Planktonic Green Algae Community
When a tank's water takes on a green tint — the classic "green water" that clears your view of the back glass — this is what is suspended in it. Planktonic green algae are the mixed community of small, single-celled green algae that dominate the open water of temperate freshwater: taxa like Scenedesmus, Desmodesmus, Ankistrodesmus, Chlorella, Monoraphidium, and Pediastrum. They are tiny, spherical to spindle-shaped cells that divide quickly — the fastest-growing floating algae in the model, capable of close to two doublings a day at a warm optimum near 28 °C.
Fast, soft, and warm-loving
Speed is their whole strategy. Given light, warmth, and nutrients they bloom rapidly, which is why a fertile, brightly lit tank can go green in a matter of days. They have no structural defence — no shell, no toxin, no slime scaffold — so their small, soft cells are excellent food for filter-feeding grazers like Daphnia, which can crop a green-water bloom hard once their numbers build. This makes planktonic green algae a cornerstone of the open-water food web: they convert light and dissolved nutrients into grazer-edible biomass faster than anything else in the water column.
Their warmth preference also sets the timing of when they matter. In the cool conditions of a freshly set-up tank, cold-adapted centric diatoms hold the water column; as the tank settles to its running temperature the green algae come into their own and tend to take over the planktonic niche.
What they need and what they leak
These greens carry a moderate carbon concentrating mechanism — enough to use a useful share of the water's bicarbonate on top of dissolved CO₂, though far less capably than cyanobacteria or strong bicarbonate-using plants. Like all algae they excrete a small fraction of their fixed carbon as dissolved organics, feeding the heterotrophic bacterial loop, but they invest little in it — they are spendthrift growers, not scaffold-builders. They are a freshwater community with limited salt tolerance, comfortable in nearly fresh water and stressed as it turns brackish.
Because the cells are unicellular and unsticky, they attach only weakly to surfaces and are easily dislodged, but they also disperse readily — a substantial fraction of any surface growth drifts off to colonise elsewhere rather than staying put. When cells die, some of the floating remains break up into fine suspended particles while the rest clumps and settles; dead surface-attached cells mostly slough off and sink. They tolerate a wide span of temperature, from near-freezing to the low forties Celsius, and a broad pH range, with a base mortality of a few percent a day. The exact growth rate, temperature and pH limits, and mortality are tabulated in the Parameter Reference.
Further reading
- Centric Diatom Community — the cold-water competitor that holds the open water in a new tank
- Benthic Green Algae — the surface-attached green cousins that coat rocks and glass
- Producers — how all the algae and plants fit together
- Food Web — how green algae feed the open-water grazers
- Parameter Reference — the growth rate, thermal and pH limits, and mortality behind this page, with citations
Key references
- Griffiths, M.J. & Harrison, S.T.L. (2009). Lipid productivity as a key characteristic for choosing algal species for biodiesel production. Journal of Applied Phycology 21, 493–507.
- Mayo, A.W. (1997). Effects of temperature and pH on the kinetic growth of unialgal Chlorella vulgaris cultures. Bioresource Technology.
- Becker, E.W. (2007). Micro-algae as a source of protein. Biotechnology Advances 25, 207–210.