, is an
essential component in the
modeling and/or measurement of UV effects on phytoplankton.
Bio-optical models can be used to predict the penetration of spectral
irradiance in the water column.
They take as input the spectral irradiance above the air-water
interface, compute the transmittance through this interface, and permit
the estimation of biologically effective irradiance as a function of
depth in natural waters (see Smith 1989 [Ray Smith PCPB 1989]
for a review).
The inherent optical properties of water are a key element in these models.
Recently Kirk [Kirk 1994 natural waters, Kirk Williamson 1994] has
reviewed the optics of UV-B radiation in natural waters and showed
that the fundamental optical properties of water in the UV region of the
spectrum are still a matter of controversy.
He points out that the absorption coefficient for pure water at 310nm,
a(310nm), as determined by Quickenden & Irvin (1980) [Quickenden
Irvin 1980, Boivin Davidson Storey Sinclair Earle 1986]
and Boivin et al. (1986)
is about an order of magnitude lower than that
estimated by Smith & Baker (1981).
[Ray Smith Baker clearest natural waters 1981]
Kirk suggests that earlier reported higher absorption coefficients
can be attributed to dissolved oxygen and trace organic materials
which are notoriously difficult to remove completely from water.
From an ecological perspective the important issue is our ability to know
the optical properties of the clearest natural water in
order to construct accurate analytical models of spectral attenuation.
Since dissolved oxygen and trace organic materials are usually present in
natural waters, the higher absorption values published for clear natural
waters should be appropriate for ecological studies,
but the potential for lower absorption as determined in laboratory studies
should not be overlooked.