ANTARES is a project leading towards the construction and deployment
of a neutrino telescope in the deep Mediterranean Sea. The
telescope will use an array of photomultiplier tubes to
detect the Cherenkov light emitted by muons resulting from the
interaction with matter of high energy neutrinos. In the vicinity of
the deployment site the ANTARES collaboration has performed a series
of in-situ measurements to study the change in light transmission
through glass surfaces during immersions of several months. The
average loss of light transmission is estimated to be only $\sim$~2\% at the
equator of a glass sphere one year after deployment. It decreases
with increasing zenith angle, and tends to saturate with time. The
transmission loss, therefore, is expected to remain small for the several
year lifetime of the ANTARES detector whose optical modules are
oriented downwards. The measurements were complemented
by the analysis of the $^{210}$Pb activity profile in sediment cores
and the study of biofouling on glass plates. Despite a significant
sedimentation rate at the site, in the 0.02 -- 0.05~$\rm cm\cdot yr^{-1}$
range, the sediments adhere loosely to the glass surfaces and can be
washed off by water currents. Further, fouling by deposits of
light-absorbing particulates is only significant for surfaces facing
upwards.
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