Fish need oxygen and oxygen levels will vary within a water column. Plants and aeration are the only two ways ponds can get oxygen. And shallower ponds will be warmer. The warmer the water, the less oxygen available for fish. So extreme heat over an extended period can take a toll on the fish population.
Submerged native aquatic plants like pondweeds or elodea are great for producing oxygen – during the day. At night though, they reverse the process and deplete oxygen levels. So a pond covered extensively with pondweeds can actually stress fish populations.
Clouds may be a welcome relief at times. But extended cloud cover causes oxygen levels in ponds to plummet. In coastal areas, short-lasted rains turn to tropical storms with winds that become an added foe for fish. Winds and heavy rains stir up sediment that captures oxygen, making it unavailable to fish. Either of these in extreme simply increase stress levels that can lead to die-offs.
As was mentioned with rains, water levels that rise too quickly can rapidly change the oxygen balance in a pond’s ecosystem. Likewise, if a pond is lowered too quickly, oxygen levels are disrupted and can cause and abrupt fish die-off.
As we work to maintain a healthy balance of aquatic plant species and remove invasive plants, we take care of the environmental conditions, both current and forecasted, so as not to contribute to fish stress. If too many oxygen producing plants are removed too fast, the result will not be favorable for fish. So there are times when planned plant control is done in stages to help avoid a sudden change in water oxygen levels. The same approach is used when water levels need to be lowered in managed ponds.