The use of an analog sclerometer to estimate strength of hard substrates supporting coastal communities

In the marine environment, substrate differences such as ‘soft’ and ‘hard’ bottom result in different ecological communities. It is unclear, however, whether strength of hard bottom affects organism presence. To date, a limitation of many studies on organism-substrate interactions is the inability to measure rock strength in situ for marine substrates. Previous efforts involved measurements of rock compressive strength, which relates to the hardness of the interior rock and does not necessarily reflect how organisms interact with the rock surface. Here we present a method for estimating the surface strength, or friability, of marine rocks in the field. We used an analog sclerometer to scratch the surface of marine rocks at predetermined force levels across 38 intertidal sites. Friability ranged over two orders of magnitude (0.13–12.10 MPa) with significant differences among rock types: mudstone (mean = 0.56 MPa) had the lowest resistance, while granitic rocks (mean = 2.14 MPa) had the highest, consistent with theoretical expectations. Friability measurements indicated expected compressive rock strengths across rock types and captured differences within types (i.e., surface weathering). The sclerometer effectively measured surface rock strength on soft rocks, such as mudstone, and hard rocks, like granite. The ability to measure the strength of surfaces represents a new tool for benthic marine research, enabling investigations of substrate-organism interactions in future studies.