How strongly do benthic Foraminifera counts differ between the >125 and >150 μm size fraction

Benthic Foraminifera assemblage in two size fractions from the Pefka E section (Island of Rhodes, Greece)

Benthic Foraminifera assemblages are employed for past environmental reconstructions as well as for biomonitoring studies in recent environments. Despite their established status for such applications, and existing protocols for sample treatment, not all studies using benthic Foraminifera employ the same methodology. For instance, there is no broad practical consensus whether to use the >125 µm or >150 µm size fraction for benthic foraminiferal assemblage analyses. Here, we use Pleistocene material from the Pefka E section on the Island of Rhodes (Greece) that has been counted in both size fractions, to investigate whether a 25 µm difference in the counted assemblage fraction is already sufficient to have an impact on ecological studies. We analyse the influence of the difference in size fraction on studies of the biodiversity as well as multivariate assemblage analyses of the sample material. We find that for both types of studies, the general trends remain the same regardless of the chosen size fraction, but in detail significant differences emerge which are not consistently distributed between samples. Studies which require a high degree of precision can thus not compare results from analyses that used different size fractions, and the inconsistent distribution of differences makes it impossible to develop corrections for this problem. We therefore advocate the consistent use of the >125 µm size fraction for benthic Foraminifera studies in the future.