Aerators may be brought in to help dying fish in Biscayne Bay’s low oxygen hot spots

Aerators may be deployed in Biscayne Bay to help improve oxygen levels in hot spots where thousands of fish have died in recent days as a result of a deadly combination of high water temperatures and increased nutrients.

“It’s unfortunate to think about Biscayne Bay as an aquarium that needs an aeration system but that’s exactly what we are considering now,” said Todd Crowl, a professor and director of Florida International University’s Institute of Environment. “I’ve never seen anything like this in a coastal area.”

A fish kill that was first reported by swimmers off Morningside Park on Monday spread to other areas of the Bay this week as dissolved oxygen levels dropped dramatically. Large quantities of dead fish were reported around North Bay Village and in southern areas like Virginia Key. Images of dozens of rays that appeared to be gasping for air near Pelican Harbor Seabird Station were widely shared on social media Thursday.

Harmful algae blooms didn’t contribute to the fish mortality event, said Eric Sutton, executive director at the Fish and Wildlife Conservation Commission. The agency analyzed water samples from the bay but found no evidence of toxins that could have caused the fish kill.

Scientists and wildlife managers are still trying to determine what caused the event, but the unusually high water temperature coupled with the presence of nutrients that washed into the Bay after recent rains probably led oxygen levels to drop precipitously, Crowl said.

While the average temperature of shallow waters in that area of the Bay is 83 degrees, it reached 90 this week, he added. Last night, FIU’s Center for Aquatic Chemistry and Environment sent out a research buoy to analyze water conditions just south of North Bay Village. It barely found any oxygen in the water late last night, he added.

Fish kills may happen in summer when warmer water and higher salinity levels often lead to a drop in oxygen dissolved in the Bay, especially in shallow areas. Algae, seaweed, microbes and other microorganisms in the water also use dissolved oxygen, leaving less to go around for fish. The situation becomes more critical at night, when algae and plants aren’t producing oxygen through photosynthesis, but are still using the dwindling oxygen available.

Seagrass warning signs

Aerators are used in ponds and lakes, in aquaculture operations and wastewater treatment plants as well as fish tanks to improve water quality and maintain sufficient dissolved oxygen. There are different aeration methods depending on the goal, and the current emergency in Biscayne Bay would likely require large equipment. Surface aerators mix water around, throwing it into the air where it absorbs oxygen.

Even as scientists and wildlife managers try to alleviate the immediate mortality event, Biscayne Bay needs a long-term strategy to address worsening pollution and the effects of climate change in the ecosystem, said Miami Waterkeeper executive director Rachel Silverstein.

She said that the seagrass die-off that started around 2011 has seriously worsened conditions in northern areas of the Bay. The presence of thick grasses helps produce oxygen in shallow waters. Possible sources of nutrients that are flowing into the Bay are storm water runoff, sewage leaks, failing septic tanks and fertilizer, she said.

“That combination is probably increasing pollution in the Bay, she said. “We are seeing a lot of warning signs in Biscayne Bay, and now we are really seeing the symptoms of an unhealthy body of water.”

In an informal Zoom meeting on Friday, wildlife managers, environmentalists, city managers and policy makers got together to discuss how to improve the health of the Bay, and especially how to finance massive infrastructure projects that are needed to keep pollution from reaching its waters.

“I can’t get over the fact that Miami-Dade has 92,000 septic tanks,” said Sen. Jason Pizzo during the Zoom call. They need to be converted to sewer if the county wants to seriously and sustainably address the issues in Biscayne Bay, he said. FWC’s Sutton, Noah Valenstein, the head of the Department of Environmental Protection, and Miami Beach Rep. Michael Grieco, among others, participated in the call.

Stinky Protest

Earlier on Friday, youth climate activists left a smelly message at Miami City Hall — a call to action in the form of a dozen dead fish, freshly harvested from Morningside Park.

“This is a sign that if we do not take action we will be the next ones in line. It’s not just the marine life,” said Emily Gonzalez, an 18-year-old climate justice organizer.

The activists hope the rotting sea life will push officials to quickly enact policies to protect Biscayne Bay from the well-documented threats of pollution from septic tank runoff, global warming and seagrass die-off.

“The plan is to continue escalation until we see serious action,” said Brandon Hudspeth, a 26-year-old climate justice organizer.

Hudspeth and Gonzalez are both affiliated with the Miami chapters of Extinction Rebellion and Fridays for Future, climate groups that use attention-grabbing actions to advocate for eliminating the emissions of carbon dioxide.

Youth activists with these and other organizations successfully convinced Miami to declare a “climate emergency” last year, but Gonzalez said they’re not happy with the progress the city has made since then. Addressing the root causes of climate change, she said, would stop future fish die-offs and preserve a healthier Biscayne Bay.