Ecological Indicators (Nov 2021)
An efficient method to wash out the particulate matter trapped by aquatic mosses
Abstract
Aquatic macrophytes capture trace elements from water and store them in their tissues, which make them good biomonitors for these substances. But they also capture suspended particulate matter that could bias the concentration measurements in the moss tissues. To avoid this problem, many biomonitoring studies using mosses include a washing step in the sample preparation procedure.However, no data is available on the quantities and variability of the particulate matter trapped by the moss, and the washing procedure has not been standardized between studies (washing time varied between 10 s to 30 min, divided in up to 7 washing steps). To study this, we extracted the particulate matter from samples of the moss Fontinalis antipyretica Hedw. collected in three rivers in NW Spain, washed them for 1, 2 and 5 min and collected the particulate matter extracted in each step. We also measured the contents of Al, As, Ba, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb and Zn in the tissues of the washed and unwashed mosses. We made a second experiment washing the samples only for 15 + 45, 30 + 30 and 45 + 15 s to determine if shorter washing times were adequate for cleaning the moss. In a third experiment we estimated the contents of particulate matter in moss samples collected monthly over a two-year period in two rivers in the same area.The results showed that a) particulate matter can be as high as 40/% of the total weight of the sample, and that there were large variations within and between sampling sites, and over time. These results showed the need of washing the samples prior to their analysis; b) that two washings are needed for the removal of most of the particulate matter, but that 30 + 30 s are long enough to clean the moss, and c) that a 20:1 (or larger) water:moss relation is recommended to avoid saturation of the washing water, which would reduce the efficiency of the procedure.We did not find significant losses of trace elements along the washing process, except for Cr in the three sampling sites, and Al, Fe and Pb in some of them. 1 min total washing time seemed the correct option both from the point of view of reducing the handling time of the samples and to prevent possible element losses.
