This paper synthesises insights and offers new quantitative analyses of data gathered during 40 years of stream research by Danish researchers and international associates. Lowland Danish streams mostly drain fertile cropland and contain high nutrient concentrations saturating maximum yield and growth rate of plants. Concentrations of carbon dioxide (CO₂) are variable, although usually high, supporting a range of wetland and permanently submerged species. All terrestrial and most amphibious species are obligate CO₂ users, while the majority of permanently submerged species can supplement their inorganic carbon demand with bicarbonate. In lake outlets, the average CO₂ concentration was close to air saturation during summer, whereas sites with no lake influence were 9-fold supersaturated. The 20% increase of atmospheric CO₂ concentrations over the past 40 years has marginal influence on CO₂ concentrations in the streams. In lake outlets with low CO₂ concentrations, calculations on 33 stream species showed essentially no underwater photosynthesis by temporarily submerged terrestrial species, low rates by amphibious species, and high rates by permanently submerged bicarbonate users. Underwater photosynthetic rates increased at sites with high CO₂ concentrations (no lake influence): they were lowest for terrestrial wetland species (mean 1.8 mg O₂ g dry weight⁻¹ hr⁻¹), followed by homophyllous (3.0) and heterophyllous amphibious species (5.6), and highest among permanently submerged species (15.0). Terrestrial and amphibious species grew very slowly when CO₂ levels were low, but rapidly in CO₂ rich water, or when in contact with air above the water's surface. Decreasing CO₂ concentrations from upstream to downstream caused lower photosynthesis rates of amphibious species, while photosynthesis by bicarbonate users was consistently high. The relative abundance of terrestrial and amphibious species decreased significantly as CO₂ resources decline from upstream to downstream, while the abundance of permanently submerged species increased as streams progressed. However, plant abundance as a function of CO₂ concentrations did not differ markedly among the plant groups. We conclude that photosynthesis and growth of species of different plant types under controlled experimental conditions resembling high in situ nutrient availability are closely related to inorganic carbon supplies, while their representation in plant assemblages is influenced by the spatially and temporally highly diverse ecological conditions from upstream to downstream in the mostly CO₂-rich lowland streams. Submerged terrestrial and amphibious species restricted to CO₂ use for photosynthesis can partly escape the slow gas diffusion under water by growing in very CO₂-rich streams and having apical leaves in contact with air. Thus, strong restrictions on their growth and existence should be identified at sites where CO₂ concentrations are consistently low and no air contact is possible, while co-limitation by nutrients is likely to be marginal. Generally, rising atmospheric CO₂ is irrelevant to photosynthesis of permanently submerged plants because of their ability to use bicarbonate and the high CO₂ supersaturation in most stream sites. In contrast, photosynthesis and growth of amphibious and terrestrial plants under water are highly dependent on CO₂ concentrations varying from close to air saturation in lake outlets to supersaturation in headwaters.