The province of PEI has been monitoring dissolved oxygen in some Island estuaries since 2015. This dataset contains data collected from 2015 to 2021. Monitoring results from later years can also be found on the Prince Edward Island open data portal.This dataset includes raw data from 20 Island estuaries from across PEI; mapped locations of monitoring sites; and downloadable raw data. The data includes dissolved oxygen concentration, dissolved oxygen saturation, water temperature and instrument depth. Data is usually collected between May and October of each year. 18 estuaries are monitored on a 3 year rotating cycle with at least 6 of these 18 monitored in any given year. Another 2 estuaries are monitored every year. 2 locations are monitored in each estuary; the Upper Estuary location represents the upper 10% boundary of the estuary’s surface area, is closest to the freshwater inputs and should display signs of eutrophication if they are present; the Mid Estuary location represents the mid-point or 50% boundary of the surface area of the estuary. The Mid Estuary site may have two loggers, one placed about 0.5 m the bottom substrate and one placed about 0.5 m from the surface of the water. Data may be used to determine hourly fluctuations in oxygen, the degree of eutrophication, and the presence and duration of anoxia and hypoxia. Temperature and instrument depth (not the same as water depth at the site) data are also collected and available for use.