PhytoNumb3rs: An easy-to-use computer toolkit for counting microalgae by the Utermöhl method.

Phytoplankton is recognised as a biological quality element and a biological descriptor in European Directives (WFD, MSFD) and several national laws. To determine the biodiversity and abundance of phytoplankton assemblages, the Utermöhl method, using an inverted light microscope, is the most widely used. Nevertheless, the Utermöhl method, standardised as UNI EN 15204 (2006), is time-consuming and labour intensive. We developed and tested PhytoNumb3rs, a tool for supporting microscope-based analysis of phytoplankton. PhytoNumb3rs is designed to make cell density calculation easier and to take account of qualitative and quantitative aspects regarding data precision, bias and method sensitivity. Specifically, it makes it possible to assess the performance characteristics of single analyses. PhytoNumb3rs is a user-friendly computing system based on a combination of Excel® spreadsheets. It allows the user to count phytoplankton cells, at various magnifications using various counting strategies, and to obtain the relative cell density, uncertainty and detection limit for each taxon. PhytoNumb3rs enables analysts to adopt a suitable counting strategy, run the analyses and easily perform all the steps associated with conventional microscope-based phytoplankton procedures (counting, data entry, computation, and data storage in a structured database). It allows the user to calculate the performance characteristics associated with the analytical results in accordance with UNI EN 15204 (2006) quality control procedures. Finally, it represents a useful tool for data harmonisation and data standardisation which in turn, are needed in order to increase the quality, comparability and accessibility of intra- and inter-laboratory data over time.