Thousands of fish turned up dead in Biscayne Bay. Coral bleaching might be next

Close up of polyps are arrayed on a coral, waving their tentacles. There can be thousands of polyps on a single coral branch.
Credit: Wikipedia

Fish may not be the only victims of the pollution and hot temperatures that drove oxygen to insufficient levels in Biscayne Bay and led to a mortality event that shocked Miami residents last week.

Coral reefs in the bay risk bleaching if  don’t improve soon, scientists said. Prolonged periods of high ocean temperatures cause coral to expel the algae that live inside them, leaving them more vulnerable to stressors like pollution and a deadly disease that’s ravaging reefs in Florida.

“It’s a one-two punch for corals,” said Chris Langdon, director of the Coral Reefs and Climate Change Laboratory at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science. “Heat is breaking records and there’s more nutrients flowing into the bay, so we are watching closely for signs of stress on the corals.”

Even if the reefs are offshore and not near coastal areas where the recent fish kill happened, worsening conditions in Biscayne Bay could affect the patches of the Florida reef tract that are already under pressure from ocean acidification, dredging and heavy boat traffic, he said.

Early last week, thousands of  were spotted floating in different locations in the northern part of Biscayne Bay as water temperatures reached about 90 degrees and dissolved oxygen dropped to levels that made it impossible for fish to survive. The fish kill was first observed by residents swimming near Morningside Park, and later spread to other parts of the bay.

While environmental authorities tested the water and didn’t find evidence of toxic algal blooms, scientists think that chronic pollution and a seagrass die-off a few years ago created the backdrop for a “perfect storm” when temperatures rose very fast. Low wind, which reduced water circulation, and above-average rainfall in the Miami area also increased nutrient discharges from the Little River and other canals that feed into the bay.

Fish kills happen when warmer water and higher salinity levels lead to a drop in dissolved oxygen, especially in shallow areas. If algae blooms occur as a result of increased nutrients in the water, there’s more life using the oxygen in addition to fish. At night, when the algae aren’t producing oxygen through photosynthesis, the situation can reach critical levels, with fish, algae and all other microorganisms breathing but no oxygen being produced.

For corals, high  and pollution cause them to expel the algae that live inside them, providing food and giving them their bright colors. In hot temperatures, the algae become toxic and the corals basically spit them out. Without the algae, the corals weaken, turn bone white and become more susceptible to disease and death.

Water temperatures at Virginia Key reached 92.5 degrees Fahrenheit on July 2, the highest  since the station was installed in 1994, tweeted Brian McNoldy, a senior research associate at Rosenstiel.

That record was unusual, because peak temperatures are usually seen in August, said Mark Eakin, a coordinator for NOAA’s Coral Reef Watch. This month, NOAA satellites have already recorded temperatures as high as 88 degrees Fahrenheit in Biscayne Bay, a mere 2 degrees from the record set on Aug 17, 2017.

Eakin noted the satellites aren’t specific enough to detect temperature changes within north Biscayne Bay, where the bulk of the fish kill occurred. But the satellites do help scientists make predictions, and right now Biscayne Bay is expected to get even hotter.

In a few weeks, NOAA predicts the Bay could be warm enough for serious coral bleaching. This week, Biscayne Bay is under a coral bleaching warning, but Eakin said it could escalate to alert level 1 as soon as next week. At that point, 10% of the coral or more could begin to bleach.

“If the temperatures continue to rise as they’re forecast to, we’re likely to see coral bleaching increase across South Florida,” he said.

The Keys has been under a level 1 alert for weeks and could jump to a level 2 soon, where “you also start to see significant coral mortality,” Eakin said.

However, he said, after years of decimation from heat, pollution and other human-caused factors, the most sensitive corals have all but died off, leaving only the hardiest and most heat-resistant corals behind.

“They’re capable of surviving all kinds of punishment,” he said. “That’s why you don’t see a lot of corals in Biscayne Bay, they’re the ones that can take a lot of abuse.”

Last week, coral reefs in the Keys shifted from a bleach warning to a level 1 alert, which means some corals are starting to bleach, according to NOAA’s Coral Reef Watch predictions.

A Mote Marine Lab report issued Friday found there were 15 confirmed reports of partial bleaching in the first two weeks of August. At most sites, fewer than 10% of the reef was affected, although several inshore reefs saw more than 75% affected.

Cory Walter, a staff biologist at Mote, said she’s seen some minor bleaching across the marine sanctuary she patrols. In most places, less than 10% of the reef is affected. But in some inshore reefs, like Newfound Harbour Key, the damage currently covers around 75% of the reefs.

NOAA predictions show a brief reprieve in the coming weeks, then seven weeks straight at alert level 1 conditions. Hot water for that long is bad news for corals, Walter said. In 2014, during a warm summer in South Florida, a bleaching event claimed patches of the 220-mile Florida reef tract that stretches between the Dry Tortugas and Fort Pierce.

“It’s possible we could see a mass bleaching event if these temperatures don’t drop,” she said. “If it’s really bad, it almost looks like it snows on the reef because everything turns white.”

But a strong wind—or a tropical storm brushing by—could be enough to churn the water and cool it down to more coral-friendly temperatures. And with 2020 predicted to be the most active hurricane season on record, that could happen as soon as the end of the week.

Another spot of good news for Florida’s heat-stressed corals: giant corals restored by Mote spawned for the first time last week.

Corals only attain sexual maturity at a certain size, which can take decades to reach. Mote grew more corals in a lab and fused them onto the naturally occurring , speeding up the process to just five years.

Also, the corals Mote grew were specially selected to be resistant to high temperatures and disease. Now, they’re ready to reproduce.

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