ABSTRACT

Analysis of aerial photographs and nearshore surficial sediment samples from the northern islands of the west-central barrier system of Florida indicates that 1) seagrass beds in the nearshore zone have controlled onshore/longshore sand transport, and 2) resulting sedimentary accumulations within nearshore seagrass beds make differentiation between nearshore and backbarrier facies difficult.

Between 1957 and 1967, an extensive seagrass community occupying the nearshore zone off Anclote Key disappeared, thus allowing the sudden and rapid onshore and longshore transport of sand. The 1,000-year-old barrier island lengthened 30 percent by recurved spit growth in a 15-year period from 1967-82. Although there are no direct observations, four possible causes of seagrass mortality have been postulated, which are as follows: 1) physical destruction by storms; 2) infection by pathogens; 3) decline of water quality due to human development; and 4) overgrazing by sea urchins.

Because of the ability of seagrasses to trap fine-grained sediments, contribute organic matter, and provide for low-energy, sheltered molluscan biocoenose, strong similarities remain between surficial sediments of recently devegetated, active nearshore zones and backbarrier/lagoonal facies. The stable carbon isotopic ratios and the molluscan assemblages within the ubiquitous fine quartz sands, in particular, are similar within these two normally disparate environments.

This study indicates that the development and destruction of benthic floral communities should be considered as a process that generates episodic/cyclic depositional events in the sedimentary record. Additionally, such changes in seagrass communities should be expected to present a blurred distinction between nearshore and backbarrier sedimentary facies.