Crusting algal and/or bacterial (<i><a href="https://en.wikipedia.org/wiki/Rivularia" class="extiw" title="w:Rivularia">Rivularia</a></i> ?) photographed in August 2016 on the bottom of the river "Sèvre Niortaise" in a shallow and well lit place, called "Roman Bridge of <a href="https://en.wikipedia.org/wiki/Azay-le-Br%C3%BBl%C3%A9" class="extiw" title="w:Azay-le-Brûlé">Azay-le-Brûlé</a> "(France). On these crusts various invertebrates, including <a href="https://en.wikipedia.org/wiki/gammare" class="extiw" title="w:gammare">gammares</a> and many <a href="https://en.wikipedia.org/wiki/Theodoxus" class="extiw" title="w:Theodoxus">w:Theodoxus</a> can be observed. This section of the river is rich in crusting areas that typically form it from horizontal carpet aquatic of mosses (bryophyta) or vertical carpet of <a href="https://en.wikipedia.org/wiki/Marchantiophyta" class="extiw" title="w:Marchantiophyta">Marchantiophyta</a>. <br>Note: Not very thick crust (more than a few cm thick) are found on the battery or the medieval bridge on rocks a priori old, suggesting that the phenomenon is recent.<br>Another note: the crusts are not on lead sinkers (metal toxic and <a href="https://en.wikipedia.org/wiki/ecotoxicity" class="extiw" title="w:ecotoxicity">Ecotoxical</a>) neither on some rocks or on pieces of dead wood (to be confirmed by  more attentive observations).
<a href="//commons.wikimedia.org/wiki/User:Lamiot" title="User:Lamiot">Lamiot</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>