The Salton Sea a large lake spanning 343 mi2in Southern California. It was accidentally created when a levee broke from the Colorado river, filling this formerly dry valley. Particularly in the summer, because the weather is warm (and sun very strong) in southern California, this heats the surface of the water. When this warm water is mixed throughout the depths, it can heat up the water and thus decrease the amount of dissolved oxygen (DO). Just as humans require oxygen to breakdown food and create energy, fish also need oxygen. It is well known that fish require a dissolved oxygen concentration of approximately 5mg/L to survive. In lakes, it is expected that at the top of a lake there will be more dissolved oxygen than at the bottom of the lake due to the different amount of electrons and the pH of the water. The Salton Sea also contains a significant amount of dead fish due to increasing salinity of the water. When there is a lot of organics in the water (such as dead fish), bacteria will use more dissolved oxygen which is required for them to consume this organic material.
In this lab, we set out to measure the dissolved oxygen of the Salton Sea. We used a field-deployable dissolved oxygen meter and used paddle-boards to measure the dissolved oxygen at different depths of the sea. In addition to the original reading, a reading was taken back in the lab and taken after 5 days to allow any organic material in the water to decay thus use up oxygen. The samples were originally kept in the fridge until the first reading back in the lab was taken. But then for the five-day period the bottles were kept in a dark, room temperature environment to allow for bacteria to consume the organics. We calculated the biological oxygen demand (BOD), which the higher the BOD, the greater amount of organic material (including dead fish) present that bacteria are eating and using up oxygen in the process. We found that for the initial DO readings, at every depth there was enough oxygen (above 5mg/L) for fish to survive (between 7mg/L at deep part – 15mg/L at shallowest measured). However, at deeper levels, with additional added organics using up oxygen, this could easily drop to below the survivable levels for fish. While BOD the data was somewhat variable, there was a trend towards fairly high levels of BOD compared to other lake water. This indicates that there is a lot of organics in the water, which can deplete the already low levels of dissolved oxygen even further when bacteria consumes it.
I think understanding the dissolved oxygen and the BOD of the Salton Sea is extremely important to both understanding fish’s availability to survive (based on the dissolved oxygen) and the amount of organics that contribute to the decrease in dissolved oxygen. The Salton Sea is an important spot for migratory birds. The fish feed many of these birds and support the ecosystem at large. We must understand the environmental issues of this critical ecosystem in order to help address these challenges. Understanding why the Salton Sea is not a fish’s first vacation spot is the first step towards making it a more viable ecosystem in the future.