Light attenuation by limestone rock and its constraint on the depth distribution of endolithic algae and cyanobacteria

To understand how light attenuation in limestone rock constrains the distri bution and abundance of endolithic photoautotrophic organisms, we examined light-level profiles in 25 rock samples containing endolithic algae and cya nobacteria. We collected samples from three representative cliff sites alon g the Niagara Escarpment, Canada, using methods that allowed us to make com parisons among as well as within sites. Photosynthetically active radiation profiles were generated by manufacturing cavities in the bottom of thick s labs of surface rock and enclosing radiation sensors in the cavities. The t hickness of the flat layer of limestone between the sensor and a light sour ce was then abraded in steps of ca. 0.5 mm. An exponential curve was fitted to the measurements for each sample to allow statistical comparisons of th e light levels at different depths. We examined the correlations between ph ysical characteristics of the rock and light attenuation and between light attenuation and the maximum depth, relative biomass, and taxonomic richness of endolithic algae and cyanobacteria. The depth at which 0.01% of availab le light remained varied from 2.1 to 4.5 mm at different sites, but there w as large within-site variability on intermediate and small spatial scales. In contrast, the maximum depth of photoautotrophic endoliths was homogeneou s within sites but different among sites (1.1-3.5 mm). Although the endolit hs all have access to light, there was no significant correlation between t he limit of their depth penetration and threshold quantum flux densities, i ndicating that the mechanisms controlling distribution and abundance of end olithic photoautotrophs are as complicated as those found for aquatic algae and cyanobacteria.