Sarah Boehm: The Dead Zone, July 5, 2013

NOAA Teacher at Sea
Sarah Boehm
Aboard NOAA Ship Oregon II
June 23 – July 7, 2013 

Mission: Summer Groundfish Survey
Geographic area of cruise: Gulf of Mexico
Date: July 5, 2013

Weather at 19:13
Air temperature: 26°C (79°F)
Barometer: 1017mb
Humidity: 93%
Wind direction: 135°
Wind speed: 18 knots
Water temp: 27°C
Latitude : 28° 44’ N
Longitude: 85° 32’ W

Science and Technology Log

Mr. Cummiskey, the other science teacher at CDCPS, asked if we saw an influence from farming along the Mississippi River in the Gulf ecosystem. At first it seems crazy that something happening over a thousand miles away can have an impact on an ecosystem as vast as the Gulf of Mexico, but it really is happening and part of our research is to monitor the effects. The first clue I had that something was changing was the color of the water. In the deep waters off Texas the water was a beautiful clear blue. As we got closer to the Mississippi delta the sea water turned a murky brown–a mix of mud brought down by the river and the phytoplankton that was thriving in the nutrient dense waters. Just like eating too much food is bad for people’s health, too many nutrients is actually bad for an ecosystem.

Each time we get to a sampling station we start by taking measurements of the water quality with the CTD (conductivity temperature and depth). From the bridge the officers control the ship to keep it in one place. Then the deck crew uses a winch and pulley system to move the heavy CTD equipment overboard and down into the water almost to the sea floor. All the way down and back up the machine is taking dozens of readings a second that are transmitted back to a computer in the dry lab.

The CTD records the depth, water temperature, the salinity (how salty the water is), and the dissolved oxygen. We are most concerned with the oxygen level because it greatly impacts the organisms living in the water. Fish and marine invertebrates breathe oxygen molecules that are mixed in with the water. Without enough dissolved oxygen in the water they will suffocate and die. Healthy levels in the Gulf of Mexico are 4 to 6 milligrams of O₂ per liter of water.  If there is less than 2 mg/L it is considered hypoxic, meaning there is not enough oxygen. This map uses the data we have collected this cruise to show dissolved oxygen levels in the bottom waters of the Gulf. The green and yellow colors shows the healthy areas, the orange areas are hypoxic.

Click on the map for a larger version. The map is updated as new data comes in.

See those orange areas in close to the coast of Louisiana? That is known as the Dead Zone. Runoff of fertilizer and other nutrient sources wash down rivers and out to sea where they contribute to algae blooms. When the algae dies it sinks and is decomposed, a process that uses up a lot of oxygen. Check out this video to learn more. All my 6^th graders should notice similarities between this situation and the virtual pond we worked with this spring.

Hypoxia video

Not only do the oxygen levels change, but the composition of the fish trawls changed dramatically too.  At station #144 we had an oxygen reading of 3 mg/L and an average sized trawl (26 kg) with a variety of species. At station #146 we had an oxygen reading of 1 mg/L (which is hypoxic) but pulled up a huge net of fish that filled 18 buckets. The total weight was 340 kg, but over 300 kg was just two species – croaker and butterfish. We were surprised by this catch and so did another oxygen reading and found while our nets started in hypoxic waters, during the 30 minute trawl we moved into better water with 3 mg/L of oxygen .  At station #147 we had a very low oxygen reading of only 0.2 mg/L. Our trawl only brought up 1.7 kg, most of which were jellies and crabs with just a few little fish.  There just wasn’t enough oxygen to support more life. Why was station #146 so huge? As the low oxygen waters spread out from the Mississippi River delta, critters were fleeing the hypoxia zone and moving to better water. So along the edge of the dead zone is an area with high population density; the oxygen refugees and the fish swooping in to eat them.  However, not all creatures can move themselves out of the way. Creature like bivalves and gastropods (clams and snails) don’t have the capability to move much and so get caught in the annual hypoxic zone of the Gulf.

Hypoxia zones caused by nutrient runoff from fertilizer and other man-made sources do not just happen in the Gulf of Mexico. They have also been recorded in the Chesapeake Bay, Long Island Sound and at the mouths of rivers around the world. They can also happen in fresh water ponds and lakes.

The CTD is our main method of recording oxygen levels, but we need to make sure it is functioning properly. So each day we also take a water sample and use a titration method to find the amount of dissolved oxygen. Check out the colorful chemical reactions in this video.

Personal Log

People, like fish, need oxygen and water to survive. Out on the ship oxygen in the air is easy to come by, but fresh water is another story. We are surrounded by water of course, but cannot drink the salt water. I tracked down out Chief Engineer, Sean Pfarrer, to find out more about where all the fresh water on board comes from.

Down in the engine room there is a reverse osmosis machine that processes salt water and turns it into fresh water. The salt water is pumped into the machine under 950 psi of pressure. The pressurized water is forced through a selectively permeable membrane that lets water molecules through, but not the larger salt molecules. (My 6^th graders should find this all sounding familiar) The super salty water left behind is pumped back out to sea, and the fresh water is used on board. Our sinks, showers and laundry all use fresh water. We go through about 1,000 gallons a day, which is close to the 1,200 gallon limit of the RO system (but only about half what 30 average Americans would use on land). To conserve fresh water the heads (toilets in sailor speak) flush with salt water.

Which brings me to one of my favorite science teacher topics – poop. Thirty people over the course of fifteen days generate a fair amount of waste. What happens to all that poop? Just emptying it into the water would be harmful to the marine environment, so we have a little waste water treatment system right on board. When you flush, it all goes down to the marine sanitation device where poop eating bacteria consume our waste.  The waste water then passes by chlorine tablets that kill any bacteria before it gets dumped into the sea. I’ll admit I’m a little fascinated by the systems and technology that keeps our floating community operating in a rather comfortable fashion.

We completed our science work this afternoon and are now heading back to port. Check out the Ship Tracker to see where we have been.

CDCPS Science Students:

How did sailors long ago during the age of exploration deal with the drinking water problem?

What do you think we could do to lessen the hypoxia problem in the Gulf?