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Limnological terms, definitions, acronyms and concepts

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

Updated: October 20, 2013      Limnology



Biological communities are supported by and based on the photosynthetic production of organic matter derived from an external source.

Aquatic ecosystems

... consist of entire drainage basins: the nutrient and organic-matter content of drainage from the catchment area is modified in each of the terrestrial, stream, and wetland-littoral components, as well as in the lake or reservoir per se. Productivity is generally low to intermediate in the terrestrial components, highest in the wetland interface region between land and water, and lowest in the open-water portion of the lake. (See also Lakes- zonation, Littoral zone, Macrophytes and Production).


Biological communities are supported by and based on the photosynthetic production of organic matter produced within a community.


In most cases, energy required for growth and maintenance of organisms enters ecosystems as light and is converted to chemical energy by plant photosynthesis.


... originating from the Greek word for "bottom", is now nearly uniformly applied to animals associated with substrata.


... is the weight of all living material in a unit area at a given instant in time. (See also Standing crop and Production).


.... is derived from two Latin words; caro meaning flesh and vorare meaning `to swallow'. A carnivore is a flesh-eating or predatory organism. Organisms imply animals as well as herbs (e.g. Venus flytrap is a herb but is a predator). Plants for the most part are neither herbivores nor carnivores, they're termed producers since they essentially manufacture their own food through photosynthesis.

Chlorophyll a

Chlorophyll a is quite often used as a surrogate measure of the amount of phytoplankton in a water sample. Comparing water bodies on the basis of chlorophyll a content implicitly assumes the algae are composed of equivalent amounts of chlorophyll though. The chlorophyll content of algae is usually about 0.5-1.5% of the dry wt. But increased amounts, up to 6% have been recorded in algae culture in weak light. Chlorophylls are "tetrapyrrolic molecules with a central magnesium atom and two ester groups", hence the need for micronutrients by plants and animals.

Chlorophyll a is the "master pigment" in bluegreen algae and higher plant photosynthesis (apparently some photosynthesizing bacteria can do it without chlorophyll a). It is chlorophyll a that ultimately captures energy from light (photons) and packages it as energy in chemical bonds for use by plants and eventually animals. There are other "accessory pigments" (such as chlorophylls b, c, and d, carotenoids, phycoerythrins, phycocyanins, and xanthophylls) which can trap light energy at shorter wave lengths and pass it along to chlorophyll a which absorbs at longer wavelengths. It is the unique combination of accessory pigments with chlorophyll a that help to distinguish certain groups of algae and higher plants from one another. For example, Euglenophyta are characterized by the presence of chlorophyll a and the accessory pigments b-carotene and the xanthophyll, lutein.

(See also Production and Phytoplankton).


..... is composed of two Greek words- kopros, meaning "dung or excrement", and pelos, meaning "mud". It is now coined to denote a mixture of humus material, fine plant fragments, algal remains, grains of quartz and mica, diatom frustules, exoskeleton fragments from aquatic arthropods, and spore and pollen relics.


Living organisms constitute only a very small portion of the total organic matter of ecosystems. Most organic matter is nonliving and is collectively called detritus.

Gas bubble disease

... occurs when certain aquatic organisms, especially fishes, are exposed to water high in dissolved gases (including oxygen). Lethal concentrations can occur when fish are exposed for several hours to waters containing total gases in excess of 115% saturation. Bubbles form in tissues and eventually gas emboli (i.e. bubbles) accumulate in gill capillaries causing death by anoxia. Super-saturation of water with atmospheric gases can occur when water falls over high dams, and this has been known to cause gas bubble disease in stream fishes below high dams. Mortality could also occur where pond DO conc. increase substantially during a period of high photosynthesis.


Bluegreen algae (Cyanobacteria) and actinomycetes (fungi) produce an odour causing compound called "geosmin" (rhymes with Jasmine), and smells "earthy-musty".


This word is used to denote the "common lacustrine sediment of eutrophy".


.... is derived from two Latin words; herba meaning `vegetation' and vorare meaning `to swallow'. A herbivore is an animal that feeds exclusively on plants.


A specialized cell characteristic of certain species of cyanobacteria (bluegreen algae). It is in this cell that nitrogen fixation occurs, the conversion (chemical reduction) of gaseous nitrogen to ammonia. It is believed that the thickened cell wall of the heterocyst allows the cell to maintain the internal anaerobic environment necessary for fixation to occur. It is this capability that allows certain cyanobacteria (notably Anabaena and Aphanizomenon) to bloom under conditions of dwindling nitrogen concentrations.


... are organisms that derive their nourishment from existing organic substances. Heterotrophs can be herbivores, carnivores, omnivores or detritivores.


Hydrilla (Hydrilla verticillata Royle) is an introduced noxious aquatic plant in North America. This submersed aquatic plant is found in most tropical areas of the world including Africa, Asia, India, Australia, and was brought into the US by the aquarium industry in about 1959. It is easily the most problematic aquatic plant in Florida, and is probably rivalled only by Eurasian water milfoil (another introduced submersed aquatic plant) for the title of most problematic aquatic plant in North America.

Lakes- eutrophication and nutrients

The term eutrophication is synonymous with increased growth of the biota of lakes, and that the rate of increasing productivity is accelerated over that rate which would have occurred in the absence of perturbations to the system. The measurable criterion of accelerated productivity is an increased quantity of carbon assimilated by algae and larger plants per given area.

Under a large majority of lake conditions, the most important nutrient factors causing the shift from a lesser to a more productive state are phosphorus and nitrogen. Typical plant organic matter of aquatic algae and macrophytes contains phosphorus, nitrogen, and carbon in approximately the ratios:

1P : 7N : 40C per 100 dry weight or

1P : 7N : 40C per 500 wet weight.

If one of the three elements is limiting and all other elements are present in excess of physical needs, phosphorus can theoretically generate 500 times its weight in living algae, nitrogen 71 (500:7) times, and carbon 12 (500:40) times.

Lakes- origin

Most natural lakes were formed by catastrophic events:

Other natural lakes form by gradual events:

Variation in basin morphology is great; most lake basins approximate an elliptic sinusoid shape. The mean depth, Z of lakes is about half (0.46) of the maximum depth, Zm (Wetzel 1983).

Lakes- zonation

The lake is separated into the open-water pelagial zone and the littoral zone, the latter consisting of the bottom of the lake basin colonized by macrovegetation. The sediments free of vegetation that lie below the pelagial zone are referred to as the profundal zone. The littoriprofundal zone is the transitional area of the sediments occupied by scattered benthic algae. (See also Littoral zone and Macrophytes).

Lakes- Morphology

Lakes- Toxic and Potentially Hazardous Substances

It is recognized that along with an increased trophic response, other harmful effects of certain substances are part of the overall problem of man-made (cultural) eutrophication. Some of these substances such as trace elements were always present in low quantities in aquatic systems supplied in the basic natural load, but with accelerated eutrophication, the increased amounts supplied, accumulated and recycled in the aquatic system cause problems. Other substances, mainly organic compounds of an anthropogenic nature, originating from pesticides, paints and other chemicals, also enter into water courses and add to the problem. These substances are usually found in very low concentrations in water but they can accumulate in animal tissues and persist in a water body.

Limnology and demophoric growth

In broad terms, limnology is the study of the functional relationships and productivity of freshwater communities as they are affected by their physical, chemical, and biotic environment, and includes standing (lentic) and running (lotic) waters.

A fundamental feature of the earth is an abundance of water, which covers 71% of its surface to an average depth of 3800 meters. Over 99% of the immense mount of water of the biosphere occurs in the oceans and polar ice deposits. The turnover time of this water is very long. Most of the remaining water occurs in freshwater lakes; its renewal time is much shorter than in marine systems.

Finite freshwater resources are being exploited and degraded at an alarming rate by the activities of humankind (Wetzel 1983).

Littoral zone

... is an interface zone between the land of the drainage basin and the open water of lakes. Most lakes of the world are relatively small in area and shallow. In such lakes, the littoral flora contributes significantly to the productivity, and may regulate metabolism of the entire lake ecosystem.

Wetland and littoral regions of freshwater ecosystems are commonly intensely metabolically active owing to the presence of aquatic macrophytes. Phytoplankton productivity is generally lower I littoral zones containing stands of aquatic macrophytes largely from a competition for nutrients (including carbon) by submersed macrophytes, and by a reduction of light by macrophyte foliage. (Also see Macrophytes and Lakes- zonation).


The term aquatic macrophyte generally refers to the macroscopic forms of aquatic vegetation, and encompasses macroalgae (e.g. the alga Cladophora, the stoneworts such as Chara), the few species of mosses and ferns adapted to the aquatic habitat, as well as true angiosperms. Four groups of aquatic macrophytes can be distinguished as follows:

(See also Littoral zone).


... is a group of algae found aggregated in the littoral zone which is neither strictly attached to substrata nor truly suspended. The metaphyton commonly originates from true floating algal populations that aggregate among macrophytes and debris of the littoral zone as a result of wind-induced water movements. (See also Periphyton and Phytoplankton)


... comes from two Latin words; omni meaning `all' and vorare meaning `to swallow'. They consume both plants and animals.


refers to microfloral growth upon substrata in fresh waters. A much more explicit manner of expression is to refer to the organisms with appropriate modifiers descriptive of the substrata upon which they grow in natural habitats. These algal communities can be classified into,

(See also Metaphyton and Phytoplankton).


The phytoplankton consist of the assemblage of small plants having no or very limited powers of locomotion; they are therefore more or less subject to distribution by water movements. Certain planktonic algae move by means of flagella, or possess various mechanisms that alter their buoyancy. However, most algae are slightly denser than water, and sink, or sediment from, the water. Phytoplankton are largely restricted to lentic ("standing") waters and large rivers with relatively low current velocities. (See also Periphyton, Metaphyton and Zooplankton).

Composition of the Algae of Phytoplanktonic Associations:


Blue-green algaeCyanophyta or Myxophyceae
Green algaeChlorophyta
Yellow-green algaeXanthophyceae
Golden-brown algaeChrysophyceae
Brown algaePhaeophyta
Red algaeRhodophyta

Production and Productivity

Production refers to new organic matter formed over a period of time plus losses to respiration, excretion, secretion, mortality, grazing, and predation. All living organisms obtain the energy of life by combustion of organic matter. Autotrophs capture solar energy radiating through air or water and store (`fix') captured energy as environmental redox potential (Eh) between the photosynthetic products, oxygen and organic matter. Autotrophs essentially "make their own fuel" in a process called synthesis or production per the following equation which is the reverse of respiration:

nCO2  +  nH2X  --->  (CH2O)  +  X
Reduction      nCO2 ---> n(CH2O)
Oxidation     nH2X ---> nX

The "photosynthetic process" (phototrophy) is also an oxidation-reduction reaction, but uses solar energy to reduce CO2 to organic matter. In photosynthesis, X is oxygen and water is oxidized to oxygen. Photosynthesis is not the only process which produces organic matter. Chemolithotrophy synthesizes organic matter in the absence of light, and where for example X is sulfur, hydrogen sulfide is oxidized. Organisms which do not produce their own fuel are dependent on organic matter produced by autotrophs (heterotrophs).

Productivity usually refers to an average rate of production over a distinct period of time (e.g. day, year).

(See also Biomass and Standing Crop).


The word recalcitrance connotes contumacious reduction in capabilities for microbial degradation, usually related to the chemical structure of the organic matter and is used in preference to refractory.


Although interpretations vary, the term refractory often implies chemical resistance and nondegradability, which is not true in these natural ecosystems.


The respiration process can be simply depicted as follows:

(CH2O)n  +  X  --->  CO2  +  H2X
Reduction      nX ---> nH2X
Oxidation     n(CH2O) ---> nCO2

where (CH2O)n represents fuel (organic matter), and "X" represents the substance used to combust it (oxidant). Respiration is an "oxidation-reduction" reaction where organic matter is "oxidized" to CO2 and another substance "X" is "reduced". All organisms do this. The only differentiating feature is the substance (X) used to accept transferred energy ("terminal electron acceptor" [TEA]). Organisms that require oxygen to combust organic matter (such as humans) perform aerobic respiration. Many organisms do not require oxygen to combust organic matter. Anaerobic respiration is the process by which organic matter is combusted (oxidized) using an alternate TEA. The alternate can be a variety of substances (X) which become reduced, and these alternates in the sequence in which they are used after oxygen is depleted are first, nitrate reduction (NO3--->N2 @ Eh approx=220 mv), manganese reduction (Mn+3--->Mn+2 @ Eh approx=200 mv), iron reduction (Fe+3--->Fe+2 @ Eh approx=120 mv), sulfur reduction (S0--->H2S @ Eh<-75mv), and fermentation [(CH2O)n--->CHn(CH4) @Eh<-75mv].

Standing crop

... is the weight of organic material that can be sampled or harvested at any one time from a given area. It does not necessarily include the whole population, because certain species or inaccessible parts of the sampled species may be omitted by the sampling procedure. For this reason, such measurements should be abandoned in limnology and only biomass should be used. (See also Biomass and Production).



Animals of fresh waters are extremely diverse, and include representatives of nearly all phyla. The zooplankton include animals suspended in water with limited powers of locomotion. Like phytoplankton, they are usually denser than water, and constantly sink by gravity to lower depths. The distinction between suspended zooplankton having limited powers of locomotion, and animals capable of swimming independently of turbulence-the latter referred to as nekton-is often diffuse. Freshwater zooplankton are dominated by four major groups of animals: protozoa, rotifers, and two subclasses of the Crustacea, the cladocerans and copepods.

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