The “crème brûlée” sampler: A new high-resolution method for the fast vertical sampling of intertidal fine sediments

• Sampling the photic zone of aquatic sediments is a methodological challenge.
• A contact corer for sampling sediment vertical sections as thin as 250 μm was designed.
• Its usefulness was demonstrated by studying microphytobenthos biomass dynamics.
• Sampling was fast and reproducible, with a negligible effect of compaction.
• This is a promising tool for characterizing microbial activities in sediment top layers.

The reduced depth of the photic zone in most intertidal and shallow coastal sediments makes the characterization of its properties a considerable methodological challenge. Available methods for sediment sampling present insufficient resolution (ca. 2 mm) for sedimentary biofilms or require considerable post-sampling handling and operator expertise. In this study, we describe a new cryo-sampling device allowing the collection of sediment vertical sections as thin as 250 μm in a fast, inexpensive and reliable way. The method was validated by quantifying common sediment physical properties (mass, water content) of samples of different thicknesses (from 250 μm to 2 mm) collected from different sampling sites (mud and sandy-mud sediments) by operators with different levels of training. Its usefulness was demonstrated by characterizing the short-term dynamics of microphytobenthos biomass (chlorophyll a content) during a typical vertical migratory event. Results emphasize the importance of choosing the sample thickness matching the dynamic processes under study in intertidal sediments. This device opens promising perspectives in studies susceptible to generate a large number of samples and where the fine-scale properties of the surface sediment are of importance.