Figure 1. Jennifer Prentice deploys a profiling UV radiometer to measure UV light penetration in the equatorial Pacific Ocean during a research cruise aboard R/VWecoma. Photo by A. Hanson.
 

Alfred K. Hanson, Jr.
Assistant Marine Research Scientist, GSO

Alfred K. Hanson, Jr. earned a BS in chemistry from the University of Hartford (1971), an MS in organic chemistry from the University of Connecticut (1974), and a PhD in chemical oceanography from URI (1981). He has been on the professional research staff at GSO for 13 years. His research interests include regulation of thin-layered algal blooms by steep nutrient gradients and the influence of UV radiation on chemical reactions and biological processes in marine waters.

Ozone gas in the stratosphere helps shield the earth from harmful solar ultraviolet radiation by absorbing damaging UV-B rays (wavelength range 280-320 nm). Releases to the atmosphere of human-made chlorofluorocarbons (CFCs) and other ozone-depleting substances are causing a downward trend in stratospheric ozone levels over most of the world. The depletion is generally more severe at higher latitudes, farther from the equator, with the greatest losses (up to 60 percent) in the polar regions. Satellite measurements indicate that ozone levels have dropped, on average, about five percent per decade over North America since 1979. Although the production, usage, and atmospheric release of CFCs has declined since the Montreal Protocol, an international treaty enacted in 1987, ozone loss is projected to continue for some time. This is due to the gradual CFC phaseout schedule and the long average lifetimes of CFCs in the atmosphere. The continued loss of ozone will result in higher levels of UV-B radiation reaching the earth's surface. The increased exposure to UV-B may be harmful to human health and have detrimental effects on both terrestrial and marine ecosystems.
     Within marine ecosystems, there is considerable evidence that continued depletion of ozone will lead to increased penetration of UV-B into the upper layers of the ocean resulting in damage to various forms of marine life. UV-B exposure can significantly affect phytoplankton, the single-celled organisms at the base of the marine food web. Excessive UV-B radiation can impair photosynthesis, inhibit phytoplankton growth rates, and cause lethal DNA damage. Zooplankton, the microscopic animals that consume phytoplankton, are also threatened by UV-B exposure due to direct physiological impairment or, indirectly, by limitation of their food resources.
     During the past few years I have had the opportunity to work with two GSO students who are investigating the effects of UV-B on marine plankton for their doctoral research. Jennifer E. Prentice has focused her research on UV-B effects on phytoplankton, and Elena B. Martin-Webb has investigated the effects of UV-B on microzooplankton. Our general research approach can be divided into two components. The first involves documenting the actual penetration of solar UV-B into marine waters at various locations and times of the year. We do this by deploying profiling UV spectral radiometers, instruments that take underwater measurements of UV radiation at different wavelengths (see Fig. 1). The second component involves conducting controlled UV-B exposure experiments, using samples of seawater that contain natural populations of phytoplankton and zooplankton. The seawater and plankton samples are exposed to sunlight but with different levels of UV-B radiation. Special UV-B absorbing plastic materials are placed over some samples to remove all the UV-B, and UV lamps have been used to increase the exposure to UV-B to above-natural levels for other samples. We have conducted UV-B measurements and exposure experiments in various marine environments including Narragansett Bay, the Pettaquamscutt Estuary, Georges Bank, Puget Sound, and the equatorial Pacific Ocean.
     The results of these investigations have led us to the general conclusion that potentially harmful UV-B radiation may already penetrate to ecologically significant depths in many marine environments. For example, the penetration of solar UV-B radiation varied from greater than 50m in the clear waters of the equatorial Pacific to less than 1m in the turbid waters of Pettaquamscutt Estuary. The UV exposure experiments that were conducted at these different locations indicated that the plankton, present within the depth range of UV-B exposure, were adversely affected by the UV-B radiation. Small increases in UV-B exposure levels may be deleterious to phytoplankton and microzooplankton. Our findings are consistent with a general consensus that has been building towards the view that even current levels of UV-B influence both the survival and distribution of planktonic organisms in near-surface waters.
     The long-term ecological consequences of continued ozone depletion, increased UV-B exposure, and plankton loss in marine waters remain unknown. Adverse impacts on planktonic organisms can have dramatic global consequences because these organisms account for more than half of Earth's biomass each year. While some plankton species may adapt to higher UV exposure, others will not. Any sustained losses in the quantity and quality of marine plankton are expected to have a direct and negative impact on the global marine food supply.

Recommended Reading
Hanson, A.K., Jr. Exploring the Chemical Activity in the Sea that is Caused by the Sun, Maritimes, May 1987.
Kerr, J.B., C.T. McElroy. Evidence for large upward trends of Ultraviolet-B radiation linked to ozone depletion, Science, 262: 1032, 1993.
Madronich, S., Chapter 1: The atmosphere and UV-B radiation at ground level, in Environmental UV Photobiology, edited by A.R. Young, L.O. Bjorn, J. Moan and W. Nultsch, p. 1-39. Plenum Press, New York, 1993.
Smith, R.C and J.J. Cullen. Effects of UV radiation on phytoplankton, Reviews of Geophysics, Supplement, 1211-1223, 1995.
USEPA Ozone Depletion: www.epa.gov/ozone/science/ozone_uv.html

Vassiliev, I.R., O. Prasil, K.D. Wyman, Z. Kolber, A.K. Hanson, J.E. Prentice and P.G. Falkowski. Inhibition of PS II photochemistry by PAR and UV radiation in natural phytoplankton communities. Photosynthesis Research, 42: 51-64, 1994.