Spectral indices calculated using second derivative value standardized to the Chl c red absorption band (<i>δδ</i>₆₃₂) and explaining more than 40% of the variability (R² > 0.4) of the LUE estimated by PAM-fluorometry using <i>Navicula phyllepta</i> (<i>N</i>. <i>phyl</i>) and <i>Biremis lucens</i> (<i>B</i>. <i>luce</i>) data set.

The lowest values of RSME (= Root Mean Square Error), in bold, to predict LUE for Entomoneis paludosa (E. palu) Planothidium delicatulum (P. deli) and Plagiogrammopsis vanheurckii (P. vanh) are those selected (Eqs 8 to 13). ***: linear regression p ≤ 0.001; n.t.: not tested.