Algae Blooms and Fish Kills — Sebastian, Florida

The Indian River Lagoon off Sebastian has seen several different kinds of algae events, each with a different origin. A lagoon-wide phytoplankton bloom from 2011 into 2012 clouded the water and stripped seagrass from the shoals inside Sebastian Inlet. A brown tide that FWC first documented in July 2012 centered on the northern lagoon. In October 2018 an Atlantic red tide reached Sebastian Inlet and the Indian River County beaches, and in July 2023 a bloom of the dinoflagellate Pyrodinium was linked to a fish kill along the Sebastian waterfront.

Most of the fish kills recorded in the lagoon near Sebastian share one mechanism. The Indian River Lagoon National Estuary Program, which publishes as One Lagoon, explains that a fish kill happens when dissolved oxygen in the water drops below what fish need to survive, and that large algal blooms are the most common trigger because decay consumes oxygen as a bloom ages and dies. The red tide of 2018 was the exception, an ocean bloom whose dead fish drifted into the lagoon through Sebastian Inlet.

Red tide is caused by the alga Karenia brevis. According to FWC, K. brevis is found year-round at background concentrations of 1,000 cells per liter or less, and its blooms are most common off Florida's Gulf coast, but they do occur along the southeastern Atlantic coast as far north as North Carolina. In October 2018 one reached the Sebastian area. CBS12 reported on October 16, 2018 that water tested at Sebastian Inlet State Park Beach yielded high levels of red tide, while Wabasso Beach Park and Sexton Plaza Beach tested at medium levels, according to FWC.

The same day, Sebastian Daily reported that FWC had confirmed red tide in three areas of Sebastian and Vero Beach, the most severe at the Indian River-Brevard County border, with the others near Wabasso and John's Island in Vero Beach, and that Indian River County and City of Vero Beach beach access points were closed until further notice. On October 17, 2018, FWC confirmed a low presence of red tide in two small areas of the Indian River Lagoon, a reading of 10,000 to 100,000 Karenia brevis cells per liter.

Most of the dead fish that later appeared along the Sebastian shoreline came from the ocean. Sebastian Daily reported that lagoon samples taken October 23 and 24 tested high north of Sebastian near Micco at the county border and medium in two areas east of Sebastian, while officials testing near Riverview Park and along the shorelines where dead fish were washing ashore found no red tide present. The fish, mostly mullet, had died days earlier and entered the lagoon through Sebastian Inlet. On October 28, 2018, all beaches in Indian River County were still closed, although conditions at the inlet were improving after a cold front.

In late July 2023 dead fish appeared along the lagoon shoreline in Sebastian. WFLX reported on August 2, 2023 that hundreds of dead fish had been photographed around a dock on the Indian River Lagoon in Sebastian, and that the event was reported to FWC as an active fish kill. Fish were seen washing up just south of the Sebastian Yacht Club, and the station reported snook, flounder and sheepshead among the species.

The Florida Oceanographic Society told WFLX that the Sebastian fish kill was caused by a Pyrodinium bloom the area had been dealing with for weeks, the same bioluminescent alga that paddlers seek out at night, and that during certain blooms oxygen in the water can drop so low that fish suffocate. The station cited runoff from flooding in Fellsmere the previous Friday and a month in which 25 of July's 31 days were under a heat advisory.

Local coverage framed the event differently. Sebastian Daily called it a minor fish kill of the kind that can happen in the Indian River Lagoon during hot summer days, reported water temperatures of 85 degrees with some local readings of 87, and disputed claims that seagrass loss had raised water temperature. The paper pointed to an oyster farm at the City of Sebastian's Fisherman's Landing project and the city's earlier septic-to-sewer grants as local lagoon measures.

The longest-lasting effects near Sebastian came from the 2011 bloom. FWC reports that a long-term phytoplankton bloom of Pedinophyceae algae clouded the Indian River Lagoon from March 2011 to as late as March 2012. In early July 2012, after reports of discolored water in the northern lagoon, FWC, the St. Johns River Water Management District and the University of Florida identified a brown tide of Aureoumbra lagunensis, the first bloom of that alga FWC had documented in state waters. FWC notes brown tides can cut water transparency to less than a foot, shading seagrass, and that fish kills can occur when the algae deplete oxygen.

The Sebastian Inlet District has tracked the aftermath on the flood shoals just inside the inlet. Its 2024 seagrass report says pre-2011 seagrass coverage on the six monitored shoals varied between about 110 and 115 acres, and that seagrasses across the lagoon have struggled to recover following the massive 2011 algal bloom. The west side of one Sebastian Inlet shoal was largely barren after 2011. By 2024 coverage on the six shoals totaled 115.38 acres, and the District credits tidal flushing from the Atlantic through Sebastian Inlet for a steadier recovery than in much of the estuary.

One Lagoon gives the oxygen thresholds that matter in the Indian River Lagoon at Sebastian: most aquatic life needs about 5 milligrams per liter, organisms come under stress between 3 and 5, and below that many species can no longer survive. Warm water holds less oxygen than cool water, calm summer weather limits mixing, and decaying muck on the lagoon bottom consumes more. Fish can usually swim away from a local low, so most kills stay small; widespread kills happen when a bloom collapses across miles of lagoon.

Location along the lagoon shapes the risk. One Lagoon lists wastewater and septic effluents, stormwater runoff, algal blooms, fish kills and muck as major stressors in the northern and central Indian River Lagoon, the stretch that includes Sebastian. Its algal bloom guidance ties blooms to warm water and excess nitrogen and phosphorus. Distance from an inlet matters too: WFIT reported in 2019 that it is almost 90 miles from Sebastian Inlet north to Ponce de Leon Inlet, so water in the southern lagoon is refreshed almost daily while parts of the northern end can take more than a year.

Fish kills in the Sebastian stretch of the Indian River Lagoon are logged through FWC. The FWC Fish Kill Hotline is a citizen science project that started in 1995, has received over 40,000 reports, and takes about 2,000 reports of fish kills and disease each year by phone at 1-800-636-0511 or online. FWC asks for the location, the number and species of fish, and contact details. The 2023 Sebastian event reached FWC this way, as WFLX reported, and the hotline is also how FWC is alerted to blooms such as red tide.

One Lagoon notes that the hotline is for reporting only and that local municipalities remove dead fish. During the October 2018 red tide, Sebastian Daily reported that wind and current moved some dead fish out of areas hard to reach by cleanup crews and onto Sebastian shorelines such as the Main Street boat ramp area.