Intermediate ice scour disturbance is key to maintaining a peak in biodiversity within the shallows of the Western Antarctic Peninsula

Abstract

Climate-related disturbance regimes are changing rapidly with profound consequences for ecosystems. Disturbance is often perceived as detrimental to biodiversity; however, the literature is divided on how they influence each other. Disturbance events in nature are diverse, occurring across numerous interacting trophic levels and multiple spatial and temporal scales, leading to divergence between empirical and theoretical studies. The shallow Antarctic seafloor has one of the largest disturbance gradients on earth, due to iceberg scouring. Scour rates are changing rapidly along the Western Antarctic Peninsula because of climate change and with further changes predicted, the Antarctic benthos will likely undergo dramatic shifts in diversity. We investigated benthic macro and megafaunal richness across 10–100 m depth range, much of which, 40–100 m, has rarely been sampled. Macro and megafauna species richness peaked at 50–60 m depth, a depth dominated by a diverse range of sessile suspension feeders, with an intermediate level of iceberg disturbance. Our results show that a broad range of disturbance values are required to detect the predicted peak in biodiversity that is consistent with the Intermediate Disturbance Hypothesis, suggesting ice scour is key to maintaining high biodiversity in Antarctica’s shallows.

The Extremes of Disturbance Reduce Functional Redundancy: Functional Trait Assessment of the Shallow Antarctic Benthos

“…Recent work along the WAP narrowed the bathymetry of the peak in biodiversity to between 50 and 60 m depth, coincident with the depth of intermediate levels of ice scour disturbance, confirming expectations that ice scour may maintain high biodiversity across these depths (Robinson et al, 2021). Assuming that IDH does indeed explain the diversity-disturbance pattern seen across this depth gradient, the mechanisms behind this observation should be reflected in assemblage functional structure (Houseman and Gross, 2011;Villéger et al, 2011).…”

“…At each of the three sites, 50 random areas of seabed were recorded at every 10 m depth interval from 10 to 100 m depth (total 1,500 images), collected between February and June 2016, using a modified mini-ROV, DeepTrekkerG2. Macro/megafauna (> 0.5 mm) were originally described across this depth gradient by Robinson et al (2021) linking peaks in biodiversity to intermediate ice scour rates, with each specimen with the sample image being counted and identified to into morphotypes. Here, morphotypes are defined as a species or group of species that a have a distinct or conspicuous morphology that allows identification and catergorisation from other species across all sample images.…”

“…In FRic, the volume of multidimensional functional space occupied by all OTUs in the assemblage changed significantly with depth [correlation: r 2 = 40.1, F (1479) = 330.39, p < 0.001, Figure 4B]. OTU richness also varied across depth with 5.77 OTU richness at 10 m depth, peaking at 60 m depth with an OTU richness of 22.04 at decreasing to 14.77 OTU richness at 100 m depth [for full details see Robinson et al (2021)]. Changes in FRic comprised a sharp increase with depth, peaking at 50 m depth, which was followed by a slight decrease.…”

“…Ice scour rates are predicted to shift dramatically in the Western Antarctic Peninsula (WAP) over the next century (Rogers et al, 2020) owing to calving from ice shelf collapse and glacier retreat (Morley et al, 2020), and reductions in both the duration and extent of sea ice (Parkinson, 2014). Throughout this study depth is used as a proxy for disturbance, however, depth is reported as many other environmental factors vary across 10-100 m depth range and were considered, for a part of this work [for a detailed description of environmental factor and their relative influence see Robinson et al (2021)].…”

Response to concerns regarding the role of Antarctic blue carbon habitats in a global context

“…However, Zwerschke et al (2022) focuses on deeper depths (104-515 m) which are representative of the majority of the Antarctic continental shelf. There, iceberg scour is decreased by a factor of ~50 to 4 scours/km 2 since the area has become ice free (Robinson et al, 2021).…”