Updated publication reference for PubMed record(s): 32424245. In order to understand how Phormidium mats establish, and the role of associated taxa in their development, we collected biofilms over a 19-day growth period during a nitrate-induced bloom event in the Wai-iti River for proteogenomics analysis. At the onset of a late summer bloom, cobbles from the Wai-iti River (Nelson, New Zealand) were removed, cleared of incipient growth with sterile sponges, and placed back into the river. Clearing was gentle as seeding from the pre-existing rock surface is important for bloom establishment [24]. Five pre-cleared cobbles were collected at each of 3 time points to capture the first 3, 6 and 9 days of growth (Table S1). Additional cobbles that contemporaneously developed biofilms were collected at days 12 and 19.