Climate change and glacier retreat drive shifts in an Antarctic benthic ecosystem

Abstract

The Antarctic Peninsula (AP) is one of the three places on Earth that registered the most intense warming in the last 50 years, almost five times the global mean. This warming has strongly affected the cryosphere, causing the largest ice-shelf collapses ever observed and the retreat of 87% of glaciers. Ecosystem responses, although increasingly predicted, have been mainly reported for pelagic systems. However, and despite most Antarctic species being benthic, responses in the Antarctic benthos have been detected in only a few species, and major effects at assemblage level are unknown. This is probably due to the scarcity of baselines against which to assess change. We performed repeat surveys of coastal benthos in 1994, 1998, and 2010, analyzing community structure and environmental variables at King George Island, Antarctica. We report a marked shift in an Antarctic benthic community that can be linked to ongoing climate change. However, rather than temperature as the primary factor, we highlight the resulting increased sediment runoff, triggered by glacier retreat, as the potential causal factor. The sudden shift from a "filter feeders-ascidian domination" to a "mixed assemblage" suggests that thresholds (for example, of tolerable sedimentation) and alternative equilibrium states, depending on the reversibility of the changes, could be possible traits of this ecosystem. Sedimentation processes will be increasing under the current scenario of glacier retreat, and attention needs to be paid to its effects along the AP.

Keywords: Antactic fjords; Antarctica; Ascidians; Benthic ecosystems; Filter feeders; Glacier retreat; Sedimentation; Sudden changes; Thresholds; climate change.

Fig. 1. Map of Potter Cove.

Picture of 2010 showing glacier retreat since 1995 and the gradient of sediment influence in different areas of the cove, represented by the color gradient. Rectangles are sampling stations with their depth sampling profiles, 15, 20, 25, and 30 m. The Inner Station (I) is most exposed to sediments carried by (a) Potter and (b) Matías creeks. M, Middle Station; O, Outer Station. Inner and Middle stations are located in the inner cove (c), sheltered from the entrance of big icebergs by a sill at 26- to 28-m depth (d; red dashed line). Outer Station is in the outer cove (e). (c) PO3 sediment core location (bathymetry of Potter Cove shown in fig. S1).

Fig. 2. Species coverage and sediments at Potter Cove.

Percentage cover of the different taxa at the Inner Station and at 25- to 30-m depth of the Middle Station, the areas that showed marked shifts in benthic communities at Potter Cove (see details for all the stations and depths in fig. S2). At the Inner Station, trends of decreasing abundance in ascidians and increased pennatulids, sponges, and mobile taxa are visible. Note that at 30-m depth, there are less taxa present in 1998 than in 1994 or 2010. The Middle Station exhibited a shift from macroalgal to zoobenthic dominance, particularly pennatulids, sponges, and ascidians. In 1998, only transects at 20- and 30-m depths were sampled. The lower part of the graph shows mass accumulation rates of bulk sediments (MAR_BS), total suspended particulate matter (SPM), and percentage of organic matter (OM) in the time lapse of the benthic surveys. MAR_BS and SPM peaked between 1994 and 1998 (gray shadow bar, which could be a threshold for this ecosystem), and the MAR_BS values were also the highest registered during the last century (see fig. S3).

Fig. 3. Shifts of benthic assemblages at Potter Cove between 1994 and 2010.

nMDS ordination of relationships among stations, depths, and year, showing high similarity between the Outer Station and the shallower depths (15 to 20 m) of the Middle Station across years. In contrast, the Inner Station and deeper areas of the Middle Station (25 to 30 m) showed a marked change. Analyses were performed using Bray-Curtis similarity index.

Fig. 4. Community shifts: Conceptual and mathematical models of ecosystem dynamics under environmental changes.

(A to C) The plots represent (A) a linear response to a gradual environmental shift with two different communities at both extremes of environmental conditions; (B) a sudden change between states, under which communities can absorb changes in environmental factors until a threshold limit pushes the system from one state to the other; and (C) thresholds with hysteresis occur when different communities can be present under the same environmental conditions. C1, community 1; C2, community 2. (D to F) Benthic shifts and three possible processes showing diversity variation with time. (D) Classical succession: Community 1 is characteristic of early successional stages, and community 2 represents mature stages. (E) Gradual replacement between communities (for example, in an ecosystem displacement process). (F) Sudden change between communities due to a nonlinear response to environmental conditions. (G) A simple simulation of a multispecies spatial competition model under sedimentation based on Levins competition model.