Data quality analysis of phytoplankton counted with the inverted microscopy-based method

Abstract

Phytoplankton standing stock largely characterizes the structure and function of aquatic ecosystems and thus, it needs to be well quantified for measuring the ecological function. Inverted microscopy method (i.e. Utermöhl's counting method) has been widely applied, but not in developing countries limited by investment for inverted microscopy. Phytoplankton samples collected from three reservoirs with distinct trophic states and three treatments of experimental enclosures were used to demonstrate data quality in using the method. The potential effects of the counting individual and cell abundance, community biomass and cell density in replicates of water samples on the data quality were statistically analyzed. As suggested in the original method by Lund (1958), counting 400 individuals for each plate of meso-and eutrophic water sample is required to balance the stability of data, but we found counting 500 individuals for oligotrophic water is needed. When collecting replicate water samples is possible, especially those used for assessing the water quality, three or five replicates of water samples are strongly suggested to be collected to reduce the standard deviation, especially in oligotrophic water. There was no significant difference in Simpson index between two quantitative methods, indicating that both methods have resulted in a similar measurement of phytoplankton diversity. There were significant differences in both measured biomass and cell density between the two methods because of cells loss during concentrating samples. Compared to the concentrated water sample-based method, inverted microscopy method takes a shorter total time for counting and sedimentation, and is preferred for use in the emergency monitoring.