October 31, 2018

MARATHON — The Florida Keys marine environment likely will need decades to recover from Hurricane Irma, scientists told marine advisers Oct. 16.

“Sobering,” said Bruce Popham, chairman of the Florida Keys National Marine Sanctuary Advisory Council, about post-hurricane survey reports on mangroves, sponges and water quality.

Mangrove forests that lay in the Lower Keys path of Irma in September 2017 endured “extensive canopy damage from high winds,” typically losing more than half their canopy cover, Kara Radabaugh of the Florida Fish and Wildlife Conservation Commission said in a briefing to the Sanctuary Advisory Council, meeting in Marathon.

Mangroves that once had 85 to 100 percent canopy cover were left with about 40 percent cover, she said, noting that larger trees took the most severe hits.

“Canopy cover recovered from 40 percent to 60 percent within two to four months,” Radabaugh said, “but recovery plateaued.”

White and black mangroves have dormant “epicormic growth” buds embedded in their bark that can blossom, but red mangroves often found along Keys shorelines do not regenerate as quickly. Many trees that appeared to have survived the hurricane died in following months.

Some mangroves will die if hydrology patterns were changed by the hurricane, leaving trees with too much or too little water.

One factor causing some mangrove deaths is a heavy layer of sediment delivered by storm surge and left behind when waters recede.

“The sediment in the forest looks like a layer of gray carpeting,” nearly four inches thick in spots, Radabaugh said. “This mud doesn’t allow air to pass through it. It smothers the roots and, if thick enough, smothers the tree and eventually kills it.”

One benefit, she noted, “Storm deposits help the elevation of a forest keep pace with sea-level rise.”

Mangroves can recover if they have live seedlings, along with proper elevation and water levels. But the mangrove may take 10 to 15 years to mature, Radabaugh said.

Bay sponges

“There are millions of sponges that died in Florida Bay because of Hurricane Irma,” John Hunt of the Florida Fish and Wildlife Research Institute branch in Marathon told sanctuary advisers.

Hunt focused on four Middle Keys sponge nurseries established on the gulfside to restore numbers of natural sponges in Florida Bay lost to bacteria from blue-green algae blooms.

Of about 8,000 cuttings of six sponge species seeded in the nurseries, about 84 percent had survived before Irma. In the storm’s wake, only 3,000 survived.

The shallowest sponge nursery, off Burnt Point, suffered the highest mortality and was abandoned.

“The water left for quite a long time,” Hunt said. “You can see sponges, but these sponges are just a skeleton. … These sponges are dead but haven’t decomposed.”

He estimated natural recovery of the bay’s sponge population probably would take 10 to 15 years, if the area can avoid future catastrophic events like algal blooms or more storms.

Seagrass losses

Jim Fourqurean, director of Florida International University’s marine science center, told the council that the recovery rate for Florida Bay seagrass has been largely established at a quarter-century.

Following a seagrass die-off in the 1980s, Fourqurean said, monitoring showed “it takes about 25 years for seagrasses in Florida Bay just to get back to where they were before the die-off event. And that was the year we had another die-off event.”

In the month after Hurricane Irma, water quality measurements revealed “really high, almost unbelievably high, chlorophyll concentrations” in the bay.

“After the hurricane we had a big algal bloom, but it’s calmed down and seems to be heading back to normal. Current trends show improvement. … Water quality impacts have largely subsided,” Fourqurean said. “We know there are still algae blooms floating around the bayside.”

In the Lower Keys backcountry where Irma crossed, “significant decreases in seagrass biomass” were recorded, he said. Many of these areas were similarly affected by Hurricane Georges in 1998.

“Many sites that were devastated by Georges in 1998 also were impacted by Irma,” Fourqurean said. “Georges and Irma had almost the same track. … I’m sure all of you know a place where there was grass patch but isn’t there now.”

Management plan

Staff with the Florida Keys National Marine Sanctuary reported the current timeline for release of the sanctuary management plan’s draft environmental impact statement, which will outline possible alternatives and the effects of each, points toward “late spring.”

Marine zoning maps and regulations for the 2,900-square-mile sanctuary have not changed significantly since 2000 when the Dry Tortugas ecological reserves were added. Other marine zones from Key Largo to Key West have existed in present form since 1997.

kwadlow@keysnews.com