Final answer:
Dissolved oxygen levels decline in water with dead fish due to increased oxygen consumption by the microorganisms that decompose the fish remains. Elevated temperatures decrease oxygen solubility, and excess nutrients can fuel harmful algal blooms that also deplete oxygen.
Step-by-step explanation:
The dissolved oxygen levels went down in columns with dead fish because the process of decomposition consumes oxygen. When fish die, their bodies become a source of organic matter for bacteria and other decomposers. As these microorganisms break down the fish remains, they use up oxygen in the water. This increased biological demand for oxygen can reduce the overall level of dissolved oxygen, which is essential for the survival of aquatic life. Moreover, factors such as high temperatures can exacerbate this issue by further decreasing the solubility of oxygen. In bodies of water like the Chesapeake Bay, global warming leads to higher temperatures, which, in turn, reduces oxygen solubility by about 1.1% for every 1.5°C increase in water temperature.
Additionally, the decomposition of excessive algae, stimulated by an influx of nutrients like phosphorus and nitrogen from sources such as fertilizer runoff, also depletes the dissolved oxygen. This leads to 'dead zones' where aquatic life struggles to survive. Finally, dense growth of algae can create a barrier at the water's surface, preventing oxygen from mixing into lower water levels and blocking light needed for photosynthesis. This can cause anaerobic conditions, further depleting oxygen and significantly altering the ecosystem.