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Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

Systematic Mapping of Bedrock and Habitats along the Florida Reef Tract—Central Key Largo to Halfmoon Shoal (Gulf of Mexico)

USGS Professional Paper 1751

by Barbara H. Lidz, Christopher D. Reich, and Eugene A. Shinn

Introduction:
Table of Contents
Project Overview
Project Objective
Geologic Setting
Primary Datasets
Primary Products - Overview Maps & Evolution Overview:
Bedrock Surface map.
Introduction
Depth to Pleistocene Bedrock Surface
Reef & Sediment Thickness
Benthic Ecosystems & Environments
Sedimentary Grains in 1989
Summary Illustration Index Map
Evolution Overview
Tile-by-Tile Analysis
Satellite image of the Florida Keys showing location of tiles.
Organization of Report
Tiles: 1, 2, 3, 4,
5, 6, 7/8, 9/10,
11
Summary
Acknowledg-
ments
References
Disclaimer
Related
Publications

Primary Products

Overview Map—Sedimentary Grains in 1989: Carbonate sands in a reef ecosystem are composed solely of skeletal grains (e.g., Enos, 1977). Different organisms occupy different niches depending upon location, e.g., quiet nearshore waters vs. offshore surf zones, or opposite protective landmasses vs. open tidal passes. In Florida, sands are formed in place and essentially remain in place (Ball, 1967). With the exception of offshelf transport in the southwest, intense wave action of storms and hurricanes redistribute but do not remove the sands far from their place of origin.

Florida's sands consist of three primary types of skeletal grains: those of the calcified green alga Halimeda, molluscs, and corals (Fig. 12; see Sedimentary Grains map and Open-File Report 97-453; Ginsburg, 1956; Swinchatt, 1965; Lidz and Hallock, 2000). In terms of percentages, coral grains are the most important. Coral sands are produced when corals are weakened by environmental (disease, turbidity, or contaminants) or mechanical damage (divers, anchors, storms, or ship groundings) and their injured skeletons become targets for attack by other organisms (bacterial diseases, grazing parrotfish, and boring algae, barnacles, fungi, molluscs, and sponges). Butterflyfish eat coral polyps, and the sea urchin Diadema antillarum grazes algae and keeps corals clean; in so doing, they both also chew off bits of coral skeleton.

Sedimentary Grains in 1989 map: Thin-section analyses of surface sediments collected shelf-wide in 1989 indicate three types of skeletal grains dominate sands off the Florida Keys: Halimeda, coral, and mollusc. (A) In 1989, Halimeda grains were dominant along the entire inner shelf, mollusc grains were dominant along portions of the outer shelf off the upper Keys, and coral grains were dominant along the outer shelf off the middle and lower Keys. (B) The highest proportions of Halimeda grains were found in inner-shelf sediments off the northern upper Keys and the lower Keys. (C) Mollusc grains generally composed <10% of skeletal grains off the middle Keys but were ~20% elsewhere. (D) Coral-grain counts were highest off the middle and lower Keys and correlated with reef decline as observed in the field (E). Diagonal red lines show general island divisions into the upper, middle, and lower Keys.
Sedimentary Grains in 1989 map: Thin-section analyses of surface sediments collected shelf-wide in 1989 indicate three types of skeletal grains dominate sands off the Florida Keys: Halimeda, coral, and mollusc. (A) In 1989, Halimeda grains were dominant along the entire inner shelf, mollusc grains were dominant along portions of the outer shelf off the upper Keys, and coral grains were dominant along the outer shelf off the middle and lower Keys. (B) The highest proportions of Halimeda grains were found in inner-shelf sediments off the northern upper Keys and the lower Keys. (C) Mollusc grains generally composed <10% of skeletal grains off the middle Keys but were ~20% elsewhere. (D) Coral-grain counts were highest off the middle and lower Keys and correlated with reef decline as observed in the field (E). Diagonal red lines show general island divisions into the upper, middle, and lower Keys. Data from Lidz and Hallock (2000).

Halimeda grains dominate The Quicksands (Hudson, 1985). Sedimentary data for The Quicksands were not included in the map because they are not contiguous with the keys data. Sediment sampling and analyses have not been conducted between Key West and the Marquesas Keys. Halimeda- and coral-grain percentages in The Quicksands are contoured in Shinn et al. (1990). [larger version]

Coral grains dominate sands where coral health is poor (Lidz and Hallock, 2000). Halimeda and mollusc sands dominate where coral health is better. Throughout the reef tract, soft-coral/algae/sponge communities influenced by turbid inshore waters are replacing reef-building corals that require clear oceanic waters. Geographically, the healthiest reefs are found off landmasses, and the least healthy occur off the widest tidal passes. Tidal passes link turbid, nutrient-enriched and winter-chilled waters from Biscayne Bay, Florida Bay, and the Gulf of Mexico to the clear, nutrient-poor warm waters of the reef tract (e.g., Roberts et al., 1982). Spatially, field observations show a dual seaward and southwestward trend in direction of reef decline along the reef tract. The coral-grain component of the sand mimics this trend in that grain percentages increase in both seaward and southwestward directions (Lidz and Hallock, 2000).

Based on coral-grain percentages obtained from sediment samples collected in 1989, (D) and (E) on the Sedimentary Grains map show a general guideline correlation between reef health and sand composition: healthiest reefs tend to co-occur with sands of <10% coral grains, declining reefs tend to co-occur with sands of <30% coral grains, and senescent (non-accreting or essentially dead, the senile reefs of this report) reefs tend to co-occur with sands of >30% coral grains (Lidz and Hallock, 2000). This correlation does not hold near Miami, where reefs are senescent yet coral-grain percentages are low. The cause is the lengthy absence of the primary biological source of coral sands, Diadema antillarum (Lessios et al., 1984), and the proliferation of algal-molluscan communities due to nutrient enrichment of the water (e.g., Hallock and Schlager, 1986; Hallock, 1988). Sands in approximately two-thirds of the study area from Miami to Key West contain up to or greater than 30% coral grains. In some areas, notably along the outer margin off the middle and lower Keys, coral-sand grains exceed 60%. These elevated percentages correlate with the observed decline of coral reefs.

Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

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