National Aeronautics and Space Administration

Wallops Flight Facility

Ocean Primary Productivity

Satellite-Based Global Ocean Primary Productivity

Throughout the history of earth, physical forces acting on the oceans and the chemical processes taking place therein have never been at steady state, but rather have constantly changed. In both coastal and open-ocean regions, physical-chemical conditions dictate biological productivity and its distribution, while biological processes in turn have feedbacks on these environmental forcing. In the past century, human activities have escalated to a point where even global climate is affected. Without doubt, the combined effects of natural and anthropogenic climate variability will influence coastal ocean productivity in the decades to come. The question is, can we measure or predict these changes and forecast their influence on issues of national interest?

Coastal waters are important to the economy and security of our nation at a variety of levels. Interestingly, the growth, composition, and distribution of phytoplankton are relevant to each of these levels. Phytoplankton form the base of the marine food web and thus play a pivotal role in regulating fisheries potentials. Phytoplankton are also the primary biological conduit through which atmospheric CO2 is removed from the atmosphere and deposited in coastal sediments. Changes in species composition can result in toxic phytoplankton blooms that damage fisheries and influence recreational activities, while the abundance of phytoplankton is critically linked to the optical properties of near-shore regions and thus of military interest in terms of homeland security. The focus of the Ocean Primary Productivity Group (OPPG) is to evaluate key environmental-physiological relationships that permit the optimum use of NASA’s remote sensing observations to address our country’s scientific, economic, and security issues in the coastal zone.

With respect to ocean biology, remote sensing technology has largely focused on quantifying near-surface phytoplankton chlorophyll concentrations. To address the issues described above, these measurements of chlorophyll standing stocks must be interpreted in terms of rates of primary production or growth, as well as in terms of phytoplankton abundance (i.e., number or carbon biomass). Conversion of chlorophyll concentration into phytoplankton photosynthesis or carbon biomass is a challenging scientific problem that is a focus of the OPPG. Our approach to this problem has been through parallel field, laboratory, and modeling efforts.

Figure 1. Ocean productivity modeled with the VGPM, combined with land production estimates.

Primary Productivity Modeling

In collaboration with scientists at Rutgers University, we have developed a commonly employed model of water-column photosynthesis: the Vertically Generalized Production Model (VGPM). The VGPM was originally based on an analysis of field productivity measurements from the Mid- and North-Atlantic regions of the United States, but has been commonly applied to global analyses (Figure 1). Although the quantification of ocean productivity is a central focus, our more important goal is to develop the capacity to detect changes (local and global) in the base of marine food web (Figure 2). Achieving this goal requires a considerable reduction in the uncertainties associated with productivity estimates. We are currently in the process of expanding the VGPM to better resolve water column primary production with time, depth, and spectral irradiance and to integrate concepts of iron- and light-limitation from the studies. Specific foci of this effort are described below.

Figure 2. Changes in global ocean and net photosynthesis (NPP) resulting from an El Nino to La Nina climate change. Ocean NPP estimated with the VGPM. Land NPP estimated with the Carnegie-Ames-Stanford Algorithm (CASA)

Iron Limitation

The limitation of phytoplankton growth by the availability of iron is a prevalent condition in both open ocean and coastal waters. We are studying iron limitation using a newly discovered, stimulated-fluorescence signal that appears diagnostic of iron stress. In collaboration with scientists at Brock University in Canada, laboratory experiments are being conducted to identify the physiological mechanism underlying this diagnostic signal. In addition, field measurements are being conducted to test the relationship between iron concentrations and the appearance of the signal. An attractive aspect of the diagnostic of iron limitation is that it can potentially be retrieved remotely by pump-and-probe technologies currently being developed at the Wallops Flight Facility.

Figure 3. Two cultures of a marine algae grown at high (left) and low (right) light. Both have equal photosynthetic rates and cell numbers. Quantifying coastal photosynthesis requires estimates of such physiological changes.

Light Limitation

The critical physiological variable required for converting near-surface chlorophyll concentrations
into water-column photosynthetic rates is the light-saturated, chlorophyll normalized assimilation
efficiency, Pbmax. We have recently demonstrated, using field data from around the world, that
variability in Pbmax is largely due to light-dependent changes in cellular chlorophyll (e.g., Figure
3). We have also shown that this process, termed photoacclimation, causes parallel changes in the
algal carbon to chlorophyll ratio (θ). The importance of this demonstration is that θ, unlike Pbmax,
can be retrieved from remote sensing data. In collaboration with other scientists at the Wallops
Flight Facility, a primary near-term focus of our research will be to develop passive or active
techniques for derived θ from space. In parallel, we are also conducting laboratory studies with
phytoplankton monocultures to further resolve the relationship between Pbmax and θ.

Mixed Layer Lidar

Quantifying surface mixed layer depths (MLD) from remote sensing measurements could make a tremendous contribution to our understanding ocean processes and their temporal variability. One potential method for measuring MLDs is to use lidar technology to probe the surface ocean for depth-dependent changes in particulate scattering. Based on field measurements, it appears that the base of the mixed layer is associated with a rapid change in the concentration of suspended particulates. This region of change in particle load should be detectable with a lidar. A prototype instrument for making such measurements is currently under construction at the Goddard Space Flight Center.

Future Directions

1)        Developing an effective, active approach to optically retrieving quantitative measurements of particulate organic carbon (POC) concentrations in coastal and open ocean regions. Our approach will be based on relationships between POC and beam attenuation. This effort will involve a significant field component to develop remote sensing algorithms and to calibrate/validate resultant remote sensing products.

2)        Continue efforts to remotely retrieve information on surface mixing depths. These efforts will entail both potential methods for directly measuring mixing depth (e.g., the Mixed Layer Lidar) and methods for deriving mixing depths from related indices, such as algal carbon to chlorophyll ratios.

3)        Conduct laboratory studies to resolve mechanistic relationships between environmental focings and variability in phytoplankton chlorophyll-specific, light-saturated photosynthetic rates.

Contributors:

Lead Investigator: Michael Behrenfeld
Collaborators: Kirby Worthington, and Donald Shea