A Sargasso Sea Study of Ocean Productivity Under Global Warming
2013-15 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 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.
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.
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
- Woods Hole Oceanographic Institution
- ETH Zurich
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)
- Airborne Iron May Have Helped Cause Past Ice Ages - AAAS.org (March 20, 2014)