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Sargasso Sea Study of Ocean Productivity Under Global Warming

A Sargasso Sea Study of Ocean Productivity Under Global Warming

2013-15 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 pursue these questions is to use modern patterns to gain insight into future changes. For example, heading south from Bermuda to Puerto Rico, there is a clear decline in fall mixed layer depth, such that the more southern waters provide an analogue for future conditions near Bermuda. The field sampling also provides opportunities to advance the Sigman group’s use of foraminifera (calcifying zooplankton) to reconstruct past changes in ocean nutrient cycling, based on deposits in the North Atlantic Ocean.

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.
Figure 2
Figure 2. Bermuda coral after being cored to retrieve a 200 year record.
Coral sampling sites on the Bermuda Pedestal
Coral sampling sites on the Bermuda Pedestal

Rising CO2 and global warming are also expected to have dramatic effects on the ocean’s coral reefs. First, the CO2 that dissolves in surface waters reduces the supersaturation of the mineral aragonite, with which corals build their skeletons. Second, the warming may change the physiology of the corals and their symbiosis with algae growing inside their tissues. Third, the warming-driven circulation changes described above and other human activities may affect the supply of nutrients to reefs, with different changes on coastal and open ocean reefs. Since coral skeletal growth relies on the productivity of the coral, nutrient cycling is central to reef building.

The shallow pedestal around Bermuda provides an ideal setting for understanding the nutrient cycling of a coral reef and its exchange with the open ocean. Projects focus on both the modern nutrient cycling of the reef and the reconstruction of past nutrient conditions on the reef through the collection of coral cores.


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.  

Jack Zhou '15
Jack Zhou, Class of 2015, with Bermudan coral. (Courtesy Tony Wang)

Collaborating Institutions

  • Bermuda Institute of Ocean Sciences
  • Bigelow Laboratory for Ocean Sciences
  • Woods Hole Oceanographic Institution
  • ETH Zurich

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

Joan Cannon '15
Sean McIntee '15
Tyler Tamasi ‘15
Jack Yujie Zhou '15