Penetration of ultraviolet radiation in the waters of the estuary and Gulfof St. Lawrence

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.