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
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
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
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.
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.