ELISA determination of levels of microcystin in environmental water samples in a study of Cyanobacteria harmful algal blooms in northern tributaries to the Albemarle Sound of northeastern North Carolina coastal plain. Collections (N = 205) were made at 15 sites between 2025-01-13 and 2025-11-19. This dataset is part of a larger study of weather, water, and cyanotoxin measurements that will be published later this year. These particular microcystin measurements have been used in a preparation of a manuscript, “A Multisensor AI Framework for Detecting Cyanobacterial Harmful Algal Blooms”, that seeks to identify microcystin producing blooms by AI analysis of satellite images.