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Pacific Equatorial Upwelling

submitted by Jason Roberts

Figure 1. Primary productivity of the central Pacific Ocean, with the Pacific equatorial upwelling highlighted as a likely EBSA

Oceanographers estimate production of phytoplankton ("primary production”) worldwide from satellite observations. Using these data, we can identify an area of high productivity around the Pacific equatorial upwelling

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Scientific Background

At the beginning of many marine food chains are single-celled, microscopic plants called phytoplankton. Through the process of photosynthesis, phytoplankton use chlorophyll and the sun’s energy to convert carbon dioxide and water to organic compounds for growth and reproduction. The generation of new plant material by photosynthesis is called primary production. Oceanographers use estimates of primary production as the most basic measure of the biological productivity of the ocean.

Primary production does not occur uniformly throughout the ocean. The rate of production depends mainly on the quantity of phytoplankton already in the water, the availability of light and required nutrients such as nitrogen and phosphorus, and the water temperature. Light availability is regulated mainly by geographic location and the annual solar cycle. Primary production only occurs in the euphotic zone, the layer of the ocean that light can penetrate. Nutrient availability and water temperature are regulated by the flow of ocean currents. Patterns in light and ocean currents lead to patterns in primary productivity. In this illustration, we highlight one such pattern known as the Pacific equatorial upwelling (Pennington et al., 2006).

The central Pacific Ocean receives a large amount of light throughout the year but is far from land-based sources of nutrients. Nonetheless it sustains a high level of primary productivity due to an oceanographic phenomenon called the equatorial divergence. In this phenomenon, physical processes caused by winds, currents, and the rotation of the Earth force water near the surface to flow away from the equator. To replace it, nutrient-rich water is drawn up from the depths, bringing a steady supply of nutrients to the euphotic zone and producing a band of consistently high primary productivity at the equator.

How the area of high biological productivity was identified

There are several methods available to identify areas of high primary productivity.  In this example we visually estimated the boundary of the high productivity area based on a map of mean annual primary production using a Geographic Information System (GIS).  This method is easy to implement and to interpret. One alternative method is to use a GIS to identify areas that exceed a specified threshold value based on ecological considerations. Another is to review the scientific literature and look for definitions of oceanographic features that correspond to regions of high productivity.  Such definitions might come in the form of geographic coordinates or threshold values for chlorophyll concentration, primary productivity, or sea surface temperature. For example, Pennington et al. (2006) specify that the Pacific equatorial upwelling occurs in the region 3° N—3° S, 90—140° W. This definition is wider in the latitudinal direction and narrower in the longitudinal direction than the rectangle than we drew, 2° N—2° S, 101—178° W.

Sources of data

For this illustration, we used primary productivity data from the Vertically Generalized Production Model (VGPM) by Behrenfeld and Falkowski (1997). The VGPM estimates the net primary productivity for a euphotic volume of water as a function of surface chlorophyll concentration, surface temperature, the length of the day, the flux of photosynthetically active radiation (a measure of the quantity of sunlight important for plant growth), and the depth of the euphotic zone (which itself is estimated from chlorophyll concentration).

Because these parameters can be estimated by high resolution satellite sensors, detailed maps of the VGPM can be calculated for the entire planet on a daily basis. Behrenfeld’s laboratory at Oregon State University publishes free monthly global VGPM maps using data gathered by several satellites. For the data used in this illustration, the VGPM was estimated from data collected from 1997-2007 by the SeaWiFS sensor on the NASA SeaStar satellite and the AVHRR sensors on the NOAA Polar Operational Environmental Satellites.

Oceanographers are continually improving methods for estimating primary productivity. While the VGPM represents the current “industry standard” (MJ Behrenfeld, personal communication), newer models may provide more accurate estimates. Behrenfeld provides two alternative models on his website.

Oceanographic data are often difficult to import into GIS programs. Marine Geospatial Ecology Tools (Roberts et al., in review), a collection of free tools published by Duke University Marine Geospatial Ecology Lab, can assist with this job.

Important considerations

Primary productivity is affected by physical phenomena that operate across a range of space and time scales. Fronts, eddies, and other small-scale dynamic processes can stimulate productivity in small regions for days to months (Willett et al., 2006). The annual solar cycle drives distinct seasonal patterns in productivity (Behrenfeld and Falkowski, 1997), especially in regions poleward of the tropics. Large-scale episodic phenomena such as the El Niño Southern Oscillation (ENSO) can force regional episodes of high or low productivity (Behrenfeld et al., 2001). Finally, global climate trends influence primary productivity on a global scale (Behrenfeld et al., 2006). When designating an EBSA on the basis of primary productivity, it is important to understand the phenomena that affect primary productivity in the given region of interest.

Regions having high primary productivity do not always have high productivity of animals higher in the food chain, such as fish or marine mammals. Phytoplankton drift passively with the currents. When a phytoplankton bloom occurs, days or weeks may pass before grazing animals multiply to significant numbers or arrive from elsewhere to consume it. Many of these grazers are zooplankton, which also drift passively. By the time these grazers have themselves been consumed by predators further up the food chain and the density of these predators has reached its peak, the food web may have drifted quite far from the bloom's original location.

For best results when addressing these considerations, consult oceanographers and biologists familiar with the given region and species of interest.

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