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. This is the crucial difference between a benthic alga and a true planktonic one, and the two are mirror images: for free-floating phytoplankton the water column is the home where they grow and bloom, and settling out is a loss; for a benthic alga the water column is the opposite — a transient dispersal and loss compartment that cells pass through on their way to a new surface or to death and detritus. Those drifting cells are alive, but they are propagules in transit, not a blooming open-water population: stripped of the biofilm microenvironment and drifting through ever-changing light, they barely photosynthesise and either re-settle within hours or die. So this floating pool stays genuinely small — it never builds into a green-water bloom the way phytoplankton do. (Earlier versions of the model let it photosynthesise like a free-living phytoplankter, which let it bloom and tint the water green — a benthic alga masquerading as plankton; it is now correctly held as a small transient.) The community also spreads sideways across adjoining surfaces through EPS-mediated creep that never passes through the water column at all.

Because almost all the growth happens on surfaces, the standing crop of benthic green algae is set by how good a home those surfaces are — their light, their nutrients, and how hard grazers crop them. The EPS-bound film is retentive: in still aquarium water (no flow-driven scour) it clings tightly, losing biomass mainly to slow senescence and grazing rather than continuously shedding into the water. This is what lets a thin film hold its place on a well-lit wall on its own surface growth. The flip side is that where surfaces are dim — under a dense floating canopy of duckweed, or shaded by tall stem plants — benthic green algae are genuinely light-starved and stay sparse. That is the heavily-planted tank working as intended: the plants win the competition for light, and the glass stays clean. A bright, open, lightly-planted tank is where the green film flourishes.

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. A flat bed like sand sits at one depth and gets the light at that depth, but a wall is different: glass runs the full height of the tank, so the film on it sees the average light over that whole span — and because light fades exponentially with depth, that average is dominated by the bright strip near the waterline. A wall is therefore much better lit than its mid-depth alone would suggest, and is the place a film holds on best in a shaded tank. 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.