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How do nitrates affect the health of aquatic animals?

Fish and aquatic insects can be affected indirectly by increased nitrate concentrations in the water.  

Basically, any excess nitrate in the water is a source of fertilizer for aquatic plants and algae.  In many cases, the amount of nitrate in the water is what limits how much plants and algae can grow.  If there is an excess level of nitrates, plants and algae will grow excessively.

Excess plants in a body of water can create many problems.  An excess in the growth of plants and algae create an unstable amount of dissolved oxygen.  During the day, there will be usually be high levels of dissolved oxygen, and at night the levels of oxygen can decrease dramatically.

  • This will create stressful conditions for fish.  If they are stressed for a significant part of the day, they will not behave normally or reproduce.  If the conditions persist for a long period of time, the stressed fish species may choose to leave that area or die off.
  • Excess algae or plant growth is also unsightly.  If you've ever been to a beach where mats of rotting algae wash up on shore or the bottom of the lake is teaming with weeds, it's probably because excess nitrates are available for plant growth.  Several years ago, excess nitrates in Lake Erie created such a problem; fortunately, thanks to the efforts of concerned citizens, Lake Erie is today a much healthier aquatic environment.
Excess plants and algae will also create conditions where organic matter accumulates.  High densities of algae will create a condition where sunlight cannot reach very far into the water.  Since plants and algae require some sunlight, plants and algae not receiving sunlight will die off.  These dead plant materials will settle to the bottom of the water and bacteria that feed on decaying organic material will greatly increase in numbers.  These bacteria will consume oxygen and, therefore, the level of dissolved oxygen in this water will fall to levels that are too low for many aquatic insects and fish to survive. 
  • Also, this can cause extreme changes in habitat.  Fish that need gravel or sand for spawning may find nothing but mats of vegetation and muck.

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