Long Term Marine Biodiversity Monitoring in Coastal Antarctica: Are Fewer Rare Species Recruiting?

The physical environment of nearshore Southern Ocean (coastal Antarctica) is altering rapidly in response to climate change, but also has other long cyclicity due to El Nino Southern Oscillation and Southern Annular Mode. Detecting biological responses to such physical change, which is complex in time and space, is very challenging not least because of remoteness, difficulty of access, frequency of iceberg destruction and short funding cycles but also the paucity of research stations with SCUBA (or ROV) facilities. At one of those few, Rothera, Adelaide Island on the West Antarctic Peninsula, we immersed arrays of artificial substrata (settlement panels) for 1 year repeatedly for over two decades. Whilst many 'mature assemblages' are monitored at nearshore sites around the world, there are few of similar duration for recruitment and colonisation. We report the variability in annual biodiversity descriptive statistics with the crucial context of also recording adjacent long (here defined as > 1 decade) term seabed disturbance and biophysical oceanography at Rothera. We ask how variable is annual colonisation, recruitment and early community development in Antarctica's shallows, what aspect of recruitment changes over two decades and in what way? Of 40 recorded, most species recruiting to our panels at 12 m depth at Adelaide Island (67.568° S, 68.127° W) were rare, comprising cheilostome and cyclostome bryozoans, polychaetes, calcarea and demosponge sponges, hydroid cnidarians and ascidians. The most striking finding was a sustained decrease in total richness of recruits over time, mainly due to loss of rare species. Unlike losses of seasonal sea ice, iceberg disturbance and benthos mortality, such findings are unlikely to be climate-forced responses. This raises important questions of whether this is a chance finding, (the data only spans 20 years), driven by a recent complex of stressors and most of all is losing rare species a wider polar problem?