Mag. 400x
The additional spines, highlighted in photos 1 & 2, make this specimen distinct from M. rotata. The pattern and form of the spines and indentations in the semi-cells, to my eyes, most closely resembles images of M. compereana as seen here: http://www.digicodes.info/Micrasterias_compereana.html. Another possibility is M. papillifera as seen here http://protist.i.hosei.ac.jp/PDB/Images/Chlorophyta/Micrasterias/papillifera/sp_03.html. Both species have these additional spines.
A water sample was taken on 7/14/2021 using a 10 micron dip net to enrich for microorganisms.