A Sargasso Sea Study of Ocean Productivity Under Global Warming
2013 Seed Grant
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.
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.
- Bermuda Institute of Ocean Sciences
- Bigelow Laboratory for Ocean Sciences
Related Media and Press Coverage
- Earth's wobble 'fixes' dinner for marine organisms - News at Princeton (September 13, 2013)
- In the open ocean, eukaryotes matter! - Department of Geoscience (September 26, 2011)