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Q & A
Alkalinity & Trout Streams
Question
I noticed on the Class A Wild Trout Waters list there is a total alkalinity column for each water. Some levels are very low (2) and some are very high (17). When chosing a water what is an ideal T-Alk level for wild trout?
Answer
Alkalinity is a measure of the buffering capacity of water, or the capacity of water to neutralize acids. Knowing the alkalinity of a stream is important in determining its ability to neutralize acidic pollution from rainfall or from wastewater effluent. Alkalinity is not the same as pH, but is related to pH since it refers to the ability of water to resist changes in pH. The greater the alkalinity, the greater the buffering capacity and the greater the resistance to changes in pH. The presence of buffering materials in water helps to neutralize acids as they get added to the water through rainfall or discharges.

These buffering materials are mostly made up of bicarbonate and carbonate. Carbonates and bicarbonates get added to streams as the water within a stream's drainage area passes through soil and rock that contain these materials, primarily calcite (Calcium Carbonate). Where limestone and carbonate-rich soils are predominate, streams will often have a high alkalinity. Where igneous rocks and carbonate-poor soils are predominate, waters will have low alkalinity.

Because alkalinity varies greatly depending on geology, there are no general standards for alkalinity. In other words, alkalinity is best used to describe the stream conditions where fish live, rather than be an accurate predictor of what the trout population will look like. Streams with high alkalinity typically have much higher productivity since, under high alkalinity conditions, more nutrients become available to the food chain.

However, it should be noted that the quality and quantity of available habitat is a more important factor in determining the overall density of trout fishery. You can have a stream with high alkalinity, but if it lacks quality habitat, the density of trout population may still be quite low. On the other hand, streams with low alkalinity and excellent habitat can have outstanding trout populations.

Alkalinity levels between 20 and 200 parts per million (ppm) are typical of freshwater streams in Pennsylvania. Levels less than 10 ppm indicates the stream has a poor buffering capacity and may be susceptible to changes in pH from natural and man-made acidity sources. However, as previously mentioned, this does not necessarily mean that the trout population will be poor, but continued input of acidity into streams with poor buffering capacity may begin to negatively effect trout populations.

Keep in mind that all of the streams on the Class A list all have exceptional trout populations, regardless of their alkalinity level. Alkalinity should be used to give you some information about the type of stream on the list. For example, streams with high alkalinity tend to flow through large valley areas with limestone geology present, while lower alkalinity streams tend to be located in more mountainous terrain with very little influence from limestone geology.

Additionally, as you look through the list you will note a tendency for brook trout to predominate these low alkalinity streams. Brook trout tend to be better suited to cope with low alkalinity conditions and more drastic fluctuations in pH than do brown trout.

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