The Sandy Beach
The beach comprises the strip of land where the ocean laps onto the land, forming the shoreline. Because the water level varies with the tide (0 – 2.7 m in Georgia), the shoreline moves up and down against the shoreline forming several zones. The area between high tide and low tide level, termed the intertidal zone, is the backshore and foreshore; the area above high tide level, the supratidal zone, and forms an area we’ll call the back-beach. The intertidal area, between high and low tide forms the foreshore, comprised of the upper foreshore and the lower foreshore; the area below low tide level is called the subtidal zone. The foreshore in Georgia is comprised of the upper foreshore with a slope of about 2o, separated from the lower foreshore with its slope of about 1o, by the first appearance of the burrows of the Carolinean Ghost Shrimp at mid-tide level.
Beach morphology of barrier islands in Georgia. The backbeach lies behind the spring high tide line, the beach between high and low tide lines, and the subtidal below low tide level. The boundary between upper and lower forebeach is the highest carolinean Ghost Shrimp burrows. (Modified after Rodrigues & Shimizu, 1995).
1. Back-Beach Habitat
The back-beach is the area between normal high tide levels and the storm high tide level, i.e. the area swashed by storms. Normally a fringe of vegetation gains a foothold on this area, only to be washed out by the next storm. The storm high tide level is commonly marked by a storm wrack line, an erosional scarp, or the lowest vegetation on a beach.
Upper foreshore and back-beach, North Beach with washed in Spartina wrack mat.
Backbeach on South Beach showing high tide line and storm high tide line marked by scarped dune ridge.
Backbeach on North Beach with dune ridge forming as wind blows sand from beach into vegetation on dune front.
The back-beach is inhabited by a few permanent residents, including abundant Ghost Crabs, less common voles, and some insects, and by a host of transient visitors who often leave track evidence of their ephemeral visits. This zone is used by shore birds and sea turtles as nesting habitat, a game trail by hogs, and as a resting area by deer.
2. Upper Fore-beach Habitat
The upper forebeach or upper foreshore comprises the area from normal high tide levels to the change in slope commonly found on Georgia beaches, at about mid-tide level marked by the highest Ghost Shrimp burrows.
This zone is a virtual desert environment, fully exposed to weather and sunlight most of the time. Inhabitants either live in the sand or migrate in and out of the environment. The most abundant organisms are the haulostoid amphipods (sand fleas) that bioturbate the substrate and destroy the backbeach laminations formed by swashing waves. Burrows of Atlantic Ghost Crabs, Ocypode quadrata (Fabricius, 1787), are abundant and marked by volcano-like piles of burrowing pellets at the entrances to their J-shaped burrows that are excavated a meter or two into the backbeach sand. Haustorid amphipods, also known as sand fleas, inhabit the upper forebeach and the lower forebeach, and are active bioturbators of the beach.
Paired Ghost Crabs copulating on the rain-spackled backbeach, St. Catherines Island.
Burrow of Ghost Crab through backbeach heavy mineral deposit, but penetrating into quartz sand.
Bird tracks crossing hatchling crawlways on backbeach indicates the visitation by large birds.
3. Lower Forebeach Habitat
The lower forebeach (foreshore) has a gentler slope on Georgia beaches. Its upper margin is also usually marked by the highest Carolinean Ghost Shrimp burrows. This desert-like environment again hosts organisms capable of burrowing or migration, although these effects diminish the lower one is on the beach, because the length of exposure decreases. As the low tide line is approached, the number and diversity of inhabitants increases.
Burrows of the Carolinean Ghost Shrimp are found from mid-tide level into subtidal waters.
When Carolinean Ghost Shrimp burrows erode, they stand in relief as the “trace fossil” Ophiomorpha.
Many invertebrates cruise under the sand to feed, like this Sand Dollar.
A specimen of Dinocardium robustum (John Lightfoot, 1786), the Atlantic Giant Cockle washed onshore.
Crepidula fornicata (Linnaeus, 1758) on Oliva sayana (Ravenel) (Courtesy of Jacksonville Shell Club at http://www.jaxshells.org/jsc.html)
A small shark foraging in the surf as a fish makes its escape. The subtidal zone is home to many invertebrates and vertebrates.
The upper foreshore hosts abundant haustorid amphipods, worms, interstitial organisms, and Carolinian Ghost Shrimp. Near the low tide water line it is common to find all the above plus sand dollars, sea stars, stomatopods, gastropods (Busycon, Oliva, Polinices, Sinum, and Turtitella), bivalves (Dinocardium, razor clams, tellinids, Atlantic Pen, etc), arthropods (the crabs and horseshoe crab) and washed-in fauna from the shallow subtidal habitat.
Blood Arcs and Giant Atlantic Cockle washed ashore onto the upper foreshore forming a small shell bed.
Knobbed Whelks are abundant and characteristic shells of the Atlantic (Jacksonville Shell Club).
Donax variabilis is a small surf clam that migrates with the tide. (Jacksonville Shell Club).
4. Tidal Sand Flats or Shoal Habitat
In areas where high velocity currents slow, as at the shoulders of the Island and low in tidal channels like the edges of McQueens Inlet, mud is deposited along with the sand, forming fluidized, muddy sand tidal flats at mid-tide levels. These tidal flats tend to be broad and level, and covered with flood or ebb structures and abundant ripple marks.
St. Catherines ebb delta forms a large tidal flat on the NE shoulder of the Island.
The fluidized sand of the tidal flat is home to small Ghost Shrimp that excavate straight burrows.
Surface sample from ebb delta tidal flat showing Scolithus burrows of Georgian Ghost Shrimp.
The fluid-like substrate is home to a host of worms, small Georgian Ghost Shrimp (Biffarius biformis (Biffar, 1971))gastropods (Busycon carica,Busycoptypus canaliculatus,Oliva sayana,Sinum perspectivum, and Terebra disolocata), bivalves, the Stone Crab, Menippe mercenaria, and adult and juvenile horseshoe “crabs,” Limulus polyphemus. The abundance of B. biformis, present in densities up tp 480 burrows/m2, helps keep this sediment bioturbated and fluidized. Evidence of visitors while the flats are submerged is afforded by shallow “vacuum” depressions of stingrays, crawlways of hermit crabs, snails, and horseshoe “crabs.”
Last updated: 7/28/2016