Aquarists from the New England Aquarium frequently travel to Belize as part of a long-term research program by Aquarium scientist Randi Rotjan, PhD, to monitor coral health near Carrie Bow Cay. Today's post by Peter Gawne is part of a series about these ongoing research efforts.

In 1996 the cultural branch of the United Nations—the United Nations Organization for Education, Science, and Culture (UNESCO)—declared Belize’s barrier reef system to be a Natural World Heritage Site. The barrier reef of Belize was selected as an area of global significance due to its natural beauty, diversity of reef types, presence of endangered and unique species, and for providing a classical example of the evolutionary history of reef systems.

By Ian Morton from Punta Gorda, Belize (Belize's Barrier Reef) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

The barrier reef of Belize is the largest in the northern hemisphere and the second largest in the world (Australia’s Great Barrier Reef is the largest). There are seven protected sites which constitute the Belize Barrier Reef Reserve System. Together, these protected sites comprise 12 percent of the entire barrier reef system.

I had the privilege of visiting three of the seven protected sites on this trip to Belize: Blue Hole Natural Monument, Half Moon Caye Natural Monument and South Water Caye Marine Reserve. Despite being within the same 190-mile reef system, the sites are quite distinct, and each holds its own unique natural treasures.

The Great Blue Hole Natural Monument lies at the center of Lighthouse Reef, one of three atolls that lie seaward of the barrier reef.  The Great Blue Hole is the most famous dive site in Belize and is located 43 miles from the mainland.  This enormous submarine cave (over 980 feet across, and 400 feet deep) was originally formed during periods of glaciation, between roughly 153,000 to 15,000 years ago.  During this period sea levels were 330-390 feet lower than at present.  At the end of the last ice age, as sea levels rose, the cave was inundated with seawater, creating the sharply vertical Great Blue Hole.

A diver begins to ascend after viewing the stalactites at the Great Blue Hole

The Great Blue Hole is a deep dive, taking us greater than 130 feet below the surface to view stalactites formed beneath the ledges in the enormous cave.  The water clarity was fairly poor by Belizean standards, but that just contributed to the unusual feel of the site.  As we wove our way through stalactites Caribbean reef sharks (Carcharhinus perezi) sporadically materialized out of the gloom for just a moment, and then quickly were absorbed back into the blue.

Magnificent frigatebirds (Fregata magnifiscens) are but a few of the many birds perched atop the canopy at Half Moon Caye.  Males possess a scarlet throat pouch, which they inflate during breeding season as a mating display.

Slightly farther south, Half Moon Caye Natural Monument is located at the southeast corner of Lighthouse Reef Atoll.  While Half Moon Caye is surrounded by waters that are remarkable for their life and diversity, the real treasure of Half Moon is the red-footed booby (Sula sula) colony.  Half Moon Caye has a population of around 4,000 breeding boobies, and is considered the only viable breeding colony in the western Caribbean.  These large seabirds perch in the littoral forest canopy amidst brown boobies (Sula leucogaster) and magnificent frigatebirds (Fregata magnifiscens).

Despite their protected status, marine reserves face many obstacles: among them, invasive species (including lionfish, Pterois volitans, pictured), unregulated fishing,and development.

Stay Tuned for part two of Pete's travels along Belize's marine World Heritage Site. And catch up on previous trips to Belize—lots more amazing pictures!

• See the beauty of hermit crabs and ride out a tropical storm during their 2013 trip
• Learn more about threats to corals, plus signs of a late-night visitor to Carrie Bow Cay, in 2012
• See what other researchers are up to at the research station in 2011
• And read the exciting post where the marine protected area was announced in 2010