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

Downwelling ultraviolet (UV) irradiance (290-400) nm was measured at 1-nm intervals in the surface waters of the estuary and Gulf of St. Lawrence, Canada, 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 instrument's noise equivalent irradiance (NEI) level of 1 X 10 -5 Wm -2 nm -1 just below the sea surface were measured at wavelengths of >294 nm. At 310 nm, the diffuse attenuation coefficient (K d ) ranged between 0.7 and 4.5 m -1 , with corresponding 10% depth penetrations between 4 and 0.5 m. This represents close to half of the summer mixed-layer water column in the Gulf of St. Lawrence. The following correlation coefficients 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 Chl 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.