Bhavna Rawal: Conductivity, Temperature, Depth (CTD) and Water Testing, August 7, 2012


NOAA Teacher at Sea
Bhavna Rawal
Aboard the R/V Walton Smith
August 6 – 10, 2012

Mission: Bimonthly Regional Survey, South Florida
Geographic area: Gulf of Mexico
Date: Aug 7, 2012

Weather Data from the Bridge:
Station: 6.5
Time: 21.36 GMT
Longitude: 080 17’ 184
Latitude: 250 3’ 088
Water temp: 29.930 oC
Wind direction: East
Wind speed: 8 knots
Sea wave height: 3 ft

Science and Technology log:

Hello students! We know how to do water testing in our lab class using the testing kit. Today, I am going to explain to you the way ocean water is sampled and tested in the South Florida coastline.

Our 5 day cruise consists of over 80 stations along the Atlantic and Gulf coast of Florida.  At each station we take water samples, and at about 20 of the stations we tow nets to catch fish, seaweed or plankton and sometimes scuba dive to recover the instruments mounted on the seafloor.

Our journey begins at station #2 at Dixie shoal, which is near Miami; you can see this on the South Florida bimonthly Hydrographic survey map below (see fig).

South Florida Bimothly Hydrographic Survey map

South Florida Bimothly Hydrographic Survey map

At each station we performed CTD (conductivity, temperature and depth) operations. The CTD is a special instrument to measure salinity, temperature, light, chlorophyll and the depth of water in the ocean. It is an electronic instrument mounted on a large metal cage that also contains bottles to collect samples.  These bottles are called niskin bottles and every oceanographer uses them.  They are made of PVC and are specially designed to close instantaneously by activation from the computer inside the ship. Collecting water samples at various depths of the ocean is important in order to verify in the lab that the instruments are working properly. Each bottle has an opening valve at the bottom and top to take in the seawater. The top and bottom covers are operated by a control system. Once a certain depth is reached, the person sitting at the control system triggers and it closes the bottles. You can control each bottles through this system to get a pure water sample from different depths. For example, when the ocean floor is 100 meters deep, water is sampled from the surface, at 50 meters deep, the very bottom.

Hard hat and life vest on and ready for CTD

Hard hat and life vest on and ready for CTD

The CTD instrument is very large, and is operated by a hydraulic system to raise it, to place it and lower down into the ocean. Rachel (another fellow member) and I were the chemistry team; we wore hard hats and life vests while we guided the CTD in and out of the water. This is always a job for at least two people.

Guiding CTD in and out of water

Guiding CTD in and out of water

The team usually closes several bottles at the bottom of the ocean, in the middle layer and surface of the ocean. We closed the bottles in the middle layer because the characteristics of the water are different from at the bottom and the surface.  Remember, the ocean water is not all the same throughout, at different depths and locations it has different chemical characteristics. We closed two bottles per layer, just in case something happened with one bottle (it is not opened properly, for example) then the other bottle can be used.

Taking water sample out of CTD bottles

Taking water sample out of CTD bottles

Rachel and I took water samples from the CTD bottles and used them in the lab to conduct experiments. Before I explain the analysis, I want to explain to you the importance of it, and how a “dead zone” can happen. Remember phytoplankton need water, CO2, light and nutrients to live and survive. The more nutrients, the more phytoplankton can live in water. As you all know, phytoplankton are at the base of the food chain. They convert the sun’s energy into food. Too many nutrients mean too much phytoplankton.

  1. If certain species of phytoplankton increase, it increases the chance of a harmful algal bloom. Too much of one kind of plankton called the dinoflagellates can release toxins into the water which harms the fish and other ocean life and it can even cause people to feel like they have a cold if they swim in the water that has those plankton.
  2. Large amounts of plankton die and fall to the sea floor, where bacteria decompose the phytoplankton. Bacteria use available oxygen in water. The lack of oxygen causes fishes and other animals die. The zone becomes ‘the dead zone’.
    We prepare the sample for nutrient analysis to measure nutrients such as nitrate, nitrite, phosphate, ammonium and silicate in the water.
    We also prepare the sample for chlorophyll analysis. In the lab, we filter the phytoplankton out of the water. Phytoplankton contains special cells that photosynthesize (chloroplasts) which are made of chlorophyll. If we know the amount of chlorophyll, we can estimate the amount of phytoplankton in a given area of the ocean.
filtering the phytoplankton out of the water

Filtering the phytoplankton out of the water

Preparing the sample for nutrient analysis

Preparing the sample for nutrient analysis

Phytoplankton needs carbon dioxide to grow. Carbon dioxide analysis is useful because it provides an estimate of total carbon dioxide in the ocean.  It is also important in understanding the effects of climate change on the ocean.  If you increase the amount of CO2 in the atmosphere (like when you drive cars), it enters into the ocean.  If you think about a can of soda it has a lot of CO2 dissolved into it to make it fizzy, and it also tastes kind of acidic.  This is similar to when CO2 dissolves into seawater.  When the ocean becomes more acidic, the shells of animals become weaker or the animals cannot produce the shells at all.

Colored dissolved organic matter (CDOM) analysis informs us where this water comes from.  The dissolved organic matter comes from decomposing plants, and some of these dead plants entered the water through rivers.  You can tell for example that water came from the Mississippi River because of the CDOM signal.  You can then follow its circulation through the ocean all the way to the Atlantic.

From the CTD instrument, we measured temperature, light, salinity, oxygen etc. and graphed it on a computer (see figure) to analyze it.

Measured temperature, light, salinity, oxygen etc. and graphed it

Measured temperature, light, salinity, oxygen etc. and graphed it

Generally, I see that ocean surface water has high temperature but low salinity, low chlorophyll, and low oxygen. As we go deeper into the sea (middle layer), temperatures decrease, dissolved oxygen increases, chlorophyll and salinity increases. At the bottom layer, chlorophyll, oxygen, temp and salinity decrease.

Personal Log:

I arrived on the ship Sunday evening and met with other people on the team, tried to find out what we are going to do, how to set up, etc. Asked so many questions… I explored my room, the kitchen, the laundry, the science lab, the equipment, etc. Nelson (the chief scientist) gave me a really informative tour about the ship, its instruments and operations. He showed the CTD m/c, the drifter, the wet lab etc. He also gave me a tour of a very important instrument called the “flow-through station” which is attached to the bottom of the ship. This instrument measures temp, salinity, chlorophyll, CDOM, when the boat drives straight through a station without stopping. I was really stunned by how precise, the measurements taken by this instrument are.

flow-through station

Flow-through station

The next morning, Nelson explained that if we have enough tide the ship would leave. We had to wait a bit. As soon as we got the perfect tide and weather, R/V Walton Smith took off and I said ‘bye bye’ to Miami downtown.

‘bye bye’ to Miami downtown

‘Bye bye’ to Miami downtown

I learn so much every day in this scientific expedition. I saw not only real life science going on, but efficient communication among crew members. There are many types of crew members on the ship: navigation, technology, engineering, and scientific. Chief scientists make plans on each station and the types of testing. This plan is very well communicated with the navigation crew who is responsible for driving the ship and taking it to that station safely. The technology crew is responsible for efficient inner working of each scientific instrument. 10 minutes before we arrive on a station, the ship captain (from navigation crew) announces and informs the scientific team and technology team in the middle level via radio. So, the scientific team prepares and gets their instruments ready when the station arrives. I saw efficient communication and collaboration between all teams. Without this, this expedition would not be completed successfully.
I have also seen that safety is the first priority on this oceanic ship. When any crew member works in a middle deck such as CTD, Net Tow etc, they have to wear a hard hat and life jacket. People are always in closed toe shoes. It is required for any first timer on the boat to watch a safety video outlining safe science and emergency protocol. People in this ship are very friendly. They are very understanding about my first time at sea, as I was seasick during my first day. I am very fortunate to be a part of this team.

The food on the ship is delicious. Melissa, the chef prepares hot served breakfast, lunch and dinner for us. Her deserts are very delicious, and I think I am going to have to exercise more once I come back to reduce the extra weight gained from eating her delicious creations!

Watch TV, play cards and have dinner together

Watch TV, play cards and have dinner together

My shift is from 5 a.m. to 5 p.m. and I work with Rachel and Grant. After working long hours, we watch TV, play cards and have dinner together. I am learning and enjoying this expedition on the ship Research Vessel Walton Smith.

Question of the Day:

Why we do water testing in different areas of river and ocean?

New word:

Colored dissolved organic matter (CDOM)

Something to think about:

How to prevent dead zone in an ocean?

Animals Seen Today:
Two trigger fishes
Three Moon Jelly fishes
Five Crabs

Did You Know?
In ship, ropes called lines, kitchen called galley, the place where you drive your ship is called bridge or wheel house.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s