  Timing of sampling is critical when attempting to evaluate environmental effects to biological organisms after a nourishment project.  We will follow the general protocol recommended by Nelson (1993) of monthly sampling for at least three months prior to nourishment, bi-weekly sampling after nourishment for two months, followed by monthly sampling for an additional 9-12 months. The timing of beach nourishment activities will likely vary among study sites with nourishment projects expected to occur in the winter and early spring of 2006 and 2007.  We will begin sampling all project and control beaches in the fall of 2005 and continue sampling at monthly intervals.  Project beaches will not be monitored during active beach nourishment.  Control beaches, however, will be monitored during the period in which project beaches are being nourished to provide temporal data on abundance and size distributions of the target species for unnourished beaches.  Impacted beaches will be monitored for one year post-project.