Size Matters: Analyses of Benthic Foraminiferal Assemblages Across Differing Size Fractions

Elena Lo Giudice Cappelli,William E N Austin

doi:10.3389/fmars.2019.00752

Elena Lo Giudice Cappelli, William E N Austin

Open Access

https://doi.org/10.3389/fmars.2019.00752

Copy DOI

Abstract

Benthic foraminiferal assemblages are the object of numerous studies spanning from (palaeo)environmental reconstructions to biomonitoring; however, the establishment of a procedure to standardize these studies remains a recent achievement. Not all studies based on benthic foraminiferal assemblages adopt the same methodology, which potentially hinders the use and comparison of samples prepared prior to the creation of a standard protocol or, indeed, without the knowledge of it. One of the main issues is to understand and possibly quantify the influence of different size fractions on foraminiferal biodiversity and richness. In this study, we analyzed benthic foraminiferal assemblages from the west coast of Shetland (Scotland), which were deliberately prepared without following the standard procedure, and were instead picked from the size fractions 63-150 m and > 150m. Based on assemblage composition, biodiversity indices and multivariate analyses of the data, we assessed the quality and precision of the environmental information that could be extrapolated from these samples. We found that general biodiversity trends remain the same regardless of size fraction, whereas the assemblage internal composition is significantly different between size fractions, with the small fraction retaining a greater degree of environmental sensitivity. We recommend compiling the two sample sets to produce a more holistic and detailed picture of environmental change and generate high-resolution environmental reconstructions. Nevertheless, we conclude that benthic foraminiferal assemblages picked from the large size fraction (> 150 µm) still provide useful information on prevailing environmental conditions and remain useful for an overview of environmental change in these coastal settings.

Highlights

IntroductionIn the past few decades, benthic foraminiferal assemblages have been extensively used to reconstruct current and past environmental conditions based on their composition, species diversity and richness, using a range of indices, statistical tests and transfer functions (e.g., Buzas, 1969, 1970; Gibson and Buzas, 1973; Schroeder et al, 1987; Poole et al, 1994; Austin and Kroon, 1996; Lei et al, 2017; Weinkauf and Milker, 2018)
Our overall aim was to follow the standard protocol described in Schönfeld et al (2012) in its parts that more generally apply to benthic foraminiferal assemblage studies and provide an opportunity for data comparisons
In the small size fraction, benthic foraminiferal assemblages are on average composed of 86% hyaline species, 9% agglutinated, and 1% porcelaneous species

Summary

Introduction

In the past few decades, benthic foraminiferal assemblages have been extensively used to reconstruct current and past environmental conditions based on their composition, species diversity and richness, using a range of indices, statistical tests and transfer functions (e.g., Buzas, 1969, 1970; Gibson and Buzas, 1973; Schroeder et al, 1987; Poole et al, 1994; Austin and Kroon, 1996; Lei et al, 2017; Weinkauf and Milker, 2018). A protocol has been designed to standardize the study of benthic foraminiferal assemblages in biomonitoring (Schönfeld et al, 2012), which resulted in the suggestion of the routine use of the >125 μm size fraction, with the recommendation of analyzing the smaller size fraction (63–125 μm) only to cases where the >125 μm fraction is insufficiently informative Applying this size “threshold” at 125 μm has a twofold effect; first, it allows, where necessary, for smaller species that are absent in the larger size fractions (125– 150 μm) to be accounted (Weinkauf and Milker, 2018), improving the representation of the entire assemblage. The question remains regarding how best to treat valuable samples that were processed either prior to the establishment of this protocol or without the knowledge of the protocol to enable their use; for example in (palaeo)environmental reconstructions the 125 μm mesh size is by no means standard (Jian et al, 1999; Fontanier et al, 2002; Goineau et al, 2011; Lei et al, 2017)

Methods

Results

Conclusion