Spectral light attenuation and phytoplankton distribution during a daily cycle in the reservoir of La Concepción, Southern Spain

Abstract

The spectral light field and the concentration of phytoplankton, total particulate material larger than 0.7 μm and chlorophyll were analyzed during a daily cycle in a stratified reservoir (La Concepcion, Southern Spain). Vertical attenuation coefficient (K d ) at different wavelengths depends on solar elevation and both phytoplankton and total particle concentration. Kd at blue and red wavebands but no in green, yellow-orange increased between 11:00 to 15:00 and it was related to the aggregation of phytoplankton at 12m depth. The product of K d and Secchi Disk (SD) depth was inversely correlated with chlorophyll. Chlorophyll per cell presented a daily cycle of acclimation to the changes in the irradiance with maximum level around noon at the maximum phytoplankton density (8-12 m depth). This pattern was similar to that of the spectral attenuation coefficient between 400-575 for the water column. Two phytoplanktonic aggregations were found at 10 % (SD) and 1% (2 SD) depending on the incident light. A chlorophyll maximum was located at 1% of incident light. The chlorophyll maximum at daylight was caused by motile dinoflagellates (Peridium and Ceratium) and diatoms (Fragilaria), but during the night only dino-flagellates remained at this depth and they were the only ones responsible for the chlorophyll maximum as well as for the particulate material concentration. The two main phytoplanktonic groups, large dinoflagellates and diatoms affected the light attenuation in different ways. In the samples dominated by dinoflagellates, with high chlorophyll content per cell, the increase in chlorophyll concentration produced a small increase in the light attenuation due to the package effect. However, in the samples dominated by diatoms, a good positive linear correlation between chlorophyll concentration and attenuation coefficient was found. The ecological implications of the short-term variations in the bio-optic properties in the reservoir are discussed.