Comparing fixed count and observed/expected taxa approaches in diatom-based water quality assessment: implications for monitoring efficiency and accuracy

Abstract

Diatom-based indices are widely used for freshwater bioassessment, yet methodological variability, particularly in count strategies, can influence ecological classification outcomes. This study compares a traditional fixed count (FC) approach with an observed/expected taxa (OE) strategy across five riverine sites in the Eastern Cape, South Africa. A total of 230 diatom taxa were identified. Comparisons between the two approaches revealed statistically significant differences in some ecological status classifications, although both methods captured similar spatial trends across the study area. The OE approach required counting fewer valves per sample, reducing analysis time without compromising assessment accuracy. While the OE taxa approach may provide a time-efficient alternative to FC methods, the differences detected between methods underscore the need for a critical evaluation of their limitations and further testing before routine application.