OC471-01

Ship

Cruise Party

Departure: Apr 23, 2011

Arrival: May 13, 2011

Operations Area

Science Objectives

The diazotroph Trichodesmium spp. constitutes a major pathway of nitrogen flow into marine planktonic ecosystems, but estimates of its impact on global nitrogen budgets vary widely. Sampling is made difficult by the fragility of the organism with the consequence that Trichodesmium spp. are difficult to manipulate in both field and laboratory experiments. Optical methods that sample the organism nondestructively are thus appealing. A recent transatlantic survey using the Video Plankton Recorder (VPR) revealed unexpectedly high abundance of Trichodesmium spp. at depth, suggesting the vertical distribution of the organism within the euphotic zone may be more uniform than previously thought (Davis, C.S. and McGillicuddy, D.J., 2006. Transatlantic Abundance of the N2-Fixing Colonial Cyanobacterium Trichodesmium. Science, 312: 1517-1520). Application of a simple bio-optical model of productivity to the observed profile of abundance suggests the depth-integrated nitrogen fixation rate could be three to five times higher than that based on the canonical profile of exponential decrease in abundance with depth. However, the observations described in Davis and McGillicuddy (2006) come from a latitude range where Trichodesmium spp. are not especially abundant. This raises a key question: is there a similar vertical distribution in waters further to the south, where Trichodesmium spp. are an order of magnitude more abundant overall? If so, are the deep populations actively fixing nitrogen? If so, the implications for the global nitrogen budget would be substantial.

To answer these questions, we will undertake two cruises to survey the waters of the southern Sargasso Sea and tropical Atlantic, where Trichodesmium spp. are commonly found in high abundance. Along-track VPR measurements will document the abundance and distribution of the organism on the scale of meters to thousands of kilometers. Standard hydrographic station work will provide for comparison of VPR-based estimates with microscope counts, as well as some additional in situ optical methods. A combination of nifH gene expression assays and direct determinations of N2-fixation rates will be made to assess whether or not the deep populations are actively fixing nitrogen. These observations will be synthesized in the context of an eddy-resolving numerical model. This will permit investigation of the mechanisms controlling the vertical and horizontal distribution and abundance of Trichodesmium spp. at multiple scales, including the enigmatic association of relative maxima in abundance with anticyclonic eddies (also described in Davis and McGillicuddy, 2006). Moreover, integration of these observations into the numerical model will facilitate revised estimates of nitrogen fixation by Trichodesmium spp. in the North Atlantic.

Science Activities

Departing from Woods Hole, we will set a course for BATS 31 40N, 64 10W.  VPR will be deployed upon crossing the shelf break.  XBTs will be deployed hourly.  VPR will be recovered each day at 0930 so CTD station work can begin promptly at 1000.  Upon completion of CTD/pump station work, VPR will be redeployed and underway sampling will recommence.

After sampling at BATS, we will proceed southward along 64W to 20N, then turning southeastward to 10N, 45W.  Alongtrack sampling is flexible, and we will adjust our course according to satellite altimetry and SST / ocean color imagery.

Additional Info

Stations:

Supporting documentation:

Shipboard Equipment

CTD/Water Sampling

Critical CTD Sensors

CTD-Related Equipment

Hydrographic Analysis Equipment

MET Sensors

Sample Storage

Navigation

Winches

Standard Oceanographic Cables

Checklist

Checklist