Spring and Your Pond

Understanding Nitrogen and Cold Water

Spring marks the transition of our ponds from a dormant ecosystem with torpid fish and inactive filters to the active biome that we enjoy throughout the summer. This transition is a stressful one for our fish, and if mismanaged, can result in increased stress, illness and death.

Perhaps the most complex changes that occur during spring wakeup are those involving the nitrifying process that converts ammonia into nitrite, then into nitrates. Most of us are aware of this process, and if we have been paying attention to our ponds as they warm up, are familiar with the spring "ammonia spike" and "nitrite spike" that occur as our fish wake up and begin to eat, followed slowly by the development of our bioconverters as the mix of aerobic and anaerobic nitrifying bacteria come online.

Ammonia is measured by most common test kits as Total Ammonia Nitrogen (TAN) which combines both ionized and non-ionized ammonia. It is the non-ionized (NH3) form of ammonia that is most toxic to our fish, and its presence as a component of TAN increases as the water becomes more alkaline (higher pH). Ionized ammonia (NH4) is considerably less toxic, and will increase as pH drops.

It is this phenomenon that protects fish that have been held in transport bags for long periods of time, and the main reason why we never pour fresh water into that bag. Water temperature also determines (in part) the ratio of ionized to unionized ammonia, with colder temperatures favoring the presence of the less toxic ionized form.

Since unionized ammonia (UIA) becomes toxic enough to stress our fish at concentrations as low as 0.05mg/L and becomes lethal at 2.0mg/L, it becomes important to be able to separate the UIA level from the TAN as our ponds warm up in order to keep our water quality and fish health optimal throughout spring startup.

The following table, borrowed from a recent KHA course prepared by Richard E. Carlson, outlines the relationship between TAN, pH, temperature and UIA.

* The "Factor" column of the chart provides a multiplier derived from a number of sources, including the University of Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences.

To use the chart, multiply the conversion factor closest to the combination of pH and temperature by your measured TAN. This will yield a rough estimate of UIA. Remember that toxic levels of UIA are anything above 0.05mg/l, so a combination of Total Ammonia Nitrogen (NH3/NH4) of 1.0mg/dl at a temperature of 72 and a pH of 8.6 would have an estimated NH3 concentration of 0.1541mg/l, easily enough to stress and possibly kill a pond full of koi.

Low temperatures are protective, but the ability to separate out toxic from non-toxic nitrogen levels as the pond warms up can head off trouble.

Bob Passovoy
President
MPKS

©2006 Robert D. Passovoy, MD

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