MEPS 207:19-31 (2000)  -  doi:10.3354/meps207019

ABSTRACT: We studied the changes in UV penetration associated with the dynamics of a shallow (mean depth = 0.6 m) coastal lagoon of South America that communicates periodically with the Atlantic Ocean. Two characteristic situations, i.e., freshwater dominance and salt-wedge intrusion were considered. Nine stations were sampled along the main axis of the lagoon, the main tributary, and the adjacent coast. The attenuation in the UV-B, UV-A and PAR wavebands were related to changes in the concentration of dissolved organic carbon (DOC), chlorophyll a (chl a), absorption (a_d) of the chromophoric dissolved organic matter (CDOM), fluorescence of CDOM (F_d), organic (OSS) and inorganic (ISS) suspended solids. The area most influenced by the marine intrusion showed the lowest DOC concentration (1.8 mg l^-1) and the highest UV penetration. In this area, the depth corresponding to 10% of the irradiance below the surface (Z_10%) accounted for 66 and 100% of the water column for the UV-B and UV-A wavebands, respectively (diffuse attenuation coefficient, K_d = 7.3 and 2.1 m^-1). The other zones of the lagoon (DOC = 5.7 to 9.3 mg l^-1) presented low UV-B penetration (K_d = 29 to 64 m^-1), and the Z_10% for UV-A accounted for 30 to 64% of the water column (K_d = 7 to 14 m^-1). Under both hydrological situations, the Z_10% for PAR reached the bottom in most parts of the lagoon (K_d = 1 to 5 m^-1). A significant reduction in DOC-specific absorption (a_d*) during the marine intrusion indicated a change in the absorption characteristics of the CDOM. The variability in K_d for UV-B radiation was mainly explained by the concentration of dissolved and particulate organic substances, while the K_d values for UV-A and PAR were best predicted by the concentration of DOC and chl a.

KEY WORDS: Ultraviolet radiation · UV-B · Dissolved organic carbon · Chromophoric dissolved organic matter · Fluorescence · Estuaries · ENSO