Q: We just got back home yesterday from a trip to the Keys and snorkeled at Looe Key as well. My daughter feels the coral looked very different, almost unhealthy, compared to the last time she visited several years ago. I wonder if she just didn't see a different part of the reef than her last trip or if indeed the reef is more stressed now than at her previous trip, and possibly becoming sick. (The last time she was there, she saw very beautiful colors, and this time, we saw dark colors as well as grey and white areas on the coral.) I realize there are environmental factors and the reef is frequently visited, possibly adding to the stress on the reef. Can you shed some light on this matter? -- Cindy

A: You are right, Cindy. You have partially answered your own question because coral reefs have been known to be seriously declining for the past 35 years or so -- not just in Florida, but around the world -- and there are several factors contributing to it.

Scientists continue to study coral reefs to discover what is causing them to die and become unhealthy. Some factors scientists have uncovered include:

• Poor water quality due to sewage that is dumped from boats and coastal cities, fertilizer runoff from coastal lawns, and gas and oil spills -- causing algae to grow in the corals.
  
  
• Direct damage to coral from divers, fishermen, boaters and storms.
  
  
• Over-fishing.
  
  
• In the Keys, reduced fresh water flow from Florida Bay (likely the result of reduced fresh water flow through the Everglades), which has led to plankton blooms and fish die-off, according to the National Oceanic and Atmospheric Administration (NOAA).
  
  
• Coral-killing diseases whose cause still isn't fully known; some scientists believe dust carried on the wind from Africa, which settles on Caribbean-area coral and deposits a fungus, is believed to affect sea fans especially hard, according to the U.S. Geological Survey.
  
  
• Rise in sea levels (thought to be caused by an increase in carbon dioxide in the environment), which takes sunlight farther away from corals.
  
  

The white and gray areas you saw on the coral is an indication of what's called bleaching. According to a branch of U.S. space agency NASA, coral bleaching is a direct result of greenhouse gas emissions that warm the surface of the ocean.

Many people depend on reefs in several ways, as you know. For example, reefs act as nurseries and breeding areas for many types of fish that people like to eat. NOAA has reported that the Keys depend on the reef there for 8,000 jobs and $106 million in income, and estimated the reef's economic value (if you could place one) at $7.6 billion.

This is money that you and your daughter likely helped to provide on your visit. So you can see that the health of Florida's reef is important to many people, not just Floridians. The coral reef tract in Florida is considered to be the most extensive in North America, and it's the 3rd largest reef system in the world after those in Australia and Belize, according to NOAA. So it's important to protect it.

People who want to help protect and restore coral reefs can join advocacy organizations like Reef Relief; let their elected officials know that the health of coral reefs is important to them; and take care when boating, diving, snorkeling and fishing near reefs. Stressing the need for a reduction in greenhouse gas emissions, water pollution and general population may help. In addition, people who live near coasts can eliminate pesticide and fertilizer use on their lawns, and push for better sewage treatment and containment.

Thanks for your concern about Florida's reefs.

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