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Sargasso Sea Study of Ocean Productivity Under Global Warming

A Sargasso Sea Study of Ocean Productivity Under Global Warming

2013 Seed Grant

Electron microscope image of a the shell of a foraminifera, a type of calcifying zooplankton, from Sargasso Sea sediments. (Courtesy Marietta Straub)

Rising CO2 is causing the atmosphere and surface ocean to warm, and there is a widely-held expectation that the tropical ocean water column will become more strongly stratified by temperature (and thus density) as warming proceeds. Primary productivity in the tropics is already limited by the supply of nutrients (particularly nitrate) from below. It is predicted that as this region becomes more strongly stratified, the upward mixing of nutrients will be further impeded, lowering tropical ocean productivity.

However, research conducted by the Sigman group near Bermuda suggests that in the summer when surface waters are warmest (most stratified), the upward supply of nitrate is actually higher than during the fall when the surface cools (Figure 1). The implication of this surprising finding is that nutrient supply (and thus productivity) in the tropics may increase as stratification strengthens. The data further suggest that higher nitrate supply will preferentially fertilize specific groups of phytoplankton, with ecological consequences for large expanses of the ocean.

The Sigman group’s strategy to test the robustness of these patterns is a space-for-time substitution: Heading south from Bermuda to Puerto Rico, there is a clear decline in fall mixed layer depth. Samples collected along this transect should thus capture changing hydrographic and biogeochemical conditions analogous to those of a warming world. Numerical modeling studies of the upper ocean are being developed to better understand the mechanism(s) and implications of upward nitrate supply. The field sampling also provides opportunities to advance the Sigman group’s use of fossil corals and foraminifera (calcifying zooplankton) to reconstruct past changes in ocean nutrient cycling, based on deposits in the Sargasso Sea and on the islands of Bermuda.

Figure 1
Figure 1. The hypothesized relationship between seasonal changes in the density structure of the upper water column and the nitrate supply to the euphotic zone. The x-axis indicates water density (top axis, increasing to the right, and red and blue lines) and the concentration of nitrate, a critical nutrient for phytoplankton growing in the sunlit upper ocean (bottom axis, increasing to the right, and solid gray lines). The y-axis indicates depth, increasing downward, and representing roughly the upper 150 meters of the water column. Shown as dashed horizontal lines are the depths of light penetration (the “euphotic zone,” dashed black line) and the bottom of the column of surface water that is homogenized by wind mixing (the “mixed layer base,” dashed gray line). a) During the summer, the low-density surface mixed layer is very shallow, making it easier for nitrate to be imported into the lower euphotic zone, where it is consumed by eukaryotic phytoplankton. b) In the fall, the surface mixed layer deepens, reducing the potential for nitrate to be imported across the base of the euphotic zone and thus decreasing nitrate-supported phytoplankton growth. It is hypothesized that, with global warming, the density structure of the upper tropical ocean will resemble that shown in panel a for longer periods of the year, increasing the total amount of nitrate imported into the euphotic zone over the course of the year.

Educational Impacts

Tyler Tamasi, Class of 2015, hard at work in the Sargasso Sea. (Courtesy Alexa Weigand)

This project is focused on the oceanic region near Bermuda, with the Bermuda Institute of Ocean Sciences (BIOS) providing a collaborative and convenient platform for combined research and education. Multiple oceanographic sampling campaigns will be undertaken aboard the BIOS research vessel, providing Princeton students a hands-on introduction to oceanography and the methods by which the ocean is explored. Beyond the central goals of this project, participation in its oceanographic cruises would give undergraduate students the opportunity to collect samples to address their own research questions, which could then be developed into Junior Projects or Senior Theses. This will engage undergraduates in both scientific field campaigns and in learning the novel methods being developed and utilized in the Sigman lab at Princeton.  

Collaborating Institutions

  • Bermuda Institute of Ocean Sciences
  • Bigelow Laboratory for Ocean Sciences

Related Media and Press Coverage

Daniel Sigman

Daniel Sigman, Dusenbury Professor of Geological and Geoophysical Sciences, Department of Geosciences

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Research Associate

Sarah Fawcett

Graduate Students

Dario Marconi
Farhan Nuruzzaman
Keiran Swart

Undergraduate Students

Sean McIntee ‘15