Brown blood cannot carry sufficient amounts of oxygen and affected fish can suffocate (窒息)despite adequate oxygen concentration in the water. Nitrite problems are typically more likely in closed, intensive culture systems due to insufficient, inefficient, or malfunctioning filtration systemsHigh nitrite concentrations in ponds occur more frequently in the fall and spring whentemperatures are fluctuating, resulting in the breakdown of the nitrogen cycle due to decreasedplankton and/or bacterial activity. A reduction in plankton activity in ponds (because of lower temperatures, nutrient depletion, cloudy weather, herbicide or chemical treatments, etc.)can result in less ammonia assimilated by the algae,thus increasing the load on the nitrifying bacteria