Freshwater isoetids exchanges a high proportion of the photosynthetically produced oxygen over the extensive root system and, therefore, they influence the redox potential (Eh) and phosphorus (P) availability in their sediments. Because isoetids rely on the sediment for P uptake, P may be a key element in controlling the distribution of isoetids. We investigated biomass and P availability to isoetids (Littorella uniflora and Isoetes lacustris) in a transect of five stations across the littoral zone in oligotrophic Lake Kalgaard, Denmark. At the two shallowest stations (0.6 and 1.0 m depth) the redox potential in the low organic rhizosphere sediment was high (>300 mV) and low concentrations of reduced exchangeable iron (Fe) and manganese (Mn) compounds in the sediment and of precipitated Fe and Mn oxides on isoetid roots (plaques) were found. The concentration of sediment P pools was low and so was isoetid P content and isoetid biomass. At intermediate water depth (1.8 m) sediment Eh was high (∼300 mV) and isoetids showed low root plaque concentrations. However, higher concentration of P pools in the rhizosphere was found at 1.8 m and isoetids showed the highest P content and biomass. At deeper stations (2.8 and 4.6 m depth) Eh was low (<100 mV) in the high organic rhizosphere and high concentrations of plaques were found. The P content in the sediment was high, however, isoetids showed low biomass and low P content. We suggest that the low P content in isoetids growing on P rich organic sediments is partly due to inhibition of the P uptake because of adsorption of P to the oxidized Fe and Mn plaques. However, ratios between oxidized Fe and Fe-bound P, 150 for plaques and 40 for sediment, suggest the isoetids are able to access some of the P that is bound in the plaques. The pools of dissolved P in the porewater were 25–1100 times lower than the estimated annual P requirement for net growth of isoetids while solid fraction P pools were 20–260 times higher than the estimated annual P requirement. Clearly, the oxygen release from isoetid roots decreases the availability of P either by keeping the entire rhizosphere oxidized (low organic sediments) or by the formation of root plaques (high organic sediments).