CRYSTAL RIVER, Fla. - Hundreds of heat-seeking manatees are packing the warm water springs at a Tampa Bay area national wildlife refuge.

That's prompted officials to close those waters to humans to safeguard the protected marine mammals.

Manatees, the gentle puppy-faced creatures, are extremely temperature sensitive. They arrive in the Lowcountry by the dozens to summer each year, but return to Florida en masse when water temperatures drop below 70 degrees.

Roughly 300 manatees have packed into the canal leading to the Three Sisters Springs at the Crystal River National Wildlife Refuge. On Tuesday, officials instituted an emergency closure of the springs to activities such as swimming and kayaking so that the manatees would not be disturbed.

The closure was scheduled to continue through Wednesday afternoon or longer. Visitor services specialist Ivan Vicente told the Tampa Bay Times that the closure will continue to keep the manatees "undisturbed for as long as possible during this cold spell."

Manatees seek out warm waters when temperatures drop. Conservationists say that from the air, you can literally see a train of them moving south along the Southeast coast in the fall.

But they are slow moving. In late October 2013, as Lowcountry water temperatures dropped below that 70-degree mark, biologists were alarmed by continued sightings of the animals here. Two manatees had turned up dead from unknown causes at the end of September in Socastee along the north coast and Hilton Head on the south coast, according the National Ocean Service marine mammal stranding program.

But the stragglers eventually did migrate, apparently; no more sightings were reported, according to the Charleston field office of the U.S. Fish and Wildlife Service.

Post and Courier reporter Bo Petersen contributed to this story.