Eutrophication can be an unexpected by product of adding fertilisers to the land, and having them end up in a water supply. When minerals such as nitrates and phosphates are washed into the system. This increase in nutrient concentration can encourage algae to glow which creates a green film ? on top of the water, oxygen depletation in the water, and a blockage for sunlight for plants that need to photo synthesise on the bottom.
Oxygen in normal atmospheric pressure of the earth is 20.9% oxygen. In water oxygen is approximately .0007% , or 7 ppm (parts per million) in water of good quality. Oxygen depletion, or when dissolved oxygen (DO; molecular oxygen dissolvedin the water) reduces to a point where it is detrminetal to the aquatic organisms in the system. Fish and marine life rely on dissolved oxygen in the water, much like land animals rely on oxygen in the air. Different fish rely on different levels of DO. Once oxygen concetraionts fall below a certain level, fish mortality may arise. The amount of dissolved oxygen can tell us a lot about water wuality.
The dissolved oxygen in water can come from photsynthesis and the atmospheric pressure. The two variables of photosysnethsis and atmospheric pressure can affect the oxygenation level. In theory photsysnesthic csan oxynegat e abody of wtaer more than atmospheiric pressure ? . The oxygen from ohtosysnetsis is produced by the plants in the water.
At higher atmospheric pressures there is less oxygen in bodies of water as there is less pressure to push oxygen molecules into the water.
A dead zone is a body of water with little or no dissolved oxygen. Water with no oxygen is called anoxic. It’s called that because aquatic organisms cannot survivie there. There is a greater chance of low DO at high temperatures, and salinities. There is a greater capacity for water to hold oxygen at a cooler tmperature. Once the water freezes and turns into ice, oxygen becomes unavailable to most organisms.
If warm water is on top, and cool water below it can limit the ability of the waters oxygen to mix, even though the water at the bottom may be cooler it may be lower in oxygen.
If herbicies or algicides end up in a lake, it may get rid of vegetation and deplete oxygen.
Bacteria then beings ot break down plants which releases more nutrients into the water. It’s called the algal bloom cycle?