First Hand reports of the effects of cold water stress in the Florida Keys
Reef Builders is proud to welcome Collin Foord to the RB writing block. Collin is one of the founders of Coral Morphologic, a long time sponsor of this blog. Through their frequent diving throughout the Florida Keys searching for cool animals to culture and capture, the Core-More team comes across some great marine life right here in our own US waters. We look forward to reading more from Collin about the U.S. coral reef beat.
During the first two weeks of January much of the United States was engulfed in a mass of frigid arctic air that broke low temperature records across the country. The effects of this arctic air were felt particularly hard in South Florida where cold-blooded animals depend on sub-tropical temperatures to stay alive. With two weeks of overnight low temperatures in the 40’s (and even lower), many of these warmth-dependent creatures suffered. On land that meant a lot of catatonic iguanas falling from trees, but in our coastal waters, death was considerably more prevalent.
I had to do some (rather cold) diving and snorkeling in the days following the coldest temps and found a mix of both heartening and disheartening news. The good news is that nearly all of the corals, zoanthids, corallimorphs, and other marinelife living on the offshore reefs made it through unscathed. The water temperature where I was diving was about 65 degrees which is the coldest I’ve ever experienced offshore, and it may have been a degree or two colder on the coldest mornings. On these reefs, 5-7 miles offshore, the cold air was buffeted by the perennial warmth of the Gulf Stream current.
Die-off was particularly severe in the shallow waters of the Florida Bay where the lack of deeper, warmer water resulted in temperatures in the mid 50’s. Popular game fish that prefer this shallow habitat suffered particularly badly. The mortality rate of snook, bonefish, and tarpon was so severe that the state of Florida has made an emergency closure to the fishery. In the protected water of the marina in Key West where we keep our collection boat, temperature was only 55 degrees in the morning. Typically there are scores of tarpon that cruise the marina waiting to eat discarded fish scraps from the commercial fishermen, but even a week after the coldest weather, the tarpon still had not returned. Multiple reports confirm ‘thousands’ of game fish carcasses piling up amidst the mangrove roots in the backwaters of the Everglades. Now two weeks after the last of the cold, the shoreline of the Florida Keys still reeks of decomposing marine life. Distasteful fish like boxfish remain uneaten by scavengers amongst the mangrove roots, their bodies now white and only identifiable by their square bony skeletons.
The intermediary habitats located between the offshore reefs and the seagrass meadows of Florida Bay displayed a range of impacts from the cold. The severity seems dependent upon how much exposure they received from the outgoing, cold water tides from Florida Bay. With the Florida Keys acting as a barrier between Florida Bay and the Atlantic Ocean, tidal currents are forced to travel through the channels that divide the islands. During the cold weather, warmer water is brought close to shore through these channels to the Bay on a high tide. On a low tide, the cold water runs back out the channels and out into Hawk Channel. Hawk Channel acts as the main thoroughfare of this colder bay water up along the oceanside of the Keys (See the map for a diagram of water movement in the Keys). On the patch reefs that dot Hawk Channel in the Lower Keys, effects from the cold were fairly minimal. Acropora cervicornis, a typically cold-sensitive species, did not seem affected on these channel patch reefs near Key West. However, according to reports from researchers in Key Largo, cold water bleaching and die-off seemed to be much more prevalent in the Upper Keys. This is likely due to the increasing accumulation of cold water running out of Florida Bay, northwards up Hawk Channel.
The greatest amount of die off that I observed occurred in the very shallow water (<5′) of the Middle Keys near a tidal channel that runs between the the bay and the ocean. This near-shore habitat is characterized by a mix of macroalgae, gorgonians, and a few hardy coral species. Here I found that about 90% of the finger coral (Porites porites) were dead. However, the much more cold-tolerant Siderastrea and Cladocora corals showed no signs of stress or bleaching. Sponge die-off was also quite severe, as evidenced by the anaerobic ‘white fungus’ mats covering a variety of species. Unfortunately, one of the largest colonies of Discosoma carlgreni I knew of in the Keys appears to be entirely wiped out with no trace left. Short spine urchins (Echinometra spp.) showed around 2/3 mortality. Particularly concerning was the total die-off of the federally protected queen conchs (Strombus gigas) in this habitat. Hopefully this widespread die-off of herbivores was relegated to only these shallow waters. A lack of these herbivores in the short-term may result in unchecked macroalgae growth, choking out suitable habitat for coral re-colonization.
In addition to the saltwater fish die-off, the cold snap was particularly painful for the tropical freshwater tropical fish farmers who experienced die-offs in excess of 50%. Furthermore, much of the freshwater tropical fish farms are located around Tampa where temperatures dropped into the 20’s for multiple nights. Unprotected fish farm ponds even froze over. On the flipside, the inability of exotic invasive freshwater fish to cope with the cold water has been a boon to the Everglades’ native fauna. Florida’s firmly established non-native cichlid and catfish populations suffered greatly. I’ll bet that Everglades wildlife officials are quite pleased that this cold weather event has likely made their fight against the much publicized Burmese python invasion much easier.
1977 was the last time Florida reefs experienced significant cold weather bleaching and die-off. That event, which claimed acres of pristine staghorn corals, preceded a series of crippling blows to our Acropora populations. The effects of white band disease and a long-spined urchin (Diadema antillarum) plague in the early 1980’s has resulted in nearly 95% loss of Acropora coverage. Fortunately, it doesn’t seem like this important species suffered much from the recent cold snap. In near-shore waters, where organisms didn’t fare as well, coral reef researchers will be afforded a unique opportunity to monitor how habitats and ecosystems adjust over the coming months and years. In the age of global warming and warm water bleaching, this cold water event demonstrates how only a few days of abnormal weather can wipe out otherwise healthy communities on our Floridian reefs. These die-offs are clearly a natural, if infrequent, occurrence that serve as a reminder that we are still a sub-tropical climate hosting tropical species at the northern limit of their distribution.