IMPACT OF UV RADIATION ON THE AQUATIC ENVIRONMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 175

the water column where they receive sufficient solar photosynthetic available radiation
(PAR). The lower limit of the euphotic zone is defined as the depth where
photosynthetic biomass production balances respiratory losses. Since this limit depends
on the light availability, the term is defined physiologically rather than physically.
Mutual shading, absorption and light scattering by other particles within the water
column limit light penetration. The euphotic zone may extend from a few decimeters in
turbid coastal waters to up to 200 m in clear oceanic waters. The transparency of a body
of water strongly depends on the wavelength: short wavelength radiation is more
strongly absorbed than blue-green light. UV-B, even though attenuated, penetrates well
into the euphotic zone 6 and affects the phytoplankton organisms.
Many phytoplankton organisms have a limited capacity to either actively swim
or use buoyancy and undergo vertical migration to move to and stay at depths which are
optimal for their growth and reproduction. This typical vertical distribution pattern of
phytoplankton is disturbed by passive mixing due to high wind and waves 20 .
The primary consumers (zooplankton) include unicellular and multicellular
organisms which feed on the primary producers. The following level in the food web are
free swimming organisms (nekton) including krill, molluscs, fishes and crab larvae,
feeding in turn small fishes, molluscs and crustaceans. The final consumers are large
fishes, birds and mammals including humans. During each transition from one trophic
level to the next the amount of biomass is reduced by a factor of about ten. In addition
to the direct effects of solar UV-B radiation, the consumers are affected indirectly when
the productivity of the primary producers decreases. To evaluate the UV-related stress
in phytoplankton the following questions need to be answered:
x What is the spectrally weighted distribution of solar radiation in dependence of
time and depth?
x What are the biological weighting functions (BWF) of the physiological
processes in ecologically important phytoplankton species?
x What is the spatial and temporal pattern of phytoplankton distribution within
the euphotic zone, affected by wind and waves?
We are far from quantitatively understanding the intricate processes, but some of the
questions have been partially answered.
Dissolved organic carbon (DOC), derived from decaying organic material, limits
the penetration of UV-B into the water column 21 . Bacteria take up DOC and recycle the
nutrients. In turn, the bacteria are food to heterotrophic nanoflagellates. Solar UV
affects the heterotrophic bacteria and flagellates which have only limited protection and
mitigating strategies. Simultaneously, short wavelength radiation breaks down the high
molecular weight DOC into smaller fragments which can then be taken up easier by
bacteria. By this process the transparency of the water column for solar UV increases 22 .
Biologically weighting functions (BWF) have been determined to quantify the
spectrally weighted sensitivity of photosynthesis to UV and visible radiation in several
phytoplankton species 23 . Most BWF have a maximum in the UV-B, but also show
significant sensitivity in the UV-A. Most BWF have roughly the same shape but in
detail vary by species and the physiological process studied 23 .
A number of models have been developed to quantify the impact of increased
solar radiation on phytoplankton productivity 23 , using physical and physiological
parameters, water column dynamics and species distribution. One major problem is that