Dry weather raises salinity in the lagoon; brown tide and calerpa presence tracked ahead of 2025 seagrass mapping

State and federal water-quality scientists updated the Indian River Lagoon Council Friday on continuing dry conditions, seagrass mapping plans and biological observations in both the northern/central lagoon and the St. Lucie estuary.

Northern and central lagoon conditions

Lauren Hall, of the St. Johns River Water Management District, reported mostly dry weather and stable depths at continuous monitoring sites and said salinities are up across the northern and central stations. "We are seeing some blooms . . . particularly in Southern Mosquito Lagoon," Hall said, identifying Aureoumbra (brown tide) as present though not at the extreme levels seen in 2012 and 2016. She noted that chlorophyll fluorescent units above roughly 10 RFU correspond with visible blooms.

Hall said dissolved-oxygen dips below hypoxia thresholds (about 2 to 3 milligrams per liter) occurred at some sites in February and March but none of the stations had fallen below that level in the last month of monitoring. She also described ongoing seagrass work: aerial photography for the official 2025 seagrass map is under way and will take about 18 months from imagery to final product. Hall said teams sampled sediments at 70 sites for seed distribution and found more Ruppia seeds than Halodule, with the Cocoa-to-Melbourne corridor showing the fewest Halodule seeds.

Calerpa and grazers

Hall described extensive calerpa prolifera beds in some areas and increasing numbers of an herbivorous sea slug (the genus Elysia) that can rapidly decimate calerpa. "I've counted up to 200 slugs per meter squared in certain locations," Hall said. She also said researchers have found a calerpa species—Calerpa ashmeadii—north of Haulover Canal that may be an uncommon sighting on the east coast; the team is reviewing historical literature and records.

Southern lagoon and St. Lucie estuary

Melanie Parker of the South Florida Water Management District presented monitoring focused on the St. Lucie estuary. She noted that inflows from Lake Okeechobee ceased in March after Corps recovery operations in December; recent inflow to the estuary has been minimal, with the most recent weekly average inflow about 81 cubic feet per second, all from tidal basins.

Parker presented salinity readings from three continuous sites and compared them to a salinity performance measure developed under the Recover/SERP program for oyster habitat. At the HR1 site in the estuary's North Fork, surface salinity was about 16 and bottom 19; US-1 Bridge salinities were about 22; A1A Bridge was about 30—Parker said HR1 and US-1 are within the 10-to-25 optimal range for oysters, while A1A is slightly high and may increase predation and disease risk.

Parker also noted oyster recruitment monitoring and said 2024 data showed early indications of spring spat settlement in April; monthly sampling continues.