The 40+ year Oceanic Flux Program (OFP) deep ocean time-series, supported by the National Science Foundation, has continuously measured the particle flux in the deep Sargasso Sea offshore Bermuda since 1978. The OFP is the longest running time-series of its kind, and unique in its focus on the ocean’s “biological pump”, a term that loosely refers to the vertical transport of material and energy from the surface to the seafloor via the “rain” of sinking particles originating from biological activities.
The formation and settling of particles through the ocean- the particle flux- is a key process that controls many aspects of ocean biology and chemistry. Particle flux provides the primary food source for all life in the vast, dark ocean interior- the largest ecosystem on earth, controls the distributions and global cycles of nutrients and many elements, regulates the ocean’s ability to adsorb excess atmospheric carbon dioxide, and efficiently cleanses the oceanic water column. Ocean particles are mainly biogenic in origin: organic and shell debris of microscopic algae and animals, zooplankton fecal pellets, and amorphous mucous material called “marine snow.” As particles settle through the water column, they are subjected to microbial degradation, animal grazing activities and chemical exchanges with the surrounding seawater. The result is a continuing evolution in particle concentration, flux and composition with depth. In total, <1% of organic material produced at the surface survives transit through the water column to become buried in deep ocean sediments. Yet this residual retains a wealth of information about ocean conditions that can be used to reconstruct the earth’s history.
The OFP time-series is located within low-productivity waters of the North Atlantic gyre, a region representative of much of the subtropical open ocean. The heart of the OFP operation is a four kilometer long subsurface deep ocean mooring that is equipped with particle interceptor traps at 500, 1500 and 3200 m depths to continuously collect sinking particles at a two-week sampling resolution and additional instruments that measure temperature, salinity, dissolved oxygen, and currents. Every six months, the mooring is recovered to retrieve the trap collection bottles and download the instrumental data, and then it is redeployed with a fresh batteries and a new set of collection bottles. Back in the lab at the MBL, the recovered flux samples are analyzed using a variety of sophisticated optical and chemical methods to determine the magnitude of flux and its detailed composition at each depth.
A central focus of OFP research is to understand the processes that control temporal variations in the particle flux and its composition on time-scales ranging from weeks to decades. This goal is facilitated by the co-location of ship-based biogeochemical time-series research programs near the OFP site, which allow detailed assessment of the linkages between the deep particle flux and the upper ocean physical and biological variables. The OFP time series has provided a unique window of how the deep particle flux varies in response to seasonal cycles in primary production, non-seasonal variability and extreme events such as hurricanes. The length of the time-series has provided new insights on the sensitivity of the deep ocean to basin-scale climate drivers such as the North Atlantic Oscillation, and is affording us a better understanding of how the ocean will respond under future climate change scenarios.