Fact Sheets

Chloride High pHNitrogenPhosphorus

Chloride Fact Sheet

The presence of chloride (Cl-) where it does not occur naturally indicates possible water pollution. Chloride itself is not dangerous. It doesn't harm macroinvertebrates until the concentration is very large (at least 1000 mg/l); concentration in the ocean is 35,000 mg/l. Typical concentrations in Massachusetts streams are about 12 mg/l.

The presence of chloride, however, can be an indicator of human-induced pollution, along with excessive nutrients or bacteria. Since chloride titrations can easily be done by the citizen volunteer, some groups test for chloride as a cheap alternative to nutrient or bacteria analyses to pinpoint the location of polluted water.

Sources of chloride include septic systems (chloride values of 50 to 100 mg/l are common in septic tank effluent), wastewater treatment plant effluent, animal waste, potash fertilizer, and drainage from road-salting chemicals. Increases in chloride, either seasonally or over time, can mean that one or more of these sources is affecting the water body. An increase in chloride from human or animal waste suggests that other nutrients are also entering the water body. Higher chloride concentrations from spring to fall may be the effect of lawn fertilizer runoff or septic systems during heavy use by summer residents. Higher values in spring after the snow melts may signify runoff from drainage basins or highways as a major source of chloride.

Near the coast, especially on Cape Cod, chloride may be high due to sea spray drifting inland.

Since lakes vary in their natural chloride content, it is important to have background data or a long term database to document changes.

Some of this information was obtained from the booklet "Understanding Lake Data" from the University of Wisconsin-Stephens Point County Extension , 1994.

High pH Fact Sheet

What causes very high pH in lakes in the summer?

Plants use carbon dioxide (CO2) during photosynthesis to produce carbohydrates (CH2O)n.

When the rate of atmospheric CO2 diffusing into the water is less than the rate of photosynthesis, aquatic plants use dissolved carbonates (H2CO3, HCO3-, CO32-) as their source of carbon. Take a look at the equations for photosynthesis in water:

nH2CO3 = (CH2O)n + nO2
carbonic acid   carbohydrate   oxygen
nHCO3-  + nH2O   = (CH2O)n   + nO2 + nOH-
bicarbonate ion   water   carbohydrate   oxygen   hydroxyl ion
 nCO32-  +  2nH2O  =  (CH2O)n   +  nO2   +   2nOH-
carbonate ion   water   carbohydrate   oxygen   hydroxyl ion

The production of the hydroxyl ion in the last two equations is responsible for the increase in lake water pH during photosynthesis. pH is also raised because photosynthesis consumes protons (H+). pHs can exceed 10 in the late afternoon in lakes undergoing photosynthesis by phytoplankton.

Nitrogen Fact Sheet

also see National Nutrient Guidance Documents

Nitrogen is second only to phosphorus as an important nutrient for plant and algae growth. A lake's nitrogen sources vary widely. Nitrogen compounds often exceed 0.5 mg/l in rainfall, so that precipitation may be the main nitrogen source for seepage and some drainage lakes (lakes with no incoming tributaries). Nitrogen may also come from fertilizer and animal wastes on agricultural lands, human waste from sewage treatment plants or septic systems, and lawn fertilizers used on lakeshore property.

Nitrogen from the atmosphere is fixed into organic matter by bacteria. Organic forms of nitrogen may also enter a lake from surface runoff or groundwater sources. Decomposing organic (plant and animal) matter releases ammonium (NH4+); at high pH (greater than 9), this is converted to ammonia (NH3). Ammonium is then oxidized (combined with oxygen) by specialized bacteria to form nitrites (NO2-) and nitrates (NO3-). Conversion to nitrate occurs more rapidly at higher water temperatures. Nitrate is usually the most prevalent form of nitrogen in lakes. Both NO3- and NH4+ can be used by most aquatic plants and algae. If these inorganic forms of nitrogen exceed 0.3 mg/l (as N) in spring, there is sufficient nitrogen to support summer algae blooms. The drinking water standard for nitrate is 10 mg/l (above that it can cause methemoglobimenia or "blue baby disease" in infants).

In some situations nitrogen can limit algae growth. This occurs when the ratio of nitrate plus ammonia to soluble reactive phosphorus is less than 10:1 (at or below this ratio, adding nitrogen will stimulate algae growth). Values between 10:1 and 15:1 are considered transitional, and values greater than 15:1 are considered phosphorus limited (at this or higher ratios, adding phosphorus will stimulate algae growth).

Trophic Status of Lakes vs Nitrate-Nitrogen levels
NO3-N (mg/l)
Trophic Level
< 0.3
0.3 - 0.5
0.5 - 1.5
> 1.5

In most cases, phosphorus is the limiting nutrient for algae. Low nitrogen levels do not guarantee limited algae growth in the same way low phosphorus levels do. Nuisance blue-green algae blooms are often associated with lakes that have low nitrogen to phosphorus (N:P) ratios. These algae can use atmospheric nitrogen gas (N2) dissolved in lake water as a nitrogen source when other sources aren't available. Therefore, lake managers control phosphorus inputs rather than try to control nitrogen inputs. If phosphorus is abundant and cannot be controlled, it might actually be better to have high nitrate levels: less obnoxious algae will outcompete the blue-greens in high nitrate waters.

Larger plants also need nitrogen and may depend on spring runoff and septic systems to recharge the sediments with nitrogen. Growth of Eurasian milfoil has been correlated with such fertilization of the sediment.

Because nitrogen fertilizers are often applied along with herbicides and pesticides on lawns, high nitrate levels can indicate other pollutants which are not economical to test in a diagnostic survey. Besides flagging harmful chemicals, high nitrate levels can also point to poor farm management practices, such as improper manure storage, and failing septic systems, which lead to bacterial pollution.

Phosphorus Fact Sheet

also see National Nutrient Guidance Documents

What Do My Total Phosphorus Results Mean?

The results we are sending are expressed in micrograms per liter (µg/l) or parts per billion. If you are used to results in milligrams per liter (mg/l), divide our results by 1000. For example, 16µg/l is equal to 0.016mg/l.

Why is phosphorus important to lake health?
Phosphorus is an essential nutrient for algae and aquatic plants (which in turn are food for micro-fauna and larger animals). Therefore phosphorus is an important element of the food chain within a lake. Phosphorus is usually present in very small amounts in a lake and is considered a ‘limiting factor' for algae and plant growth; i.e., even if there are plenty of other nutrients such as nitrates and carbonates, algae and plants will not grow if there is not enough phosphorus. Lake with low nutrient levels are said to have a low trophic level–we could say these lakes are ‘lean' and not very productive; scientists call that trophic level oligotrophic (little food). Lakes with more, but not excessive, amount of nutrients are called mesotrophic (middle food), and lakes overenriched with nutrients are called eutrophic (true or much food).

"Oligotrophic lakes are generally clear, deep and free of weeds or large algae blooms. Though beautiful, they are low in nutrients and do not support large fish populations, However, oligotrophic lakes often develop a good food chain capable of sustaining a very desirable fishery of large game fish.

Eutrophic lakes are high in nutrients and support a large biomass (all the plants and animals living in a lake). They are usually either weedy or subject to frequent algae blooms, or both. Eutrophic lakes often support large fish populations, but are also susceptible to oxygen depletion. Small, shallow, eutrophic lakes are especially vulnerable to winterkill which can reduce the number and variety of fish. Rough fish are commonly found in eutrophic lakes. Devoid of oxygen in late summer, the hypolimnions (lower lake layer) of deeper eutrophic lakes limit cold water fish and cause phosphorus cycling from sediments.

Mesotrophic lakes lie between the oligotrophic and eutrophic stages.

What is the trophic level of my lake?
Researchers use various methods to calculate the trophic state of lakes. Common characteristics used to make the determination in lakes without excessive weeds are:

  • total phosphorus concentration (important for algae growth)
  • chlorophyll a concentration (a measure of the amount of algae present)
  • Secchi disk reading (an indicator of water clarity). (Adapted from Understanding Lake Data, Shaw et al University of Wisconsin-Extension, 1994)

The Carlson's Index shown here includes all three parameters (use a straight edge to match your TP level to the Trophic State).

image of Carlson's index