1. In extremely acid mining lakes, benthic filamentous green algae (Zygnemataceae, Chlorophyta) thrive as effective competitors for limited carbon (C). These algae could supply C for microbial-mediated benthic alkalinity generation. However, biomass, productivity and impact of the acidobiontic filamentous green algae at pH ≤3 have not previously been determined.

2. Periphytic filamentous green algae was mapped by harvesting their biomass from 85 1 × 1 m quadrats in mining lake Grünewalder Lauch. Zygogonium ericetorum colonised water depths between 1.6 and 10.5 m covering 88% of total area. Biomass peaked at 5–6 m depth. Total Zygogonium biomass amounted to 72.2 t dry weight for the whole lake (0.94 km²), which corresponds to 16.1 t C and the accumulation of primary production from 2.2 years.

3. Growth of Zygogonium is moderately N, C and extremely P deficient, and seriously stressed by high rates of Fe deposition during summer. Consequently, net primary production (NPP) of Zygogonium, calculated from measured photosynthesis versus irradiance characteristics and calculated underwater irradiance (0.13 g C m⁻² year⁻¹) and in situ oxygen measurements (7.8 g C m⁻² year⁻¹), corresponds to only 0.3% and 18.1% of pelagic NPP.

4. Neither pelagic nor benthic Zygogonium primary production can supply enough C for efficient acidity removal. However, at rates of benthic NPP in summer of 21.4 mg C m⁻² day⁻¹, Zygogonium contributed 26% of the C equivalents to remove acidity associated with ferric iron, contributing at least seasonally to efficient alkalinity generation.