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.

Local processes in marine ecosystems, including coastal estuaries, modify ocean acidification caused by rising atmospheric CO2. Because ocean acidification poses a threat to shell-forming organisms, it is critical to understand how these processes affect acidification in specific regions. In the Snohomish River estuary, freshwater from river discharge deposits directly into Possession Sound, impacting the salinity and temperature of the area. River discharge in estuaries has been found to be slightly acidic, as well as a source of nutrients that fuel blooms of phytoplankton. Large phytoplankton blooms can lower the pH at depth because of the process of respiration, which releases CO2 and decreases dissolved oxygen levels. My research examines changes in pH with temperature, salinity, chlorophyll, and dissolved oxygen at different depths in Possession Sound, Washington, using data collected from January 2017 through January 2021 with a YSI EXO2 Sonde. I hypothesized that near-surface depths and sites located closer to the river would have lower temperatures and salinities correlating with lower pH. Additionally, lower dissolved oxygen at greater depths would correlate with greater amounts of chlorophyll and a decrease in pH at depth. Depths near the halocline were predicted to have alkaline pH values due to photosynthetic organisms. I analyzed data using Microsoft Excel and R Studio. Results found that with chlorophyll less than ~1.25 RFU, pH was greater than 8.0, while with lower dissolved oxygen, pH was less than 7.75. Temperatures less than 10°C corresponded with more pH values between 7.0 and 7.5, while salinity had no apparent trend. In most seasons, pH appeared to decrease slightly at greater depths. The exception to this was winter, when more acidic pH values were observed at near-surface depths. Overall these results indicate that local processes in the Snohomish River estuary are affecting changes in pH.