Kelly Dilliard: CTDs, May 26, 2015

NOAA Teacher at Sea
Kelly Dilliard
Onboard NOAA Ship Gordon Gunter

May 15 – June 5, 2015

Mission: Right Whale Survey
Geographical area of cruise: Northeast Atlantic Ocean
Date: May 26, 2015

Weather Data from the Bridge:
Air Pressure:  1021.52 mb
Air Temperature: 14 degrees C
Relative Humidity: 90 %
Wind Speed:   23.4 knots
Wind Direction:  208 degrees

Science and Technology Log:

While we are out in the Great South Channel looking for right whales, a group of scientists (under Dr. Mark Baumgartner) from Woods Hole Oceanographic Institute (WHOI) is collecting data about ocean characteristics using a CTD (Conductivity, Temperature, and Depth).  They have established a grid of drop sites spaced 5 miles apart along a transect, with the transects 12 miles apart from each other.  There are approximately 60 sites along five transects of varying length.

Chart of transect lines and stations. In this chart, the ship is not yet to the transect lines.

What is a CTD?  Well, it is a standard tool used by oceanographers to measure changes in salinity and temperature with depth.  The instrument is lowered over the side of the ship with a vice winch and it is dropped through the water column at a steady rate, about 45 meters per minute, recording both conductivity and temperature.  The “CTD” that is being used on this cruise includes an array of instruments and two actual CTDs that are attached to a cage.  Other instruments on the cage include: two fluorometers, a video plankton recorder (VPR), and an optical plankton counter (OPC).

All three of these instruments are being used to study the food source (copepods) of the North Atlantic right whale. The fluorometer is used to determine the amount of phytoplankton (food source of the copepods) in the water, which fluoresces at specific wavelengths when light is shined on them.  The amount of light flashed back to the receiver is proportional to the amount of phytoplankton in the water.  The VPR is actually a camera with a strobe light on one end that takes images of the water column as the CTD passes through.  The VPR can be used to determine abundance and types of plankton, based on size, at different depths within the water column.  The OPC contains a narrow, rectangular-shaped, open-ended box with a red light shining in the center.  The water and plankton pass through the box and create shadows when they interfere with the light.  These shadows are then counted and an abundance of plankton can be determined.  The total package of instruments and weights weighs about 475 pounds.

Various views of the “CTD” cage and the instruments on it.
CTD coming out of the water. The orange blobs in the water are actually jellyfish.

What does the CTD tell us?  The CTD itself can show us how the ocean is stratified, or layered, based on different densities.  These layers change seasonally and daily depending upon the wind strength and tidal strength.  During the spring and summer months off Cape Cod, there tends to be a freshwater surface layer representing seasonal snowmelt that has made its way to the ocean via streams.  This layer sits on top because it contains less dissolved solids and therefore has a lower density.  This water also absorbs the sunlight, which makes it warm.  The surface water can mix due to the wind.  Phytoplankton and zooplankton are often found in the mixed layer.  Below this zone is a relatively thick layer of cold water that does not tend to mix.  This layer has been referred to as the “cold pool” and originates during the winter months.  The basal layer, or bottom water, tends to be warmer, but saltier, making it denser than the cold pool.  The bottom water originates out in the Atlantic Ocean proper.  So to recap from the surface down: 1) freshwater, 2) mixed water, 3) cold pool, and 4) bottom water.  These layers can be seen on the CTD profile as temperature and salinity change with depth.  Data from the fluorometer is also plotted with depth and one can determine where the phytoplankton is abundant, often in the mixed layer.

CTD profile.  The black line represents fluorescence, the green line is temperature, and the orange line is salinity.   All values increase to the right.  The duplication is due to the instrument recording measurements on the way down (the data used) and on the way up.
CTD profile. The black line represents fluorescence, the green line is temperature, and the orange line is salinity. All values increase to the right. The duplication is due to the instrument recording measurements on the way down (the data used) and on the way up.  You can see the warm, fresh layer at the top and the rapidly changing thermocline.  There is also a warmer, more salty bottom water in this profile.  The profile above was taken from the top track line and a eastern data site (SC115).

What is a copepod?  A copepod is a type of zooplankton.  Actually they are minute crustaceans that contain an exoskeleton.  There are thousands of species of copepods, but North Atlantic right whales are particular to Calanus finmarchicus.  These copepods are kind of interesting.  They feed on phytoplankton at the ocean surface during the night in hopes of avoiding being eaten themselves.  During the day, they sink to the bottom waters to avoid predators.  When they feed in the spring months, they store up energy in oil sacks.  This allows them to hibernate at a boundary layer near the bottom during the summer months when predators are most active.  Right whales are thought to dive down to this area and still feed on them.

Calanus finmarchicus. Note the oil sacs (where the red color is in the copepod). Image from Centre of Ecotoxicology and Experimental Biology.
 Image of Calanus finmarchicus taken in-situ by the video plankton recorder (VPR). Image from Dr. Baumgartner's project website on Copepod Diel Vertical Migration.
Image of Calanus finmarchicus taken in-situ by the video plankton recorder (VPR). Image from Dr. Baumgartner’s project website on Copepod Diel Vertical Migration.

Several of the scientists that have been on these yearly North Atlantic right whale surveys will tell you that five or more years ago they would have seen lots of whales.  On this survey, we have only seen a handful of right whales.  There are many questions that remain unanswered, but most importantly, where are the right whales going once they leave areas like Cape Cod Bay.  They used to come to the Great South Channel, but that appears to be changing.  The CTD transects provide a method of studying oceanographic conditions and the location of right whale food sources on a yearly basis and provide a means of comparing ocean states from year to year.  Hopefully, with further research the question of why the right whales are not coming to this feeding ground anymore can be answered.

Please check out these project websites of Dr. Baumgartner for more information on using CTDs to study whales:

– North Atlantic Right Whale Diving and Foraging Behavior in the Western Gulf of Maine

From phytoplankton to whales: Ecological Interactions on Stellwagen Bank

Copepod Diel Vertical Migration

I would like to thank Dr. Mark Baumgartner for explaining much of what is written above and several students working under him (Hansen Johnson and Divya Panicker) for giving me a tour of the CTD instruments.

Personal Log:

We have been doing CTD transects for the past few days.  Half the group is on a CTD watch schedule and the other half, of which I am included, is continuing the watch on the fly bridge for whales.  It has been slow going, due to the frequent stops for CTD drops, with not many whale sightings.  Yesterday, (Tuesday) had especially low counts (only 6 observations).  This is also due to the weather conditions.  Strong winds blowing across the water created significant white-caps making it challenging to see blows off in the distance.  We did have a day off of sorts on Sunday when we were making our way to the first CTD transect line.  The ship strangely felt like a ghost ship as everyone caught up on sleep, laundry, and movies.

Since most of the watches that we are doing require sunlight, we are generally off from 7 pm to 6 am.  A few of us get together and play cards, specifically the game Peanut.  I had never heard of this, but am finding my life long playing of solitaire to be very helpful.  Thank you Nana!  Peanut is kind of like solitaire and can go by the name Nertz.  Each player has his or her own deck of cards.  You start with eleven cards in a pile and four single cards.  The object is to decrease your eleven card pile to zero, by either stacking the cards solitaire-style on the four single cards or by placing them on the various aces (from their own or others) out in the center.  The first person to end with zero wins and receives an extra ten points.

Want to end with a photo of some mammals.  Here is one of some white sided dolphins.

White sided dolphins swimming off the boat.

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