2019 Gulf Dead Zone smaller than forecasted – why?

The forecast for a record size Gulf Hypoxic Zone this
year got much attention in the media, followed by the actual measurement of an
area of low oxygen that was smaller than predicted. What happened, and why?

Hypoxia is a condition of low oxygen levels in the water, which is not stationary, and it can occur in fresh or saltwater. Large nutrient loads wash into the Mississippi River and its tributaries with rain and snowmelt. These nutrients wash away from farm fields and paved areas. The nutrients enter the Gulf of Mexico as the water makes its way down River. These nutrients, nitrogen and phosphorus, which are so essential for good crops, also feed large algae blooms offshore. When the algae blooms decay and sink and are eaten by zooplankton, they decompose and use up most of the oxygen in the water.

What’s left is water without enough dissolved oxygen
in it to support life. Anything that can flee the area does. Everything else
dies. There’s a reason the hypoxic zone is frequently referred to as The Dead Zone.

Each year at the end of July, a mapping cruise from
Louisiana State University and the Louisiana Marine Consortium (LUMCON) goes
out to measure the extent of low oxygen. Their trip lasts a week and provides a
snapshot of conditions in the Gulf. Funding is the limitation that keeps a more
extensive mapping from happening, though measurements are going on offshore at
other times that help provide a more complete picture.

The hypoxic zone generally dissipates in the fall as weather
systems churn up the water more frequently. If a storm or hurricane passes
through the waters off Louisiana’s coast shortly before the cruise takes place,
the water column is mixed up, and oxygen levels can rise, leading to a smaller
hypoxic area. This situation has happened on several occasions and was the case
this year due to Tropical Storm/Hurricane Barry.

This effect is temporary. Subsequent measurements show that hypoxia generally reconstitutes following a disturbance. So a smaller than forecast size of the zone in any one year doesn’t mean that the trend of growing hypoxia offshore has changed.

That’s why the Gulf Hypoxia Action Plan specified that its goal was to reduce the average annual size of the zone measured over a five year period. That measurement would show a change in the trend, not just yearly variation.

We know that the size of the hypoxic zone can shrink
significantly in drought years when river levels are low, showing that the system
will respond to reduce inputs of nutrients. The scale of those reductions in a
drought year is more significant than human management could achieve in a short
time frame, which is one reason why efforts like the Action Plan try to reach their goals over a more extended period.
The lag time between actions upstream and results in the Gulf is another, with
the size of the Mississippi River Basin being a factor.

The importance of achieving those reductions illustrates
the changes in the trend recorded over the past few years – long-term loading
of nutrients into these offshore waters have made them more sensitive to their
effects, with a higher hypoxic response to the same amount of nutrients.
Measurements of nutrients in the Mississippi River by the U.S. Geological
Survey show that conservation and management efforts over the past two decades
have helped keep those levels somewhat steady, but have not achieved the
reductions needed to reverse the trend of hypoxia in the Gulf.

The Gulf Hypoxia Task Force, composed of federal
agencies and river states, failed to meet its first target date of 2015 and
moved the timeframe for achieving a 5000 square kilometer (1950 square mile)
average annual size of the hypoxic zone to 2035. This year’s zone was 18,000
square kilometers or 6,952 square miles.

The Task Force set an Interim Target of achieving a
20% reduction in nitrogen and phosphorus loading to the Gulf by 2025. That
Target was identified as a critical milestone – if it’s achieved, then there’s
a reasonable chance of meeting the 2035 goal. We’re halfway to that milestone
date, set in 2014, and it’s safe to say that the Task Force has not marshaled
the focus of attention and resources needed to reach it.

We should hold them to that commitment – along with
Congress, which is the main source for new funding on a scale that can change
the trends in the basin and Gulf. MRN is working with our partners and other
stakeholders to make that happen, and your input to Congress, states, and the industry
is an integral part of the process. Your cards, letters, petitions, and visits
let Congress know the public is paying attention and expects results, so they
do make a difference.

Background:

This year’s Gulf Hypoxia Mapping Cruise and the others
going back to 1985 can be learned about at www.gulfhypoxia.net.

The Gulf Hypoxia Action Plan can be found at https://www.epa.gov/ms-htf/hypoxia-task-force-2008-action-plan-and-related-documents.

You can keep up with MRN’s work at https://1mississippi.org/mrn/, and on Facebook at @1Mississippi. Become a River Citizen today!