Dissolved oxygen (DO) is a vital component of marine ecosystems, providing the key life source for thousands of species of marine vertebrates and invertebrates. Oxygen’s solubility in seawater is influenced by many variables, which can make DO difficult to predict. Estuarine systems experience DO fluctuations, as DO can limit ecosystem reproduction and health. Levels below 4 mg/L induce hypoxic conditions, creating stress for marine organisms, which makes tracking DO levels over time an essential tool for monitoring marine ecosystem health. My research provides Spatial-temporal depth analysis of DO data from the years 2014 through 2021 in the Snohomish River Estuary in Everett, Washington. Temporally, I predicted DO to exhibit a seasonal trend with highs in the winter and lows in the summer and decrease yearly at all depths due to global ocean temperature increase. Spatially, I expected DO to be higher at sites closer to the Snohomish River, and slightly lower at locations further from the river, in the center of the sound. With regard to depths, I predicted DO to be higher near the surface and lower near the bottom, and the oxycline is expected to get closer to the surface over time. Data were collected using an EXO2 Sonde at five different field sites at varying distances from the Snohomish River. I analyzed data using Excel, RStudio, and ArcGIS. Results found that DO is increasing over most sites with seasonal fluctuations of higher DO in the winter, and lower in the summer. There was one hypoxic event in 2016 at Buoy, along with a yearly increase in DO that suggests hypoxic conditions in Possession Sound may not last. Spatially, DO is higher at sites closer to the mainland, contrary to my hypothesis. Continuation of research will include further analysis of Spatial-temporal data in ArcGIS and Rstudio.