Climate-linked iceberg activity massively reduces spatial competition in Antarctic shallow waters

number of spatial competitors (Supplemental information). These 'indicators' appeared stable up to 2006 but have decreased abruptly since. Mean percentage cover by encrusting fauna was >26% before 2006 and has been <23% since (%cover significantly decreased with increased ice scour; one-way ANOVA, F = 3.9, p = 0.011). The major space occupiers, bryozoans, dominated this assemblage in terms of richness (63.2 ± 0.1% species), space occupied (71.9 ± 11% colonised area) and measurable interference competitive encounters (>99%) and thus we focussed on this taxon. About 44% of recruits encountered spatial (interference or contest) competition before 2006 and has declined by 1% per year since (Figure 1; competitive encounters significantly decreased with increased ice scour; one-way ANOVA, F = 6.6, p = 0.001). However, recent changes go further than prevalence of competitive encounters. The number of species involved in competitive encounters decreased with increased ice scour; one-way ANOVA, F = 3.6, p = 0.016. The number of competitor identities involved in spatial competition at Rothera has nearly halved from means of 8.75 (±1.53) in 1997 to 5.33 (±1.15) in 2013. All the species recorded in 1997 were still present in 2013 but many have become so rare that they perform little role in assemblage dynamics, such as spatial competition. Even more fundamental than this, spatial competition has progressively changed from a complex network into a very simple structure over the course of the study period (Supplemental information; Figure S2). Each year more of the pairwise competitive permutations that occurred involved one particular (weak, pioneer) species. The 2013 survey revealed that every interaction involved just one species — amongst the simplest structures of seabed competition reported anywhere. In 2013 more than 96% of the total interactions only involved F. rugula (i.e. were intraspecific) and more than 99% of these were intransitive (end in tied rather than decided [win/loss] outcomes; the proportion of interactions being intraspecific significantly increases with ice scour, one-way ANOVA, F = 41, Life on Antarctica's coastal seabed rollercoasters between food-rich, open-water, iceberg-scoured summers and food-sparse winters, when the sea surface freezes into 'fast-ice', locking up icebergs, reducing their seabed collisions (scouring). In the last half century, there have been massive losses of winter sea ice along the Antarctic Peninsula, as well as retreat of glaciers and disintegration of ice shelves coincident with rapid recent regional warming [1]. More calving from glaciers and ice shelves coupled with less winter ice …