Whales!

August 7, 2013

Firstly, we saw whales! A large pod of pilot whales decided to come and investigate what we were doing, and treated us to a show on Aug. 5th. The day before, while the trace metal rosette and CTD were in the water, we saw water spouts from humpback whales off in the distance. Exciting times!




Pilot whale pod August 5th
(photo courtesy of Monica Mejia)

The Science...

Approximately 50% of photosynthesis occurs in the ocean. The vast majority of photosynthetic organisms in the ocean are phytoplankton, too small to be seen individually without the aid of a microscope. Yet, Phytoplankton ('phyton' from the Greek for plant) form the base of all marine food webs. Primary producers (photosynthetic organisms) use  sunlight to convert carbon (in the form of carbon dioxide) to energy, producing oxygen as a bi-product of this reaction. Thus, they are a fundamental component of the global carbon cycle.

Like terrestrial plants, marine plants require nutrients (the major nutrients are nitrate and phosphate) for growth. For many years scientists were puzzled about why there were large expanses of open ocean with a plentiful supply of the major nutrients but low productivity. About 25 years ago, John Martin, a scientists at Moss Landing Marine Labs, California, proposed that productivity in these areas was limited by the availability of dissolved iron (Fe). This theory was tested in the equatorial Pacific (one of the High Nutrient Low Chlorophyll, HNLC, regions; chlorophyll is an analog for primary productivity in the ocean), and the results indicated that iron limitation could, indeed, limit productivity and biomass in the ocean (Martin, et al., 1994, Nature, 371). Since that time it has become apparent that many other elements (e.g. Co, Cu, Zn, and others) have the potential to limit (or co-limit) primary production. This is because these elements are essential components of many of the enzymes involved in photosynthesis and/or major nutrient uptake and assimilation. In much the same way as we require a balance of nutrients and trace elements for a healthy diet, phytoplankton do too.

And this is where the trace metal group come into the picture. The trace metal component of the CLIVAR/CO₂ Repeat Hydrography Program (A16N-2003, as well as a number of other CLIVAR cruises in other ocean basins) provided an important opportunity to measure water column profiles of dissolved (dFe and dAl) and particulate Fe and Al (pFe and pAl) and other trace elements across entire ocean basins as well as the distributions of total and soluble aerosol Fe and Al. Such high-resolution profiling (60 nautical miles) revealed fine-scale features in the distributions of Fe and Al, and other trace elements, which were previously unrecognized. Coupling aerosol and water column sampling reinforces the concept that dust input significantly influences the trace element chemistry of the upper water column.

Pam (left) and Rachel tending the tag lines on the trace metal CTD (Conductivity, Temperature and Density sensors. The sensors used to monitor these fundamental parameters are mounted below the 10L (GO-Flo) water collection bottles) and rosette as it's lifted over the stern railing into the ocean. Reggie (far left) gives commands to the A-frame operator, Nick.
(photo courtesy of Monica Mejia)

The CLIVAR/CO₂ Repeat Hydrography Program is an international, interdisciplinary research program with the goal of collecting and modeling hydrographic and tracer data from the world's oceans on a roughly 10-year cycle (http://ushydro.ucsd.edu/index.html). This program monitors the invasion of anthropogenic CO₂ and macronutrient/micronutrient cycling, primary and secondary productivity, and carbon flux to the deep ocean. The data collected from previous CLIVAR cruises reveal many previously unknown features in the distributions of dissolved and particulate trace metals, as well as the aerosol chemistry of Fe that are not currently expressed in existing models.

Dr. Bill Landing mans the winch that lowers and lifts the CTD rosette, while Joe is in the clean lab van manning the deck box which controls the depth at which the GO-Flos are remotely closed, thus collecting water from that depth.
(photo courtesy of Monica Mejia)

One unanticipated result from A16N in 2003 was the identification of a large region at low latitudes greatly depleted the biogenic particulate calcium (Ca), likely the result of subsurface waters with a relatively low carbonate saturation-state. Decreases in particulate Ca may strongly impact the export of carbon in this region, allowing more recycled CO₂ to be readily mixed into the near-surface layer. Such findings provide the impetus to examine this same section a decade later to answer key questions. For example, are individual surface and subsurface trace-element features in a steady state? How might they respond to both short-term variability and decadal trends in dust inputs? The Barbados dust record (Joe Prospero, Pers. Comm.) indicates a decrease in dust flux by as much as 20-30% to the Atlantic over the last 10-15 years. Based on the 2003 trace metal data, initial estimates of residence times for both surface and subsurface trace-metal features are from <1 to <2 years. How have these features responded to variations in dust deposition (the major source of trace elements to many open ocean regions) over the intervening years? As for the carbonate system, will increasing ocean acidification cause the biogenic particulate Ca deficit to shoal or increase in range?

Dr. Bill Landing putting water-tight tape over the electrical connections in the aerosol sampler before leaving port in Reykjavik. A vacuum pump draws air over filters that are housed under the triangular lid. The aerosols are analyzed for total (bulk) trace metal loading and soluble trace metals. We are interested in the soluble fraction as it is this fraction that is the most readily available to phytoplankton.
(photo courtesy of Monica Mejia)

On CLIVAR A16N 2013, the trace metal team consists of:

Leg 1:

• Rachel Shelley (postdoctoral associate and leg 1 blogger, FSU)
• William (Bill) Landing (Principal Investigator and professor, FSU)
• Joseph Resing (Principal Investigator and professor, Univ. Washington and PMEL/NOAA)
• Pamela Barrett (Graduate Student, Univ. Washington)

Leg 2:

• Pamela Barrett
• Peter Morton (postdoctoral associate and leg 2 blogger, FSU)
• Randy Morton (volunteer, FSU)
• Nathan Buck (technician, PMEL)

The rain sampler is installed and ready to go. When it rains, an infrared sensor signals the lid to come off the bucket. The bucket contains a plastic funnel attached to a LDPE bottle which collects the rain. After the rain stops, the rain is collected and stored for trace metal determination by ICP-MS at the National High Magnetic Field Laboratory, FSU.
(photo courtesy of Monica Mejia)
 

The FSU group are collecting samples of marine aerosols, rainwater and dissolved trace metals in the water column (down to 1000 m depth). All our analysis will occur in the lab at FSU. The Univ. Washington group are collecting samples for particulate trace metals in the water column and analyzing water column samples for dissolved Fe and Al on board the ship. We are also collecting samples for other researchers, e.g. mercury in seawater, and pteropods (a taxa that are very sensitive to a decrease in the decrease in the carbonate saturation state of the ocean).