Foram-AMBI: A sensitivity index based on benthic foraminiferal faunas from North-East Atlantic and Arctic fjords, continental shelves and slopes

Highlights • Foraminiferal species were assigned to ecological groups according to AMBI procedure. • The benthic foraminifera-based Foram-AMBI was calculated using the AMBI formula. • Foram-AMBI was successfully tested on independent data sets. • Foram-AMBI correlates well with Shannon's diversity and TOC in test data sets. • Foram-AMBI reflects a stress gradient caused by increased organic carbon. The present study follows up the FOraminiferal BIo-MOnitoring (FOBIMO) working group's aim to explore methods which will improve the usefulness of benthic foraminifera in environmental monitoring. An internationally well-established marine biotic index, AMBI, commonly applied to assess ecological quality status was adapted for use on benthic foraminifera. As required by the AMBI formula, species were assigned to one of five ecological groups according to their sensitivity/tolerance to conditions along an increasing stress gradient (here increasing organic matter enrichment). For the assignments, we used 19 published data sets on fully marine NE Atlantic and Arctic fjord, continental shelf, and slope assemblages for which total organic carbon (TOC) data were available. Assignments were based on the relative abundance of the different species along associated TOC gradients. Of the 128 assigned species, the majority was assigned to Groups I–III dominating in low to moderately organic enriched environments with a high to good ecological quality status. Groups IV and V, representing strongly organically enriched environments with a moderate to poor ecological quality status, had 1 and 2 species, respectively. The resulting foraminifera-based Foram-AMBI was calculated using the AMBI formula and tested on four independent foraminiferal data sets from the same geographical region. The validation included correlations of the Foram-AMBI with Shannon's diversity (H′log2) as well as with the organic carbon content in the validation data sets. In two validation data sets from the Norwegian Skagerrak coast, a high proportion of the assemblages consisted of assigned species. The results showed a good correlation between the Foram-AMBI and both the TOC and Shannon's diversity H′log2. In two more southern validation data sets all TOC values were low and the abundance of unassigned species was too high for the Foram-AMBI to provide trustworthy results. The Foram-AMBI of the two validation data sets with high abundance of assigned species clearly reflected an increasing organic carbon-induced stress gradient. Hence, this first attempt to apply the AMBI formula on benthic foraminiferal data shows promising results. However, to improve the applicability of Foram-AMBI, there is a need to assign more species by obtaining data from studies along wide organic carbon pressure gradients, particularly from the southern North Sea and southwards.