Taxonomic and functional diversity of benthic foraminifera as a promising proxy for tidewater glacier retreat

Abstract

Ice sheets and glaciers globally are losing mass at increasing rates, with the strongest changes occurring at tidewater glaciers. These are rapidly retreating and often transitioning to land-terminating with implications for sea level change and ecosystem functioning. However, reconstructions of past tidewater glacier dynamics are currently limited to short time scales (i.e. decades), which affects the accuracy of future predictions. Given the strong response of benthic marine biodiversity to tidewater glacier-induced environmental gradients, small fossilizing organisms such as foraminifera may prove to be a reliable ecological proxy for glacier termini positions on longer time scales. Here, we test living benthic foraminiferal taxonomic and functional diversity along a decreasing gradient of environmental stress off the Kronebreen tidewater glacier front in Kongsfjorden, Svalbard. Additionally, we test four traditional size fractions used for foraminiferal analysis (i.e. >63, >100, >125 and >150 μm) to statistically define the best compromise providing reliable information with minimal analytical time. Both taxonomic and functional diversity increased with increasing distance from the glacier front. The observed diversity patterns were consistent across size fractions, although some were not detected by the largest size fraction (>150 μm). The size fraction >125 μm seems to represent the best compromise to obtain reliable ecological information. We hypothesize that reduced glacier-induced disturbance away from the glacier front increases the availability of ecological niches, which in turn allows the establishment of functionally diverse species. Conversely, only few opportunistic species with reduced functional diversity were associated with the highly unstable environment near the glacier front. The progressive benthic foraminiferal diversity loss observed towards the glacier front is a potential ecological proxy for tidewater glacier dynamics in historical and palaeo-reconstructions. Its effectiveness and robustness to seasonal variability and taphonomic processes should be tested on historical records.