Non-detrital Si concentrations as an estimate of diatom concentrations in lake sediments and suspended material

This paper evaluates a method, based on normalization with Al, to estimate the concentration of diatoms in suspended and deposited sediments. Provided that Al exists primarily in the detrital phase, one can calculate the frac tion of non-detrital Si (i.e. biogenic Si). Suspended matter and sediment i n two lakes in northern Sweden have been analyzed. Results from the normali zation method are compared with microscope count data. The results from the two methods differ somewhat. Partly dissolved diatom frustules may explain part of this discrepancy. These remnants can hardly be recognized as diato ms. Therefore, they are not counted during the microscopic analysis, althou gh they contribute to the concentration of biogenic Si. Another factor is t he differences in Si content among different types of diatoms. We show that the calculated concentrations of non-detrital Si are good estimates of the diatom concentrations. By using normalization as a tool, reasonable estima tes of the stocks of biogenic Si can be obtained rapidly. Using these estim ates as a guide, sites where considerable changes in diatom productivity an d/or dissolution rates have occurred can be identified. (C) 2001 Elsevier S cience B.V. All rights reserved.