Shifts in benthic megafauna communities after glacial retreat in an Antarctic fjord

The Antarctic seabed, despite harsh glacial conditions covering over 95% of the continent, supports diverse flora and fauna. Benthic megafauna are a dominant component of the Antarctic biota, significantly contributing to ecosystem functioning and serving as indicators of environmental change. However, the impact of glacial retreat – a significant consequence of global warming – on these communities remains poorly understood. Marian Cove, on the West Antarctic Peninsula (WAP), provides an ideal study area due to its substantial glacial retreat (approximately 1.9 km from 1956 to 2017) and well-established benthic megafauna assemblages, largely composed of filter feeders. Previous research in the area highlighted the impact of deglaciation on ascidians, linking their distribution to sedimentation and ice-scouring. This study aims to address the knowledge gap regarding the effects of glacial retreat on Antarctic nearshore benthic megafauna by examining the distribution of megafauna at sites with varying deglaciation times, assessing the structural and functional diversity of these communities, and analyzing how these factors change with deglaciation time and water depth. Understanding this response is crucial for predicting the future of the Antarctic marine ecosystem under continued warming.

Existing literature reveals that Antarctic benthic communities are sensitive to environmental changes. Ice-scouring, a major disturbance factor, significantly impacts community structure and function, leading to increased mortality and reduced biomass. Studies on the specific impact of glacial retreat on Antarctic benthic megafauna are scarce, often focusing on limited regions, narrow depth ranges, or specific taxa. While quantitative taxonomic diversity (TD) provides valuable information on community structure, functional diversity (FD) is crucial for assessing functional changes. Although the utility of both TD and FD is acknowledged, their combined application to studying the response of Antarctic benthic assemblages to climate change is limited. This paper addresses this gap by using both TD and FD analyses to quantitatively examine the influence of glacial retreat on community structure and function.

The study utilized underwater imagery collected using a remotely operated vehicle (ROV) in Marian Cove. Five stations (MC2-MC6) were selected, representing different distances from the glacier and thus varying times since deglaciation. Six depths (10, 20, 30, 50, 70, and 90 m) were surveyed at each station, except MC6 (50 and 70 m only), the closest to the glacier. ROV images were analyzed to identify and quantify benthic megafauna taxa. Taxonomic diversity (TD) was calculated using a taxonomic distinctness index, providing a measure of structural diversity. Functional diversity (FD) was calculated based on five biological traits (assigned numerically) and their categories, reflecting functional aspects of the community. Statistical analyses included Kruskal-Wallis tests, Mann-Whitney U tests, cluster analysis, non-parametric multidimensional scaling (MDS), indicator value (IndVal) analysis, and BIOENV analysis to determine relationships between environmental factors and benthic megafauna community characteristics. The data for stations MC2-MC5 were adapted from a previous study while MC6 data was collected as part of this research. Sediment properties, including organic matter content, were also analyzed for each station and depth.

Seventy benthic megafauna taxa were identified. Taxa numbers increased towards the outer area of the cove, particularly at 70–90 m depth. Individual density was lowest at the innermost site (MC6) and peaked at an inner site (MC5). Pioneer species dominated near the glacier, declining towards the outer area, where late-successional species were abundant. Taxonomic diversity (TD) was low at the innermost site but did not significantly differ between other stations. TD was lower at 10 m depth than at other depths. Functional diversity (FD) was lowest at the innermost site and increased with distance from the glacier, peaking at 30 m depth at the outermost station. Cluster analysis revealed four groups of benthic megafauna assemblages based on depth and distance from the glacier. Group A (10 m depth) was characterized by low diversity and abundance, dominated by motile mollusks. Group B (innermost site) had low diversity, dominated by Serpulidae spp. Group C (inner sites) had high density of pioneer species (Molgula pedunculata and Cnemidocarpa verrucosa). Group D (outermost site) had few pioneer species and abundant late-successional species. BIOENV analysis indicated that sediment composition, depth, and distance from the glacier were significantly related to benthic megafauna community structure. Silt content was most influential for community structure and density, while distance and sediment organic matter (SOM) were most influential for FD. TD was related primarily to sediment properties (gravel and sand content).

The spatial variation in benthic megafauna communities reflects the long-term successional processes following glacial retreat. Shallow areas, subjected to frequent ice-scouring and high turbidity, exhibited low diversity and abundance, dominated by disturbance-tolerant species. Deeper areas showed distinct successional stages: colonization (dominated by pioneer species), transition (decreasing pioneer species, increasing taxa), and maturing (abundant late-successional species). The increase in FD from the colonization stage to the maturing stage indicates that the succession of benthic megafauna communities progresses depending on the time elapsed after deglaciation. The study highlights that both structure and function of benthic megafauna communities are crucial for understanding responses to environmental change, as the mechanisms driving shifts in structure and function differ. Sediment composition, depth, and distance from the glacier (reflecting habitat stability and formation time) were identified as key environmental factors influencing community structure. FD was also influenced by food availability, which increased with decreasing distance to the glacier.

This study demonstrates how shifts in both structure and function of benthic megafauna communities vary with depth and distance from the glacier in a deglaciated Antarctic fjord. Frequent disturbance limits community development at shallow depths, while deeper areas show clear successional patterns. Both physical stability and food availability influenced the communities' response to glacial retreat, highlighting the need to consider both structure and function for a comprehensive understanding of environmental impacts. The results provide valuable insights into the impact of ongoing climate change on Antarctic benthic ecosystems and offer a basis for predicting future scenarios.

The study relies primarily on ROV imagery, which may underestimate infauna. Some taxonomic identifications were limited by image resolution. While the study provides a strong spatial representation of community shifts, the temporal resolution is limited to the available historical data and recent surveys.