Förbättring av den svenska kiselalgsmetoden-hantering av sedimentering

Sammanfattning: Benthic diatoms are used in Sweden for monitoring water quality in run-ning water; the method is also frequently used in other countries in Europe. One of the problems of the diatom method is the need for sedimentation of the fresh sample, to be able to decant the over standing water and add alco-hol as a preservations solution. To my knowledge, no results have been published about how long the settling time must be to ensure that the ben-thic diatoms really have settled down to avoid a loss of the diatoms when decanting the over standing water. The aim of my investigation was to improve the Swedish method where the benthic diatoms are used for water quality monitoring. I wanted to find out the appropriate time necessary to wait until all diatoms have settled down before decanting the over standing water. First, fresh benthic diatom samples were taken in the river Fyrisån and the lake Mälaren on different days. After arriving in the laboratory, I waited three different times (30 min, 60 and 180 min) for the diatoms to settle. Thereafter I counted the settled diatoms in the microscope. In the next step, I employed a statistic test to investigate if the number of settled diatoms after the different times were significantly different. I ex-pected to identify the time when all diatoms have settled down without los-ing any diatom when I poured the samples. The results of the statistic test demonstrated that it is enough to wait 30 min for diatoms to settle. This means that the recommendation for the fresh samples sedimentation time is to wait a minimum of 30 minutes prior counting. Further studies are needed to investigate the settling of diatoms since this is the first investigation and more samples needed for better understanding of the settling of benthic diatoms.