Morphological analysis
Diatom frustules were cleaned with 30% v/v hydrogen peroxide and hydrochloric acid following standard methods (European Standard EN 13946 2003). Permanent slides were mounted using Naphrax (refractive index of 1.74). At least 400 frustules were identified and counted in each sample under light microscopy using an Olympus BX60 microscope, according to Krammer & Lange-Bertalot, 1986a; Krammer & Lange-Bertalot, 1986b, Kramer & Lange-Bertalot, 1991a; Kramer & Lange-Bertalot, 1991b and Lange-Bertalot et al., 2017.