• Lakes and reservoirs can be classified into ultraoligotrophic, oligotrophic, mesotrophic, eutrophic and hypertrophic.

• This classification denotes the nutrient status of a waterbody.

   

  Table 4.1   Trophic lake classification system (concentrations in micrograms per liter).

  Category	Total phosphorous	Mean chlorophyll a	Maximum chlorophyll a	Mean Secchi depth (m)	Minimum Secchi depth (m)
  Ultraoligotrophic	less than 4	less than 1	less than 2.5	greater than 12	greater than 6
  Oligotrophic	less than 10	less than 2.5	less than 8	greater than 6	greater than 3
  Mesotrophic	10-35	2.5-8	8-25	6-3	3-1.5
  Eutrophic	35-100	8-25	25-75	3-1.5	1.5-0.7
  Hypertrophic	greater than 100	greater than 25	greater than 75	less than 1.5	less than 0.7

   

• Eutrophication is usually considered undesirable, since its effects can interfere with human use of the water resources.

• Increased productivity can be a positive feature: It can sequester carbon.

SYMPTOMS OF EUTROPHICATION

• Oligotrophic lakes and reservoirs are characterized by low nutrient concentrations, a diverse plant and animal community, a low level of primary productivity and biomass, and good overall water quality for most uses.

• By contrast, eutrophic waterbodies have a high level of productivity and biomass at all trophic levels, frequent occurrences of algal blooms, anoxic bottom waters during periods of thermal stratification, low plant and animal species diversity, enhanced growth of littoral zone aquatic plants, and poor water quality for many uses.  

  Table 4.1   Characteristics of oligotrophic and eutrophic lakes and reservoirs.

  Parameter	Oligotrophic	Eutrophic
  Aquatic productivity	low	high
  Species diversity	high	high to average
  Biomass level	low	high
  Algal blooms	rare	frequent
  Algae quantity	low	high
  Algae vertical extent	into hypolimnion	usually on surface
  Aquatic plant growth in shoreline	sparse to abundant	often abundant
  Fish types	deep-dwelling cold-water fishes (salmon, trout)	surface-dwelling warm-water fishes (perch, bass)
  Oxygen content in hypolimnion	high always	low or absent during thermal stratification
  Dissolved salt content	usually low	sometimes very high
  Mean depth	often deep	often shallow
  Volume of hypolimnion	often large	small or large
  Temperature of hypolimnion	usually cold	usually warm
  Water quality for human uses	good	often poor

   

• The worldwide distribution pattern for lakes shows a bimodal pattern.

• The largest percentage lies between 35-55^o latitude (temperate), and between 15^o North to 20^o South (tropics).

• Reservoirs are human-made, and do not exhibit this bimodal distribution.

• Most reservoirs are located in the subtropics (15-35^o latitude).

• Natural lakes occupy natural depressions, their tend to be symmetrical, and have many tributary inputs.

• Reservoirs are usually elongated with a single, large tributary.

• There is often a gradient of sediment and nutrient concentration along the body of a reservoir, which causes a gradient in biological productivity and water quality.

• Reservoirs exhibit physical, chemical, and biological gradients not normally found in natural lakes.

• Eutrophication management approach should consider differences between lakes and reservoirs.

  Tropical lake systems

• Tropical lakes and reservoirs are characterized by highly seasonal rainfall and limited temperature fluctuation (10^o or less).

• There is no annual freeze-thaw cycle.

• Oxygen depletion in the hypolimnion can occur regardless of the trophic status.

• Hypolimnetic oxygen depletion has little meaning as a trophic state indicator.

• Productivity is high; phytoplankton blooms can occur at any time of the year.

• Phosphorous concentrations of 50-60 micrograms per liter may be a more realistic boundary between mesotrophic and eutrophic.

• Tropical systems develop low N:P ratios, favoring the dominance of blue-green, nitrogen-fixing algae.

• Nitrogen may be the limiting nutrient in tropical systems.

• Other studies suggest that P may be limiting in certain cases.

• Present evidence supports the belief that eutrophication control may be performed in the same manner in tropical and temperate systems.

• Effective strategy to control eutrophication is based on reducing the quantity of nutrients entering the waterbody.

• Justus (1840) found that the yield of terrestrial crops is often limited by the materials needed by the crop in minute quantities.

• Absolute and relative quantities of essential nutrients are primary factors in regulating the algal biomass.

• Marconutrients are important, but so are other nutrients such as sodium, potassium, and other trace metals.

• Phytoplankton biomass correlates with chlorophyll concentration.

• Phytoplankton biomass is proportional to its nutrient load, at least up to a certain point, where it achieves a constant value.

• The limiting nutrient concept is based on photosynthesis.

• Control or limitation of the external nutrient supply (especially P and N) is the most effective way of controlling eutrophication.

  Some assumptions in using the limiting nutrient concept

• The average ratio of C, N, and P in plankton samples was 106C:16N:1P.

• This ratio has been widely cited for assessing limiting nutrient.

• A waterbody normally contains a number of different algal species, each with relative specific nutrient requirements.

• Diatoms appear to be superior competitors for P, but inferior for N.

• Experiments show that algal growth is regulated by the singlest nutrient in shortest supply.

• P is the only essential element that can easily be made to limit algal growth.

• Phytoplankton standing crop should be distinguished from its rate of production.

• Biomass is not the same as productivity.

• Biomass is more important than productivity in assessing eutrophication.

  N and P ratios to assess nutrient limitation

• Comparing the calculated N:P ratio to the 16N:1P reference value provides and indication of which nutrient is limiting.

• Concentrations of biologically available P of less than 5 micrograms/L indicate potential P limitation.

• Concentrations of biologically available N of less than 20 micrograms/L indicate potential N limitation.

• Atomic ratio 16N:1P corresponds to mass ratio 7.2N:1P. If ratio is less than 7N:1P, nitrogen is limiting; if greater than 7N:1P, phosphorous is limiting.

• Low N:P ratios favor blue-green algae in natural lakes in the temperate zone.