Twice now I've witnessed a yellow tide from my home on the Nanticoke River, May 22 and tonight, June 4th. Both yellow tides have been after/during rain and "bloom" seems to be on outgoing tide. In collecting a water sample mid-stream, I was under a green ash loaded with the "rust". Is this causing the yellow streams on the surface of the river or is it the Hysophmycetes from decaying leaf litter?
I apologize for the slow response. I've been reaching out to some colleagues with other organizations to see if any of them have witnessed the same phenomenon to help me answer your question, but haven't been able to connect with anyone.
Unfortunately, I can't say for sure what is happening there without a site visit. My guess is that the color you are seeing is probably from the tannins leaching out of organic materials giving it that "rusty" appearance.
I notice you also mentioned taking a sample. Are you a Creekwatcher with the Nanticoke Watershed Alliance? Beth Wasden, who coordinates that program, may have some insight for you. If you are not affiliated with that program already, you may be interested in joining.
Her contact information is: firstname.lastname@example.org
I'm sorry I couldn't offer more assistance at this time. If you'd like to reach out to me directly, I can be reached at email@example.com. Perhaps I can try to help connect you with some water quality monitoring organizations to help analyze your samples.
Soon after I replied this morning, I was forwarded several email chains from a colleague at DNREC. It seems that folks with the Nanticoke Watershed Alliance, DNREC, and the Broadkill Citizen Monitoring Program have been looking into this since May 22nd and that this "yellow tide" has been observed on the Nanticoke, Choptank, and Broadkill rivers to name a few. Maddy Lauria with the Cape Gazette wrote an article that should be of some help: http://www.capegazette.com/article/fungus-causes-delmarva-rivers-turn-yellow/134098