Diatoms were prepared using standard methods (e.g., Wilson et al. 1996).
Approximately 5 mL of wet surface-sediment or rock-scrape sample was digested in a 20-mL glass vial using a mixture of concentrated HNO3/H2SO4, which was heated for approximately 5 hours in a hot water bath (70 oC), with occasional mixing. The samples 21 were allowed to cool and settle for approximately 24 hrs, after which ~3/4 of the uppermost acid was aspirated. The samples were rinsed repeatedly with distilled water
(settling for 24 hrs between rinses) until the sample gave a pH reading of distilled water,
and then water was adjusted to a volume of 15 mL. Four hundred μl of each sample was
added to a test tube, and four serial dilutions of each sample were prepared. Each of the
four dilutions were placed on cover slips and allowed to dry for approximately 2 days and
then mounted onto glass microscope slides using Naphrax®. Diatoms were identified and
counted along transects using a Leitz DRM light microscope (DMRB) under an oilimmersion
with differential interference contrast at 1000x magnification (NA objective =
1.3). A minimum of 400 diatom frustules were counted per sample. Diatoms were
identified to the species level wherever possible using available published references
(Gasse 1996; Cocquyt 1998; Cramer 1987)