Benthic Green Algae Community

Once a tank's brown-diatom phase fades, the green takes over — the soft green fuzz and slick that build up on rocks, glass, and gravel as a tank matures. That is the benthic green algae community: the surface-dwelling green algal film of taxa like Stigeoclonium, Ulothrix, epilithic Chlorococcales, Oocystis, and Chaetophora. These are small cells and filaments that lay down an exopolysaccharide (EPS) matrix and build stratified biofilms across submerged surfaces. They are the silica-independent successor to the diatoms — and one of the most ordinary, stable features of a settled aquarium.

A floater pool feeding an attached community

Although most of this community lives stuck to surfaces, it keeps a small floating (dispersal) pool of cells that drift through the water column to colonise new surfaces and, along the way, feed filter feeders such as Daphnia, rotifers, and copepods. That floating pool stays small at equilibrium because the algae settle out of it quickly — their EPS makes them far stickier than free-living unicellular greens — but it swells after a grazing event strips the surfaces and the survivors re-seed the water. At rest, the great majority of the biomass is on surfaces and only a small fraction is adrift. The community also spreads sideways across adjoining surfaces through EPS-mediated creep that never passes through the water column at all.

Growth, light, and the succession order

Benthic green algae grow a little faster than pennate diatoms but a little slower than the fast centric diatoms — roughly one and a half doublings a day at a warm optimum. That near-match to the diatoms is deliberate and ecologically important: early-tank diatom dominance is not meant to come from a big growth-rate gap. It comes from the diatoms' better adhesion, their access to plentiful silica, and their shade tolerance. Benthic green algae are more light-demanding than the deeply shade-adapted pennates, so in the dim conditions of a new tank the diatoms hold the surfaces. The greens overtake them only after dissolved silica runs down and the diatoms lose their silicon-dependent edge — the correct succession order seen on real surfaces (Biggs 1996; Stevenson 1996). When cells die, a portion of the biomass dissolves to labile DOM and the rest splits between suspended and settled detritus.

How they inherit the surfaces

Like the diatoms, benthic green algae grow inside a thin diffusion boundary layer against each surface, where nitrogen is richer than in the open water. The model builds an enriched perceived concentration for each surface from three sources: mineralization of settled detritus by bacteria on the surface; proximity to pore water on sand or soil, where upward-diffusing ammonium is intercepted before it mixes away; and local nitrification by nitrifying bacteria within the biofilm. Each surface is enriched on its own, and the enrichment raises only the nitrogen the cells perceive — actual uptake still draws from the bulk water, so mass balance holds.

This boundary-layer enrichment is the key to how green algae replace diatoms after the silica crash. When diatoms die and vacate a surface, the detritus they leave behind actually raises the local nitrogen enrichment as it decomposes. Green algae moving onto that surface inherit a nutrient-rich microenvironment with no silica requirement to satisfy — the very same biofilm mechanism that fed the diatom film now feeds its successor, minus the limitation that crashed the diatoms.

Every surface is its own world

Growth is computed for each surface separately, not as one lumped film, because each presents a different microenvironment:

• Light. Light is attenuated down through the water column (Beer-Lambert absorption by floating algae, refractory DOM, and background turbidity) before each surface takes its share at its own depth — a shallow glass wall gets far more than a deep sand bed. Unlike the diatoms, these greens don't add extra mat self-shading within their film.
• Nutrient enrichment. Each surface carries its own enrichment, so a sand bed over soil can run nitrogen-rich while a glass wall far from the substrate stays lean.
• Carrying capacity. Each surface holds only so much before crowding throttles growth (carrying capacity). Because these cells are tiny, they find more usable area on rough or porous surfaces such as ceramic and sand — microscale crevices count as extra attachment area — than on smooth glass. This caps the biomass any one surface can carry and sets up competition for space between greens and diatoms.

Because growth is per-surface, the community can sit in quite different states on different surfaces at once — thriving on a nutrient-rich sand bed while barely present on a light-starved wall. It also means that when diatoms crash on one surface from silica depletion, green algae can move onto that specific surface while both still coexist elsewhere.

Further reading

• Pennate Diatom Community — the brown film these greens succeed once silica runs out
• Planktonic Green Algae — the free-floating green cousins of the open water
• Silicon Cycle — the silica crash that hands the surfaces from diatoms to greens
• Biofilm Nutrient Enrichment — the boundary-layer enrichment that lets surface films grow when the open water runs lean
• Algae Settlement and Surface Dynamics — how cells move between the water column and surfaces
• Producers — how all the algae and plants fit together
• Parameter Reference — every growth rate, light half-saturation, and attachment rate behind this page, with citations

Key references

• Biggs, B.J.F. (1996). Patterns in benthic algae of streams. In R.J. Stevenson, M.L. Bothwell & R.L. Lowe (eds.), Algal Ecology: Freshwater Benthic Ecosystems. Academic Press.
• Hoagland, K.D., Roemer, S.C. & Rosowski, J.R. (1982). Colonization and community structure of two periphyton assemblages, with emphasis on the diatoms. American Journal of Botany 69, 188–213.
• Stevenson, R.J. (1996). Algal Ecology: Freshwater Benthic Ecosystems. Academic Press.