Monitoring the Effects of Beach Nourishment

The coquina clam Donax variabilis is a common inhabitant of the intertidal zone of Florida's sandy beaches. Known for their highly variable color patterns (right), coquinas can be found buried just under the surface of the sand in the wave-swept area of the beach known as the swash zone. These algae filter-feeders represent a critical food source for fish, crabs and shore birds. Because their naturally high abundance and ecological importance, coquina can be used as an indicator species for beach communities.

Although they are not mollusks, other fauna, or animal life from the same region, commonly found on the beach includes the mole crab Emerita talpoida (right) and the ghost crab Ocypode quadrata (below right). Like coquina, mole crabs are filter feeders that burrow in the fluid sand of the swash zone. Ghost crabs are found on the upper portion of the beach, above high tide. These animals normally reside in burrows (below left) during the day and forage at night. Very little is known about the effects of beach nourishment on the natural populations of these fauna.

The Molluscan Fisheries group at the Fish and Wildlife Research Institute began monitoring the physical attributes and invertebrate populations of six Pinellas County beaches in 2005. Belleair Shores, Redington Beach and Fort De Soto Beach were chosen as control sites while two locations along Indian Rocks Beach and one at Indian Shores were within the construction area of the Sand Key Beach Renourishment Project.

All sites were sampled monthly from October 2005 through July 2007. A 300-meter transect was laid parallel to the water line and samples were collected at 10 random locations along its length. If cusps were present along the shore, fauna samples were distributed equally between cusps and inter-cusps (see figure). Sampling was not conducted at any site while nourishment construction was present. The diagram below illustrates the sampling scheme used in this study relative to typical beach formations and zones.

Scientists measured the vertical profile of each beach at five random locations along the 300-meter transect. Vertical profiles extended from the upper boundary (a seawall, property line, three meters into the vegetation, or the base of a dune) below sea level and the swash zone. Researchers used a surveying scope, stadia rod and transect tapes to measure the shape, width and elevation of beaches (right).  View graphs of the beach profiles

Sediment cores were collected along vertical profile transects from the upper beach, lower beach and swash zone. These samples were collected to compare changes in grain size and organic matter resulting from nourishment filling. View changes in sediment composition over time

Surveys of ghost crab burrows were conducted at five random locations along the 300-meter transect on the upper beach to estimate changes in population abundance. Each sampling area was 15 -meters wide along the 300-meter transect line. Burrows were counted to estimate population abundance. The diameter of each burrow was recorded to estimate relative size of crabs along each beach. View graphs of ghost crab populations

Ten locations along the 300-meter transect line were randomly selected for faunal sampling. A metal scoop lined with 1mm mesh was used to collect samples from the high-, mid- and low-swash zone at each of the 10 locations. Samples from the swash zonewere carefully sorted to find coquina and mole crabs. These animals were counted and measured to monitor the natural population abundance, growth and changes following nourishment. View graphs of fauna abundance.

Map of Pinellas County and Sand Key Nourishment Area

Pinellas County Coastal Management website - Beach Nourishment

For more information on crab research, visit the crustacean section.