The Helgoland Experiment - assessing the influence of methodologies on Recent benthic foraminiferal assemblage composition

Most recent benthic foraminiferal studies focus on species inventory and distribution and ecology. An integration of results from different studies is often hampered by the application of different methods. The influence of different sample treatments and analyses on the accuracy of faunal data is the subject of the present study. We compare preservation, staining, and preparation techniques to constrain the internal data variability as inferred by different methodologies. Variations produced by different persons analysing the same foraminiferal assemblage and consequences for the accuracy of foraminiferal data are addressed. We retrieved a large surface sediment sample from the southeastern North Sea near Helgoland. Twelve replicate subsamples were taken and preserved with ethanol, ethanol - rose Bengal solution, or formaldehyde. Samples were then processed with standard methods. Some samples were stained after processing with an aqueous rose Bengal solution, or the foraminifera were concentrated by floatation. Coloration of specimens that were living at the time of sampling was different between samples where rose Bengal was added together with the ethanol and samples, which were stained after washing. In the latter case, only the last two or three chambers were impregnated. The sample preserved with formaldehyde showed dissolution features affecting both arenaceous and calcareous species. In particular, the outer shell layer of miliolids was corroded, calcareous cement of agglutinated tests was reduced, and pores of rotalids were enlarged. The population density reflecting the number of recognised, stained specimens was highly variable among different preservation, picking modes, and examinators. The accuracy of picking was in the range of ±4 % (1-sigma), while the reproducibility ranged from -44 to +26 % between different examinators, which also concerns the proportions of dominant species. There was no significant difference between wet and dry picking within the 95 % confidence limits, but samples that were stained later or concentrated by floatation generally yielded a lower number of specimens. Arenaceous species and miliolids were underrepresented in samples that were stained after washing and in the floatation concentrate while Stainforthia fusiformis was seemingly better recognised therein. More data and parallel investigations involving a larger number of specialists are needed to achieve a better comparability of faunal census data