Sau reservoir’s light climate: relationships between Secchi depth and light extinction coefficient

The relationship between light extinction coefficient (kT) and Secchi depth (ZSD), as well as that between these variables and chlorophyll-a concentration (chl-a) and suspended material (SST) was studied in the Sau reservoir between 1995 and 2001. The results show that kT and ZSD are correlated by the equation kT = 1.36 ZSD, although this relationship exhibits a high inter-annual variability. Among the different factors contributing to water transparency, SST is the most correlated variable with both kT and ZSD. On the other hand, chl-a showed a weak or no correlation, either when the whole studied period was analyzed or, for most years, when they were considered individually. The cause of this low contribution of chl-a to water transparency is to be found in the large amount of detritic matter present in Sau, due both to the inputs of allochthonous particulate organic matter, as well as sediment mobilization from the bottom or the shores during the dry periods. For the period from May 2000 through December 2001, a partition of kT and ZSD was performed using the multiple regression method to assess the contribution of the several dissolved and particulate components. The considered variables were chlorophyll-a, detritic particulate organic matter, inorganic matter, and phytoplankton as particulate material, as well as the effect of water. The results show that detritic particulate matter, with a contribution that ranges from 11% to 63.5% of kT and from 20.3% to 86% of ZSD, is the most important factor influencing water transparency. This results show that the diffuse irradiance is much more important than the directional or vectorial one, and that therefore, Secchi depth is a good descriptor of the light climate, but not of chlorophyll-a concentration.