The influence of light attenuation on the biogeomorphology of a marine karst cave: A case study of Puerto Princesa Underground River, Palawan, Philippines.

Karst caves are unique biogeomorphological systems. Cave walls offer habitat for microorganisms which in-turnhave a geomorphological role via their involvement in rock weathering, erosion and mineralisation. Theattenuation of light with distance into caves is known to affect ecology, but the implications of this forbiogeomorphological processes and forms have seldom been examined. Here we describe a semi-quantitative microscopystudy comparing the extent, structure, and thickness of biocover and depth of endolithic penetration forsamples of rock from the Puerto Princesa Underground River system in Palawan, the Philippines, which is a naturalUNESCO World Heritage Site.Organic growth at the entrance of the cave was abundant (100% occurrence) and complex, dominated byphototrophic organisms (green microalgae, diatoms, cyanobacteria, mosses and lichens). Thickness of thislayer was 0.28 ± 0.18 mm with active endolith penetration into the limestone (mean depth = 0.13 ±0.03 mm). In contrast, phototrophs were rare 50 m into the cave and biofilm cover was significantly thinner(0.01 ± 0.01 mm, p b 0.000) and spatially patchy (33% occurrence). Endolithic penetration here was alsoshallower (b0.01mm, p b 0.000) and non-uniform. Biofilm was found 250 m into the cave, but with a completeabsence of phototrophs and no evidence of endolithic bioerosion.We attribute these findings to light-induced stress gradients, showing that the influence of light on phototrophabundance has knock-on consequences for the development of limestone morphological features. In marinecaves this includes notches, which were most well-developed at the sheltered cave entrance of our study site,and for which variability in formation rates between locations is currently poorly understood.