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.
