Changes in UV penetration associated with marine intrusions and freshwater discharge in a shallow coastal lagoon of the Southern Atlantic Ocean

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.