Downwelling ultraviolet (UV) irradiance (290-400) nm was measured at 1-nm i
ntervals in the surface waters of the estuary and Gulf of St. Lawrence, Can
ada, in September 1996 and June and September 1997. Measurements were made
with an Optronic Laboratories OL 754-O-PMT spectrophotometer equipped with
a fiber-optic cable and an underwater sensor. Fluxes greater than the instr
ument's noise equivalent irradiance (NET) level of 1 x 10(-5) Wm(-2) nm(-1)
just below the sea surface were measured at wavelengths of greater than or
equal to 294 Mn. At 310 nn, the diffuse attenuation coefficient (K-d) rang
ed between 0.7 and 4.5 m(-1), with corresponding 10% depth penetrations bet
ween 4 and 0.5 m. This represents close to half of the summer mixed-layer w
ater column in the Gulf of St. Lawrence. The following correlation coeffici
ents were found: 0.81 between dissolved organic carbon (DOC) and K-d, 0.73
between chlorophyll a (Chl a) and K-d, and 0.73 between Chi a and DOC. The
spectral variation of K-d suggests that yellow substance, and therefore DOC
, is responsible for UV attenuation in these bodies of water.