A biological indicator for measuring and communicating local sea level rise captured in historic photographs.

Abstract

\n This paper explores a novel approach to collecting and communicating local site-specific data on recent sea level rise (SLR) using black zone biotic levels left on historic coastal stone structures by a stable community of cyanobacteria (blue-green algae) at the Royal Naval Dockyard in Bermuda. Photographs taken at the Dockyard in 1870, 2007 and 2017 show an upward shift in this living cyanobacterial community. A spatio-temporal digital twin computed from historic and contemporary photo assets was created to test the viability of these black zone lines as a proxy for sea level rise (SLR) measurements in Bermuda. Shifts in these black zone lines when analyzed through the digital twin demonstrate an average upward shift of 2.2 mm per year between 1870 and 2007 and 2.7 mm per year between 1870 and 2017, somewhat lower than the Global estimates from the Intergovernmental Panel on Climate Change Assessment Report predictions. However, the digital twin showed a dramatic upward shift of 8.8 cm between 2007 and 2017, or 8.8 mm per year, which coincided with Bermuda s highest recorded tidal extent since 1932. Black zone cyanobacteria are highly SLR sensitive and over long time scales comparative imagery of black zones could present a proper indicator of average sea level rise. At timescales less than 10 years the black zone may be best indicative of episodic tidal extent. As SLR will continue to shift supralittoral cyanobacteria upwards in Bermuda and in warm rocky intertidal zones worldwide, tidal monitoring and black zone assessments may prove to be a useful combination in documenting and communicating the reality, extent and possible acceleration of local SLR.