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Phytoplankton (of fresh waters)

Soil & Water Conservation Society of Metro Halifax (SWCSMH)

April 10, 2015                              Limnology



PLANKTON: "We're an indolent lot... Shiftless microscopic drifters. Here in the oceans a million trillion trillion of us just float and aimlessly worship the sun. We have no brains at all. And we don't do anything at all except procreate with promiscuous abandon and generate most of Earth's oxygen. And we have no advice at all for you diligent bipeds who use your capacious intellects to so industriously befoul the seas. For about two billion years we got along quite well without you. And without us, you will suffocate".

.................. Lake Management- S.E. Jorgensen


All sound lake management must be based on knowledge of the processes and/or mass-and energy flows in the lake ecosystems. From a thermodynamic point of view a lake may be considered as an open system, which exchanges materials (wastewater, evaporation, precipitation, nitrogen, bacteria) and energy (evaporation, radiation) with the environment. The cycles of the most important elements, C, N, P, Si, O, S, Fe, are a major part of the chemical-biological processes in a lake system. n addition to the biochemical cycles some physical processes must be mentioned. The flow of energy has a great influence on the system, which means that light penetration, formation of thermocline and evaporation must be included among the essential processes of a lake. Primary production means the organic material formed, and the greatest part of production in lakes and reservoirs is caused by phytoplankton. Macrophytes might have a substantial standing crop, but phytoplankton has generally a much shorter turnover time.

Primary production represents the synthesis of organic matter of aquatic systems and the total process, photosynthesis, whose complex metabolic pathway can be oversimplified as follows:

light + 6CO2 + 6H2O ----> C6H12O6 + 6O2

Plants have photosynthetic pigments, one of which, cholorophylla is present in almost all photosynthetic organisms. Several other pigments, such as chlorophyll b, c, d and e, carotenoids, xanthophylls and biliproteins, can be found in plants.

The word "trophy" refers to the rate of organic matter supplied by or to the lake per unit of time. Lakes receiving a relatively large portion of their organic material from allochthonous (external) sources are termed dystrophic (brown water) lakes. Productivity of most dystrophic lakes is low. The difference between dystrophic and eutrophic lakes is the source of the organic matter- allochthonous and autotrophic respectively.

Lacustrine Zonation

The bottom of a lake basin is separable from the free open water, the pelagial zone, and is further divisible into a number of rather distinct transitional zones from the shore to the deepest point: Epilittoral, Supralittoral, Littoral (Eulittoral, Upper Littoral, Middle Littoral, Lower Littoral), Littoriprofundal, and Profundal zones. Periphyton, although variously used, usually refers to microfloral growth upon substrata. The term benthos is now nearly uniformly applied to animals associated with substrata.

Among the algal communities, one can readily differentiate the following:

  1. epipelic algae as the flora growing on sediments (fine, organic)
  2. epilithic algae growing on rock or stone surfaces
  3. epiphytic algae growing on macrophytic surfaces
  4. epizooic algae growing on surfaces of animals, and
  5. epipsammic algae as the rather specific organisms growing on or moving through sand.

The general word psammon refers to all organisms growing or moving through sand. A group of algae found aggregated in the littoral zone is the metaphyton, 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.


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.

Composition of the Algae of Phytoplanktonic associations

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

Blue-green algae (Phylum Cyanophyta or Myxophyceae)

[in Greek, cyano = blue-green, and myx = slime]: These occur in unicellular, filamentous, and colonial forms, and most are enclosed in mucilaginous sheaths either individually or in colonies. A majority of the planktonic blue-greens consists of members of the coccoid family Chroococcaceae (e.g., Anacystis = Microcystis, Gomphosphaeria = Coelosphaerium, and Coccochloris) and filamentous families Oscillatoriaceae, Nostocaceae, and Rivulariaceae (e.g., Oscillatoria, Lyngbya, Aphanizomenon [3-6 m], Anabaena).

Green algae (Phylum Chlorophyta)

These are an extremely large and morphologically diverse group of algae that is almost totally freshwater in distribution. A majority of the planktonic green algae belong to the orders Volvocales (e.g., Chlamydomonas, Sphaerocystis, Eudorina, Volvox) and Chlorococcales (e.g., Scenedesmus, Ankistrodesmus [2-3 m], Selenastrum [6-7 m], Pediastrum). Many members are flagellated (2 or 4, rarely more) at least in the gamete stages; in the desmids (Conjugales or Desmidiales), the gametes are amoeboid.

Whereas the planktonic Volvocales and Chlorococcales are ubiquitous in distribution among waters of differing salinity within the normal limnological range, the distribution of most species of desmids of the Conjugales is limited to low concentrations of the divalent cations, calcium and magnesium. Although not totally restricted to waters of low salinity, the desmids are most common and the species diversity is greatest in soft waters draining land forms developed in granitic or other igneous rocks, and especially in waters with a high content of dissolved organic matter. Their abundance and diversity are often greatest in bog waters that drain through deposits of the moss Sphagnum. Many desmids are distributed widely, but as a whole they are less cosmopolitan than most unicellular algae.

Yellow-green algae (Phylum Xanthophyceae)

These are unicellular, colonial, or filamentous algae that are characterized by conspicuous amounts of carotenoids in comparison to chlorophylls. Nearly all of the motile cells possess two flagella. A majority of them are associated with substrata, and many are epiphytic on larger aquatic plants. A few members are planktonic and include common genera such as Chlorobotrys, Gloeobotrys, and Gloeochloris.

Golden-brown algae (Chrysophyceae)

These have a dominance of -carotene and specific xanthophyll carotenoids, in addition to chlorophylla. Most of them are unicellular, a few are colonial; they are rarely filamentous. A number of them from important components of the phytoplankton. The unicellular species with a single flagellum (e.g., Chromulina, Chrysococcus, Mallomonas) are usually very small algal constituents of the nannoplankton (10-50 m). Larger colonial forms such as Synura, Chrysosphaerella, Uroglena, and particularly Dinobryon are widely distributed. Certain species of Dinobryon and Uroglena develop in lakes of very low phosphorus concentrations, while other species of Dinobryon and Synura have high phosphorus requirements.

Diatoms (Bacillariophyceae)

They are a most important group of phytoplankton even though most species are sessile and associated with littoral substrata. Both unicellular and colonial forms are common.

The group is commonly divided into the centric diatoms (Centrales), which have radial symmetry, and the pennate diatoms (Pennales), which exhibit essentially bilateral symmetry.

The four major groups of pennate diatoms are differentiated on the basis of cell thickenings and dilations:

  1. the Araphidineae (e.g., Asterionella, Diatoma, Fragilaria, Synedra) possess a pseudoraphe (i.e., a depression in the axial areas of the cell wall)
  2. the Raphidioidineae (e.g., Actinella, Eunotia) in which a rudimentary raphe (i.e., a slit traversing all or part of the cell wall) occurs at the cell ends
  3. Monoraphidineae (e.g., Achnanthes [10 m], Cocconeis [10 m]) which have a raphe on one valve and pseudoraphe on the other, and
  4. the Biraphidineae (e.g., Amphora, Cymbella, Gomphonema, Navicula, Nitzschia, Pinnularai, Surirella) in which the raphe occurs on both valves.


Most of these algae are naked, unicellular, and motile. This class is very small and most of the planktonic members belong to the Cryptomonadineae (e.g., Cryptomonas, Rhodomonas, Chroomonas). Dense populations of these algae often develop during cold periods of the year under relatively low light conditions.

Dinoflagellates (Dinophyceae of the Pyrrophyta)

They are unicellular flagellated algae, many of which are motile. A few species of the order Gymnodiniales (e.g., Gymnodinium) are naked or without a cell wall; but most develop a conspicuous cell wall, the Peridiniales (e.g., Ceratium, Glenodinium, Peridinium).

Although many zooplankton change size and form seasonally, only a few phytoplankton undergo seasonal polymorphism or cyclomorphosis. One example as temperatures increase is the dinoflagellate Ceratium. Presumably, these changes are of adaptive significance in that they reduce the rate of sinking out of the photic zone.

Euglenoids (Euglenophyceae)

Few species are truly planktonic. Almost all are unicellular, lack a distinct cell wall, and possess one, two, or three flagella. Their development in the phytoplankton occurs most often in seasons, strata, or lake systems in which concentrations of ammonia and especially dissolved organic matter are high. However, these algae are found most often in shallow water rich in organic matter such as farm ponds.

Brown algae (Phaeophyta)

They are mostly filamentous or thalloid, and are almost exclusively marine. The genera that are found in fresh water are attached to substrata, such as rocks. No species is planktonic.

Red algae (Rhodophyta)

None is planktonic and sparsely represented in fresh water. The thalloid or filamentous species (e.g., Batrachospermum) are nearly all restricted to fast-flowing streams of well-oxygenated, cool waters.

Seasonal succession

In spite of the number of pervasive generalizations about the common seasonal succession in fresh waters, close inspection of existing data shows a great diversity of patterns. Nevertheless simplifying briefly, the temperate pattern of succession involves a winter minimum of small flagellates adapted to low light and temperature, a spring burst of diatom activity and biomass, followed rapidly by a smaller development of green algae and a transitional lull between spring and summer. Summer populations vary in relation to the trophic status of the lakes, but can include either another diatom development in less productive lakes by late summer and early autumn or increases in nitrogen-fixing blue-green algae in eutrophic lakes.

The widely held assumption that winter productivity is insignificant is not universally valid, and rates of primary production under ice cover can constitute a very significant portion of the total annual primary productivity of the phytoplankton. Increasing light is the dominant factor contributing to the development of the spring "outburst", because water temperatures are still low. The spring maximum is frequently dominated by one species, a diatom, such as Asterionella, Cyclotella, or Stephanodiscus.

The decline of the spring maximum of phytoplankton and onset of summer populations in temperate lakes also is associated with a complex interaction of physical and biotic parameters. In many straightforward cases, reduction of nutrients in the photic zone of the epilimnion is responsible for slowing the growth of populations of the dominant as well as rarer algae. Since diatoms are often the dominant component of the spring maximum in temperate lakes, silica concentrations are often reduced to limiting levels (<0.5 mg/l) in less than two months when turbulence is low and sedimentation of diatom frustules occurs rapidly, subsequent to extensive growth of the diatoms.

As silica concentrations are reduced in productive lakes, diatom populations are often succeeded by a preponderance of first green algae and later blue-green algae. Growth in these eutrophic lakes can be so intense that combined nitrogen (NO3, NH4+) sources are reduced to below detectable concentrations in the trophogenic zone. When this happens, often by midsummer when the warmest epilimnetic temperatures occur, blue-green algae with efficient capabilities for fixing molecular nitrogen have a competitive advantage and can predominate. These lakes require, as a general rule, a reasonably sustained and heavy loading of phosphorus.

Phosphorus concentrations required for growth

Extensive investigations grouped freshwater algae into categories according to whether their tolerance fell below, around, or above 20 g PO4-P/l:

General characteristics of oligotrophic and eutrophic lakes and reservoirs in the temperate zone (Ryding and Rast, 1989)

Aquatic plant and animal productionlowhigh
Number of plant and animal speciesmanymany; can be substantially reduced in hypertrophic waters
General levels of biomass in waterbodylowhigh
Occurrence of algal bloomsrarefrequent
Relative quantity of green and blue-green algaelowhigh
Vertical extent of algal distributioninto hypolimnium (bottom waters) in thermally stratified waterbodiesusually only in surface waters
Aquatic plant growth in shallow shoreline area (littoral zone)can be sparse or abundant; if present, usually consists of submerged and emergent vegetationoften abundant; usually an increase in the presence of filamentous algae and a decrease in macrophytes
Daily migration of algaeextensivelimited
Some characteristics of algal groupsGreen algae: Desmids, Staurastrum

Diatoms: Tabellaria, Cyclotella

Golden-brown algae: Dinobryon

Blue-green algae: Anabaena, Aphanizomenon, Microcystis, Oscillatoria

Diatoms: Melosira, Fragilaria, Stephanodiscus, Asterionella

Characteristics of common major algal associations of the phytoplankton in relation to increasing lake fertility (Wetzel, 1983)

Lake Trophy
Dominant algaeOther commonly
occurring algae
OligotrophicSlightly acidic; very low salinityDesmids Staurodesmus, StaurastrumSphaerocystis, Gloeocystis, Rhizosolenia, Tabellaria
OligotrophicNeutral to slightly alkaline; nutrient-poor lakesDiatoms, especially Cyclotella and TabellariaSome Asterionella spp., some Melosira spp., Dinobryon
OligotrophicNeutral to slightly alkaline; nutrient-poor lakes or more productive lakes at seasons of nutrient reductionChrysophycean algae, especially Dinobryon, some MallomonasOther chrysophyceans, e.g., Synura, Uroglena; diatom Tabellaria
OligotrophicNeutral to slightly alkaline; nutrient-poor lakesChlorococcal Oocystis or chrysophycean BotryoccocusOligotrophic diatoms
OligotrophicNeutral to slightly alkaline; generally nutrient poor; common in shallow Arctic lakesDinoflagellates, especially some Peridinium and Ceratium spp.Small chrysophytes, cryptophytes, and diatoms
Mesotrophic or EutrophicNeutral to slightly alkaline; annual dominants or in eutrophic lakes at certain seasonsDinoflagellates, some Peridinium and Ceratium spp. Glenodinium and many other algae
EutrophicUsually alkaline lakes with nutrient enrichmentDiatoms much of year, especially Asterionella spp., Fragilaria crotonensis, Synedra, Stephanodiscus, and Melosira granulataMany other algae, especially greens and blue-greens during warmer periods of year; desmids if dissolved organic matter is fairly high
EutrophicUsually alkaline; nutrient enriched; common in warmer periods of temperate lakes or perennially in enriched tropical lakesBlue-green algae, especially Anacystis (= Microcystis), Aphanizomenon, AnabaenaOther blue-green algae; euglenophytes if organically enriched or polluted

Phytoplankton Indices (Hutchinson, 1967)

Several phytoplankton indices have been reported and related with transparency, and so the total mass of seston, and to some extent with productivity. The use of these indices, however, requires discretion; they are certainly inapplicable in some regions.
  1. Myxophycean index= (# of Myxophyceae species) / (# of Desmideae species)

  2. Chlorophycean index= (# of Chlorococcales species) / (# Desmideae species)

  3. Diatom index= (# of centric diatom species) / (# of pennate diatom species)

  4. Euglenophyte index= (# of Euglenophyta species) / (# of Myxophyceae and Chlorophyceae species)

  5. Compound index= (# of Myxophyceae, Chlorococcales, centric diatoms, and Euglenophyta species) / (# of Desmideae species)

Since there is a tendency for green and blue-green algae to be summer forms, although the diatoms may flourish at any time of year, the indices other than the diatom quotient refer only to summer collections, preferably made in June, July, and August. The diatom quotient is supposedly applicable at any time of year.

Nygaard regarded lakes containing associations giving a compound index of less than 1.0 as unproductive and those giving an index of more than 3.0 as definitely eutrophic; the intermediate values implied mesotrophy or weak eutrophy.

Phytoplankton indices for less productive and more productive groups of lakes (Nygaard's data for several Danish lakes)
Lake characteristicsMyxophyceanChlorophyceanDiatomEuglophyteCompound
Less productive, more transparent (pH<7.0, Ca<10 mg/l)0.0-0.40.0-0.70.0-0.30.0-0.20-1
More productive, less transparent (pH>7.0, Ca>10 mg/l)0.1-3.00.2-9.00.0-1.750.0-1.01.2-25

Several South Swedish lakes grouped in three geographical categories and the Chlorophycean indices
 Vaxjo and Lund District (4 lakes)Vastervik District (6 lakes)Aneboda District (4 lakes)
Secchi Disk transparency (m)0.82-0.383.05-1.416.04-4.48
Number of Chlorococcales species26-4214-284-14
Number of Desmidiae species3-156-1720-35
Chlorophycean Index2.6-141.0-3.00.2-0.5

It is also possible to make for any region a list of organisms of more frequent occurrence in unproductive waters and another list of species of more frequent occurrence in more productive waters. The ratio, in the plankton of any lake, of the number of species on one list to the number on the other can be used to assess the position of the lake in a scale of productivities.

CCME (Canadian Council of Ministers of the Environment) Guidelines (rev. 1992)

The "pea soup" or "bloom" condition as a result of the rapid increases in numbers of algae can be a natural phenomenon, but it is often due to accelerated eutrophication caused by human activities. Toxic algae are found in all aquatic environments and have been responsible for illness in humans, livestock, waterfowl and fish. Bather poisonings have occurred after immersion in lakes and ponds containing dense blooms of blue-green algae.

The most important phyla are Pyrrhophyta (dinoflagellates), Chrysophyta and Cyanophyta (blue-green algae). Algae of concern in lakes and ponds are usually the blue-greens Anabaena flos-aquae, Microcystis aeruginosa and Aphanizomenon flos-aquae. Massive blooms of freshwater algae often cause die-off of fish and other organisms when the algal populations suddenly collapse. The death and rapid decomposition of the algae quickly lead to anoxia and asphyxiation of fish and other aquatic animals.

Some odours from lake water originate from the decomposition of algae and other aquatic plant materials. Several species of algae produce offensive odours while in the active growing state. Many natural surface waters not influenced by odour-producing algae have a TON (Threshold Odour Number) of 5, whereas others with excessive algal growth may have a TON exceeding 200.

Other problem species

In addition to blue-green algae, certain of other algal species have been found to be nearly as much a problem. Asterionella species, for instance, may form blooms and are often involved in water spoilage. Dinobryon cylindricum also blooms under favourable conditions and may cause a disagreeable odour and taste in domestic water supplies.

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