Comment on Small Particles of Amorphous Silica in the Nepheloid Layer

Abstract

Sandilands and Mudroch (1983) have shown with scanning electron microscopy (SEM) that there are many particulates ranging in size from 0.2 to 0.4 p'm in the nepheloid layer of Lake Ontario. In addition, they present evidence that these particles are amorphous or biogenic silica, presumably fragments of diatom frustules. This observation is significant because, as they point out, particles of this size could pass through the filters commonly used in limnological research and therefore would not be assessed as particulate matter. This finding may be an explanation for the relatively small increases in particulate silica (collected on 0.45 p. HA Millipore filters) that we found in near-bottom waters compared to other depths in the water column of Lake Michigan (Schelske et al. 1983), whereas Chambers and Eadie (1981) have shown that the nepheloid layer is a widespread feature in the nearbottom waters. However, Sandilands and Mudroch (1983, pg. 196) incorrectly assumed that small particles of amorphous silica will be measured and contribute to the dissolved reactive silica pool of water samples. Amorphous silica will not be dissolved under the acidic conditions used to measure reactive silica. In our laboratory, we have never found a difference in reactive silica concentrations between filtered (HA Millipore) and raw water samples. In this regard it is interesting to note that preparing diatoms for microscopic analysis in boiling acid obviously does not destroy frustules. If amorphous silica is to be measured, it will require leaching in a basic solution and possibly including a correction for mineral interference (Krausse et al. 1983). The importance of diatom fragments has been