Somenos Lake: Algal Bloom

RAMP 01Somenos Lake and Marsh, located a short distance north of downtown Duncan, consist of seven-meter deep Somenos Lake, hollowed out when glaciers receded 11,000 years ago, and its surrounding marsh system.  The system, 200 hectares in size,  includes Somenos Lake and four creeks.  Somenos Creek is the outflow from Somenos Lake into the Cowichan River, while the primary streams into the lake are Richards, Averill, & Bings creeks.  Grass and shrub habitats are common, and willows are interspersed throughout.  Adjoining farmlands become flooded marsh-like areas in winter and spring.  Some of the rare flora and fauna found here include the Vancouver Island Ringlet, Prairie Lupine, Yellow Montane Violet, and Garry Oaks growing in deep soils.  The marsh area is unique for its population of trumpeter swans, with up to 1,000 swans in winter, representing 5% of the world’s population.

Somenos and its neighbouring lake, Quamichan, share similar problems:

  • excessive nutrient loading from the farms and homes which surround the lakes
  • insufficient “flushing” in summer due to reduced inflows and truncated outflows
  • increased plant growth (“algae-blooms”), and
  • eutrophication (reduced oxygen as the vegetation decomposes)
  • intensified warming

The combination of oxygen reduction at the bottom and temperature increases at the surface forces indigenous fish into the middle layer, where there is still some oxygen and temperatures are not too warm.  In most recent summers, even that habitable middle layer has disappeared, resulting in fish kills.  Accessible for collecting at the boat launching ramp and its tributary streams, Somenos Lake and its tributaries represent the most important source of pond samples in the Duncan area (adapted from IBA and Cowichan Watershed Board sites).

In 2015, the Cowichan Valley experienced drought and an extended, very hot summer.  The clear waters of Somenos Lake, our local stop on the trumpeter swan’s route northwards, turned into a green soup with a musty smell.  Described in the local newspapers as an “Algae Bloom”, it is actually a proliferation of “blue-green algae” – in reality, not true algae at all, but like many such blooms worldwide, a massive overgrowth of photosynthetic primitive cyanobacteria:20150817_133959-2

Sampling this opaque marginal  green soup revealed the lake waters to have been taken over by a three types of cyanobacteria during August through early October:


Anabaena is the major component, an attractive organism that forms spirals of vegetative cells interspersed with ovoid nitrogen-fixing heterocysts:

Anabaena, with rare Microcystis and Nostoc

One of the four genera of cyanobacteria that produce neurotoxins, Ananabaena is found both in fresh water blooms and in symbiotic relationships with plants such as the mosquito fern, where the neurotoxin possibly protects the plants from grazing.  This spiral bacterium produces several types of toxins:

“Anabaena sp. can produce several kinds of toxins. Two different neurotoxins have been described. Anatoxin-a is a potent postsynaptic cholinergic nicotinic agonist, which causes a depolarizing neuromuscular blockade. Anatoxin-a(s), chemically unrelated to the first, acts as an inhibitor of cholinesterase leading to a neuromuscular blockade. Both cause a “tetanus- like” muscle paralysis.

Neurotoxins are notoriously rapid-acting poisons. Onset of symptoms and death to the animal may occur within a few minutes to a few hours, depending upon size of animal and amount of toxic bloom consumed. Anatoxin-a toxicosis may exhibit staggering, paralysis, fasciculations (muscle twitching), gasping, convulsions, backward arching of neck in birds, and death. Anatoxin-a induced toxicosis in experimental animals may exhibit hypersalivation, tremors, fasciculations, involuntary muscle movement, diarrhea, cyanosis (tongue and mouth lining appear bluish). and death.

Members of this genus also produce Microcystins (hepatotoxins). Named for the genus in which they were originally discovered, they alter the cytoskeletal components of hepatocytes leading to intercellular dissociation causing blood accumulation within the liver and death by hypovolumic shock. Very recent experimental evidence shows that at least one of the molecular mechanisms of action is consistent with certain known carcinogens. Researchers suspect these toxins may also be possible liver carcinogens.  This could prove significant to humans following continuous, low level exposure.

Poisoning from microcystins may take 30 minutes to 24 hours to appear, depending upon the size of the animal affected and the amount of toxic bloom consumed. Microcystin toxicosis may exhibit jaundice, shock, abdominal pain/distention, weakness, nausea/vomiting, severe thirst, rapid/weak pulse, and death (see Crayton)”.

Despite its toxicity and sludgy appearance in the lake, Anabaena is actually a very pretty organism.  Unlike eukaryotic cells with their mitochondria, nucleus, and endoplasmic reticulum, bacteria cannot form chemically-separate intracellular compartments to isolate chemically-incompatible functions.  Cyanobacteria have developed interesting adaptations to solve this problem.

In low nitrate environments, Anabaena and other cyanobacteria can form heterocysts, pretty

Anabaena Heterocysts

ovoid cells specialized for nitrogen fixation:

“…vegetative cells differentiate into heterocysts at semi-regular intervals along the filaments. Heterocysts are cells that are terminally specialized for nitrogen fixation. The interior of these cells is microoxic as a result of increased respiration, inactivation of O2-producing photosystem (PS) II, and formation of a thickened envelope outside of the cell wall. Nitrogenase, sequestered within these cells, transforms dinitrogen into ammonium at the expense of ATP and reductant—both generated by carbohydrate metabolism, a process that is supplemented, in the light, by the activity of PS I. Carbohydrate, probably in the form of sucrose, is synthesized in vegetative cells and moves into heterocysts. In return, nitrogen fixed in heterocysts moves into the vegetative cells, at least in part in the form of amino acids…(see Wikipedia).”

Akinetes in Anabaena ~400X
Heterocysts in Anabaena ~400X
Heterocysts in Anabaena 600X. Note internal substructure in heterocyst.
Heterocysts in Anabaena 600X. Note internal substructure in heterocyst.

Heterocyst differentiation and the genome of Anabaena have been extensively studied as a model for cellular differentiation.  Protecting the nitrogenase enzyme from oxygen requires cell wall changes as well as biochemical sequestration:

“…Due to the necessity of keeping nitrogenase isolated from oxygen, heterocysts have developed elements to maintain a low level of oxygen within the cell. To prevent the entrance of oxygen into the cell, the developing heterocyst builds three additional layers outside the cell wall, giving it its characteristic enlarged and rounded appearance, thus the rate of oxygen diffusion into heterocysts is 100 times lower than of vegetative cells. One layer creates an envelope polysaccharide layer where the nitrogen is fixed in a oxygen-restricted milieu. To lower the amount of oxygen within the cell, the presence of photosystem II is eliminated, and the rate of respiration is stepped up to use up excess oxygen… (MicrobeWiki)”.


The cyanobacterium Microcystis makes up a minor component of the bloom, appearing as scattered balls and sheets of small cells embedded in a mucilaginous matrix:

Microcystis globes with Anabaena
Microcystis globes with Anabaena

Like Anabaena, Microcystis is well known for producing neurotoxins (including cyanopeptolin) and the hepatotoxic cyclopeptides microcystins.  Intracytoplasmic gas vesicles allow Microcystis blooms to rise to the surface for optimal growing conditions, where they form surface accumulations and are a major nuisance in affected bodies of water.  It has the unique ability to thrive in the presence of gylphosate (Monsanto’s Roundup), in fact using this herbicide as a nutrient.


Microcystis, loose Cluster. 600X.
Microcystis, loose cluster. 600X.

Multiple species of Microcystis have been described in the literature, but the cellular and colonial morphology of this genus of cyanobacterium is extremely variable – genetic classification seems more certain than any traditional morphological system, so speciation should be considered tentative at present (see Komarek).  Furthermore, chemical typing of the toxins and other metabolic products of a wide variety of Microcystis reveals that there is little correlation between structure, colony appearance, and chemical characterization.  Consequently, speciation of Microcystis by traditional morphological criteria is of debatable value (see Welker et al.).

Nostoc (I Think):

The third component of the bloom is a cyanobacterium rather like a tangled ball of string in appearance, tentatively identified as a species of Nostoc.  Note the smaller size of the vegetative cells, presence of both heterocysts and akinetes, and more delicate filament structure compared to Anabaena:

Nostoc, stack of two images. Note smaller cell size and thinner filaments compared to Anabaena on right.
Nostoc, stack of two images. Note smaller cell size and thinner filaments compared to Anabaena on right.
Nostoc tangle, about 400X
Nostoc tangle, about 400X

“…Nostoc filaments are made up of spherical or barrel-shaped cells of uniform size that are blue-green or olive green in color. The bent, kinked, or coiled filaments are long, isopolar, and held together by firm mucilage….The heterocysts are solitary, barrel-shaped or spherical, and may be intercalary or located at the ends of the trichomes. The akinetes are ellipsoidal and only slightly larger than the vegetative cells. The akinetes of Nostoc are usually located halfway between the heterocysts. This differs from Anabaena, where the akinetes are normally adjacent to the heterocysts…. (see Connecticut College).”

(Sanguinis:  So what do you expect when you pour a bunch of fertilizer in my nice clean lake and let your cows poop in it and then screw up the weather?  Gardenias and palm trees???  This was a nice neighborhood before your crowd came along).

Late October:

In mid- to late-October, temperatures finally fell, with rain and cooler, overcast days heralding the coming of winter.  Centrally, the lake cleared, but a green, soupy suspension of tiny, green knobby masses around the lake edge showed that the bloom had not entirely given up:

Bloom Colonies at Lake’s Edge
Bloom Colonies, White Background
Bloom Colonies and Duckweed, White Background, ~4X

Microscopy revealed that, with the exception of rare, fragmented organisms, Anabaena and Nostoc had entirely disappeared, the cooler temperatures favoring an almost pure culture of Microcystis, forming knobby balls, sheets, and at times, exotic ringed colonies:MICROCYSTIS COLONY 02 CE

Microcystis, approx. 40X, Various Colony Shapes

Higher powers showed the typical small vegetative cells of Microcystis, surrounded by a greenish mucilaginous matrix:

Microcystis, approx. 400X



Connecticut College Biology Notes.  “Cyanobacteria: Nostoc.”

Crayton, M. A. Quoted by Department of Ecology, State of Washington, Algae Control Program, “Anabaena sp. Identification”.

Komarek, J. and Komarkova, J. Review of the European Microcystis-morphospecies (Cyanoprokaryotes) from Nature.  Czech Phycology, Olomouc, 2: 1-24, 2002.

MicrobeWiki, Kenyon College.  “Anabaena.”

MicrobeWiki, Kenyon College.  “Microcystis aeurginosa.”

Welker, M. et al.  “Diversity and distribution of Microcystis (Cyanobacteria) oligopeptide chemotypes from natural communities studied by single-colony mass spectrometry.”  Microbiology. 2004 Jun;150(Pt 6):1785-96.

Wikipedia.   “Anabaena.”

Wikipedia. “Microcystis aeruginosa.”

Somenos Lake: Algal Bloom

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