Nitrates - How much is too much ?

[keith_r]

i've had a few arguments.. well.. discussions about nitrates with some other ap'ers..i "feel" that high nitrates can be a bad thing so i started researching a little more.. while i'm far from an expert, i've found info on different species of fish regarding what is acceptable levels of nitrates.. in one study that ohio state did, northern bluegill had no problems with nitrAtes up to 400ppm... which i think is excessive, but for the point of the study, i'm glad they did it..
but i also found that many juvenille fish are more susceptible to issues with high nitrates than adult fish..
i'd rather the levels were low and the system more in balance, if my rates shot up suddenly, i'd be looking for the cause, and if they gradually started to build up, i'd be able to adjust feed accordingly..
all that being said, i've only seen my rates over 40 once

[Earthan Group]

Hi,

Aside from direct impacts on fish health, high nitrate levels are problematic for other reasons including:

• When exposed to low dissolved oxygen levels, nitrates will be converted back to nitrites.
• High nitrate levels are believed by some researchers to be responsible for the muddy "off tastes" in some freshwater fish species.
• Elevated nitrate levels will produce nitric acid - which will lower your pH.

Gary

Generally it is difficult to get really high nitrate levels in aquaculture especially in aquaponics and pond culture. In recirc. nitrates can accumulate but generally the water exchanges keep it relatively low. If recirc are utilizing denitrification like the use of plants the exchange rates can be cut in half.

The risk to fish health are a non issue really because it is very difficult to maintain levels that may cause concern (>400mg/L)

Along with what Gary has put up as issues I would like to expand a little on those.

• The denitrification process that Gary has mentioned operates in an anaerobic environment. This process does reverse the nitrate to nitrite before it is gased off. High nitrate levels in growbeds that are loaded with solids can contribute to persistantly high nitrite readings, especially in warmer weather.
  
• High nitrate or more importantly high nutrient levels in ponds encourage what are called Men In Black (lol) or MIB (2-Methylisoborneol) and Geosmin (GSM). These are generally produced by the blue green algal species. One particular to recirc. systems is a bacteria/fungus Actinomycetes which produces the GSM mentioned above. High phosphorous levels are generally attributed to the production of these compounds which are harmless to people though generally high nutrient will encourage its presence.
  
• The elevated nitrate levels contribute to nitrite levels in the anaerobic conditions mentioned earlier. Nitrate only presents a pH issue through that non oxygen denitrifying process. So elevated nitrate can impact on the pH but it is not the primary cause. Nitrate is a base or alkaline (conjunct) of nitric acid but it is not acidic.

[GaryD]

Hi,

Thanks for the clarification around those points, Paul.

The principal concern around high nitrate levels may have less to do with fish than with plants......as this document makes clear.

The accumulation of too much nitrate in aquaponic systems is sometimes a concern as fruiting plants set less fruit and produce excess vegetative growth when nitrate levels are high.

The filter tanks in the UVI commercial-scale system have a mechanism for controlling nitrate levels through denitrification, the reduction of nitrate ions to nitrogen gas by anaerobic bacteria. Large quantities of organic matter accumulate on the orchard netting between cleanings.

Denitrification occurs in anaerobic pockets that develop in the sludge. Water moves through the accumulated sludge, which provides good contact between nitrate ions and denitrifying bacteria.

The frequency of cleaning the netting regulates the degree of denitrification. When the netting is cleaned often (e.g., twice per week), sludge accumulation and denitrification are minimized, which leads to an increase in nitrate concentrations. When the netting is cleaned less often (e.g., once per week), sludge accumulation and denitrification are maximized, which leads to a decrease in nitrate levels. Nitrate-nitrogen levels can be regulated within a range of 1 to 100 mg/L or more.

High nitrate concentrations promote the growth of leafy green vegetables, while low nitrate concentrations promote fruit development in vegetables such as tomatoes.

Gary

[GaryD]

Hi,

And then there's the effect of high nitrate levels in humans brought about by eating plants with high nitrate levels.

I took a look at what Wikipedia had to say on the matter and found.....this....

However, nitrate exposure may also occur if eating, for instance, vegetables containing high levels of nitrate. Lettuce may contain elevated nitrate under growth conditions such as reduced sunlight, undersupply of the essential micronutrients molybdenum (Mo) and iron (Fe), or high concentrations of nitrate due to reduced assimilation of nitrate in the plant. High levels of nitrate fertilization also contribute to elevated levels of nitrate in the harvested plant.[8]

My interpretation of this quote suggests that plants can take up high nitrate levels for the mere fact of being exposed to high rates of nitrate fertilisation......and also in certain other circumstances that have to with the culture of the plant.....like molybdenum and iron deficiencies and reduced sunlight......and that, if such plants are consumed, those high nitrate levels can create health issues.......especially in very young children.

Gary