UNDERWATER RADIATION MEASUREMENTS: CONSEQUENCES OF AN INCREASED UV-B RADIATION - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 185

UNDERWATER RADIATION MEASUREMENTS: CONSEQUENCES OF AN
INCREASED UV-B RADIATION
BERIT KJELDSTAD
Department of Physics, Norwegian University of Science and Technology,
N-7491 Trondheim, Norway
1. Introduction
Almost all outdoor living organisms are exposed to ultraviolet radiation (UV).
Ecosystems experience from morning to evening a strong variation of UV intensity due
to diurnal changes in solar elevation, which depends on latitude and time of the year.
These changes are the most dominant factor causing short term variation in UV
radiation on Earth. The amount of UV radiation reaching the surface at any time also
depends on atmospheric factors. Components such as ozone and particulate matter in the
form of clouds and aerosols absorb solar radiation and cause both short term and long
term variability. In the UV range scattering processes are also important and reflection
from the ground influences radiation levels measured at the surface. All changes in UV
caused by atmospheric factors, such as, for instance, ozone depletion, increased amount
of aerosols or increased cloud cover, influence of course the amount of UV in water. To
assess the amount of radiation into the water one has to consider radiative transfer in the
water and particular components which strongly absorb ultraviolet radiation, such as
humic substance, also called gelbstoff or CDOM (Chromophoric Dissolved Organic
Matter). Moreover at the air-water boundary the index of refraction changes and must
be taken into account when penetration of UV into water is investigated.
In several studies it has been shown that there is a great variability in UV
penetration both in fresh-water and marine environments, due to different water quality.
The span in diffuse attenuation coefficients retrieved from the clearest ocean water to
small lakes can be three order of magnitude 1 . A recent overview of attenuation of
ultraviolet irradiance in North European coastal waters indicated a variability of one
order of magnitude in open coastal waters 2 . Geographical variability of UV penetration
has been shown to be essential, even within the same bay or fjord system 3-4 .
This study focuses on seasonal variability of UV penetration at one specific site
within Trondheim Fjord, a fjord system in Norway. The total period investigated is four
years. The parameters taken into consideration to affect UV penetration are total organic
carbon (TOC), chlorophyll (Chla) and salinity.