Limestone biopitting in coastal settings: A spatial, morphometric, SEM and molecular microbiology sequencing study in the Mallorca rocky coast (Balearic Islands, Western Mediterranean)

• We examine the nature and spatial occurrence of biopits in coastal limestones.
• Biopits are developed by the biological activity of cyanobacteria (Rivularia sp.).
• Biopits exhibit a modal diameter of 4 mm although can enlarge by coalescence.
• The width-depth ratio reaches 2 and values lower than 1.5 are scarce.
• Biopit growth exhibits a preferential location in sheltered and gentle rock surfaces.

Biological agency on rock coasts has been widely recognised over recent decades. This study deals with the distribution and morphometric characteristics of microforms features developed by cyanobacteria (Rivularia sp.) on coastal limestone outcrops. These coastal microforms, known as biopits, have a small rounded basin shape a few millimetres in size. Environmental and geological data were collected from 100 random rock surface spots from Punta des Faralló cape (Mallorca, Western Mediterranean), from which major controls on the spatial distribution of biopits were established. Additionally, morphological data on 382 biopits determined the diagnostic morphometry of these features and their enlargement mechanisms. The results indicated that biopits exhibit a preferential location in shaded exposures and sheltered areas from prevailing winds and waves, avoiding direct insolation and desiccation. Other major controls on these microforms location and development were variables such as the rock surface slope and the distance to the coast (i.e. influence of splash and spray). Shadow spots displayed higher biopits density than other locations according to the patterns determined by environmental and geomorphological factors at the study site. Morphometric analyses showed that biopits have a width twice their depth. The average width of the microforms was 6.49 ± 2.40 mm and the average depth 2.46 ± 1.09 mm. Most frequently, the width/depth ratio was 2 or larger. This characteristic shape ratio was an additional factor that plays a role in maintaining the necessary humidity for microorganisms associated with biopits.