Bacterial Dynamics in Supraglacial Habitats of the Greenland Ice Sheet

Abstract

Current research into bacterial dynamics on the Greenland Ice Sheet (GrIS) is biased toward cryoconite holes, despite this habitat covering less than 8% of the ablation (melt) zone surface. In contrast, the expansive surface ice, which supports wide-spread Streptophyte micro-algal blooms thought to enhance surface melt, has been relatively neglected. This study aims to understand variability in bacterial abundance and production across an ablation season on the GrIS, in relation to micro-algal bloom dynamics. Bacterial abundance reached 3.3 ± 0.3 × 10⁵ cells ml^-1 in surface ice and was significantly linearly related to algal abundances during the middle and late ablation periods (R ² = 0.62, p < 0.05; R ² = 0.78, p < 0.001). Bacterial production (BP) of 0.03-0.6 μg C L^-1 h^-1 was observed in surface ice and increased in concert with glacier algal abundances, indicating that heterotrophic bacteria consume algal-derived dissolved organic carbon. However, BP remained at least 28 times lower than net primary production, indicating inefficient carbon cycling by heterotrophic bacteria and net accumulation of carbon in surface ice throughout the ablation season. Across the supraglacial environment, cryoconite sediment BP was at least four times greater than surface ice, confirming that cryoconite holes are the true "hot spots" of heterotrophic bacterial activity.

Keywords: Greenland; bacterial abundance; bacterial production; glacier algae; ice sheet.

Figure 1

A map of Greenland highlighting the camp at which sampling was conducted (A); images of surface ice with visible algal bloom (B); an example ice core (C); cryoconite holes (D); clean surface ice with no algal bloom (E).

Figure 2

Bacterial and algal abundance (mean ± SE) across the ablation season (sectioned as Pre: Pre-snowline retreat; early, mid, and late ablation periods) in snow (Sn); Ice cores (C); Surface ice (Su); and surface ice with a low (L), medium (M), and high (H) algal coverage. Numbers above bars represent the n of the sample. Letters above bars represent homogenous groups determined by a one-way ANOVA (abundance ~ algal coverage) and capital letter denote homogenous group determine by a t-test between surface ice with a high algal coverage between mid and late ablation periods.

Figure 3

Bacterial production (BP), (mean ± SE) across the ablation season (sectioned as Pre: Pre-snowline retreat; Early, Mid and Late ablation periods) in snow (Sn); Ice cores (C); Surface ice (Su); and surface ice with a low (L), medium (M) and high (H) algal coverage. Numbers above bars represent the n of the sample. Lower-case letters above bars represent homogenous groups determined by a one-way ANOVA (abundance ~ algal coverage) and a capital letter denote homogenous group determine by a t-test between surface ice with a high algal coverage between mid and late ablation periods.