Samples were filtered over cellulose acetate filters (Sartorius, pore size 0.8 µm) for scanning electron microscopy. In order to avoid mechanical stress to fragile diatom chains, syringe filters were used. Salt was washed-out by repeated rinsing with distilled water. After drying, filters were fixed by conductive tape (Plano GmbH, Germany) onto aluminium stubs and sputtered for several minutes with gold-palladium. Investigations were then performed by a scanning electron microscope of the type Quanta 200 FEG (FEI Company).