When cruise ships lumber into Key West’s shallow harbor, they can sometimes churn up a storm of sediment more potent than a hurricane, new monitoring by the city shows.
A year’s worth of tracking by the College of the Florida Keys found 32 events where turbidity measurements not only exceeded limits set by the U.S. Environmental Protection Agency but often reached levels above what hurricanes like Helene and Milton generate when they pass by, according to Patrick Rice, principal investigator for marine research at the college.
“There's 32 events that exceed those turbidity standards over the past year. So it's like having 32 hurricanes, basically,” Rice told city commissioners during an update last week.
“And none of those 32 events were an actual hurricane?” asked Commissioner Monika Haskell.
“No,” Rice responded. “Those were all associated with the cruise ships.”
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Key West hired Rice to monitor turbidity, which can damage sea life including coral and seagrass, after state lawmakers and Gov. Ron DeSantis pre-empted a local effort to limit massive ships sailing into the port where channels are only about 34 feet and most ships have a draft of 27 feet. Port Miami, by comparison, is between 50 and 52 feet deep.
The city has since enacted its own ordinance on a city-owned docks, limiting ship arrivals to one a day at the only privately owned Pier B. In March, DeSantis and the cabinet agreed to expand the lease space at Pier B for 25 years to allow larger ships.
The city launched the study as part of an effort to improve water quality with a citywide master plan to better protect waters around Key West and several smaller islands within city limits.
In addition to turbidity monitoring, testing will also begin for chemicals found in sunscreens. An effort to ban sunscreens containing oxybenzone and octinoxate was also overturned by DeSantis. Other testing will look for evidence of sewer discharges in waters and potential leaching from an old landfill on Stock Island.
While the EPA sets a turbidity limit of 29 NTUs — a unit of measurement to determine the amount of cloudiness liquid — a 2021 Florida Department of Environmental Protection study found the limit needs to be higher to protect wildlife and recommended revising the rule.
The Clean Water Act requires the state to update its water quality rules every three years to reflect new findings. In September, FDEP again postponed a revision while it continues to study the matter.
Since a monitor was installed in late October 2023 near Mallory Square, 244 ships have motored into the harbor, ranging in size from small vessels capable of holding about 40 passengers to massive ships carrying more than 3,900, the study said.
Spikes in the amount of suspended sediment coincided with the arrival and departure of the big ships, Rice said, possibly caused by how the ships maneuver in and out of the shallow harbor.
“When the ships come in, especially on a windy day, they have to use their bow thrusters,” he said. “The ship channel is not that deep. The hull, depending on the size of the vessel of course, is not far removed from the bottom of the sea floor.”
At times, monitoring showed the clouds of sand lingered and moved with the tide, dissipating as it washed out but returning later when the tide washed back in. Blocking out the sunlight or settling can damage coral and other sea life.
Sediment churned up by the Port Miami dredge in 2014 killed about a half million corals on the reef nearby. Plowing up the bottom can also stir up toxins trapped in the sand, including bacteria that produces hydrogen sulfide and algae that fuels red tide, Rice said
Monitoring found the amount of sediment can spike at both shallow and deeper depths. In December, monitors detected turbidity at more than twice the EPA limit, and nearly equal to levels measured when Hurricane Debby passed Key West about 100 miles to the west in early August. The levels were about the same for Helene and Milton. Nine hours later, turbidity measured nearly four times the limit, coinciding with a ship’s departture. In August, a ship churned up nearly seven times the allowed limit.
The tight harbor means that massive ships need to thread a narrow route in and out. The channel leading into the port is about 34 feet deep and only about 300 feet wide, with reefs rising on either side as high as 16 feet. Many of cruise ships that sail in and out are just six to seven feet from the bottom and can be as wide as 128 feet.
Dredging the harbor could eliminate some of the sediment, Rice said. But he warned that it would need to be managed carefully.
“A maintenance dredge, just removing the top layer of sediment, not making it deeper so the bigger ships can come in,” he said.
With coral reefs and seagrass meadows already stressed from rising ocean temperatures - a 2023 heat wave killed swaths of coral in the nearby Florida Keys National Marine Sanctuary including the last stands of wild elkhorn - Rice said he hopes to add more monitors to better understand how currents and tide move the turbidity around.
“The goal is to provide that information,” he said. “But for now, all I can see is by measuring the tide and seeing if it's an incoming tide or an outgoing tide.”
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