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Environmental quality and preservation; reefs, corals, and carbonate sands; guides to reef-ecosystem health and environment

April 1, 2001

In recent years, the health of the
entire coral reef ecosystem that lines
the outer shelf off the Florida Keys
has declined markedly. In particular,
loss of those coral species that
are the building blocks of solid reef
framework has significant negative
implications for economic vitality of
the region. What are the reasons for
this decline? Is it due to natural
change, or are human activities (recreational
diving, ship groundings,
farmland runoff, nutrient influx,
air-borne contaminants, groundwater
pollutants) a contributing factor
and if so, to what extent? At risk
of loss are biologic resources of the
reefs, including habitats for endangered
species in shoreline mangroves,
productive marine and wetland
nurseries, and economic fisheries.
A healthy reef ecosystem builds
a protective offshore barrier to catastrophic
wave action and storm
surges generated by tropical storms
and hurricanes. In turn, a healthy
reef protects the homes, marinas,
and infrastructure on the Florida
Keys that have been designed to
capture a lucrative tourism industry.
A healthy reef ecosystem also protects
inland agricultural and livestock
areas of South Florida whose
produce and meat feed much of the
United States and other parts of the
In cooperation with the National
Oceanic and Atmospheric Administration's
(NOAA) National Marine
Sanctuary Program, the U.S. Geological
Survey (USGS) continues longterm
investigations of factors that may
affect Florida's reefs. One of the first
steps in distinguishing between natural
change and the effects of human
activities, however, is to determine
how coral reefs have responded to
past environmental change, before the
advent of man. By so doing, accurate
scientific information becomes available
for Marine Sanctuary management
to understand natural change and
thus to assess and regulate potential
human impact better. The USGS studies
described here evaluate the distribution
(location) and historic vitality
(thickness) of Holocene reefs in South
Florida, relative to type of underlying
bedrock morphology, and their varied
natural response to rising sea level.
These studies also assess movement
and accumulation of sands, relative
to direction of prevailing energy, and
origin of the component sand grains.
Geophysical data collected with highresolution
sound-wave instruments
that provide pictures of the sediment
and bedrock are used to interpret
sediment thickness. Reef thickness is
determined by collecting limestone
rock cores by drilling. Drill cores
through reefs are used to identify the
coral species that built them and to
determine how reefs reacted to rising
sea level. These data are supplemented
by using isotope-dating techniques to
derive the carbon-14 (C14) age of the
corals and mangrove peat in the cores.
Mangrove peat forms in very shallow
water and at the shoreline but is found
today buried beneath offshore reefs.