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Last updated: January 15, 2013
South Florida Restoration Science Forum

Our Coastal Ecosystems

The Florida Keys: What is happening to the reef tract and why?

Part 3: Human and Natural Threats on Reef Health


Both human and natural threats can cause deleterious effects on coral reef health. Hurricanes sweep across shallow water depths where corals live, creating swift currents and pounding waves that can overturn, rip up, and move around rather large corals. In recent years, El NiŅo has become a household term. El NiŅo's effects on corals are regional to global in scale. The primary result is higher than normal sea surface temperatures, causing symbiotic algae living in coral tissue to be expelled and making the coral appear to turn white. This outcome is called coral bleaching and can result in the death of the coral if the high water temperatures are consistent for a long period of time.

Human threats, which can be numerous, are displayed here. A recent study using fluorescent dyes has discovered that natural ground-water flow is in the direction from Florida Bay to the Atlantic Ocean. This direction indicates that if any pollutant enters the groundwater system on land, the possibility that it is transported toward the reef tract is great. The cause for ground-water flow was determined to be the result of tidal pumping: a phenomenon caused by the tidal fluctuation on the ocean side of the Florida Keys and the lack of tides on the bay side.

Natural Threats


coral rubble
fracturing of spur formation
Spill-over of coral rubble after hurricane Georges
Fracturing of a spur formation at molasses reef after hurricane georges

hurricane track lines (Click on map above for full-sized version.)

Hurricane track lines from 1886-1992 that passed within 75 nautical miles of Miami, FL.

sediment loss at coral head
elkhorn rubble
Removal of approximately 8 inches of sand from the base of this coral head after hurricane Georges
Elkhorn rubble at Grecean Rocks after hurricane Georges

elkhorn coral branches Entire elkhorn coral branches turned over after hurricane Georges, 1998 (left) and after hurricane Donna, 1960 (below).
  elkhorn coral branches

El Niño and Coral Bleaching ...

How does El Niño affect corals

Increased sea surface temperatures during El Niño events, such as the large one during 1997/1998, can cause corals to bleach. Bleached corals have been stressed enough by higher water temperatures that they expel the symbiotic algae that live within the corals tissue. Some corals may live after the algae are expelled but many die. The symbiotic algae gives the coral its distinct coloration.
star coral
brain coral
elkhorn coral
bleached sea fan
large star coral

Human Threats

illustration of human threats

(Photos below correspond to map above.)
Photo A
septic tanks
Photo B
injection well

In the Florida Keys there are:
  • ~30,000 septic tanks
  • 5,000 to 10,000 cesspools
  • 100-1000 class V injection wells
  • a large sewage outfall off key west
  • frequent aerial spraying for mosquitos
  • boat groundings and anchor dragging on reefs
  • water from the everglades that originates along the kissimmee river watershed, lake okeechobee, and the farming areas to the north may eventually reach Florida Bay and the reef tract.

tracer sites
(Image showing tracer sites. Click on image for full-sized version.)
Research on groundwater flow in the keys has determined that:

  1. the net direction of flow is to the east/southeast (white arrows on image).
  2. flow speed is up to ~8 feet per day.
  3. direction of flow depends on the ocean tides.
  4. on average, florida bay water level is higher (~8 inches) than the ocean water level

Tidal pumping drives groundwater flow
(Click on images below for a full-sized version.)

  Florida Bay   Atlantic Ocean
Key Largo
Bay Level Constant
  Mean Tide
illustration of tidal pumping driving groundwater flow
Bay Level Constant
  Mean Tide
ocean water level and bay water level are same
Bay Level Constant
  Falling Tide
illustration of tidal pumping driving groundwater flow

A Ocean tide comes in (flood tide) and creates a head difference that causes water to flow toward Florida Bay. 

B Ocean water level and Bay water level are the same and therefore there is no head difference and no flow. 

C Ocean tide goes out (ebb tide) and is now lower than the Bay water level. This creates a head difference for water to flow toward the ocean.

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U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology
This page is: http://sofia.usgs.gov/sfrsf/rooms/coastal/flbay/threats.html
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Last updated: 15 January, 2013 @ 12:44 PM (KP)