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Biscayne Bay Hypoxia and Fish Kill

August 20th, 2020

The hypoxia (a deficiency of oxygen in water) and fish kills reported in upper Biscayne Bay last week are an increasingly common occurrence in our shallow coastal bays.  The magnitude of this fish kill and its proximity to such a large human population is a big reason why this environmental crisis is drawing so much attention.  While nutrients (mainly nitrogen and phosphorus) and organic matter from sewage, fertilizers, and other sources are a likely culprit, temperature is also playing a key role.

Biscayne Bay is divided into two sections, with downtown Miami serving as the dividing point (refer to map below).  To the south of downtown Miami, the coastline is less developed and there are natural shoreline features like mangroves.  Importantly, the southern bay is well-flushed by the Atlantic Ocean, as there are few barriers to tidal water exchange.  This keeps pollutants from accumulating to a great extent in the southern bay.  To the north of downtown Miami, the upper bay’s coastline is generally more developed and the shoreline is more “hardened” with few mangroves or other natural coastal features.  Also, upper Biscayne Bay’s shallow waters are isolated from the Atlantic Ocean by the Miami Beach barrier island.  This results in very little tidal exchange with the Atlantic Ocean, thus allowing pollutants and organic matter to accumulate over time as they would in a lake or a pond.

Water quality aside, hotter water holds less dissolved oxygen needed by fish and shellfish.  Therefore, hypoxia can be a problem in many water bodies during summer months.  Nutrients and organic matter from fertilizers, sewage spills, and leaky septic tanks can tip the balance of healthy aquatic environments by enhancing the growth of bacteria and algae.  Like fish and shellfish, most bacteria and algae consume oxygen to survive, so their presence can exacerbate hypoxia.

As our climate continues to warm, water temperatures will also continue to warm.  This is especially critical during the nighttime, when plants and algae are not producing oxygen through photosynthesis.  As nighttime water temperatures continue to rise, the competing demands for a smaller available pool of oxygen held can lead to hypoxia and fish kills like we are seeing in upper Biscayne Bay.  Having healthy seagrass can keep the water oxygenated, even during hot summer months.  However, upper Biscayne Bay experienced a major seagrass die-off in 2017 that has not recovered.

What can be done about this?  Given the vulnerability of enclosed coastal water bodies like upper Biscayne Bay and Indian River Lagoon, more work is needed to curb the loading of nutrients and organic waste (especially through leaky septic systems and sewage spills) into our coastal waters.  Seagrass restoration in upper Biscayne Bay should also be a priority, as healthy seagrass meadows provide fish habitat, a trap for nutrients and organic matter, and a critical source of oxygen to the water during summer months. At the global scale, action must be taken to reduce carbon (CO2 and methane) emissions.  Our planet continues to warm, and Florida is increasingly feeling the effects in both air temperatures (see NOAA national temperature map below) and water temperatures.

Map of Biscayne Bay, showing the lower, well-flushed section of the bay (to the south of downtown Miami) and the upper, more-developed and enclosed area of the bay (to the north).  Note the different colors surrounding the shoreline from the upper bay to the lower bay, with white and gray depicting a more developed coastline and green depicting vegetation like coastal mangroves. The location of the recent hypoxia and fish kills is identified by the red dot.

NOAA National Centers for Environmental Education map of temperature percentiles for January through July 2020 showing that nearly all of Florida has seen record high temperatures this year.

Photos clockwise from upper left: Autonomous water sampler used by Florida International University to measure water quality across upper Biscayne Bay; dead fish collected from Biscayne Bay; distressed Southern stingrays gathering near the affected shoreline of Biscayne Bay; and visualization of dissolved oxygen data collected around the Julia Tuttle Causeway, with red dots indicating areas of hypoxia.  All images provided by Florida International University’s Institute of Environment.

Here are a few more recent news articles covering the Biscayne Bay fish kill.

  1. WLRN, August 18: Biscayne Bay Needs Better Pollution Rules, An Oversight Board And A Chief Officer, Task Force Says.

  2. Miami Herald, August 15: In race to prevent more fish kills, governments deploy pumps on Biscayne Bay shores.

  3. NBC6, August 15: Miami Mayor Outlines ‘Emergency Steps’ Taken to Address Fish Kill in Biscayne Bay.

  4. Local, August 13: Stingrays desperate for oxygen part of a critical tipping point at Biscayne Bay.

  5. FIUNews, August 12: Biscayne Bay fish kill is a warning sign, researcher says.

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