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John Ryan, Postdoctoral Fellow
Monterey Bay Aquarium Research Institute

John Ryan earned an MS (1993) and a PhD (1998) in biological oceanography from URI. In the fall of 1998, he began a postdoctoral fellowship at the Monterey Bay Aquarium Research Institute (MBARI). His research focuses on the exchange of carbon dioxide between ocean and atmosphere.

The continental shelf ecosystem of the northeast United States is among the most productive in the global ocean. Away from near-coastal waters, sustenance of this highly productive ecosystem derives from photosynthesis by microscopic algae, the phytoplankton (see Fig. 1). Because these single-celled plants absorb certain colors of light and reflect others, they change the color of the upper ocean. Using instruments that orbit hundreds of miles above the ocean surface, we can measure the signature in the visible spectrum and thus characterize the abundance of phytoplankton. In minutes, satellite-borne instruments can capture conditions over a million square kilometers at a resolution of about 1 square kilometer. Satellite remote sensing of ocean color has greatly contributed to the study of phytoplankton distributions in this highly productive region, allowing us to see the oceanic forest via the microscopic trees. However, understanding ocean processes that drive high productivity requires that we combine the satellite perspective with ship-based observations of conditions below the surface.
      For the past two decades, a partnership between science and commerce has provided valuable ship-based observation of this biologically rich region. Crossing continental shelf waters on its weekly round-trip between New Jersey and Bermuda, the MV Oleander makes an ideal platform from which to regularly sample the ocean along a fixed transect (see Fig. 2). Observations of hydrographic conditions and zooplankton have been made for almost two decades aboard the Oleander (See article by Jossi, Benway, and Goulet), and observations of circulation have been made for most of the last decade (See article by Rossby and Schwartze). The Oleander is equipped to continuously measure and record properties of the water beneath its hull. The scientific apparatus on this volunteer observation ship is amenable to expansion for research projects.
      During the spring of 1997, I augmented this versatile system with a fluorometer, an instrument that measures fluorescence as an indicator of phytoplankton abundance. The motivation for this effort was to study a striking annual feature of this biologically productive region: enhanced phytoplankton abundance at the continental shelf break during late spring. Because this localized blooming develops when near-surface waters of the region are generally depleted of nutrients, its development requires a local nutrient supply from deeper waters. This can occur by upwelling or vertical mixing.
      In 1997, scientists were able to use satellite and ship observations together to study phytoplankton distributions and processes in this region. This opportunity followed a decade with no satellite ocean color observations. During early May 1997, the Ocean Color and Temperature Sensor (OCTS) detected bloom concentrations of phytoplankton following the continental shelf break for more than 200 kilometers. In Figure 3, the location of the shelf break is shown by the thick white line. Note the high concentrations (darker shading) following the shelf break east of approximately 72.5°W. This biological enhancement coincided with the propagation of frontal meanders along the shelf break. One of these meanders is outlined in Figure 3 using a contour of the 10° sea surface temperature isotherm (black contour within the white box).
      The Oleander crossed the shelf break bloom during the period of satellite detection. The arrows in Figure 3 show the direction and relative magnitude of water flow beneath the ship during that crossing. They show that shelf water flowed offshore southwest of this meander. Observations at another meander farther north showed similar offshore flow of shelf water. Offshore flow at the shelf break can cause upwelling due to the structure of the front (dynamic boundary) between shelf and slope waters (regions of shelf and slope water are identified in Fig. 2). Indeed, Oleander observations captured the bloom of phytoplankton at the shelf break. The highest abundance (see Fig. 4a) was coincident with upwelling of shelf water (see arrow in Fig. 4b). The colder, deeper shelf water is more nutrient-rich than surface waters. Its upwelling into shallower waters, where light intensity is greater, promotes phytoplankton blooms.
      The Oleander observations from spring of 1997 served a few important purposes. First, because phytoplankton growth and distributions depend on water properties and circulation, the combined physical and biological observations from the Oleander provided a window into processes influencing phytoplankton productivity. Second, the ship-based observations of phytoplankton abundance filled in where and when the satellite instrument could not. Where? Although satellite ocean color captures the big picture, it cannot resolve variability at scales less than 1km. When? Because cloud cover completely obscures viewing of ocean color by satellite, measurements from the Oleander provided biological information under those conditions. Lastly, the Oleander observations provided independent measures that complemented and augmented interpretation of the satellite observations. Independent measure of phytoplankton abundance from volunteer observation ships offers tremendous potential for much needed calibration of satellite ocean color sensors. These valuable contributions required only a simple installation of our instrument within the existing infrastructure already on the Oleander. Generous assistance from NOAA and URI scientists and Oleander engineers made this endeavor rewarding, both in success and camaraderie. Our understanding of the oceanographic dynamics influencing life in these highly productive waters has expanded in the wake of this volunteer ship, and the scientific potential of this partnership remains very promising.