Keep reading for our daily updates, and then some details about
the biological oceanography experiments we are doing.
Day 7
We left port at around 4am, and spent most of the day steaming
back to our one lonely drifter. We had a very small chance of some rain
towards the evening, but it broke up before it reached us. Once we made
it to the drifter, we did a late CTD cast, around 10pm, and then turned on the
aerosol sampler for the night.
Day 8
Repeating our sampling scheme from previous days, we turned off
the aerosol sampler in the early morning to head back to our drifter in time
for a sunrise CTD cast. Thanks to our stop at port, we now have a working
winch for the trace metal CTD and were able to get that going as well.
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| Successfully recovering the trace metal CTD after the first cast with the new winch |
As this is our last day sampling this particular spot, we decided
to make the most of it. We did two PAR casts, the normal one around noon,
and an extra around 10am. The angle of the sun may change the attenuation
coefficient, so we wanted to test this theory. We also did two more
normal CTD casts, both to get temperature and salinity data, not to collect any
samples.
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| Preparing the CTD for deployment |
Then we deployed the towfish in the afternoon and towed it in a
circle around the drifter. We finished up the day by recovering the last
drifter and heading south. We have a new spot picked out that looks like
it may get some rain in the near future. I'll have more details on that
once we arrive.
At one point, right before lunch, we were quite near the drifter
and noticed it was attracting some fish. There was a mad fishing
scramble, and we ended up catching close to half a dozen mahi. Dinner was
quite delicious, as our chef cooked up fresh teriyaki mahi with fried rice.
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| Catching Mahi |
Biological oceanography
Almost every time we do a CTD cast, we sample the water at various
depths. Once the CTD comes back up, the water is portioned into
pre-labeled bottles to be further analyzed or used for experiments. There are
several types of biological analysis we can do:
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| Collecting initial water samples from the CTD |
Measuring Nutrients. We take a sample of water and measure
the amount of nutrients in it. It gets more specific than just nitrogen
and phosphorus, as we want to know what form these nutrients are in.
Certain forms are more easily used by phyotoplankton than others.
The results are given as concentrations and can show how much of each
nutrient is currently in the water sample.
Measuring Chlorophyll. We take at least a liter of water and
filter it through a relatively small filter. Small phytoplankton are left
on the filter, and the sea water goes through. The filter is dissolved in
acetone, releasing the chlorophyll that was caught on it. Since
chlorophyll is a specific color (think of green leaves and plants), we can use
light to determine how much there is. The amount of chlorophyll tells us
how many phytoplankton there are in that water sample - this is called abundance.
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| Filtering samples for chlorophyll |
It takes both these types of measurements to give an accurate
picture of what is going on in the water. For example, low nutrients
could mean a bad area for phytoplankton to grow. But, if the chlorophyll
measurements are high, we know we took our samples right after the
phytoplankton used up all the nutrients. Likewise, if chlorophyll
measurements are high and so are nutrients, then we caught the phytoplankton in
the middle of the growing stage. They haven't yet used up all the nutrients
to grow and reproduce.
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| Processing samples in front of a different filtering apparatus |
Taking nutrients and chlorophyll straight from the CTD samples
gives us a snapshot of what is happening at each depth the CTD sampled.
However, if we also run experiments on the samples, we can learn a lot
more.
One type of experiment is an incubation experiment. In this
case, we place water samples in plastic bottles and incubate them, or allow
them to sit. Then, chlorophyll and nutrients are measured at different
time points to see how things change. In some cases, the bottles are
clear, and are kept in an environment that is similar to where the sample was
taken from. We have special clear chambers, called incubators, set up on
the deck of the ship. Each one has a different number of layers of a
black mesh over it, to set different light levels. Water is continuously
pumped through these chambers to keep them at close to the same temperature
they were sampled at. In other cases, we want to see how things progress
if there is no light input, so we use dark bottles instead of clear ones.
These bottles are also placed in the on board incubators, to keep
everything else the same.
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| The never ending labeling that goes with all these samples |
Another type of experiment is called a bioassay. Sampled
water is divided into bottles and then different types of nutrient treatments
are added. For example, we have an iron-only treatment, a nitrate and
iron treatment, and a rainwater treatment, among others. These bottles
are then incubated and chlorophyll and nutrient measurements are taken over
time. By comparing how many phytoplankton grow in the bottles with
different nutrients added, we can determine which nutrient was the limiting
factor. Since one of our treatments is rainwater, we can also see how the
growth in the rainwater bottles compares to the other treatments. This
will give us an indication of how phytoplankton might respond to a rain event
in the ocean.
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| Our incubators on the back deck |
By examining the results from all these experiments together, we
can get a good idea of what is happening and why. A lot of the
experiments are being run while we are on board, but some samples are being
refrigerated or frozen for further analysis in a normal laboratory on land.
We have a few preliminary results from our first few samples. They
still need some extra work, but there is enough information to show that things
are going well and we are getting significant results so far. This is
confirmation that the experiments are being carried out correctly and that we
chose the right ones to perform.
Thanks for reading! That's all I've got for today.
Stay tuned for the next update - on our new location, new plans, and
hopefully some rain.
More posts in this series:
Upcoming Cruise
Cruise Delays
Upcoming Cruise, Part II
Update #1
Update #2
Update #3
Update #4
Update #6
Update #7
Update #8
Update #9
More posts in this series:
Upcoming Cruise
Cruise Delays
Upcoming Cruise, Part II
Update #1
Update #2
Update #3
Update #4
Update #6
Update #7
Update #8
Update #9
