Summary• Lobelia dortmanna thrives in oligotrophic, softwater lakes thanks to O 2 and CO 2 exchange across roots and uptake of sediment nutrients. We hypothesize that low gas permeability of leaves constrains Lobelia to pristine habitats because plants go anoxic in the dark if O 2 vanishes from sediments.• We added organic matter to sediments and followed O 2 dynamics in plants and sediments using microelectrodes. To investigate plant stress, nutrient content and photosynthetic capacity of leaves were measured.• Small additions of organic matter triggered O 2 depletion and accumulation of NH 4 + , Fe 2+ and CO 2 in sediments. O 2 in leaf lacunae fluctuated from above air saturation in the light to anoxia late in the dark in natural sediments, but organic enrichment prolonged anoxia because of higher O 2 consumption and restricted uptake from the water. Leaf N and P dropped below minimum thresholds for cell function in enriched sediments and was accompanied by critically low chlorophyll and photosynthesis.• We propose that anoxic stress restricts ATP formation and constrains transfer of nutrients to leaves. Brief anoxia in sediments and leaf lacunae late at night is a recurring summer phenomenon in Lobelia populations, but increased input of organic matter prolongs anoxia and reduces survival.

* High radial oxygen loss (ROL) from roots of aquatic plants to reduced sediments is thought to deplete the roots of oxygen and restrict the distribution of those species unable to form a barrier to oxygen loss. Metal precipitates with high iron content (Fe-plaques) frequently form on roots of aquatic plants and could create such a diffusion barrier, thereby diverting a larger proportion of downward oxygen transport to the root meristems. * To investigate whether Fe-plaques form a barrier to oxygen loss, ROL and internal oxygen concentrations were measured along the length of roots of the freshwater plant Lobelia dortmanna using platinum sleeve electrodes and Clark-type microelectrodes. * Measurements showed that ROL was indeed lower from roots with Fe-plaques than roots without plaques and that ROL declined gradually with thicker iron coating on roots. The low ROL was caused by low diffusion coefficients through root walls with Fe-plaques resulting in higher internal oxygen concentrations in the root lacunae. * By diverting a larger proportion of downward oxygen transport to root meristems in L. dortmanna, the presence of Fe-plaques should diminish root anoxia and improve survival in reduced sediments.

1. The cyanobacterial genus Nostoc includes several species forming large spherical or sheet-like gelatinous colonies of ecological importance in freshwater and semi-terrestrial habitats. We tested differences in morphology, growth and metabolism across a range of temperatures (6-43°C) of three colonial species of Nostoc collected and grown in ambient water from typical localities from Denmark and Sweden: the plume-shaped Nostoc pruniforme, the rare blackberry-shaped Nostoc zetterstedtii and the semi-terrestrial, sheet-like Nostoc commune. 2. All three species grew and photosynthesised in water between 6 and 33°C in our experiments but died at 43°C. The optimum temperature of around 25°C and the critical temperature of around 33°C are markedly higher than mean and maximum temperatures in the majority of habitats of N. pruniforme and N. zetterstedtii, suggesting that their distribution does not reflect temperature preference directly. Although N. commune survives deep-freezing and heating to 70°C, sustained growth was restricted to 0-33°C. 3. Maximum growth rates were relatively high for N. pruniforme and N. commune (doubling time of 13-15 days) and extremely low for N. zetterstedtii (doubling time of 2.4 years). Rates of photosynthesis, respiration and growth were markedly higher in alkaline water than in softwater, but N. pruniforme still grew much faster than N. zetterstedtii in the same water. Declines of growth and photosynthetic rates with increasing ratio of dry weight to surface area between the species and with increasing colony size reflect higher respiratory costs relative to resource uptake. 4. We conclude that the three Nostoc species have similar temperature tolerance from 6 to 43°C, despite large interspecific differences in rates of growth and photosynthesis, colony persistence and distribution.

Summary 1. Littorella uniflora and Lobelia dortmanna are prominent small rosette species in nutrient‐poor, soft‐water lakes because of efficient root exchange of CO2 and O2. We hypothesise that higher gas exchange across the leaves of L. uniflora than of L. dortmanna ensures O2 uptake from water and underlies its greater tolerance to sediment anoxia following organic enrichment. 2. We studied plant response to varying sediment O2 demand and biogeochemistry by measuring photosynthesis, gas exchange across leaves and O2 dynamics in plants during long‐term laboratory and field studies. Frequent non‐destructive sampling of sediment pore water was used to track changes in sediment biogeochemistry. 3. Addition of organic matter triggered O2 depletion and accumulation of , Fe2+ and CO2 in sediments. Gas exchange across leaf surfaces was 13–16 times higher for L. uniflora than for L. dortmanna. Oxygen in the leaf lacunae of L. uniflora remained above 10 kPa late at night on anoxic sediments despite organic enrichment. Leaf content of N and P of L. uniflora remained sufficient to keep up photosynthesis despite prolonged sediment anoxia, whereas nutrient content was too low for long‐term survival of L. dortmanna. 4. High gas exchange across L. uniflora leaves improves its performance and survival on anoxic sediments compared with L. dortmanna. Lobelia dortmanna uses the same gas‐tight leaves in air and water, which makes it highly susceptible to sediment anoxia but more cost‐effective in ultra‐oligotrophic environments because of slow leaf turnover.

Summary 1. Arbuscular mycorrhizal fungi (AMF) commonly colonise isoetid species inhabiting oxygenated sediments in oligotrophic lakes but are usually absent in other submerged plants. We hypothesised that organic enrichment of oligotrophic lake sediments reduces AMF colonisation and hyphal growth because of sediment O2 depletion and low carbon supply from stressed host plants. 2. We added organic matter to sediments inhabited by isoetids and measured pore‐water chemistry (dissolved O2, inorganic carbon, Fe2+ and ), colonisation intensity of roots and hyphal density after 135 days of exposure. 3. Addition of organic matter reduced AMF colonisation of roots of both Lobelia dortmanna and Littorella uniflora, and high additions stressed the plants. Even small additions of organic matter almost stopped AMF colonisation of initially un‐colonised L. uniflora, though without reducing plant growth. Mean hyphal density in sediments was high (6 and 15 m cm−3) and comparable with that in terrestrial soils (2–40 m cm−3). Hyphal density was low in the upper 1 cm of isoetid sediments, high in the main root zone between 1 and 8 cm and positively related to root density. Hyphal surface area exceeded root surface area by 1.7–3.2 times. 4. We conclude that AMF efficiently colonise isoetids in oligotrophic sediments and form extensive hyphal networks. Small additions of organic matter to sediments induce sediment anoxia and reduce AMF colonisation of roots but cause no apparent plant stress. High organic addition induces night‐time anoxia in both the sediment and the plant tissue. Tissue anoxia reduces root growth and AMF colonisation, probably because of restricted translocation of nutrient ions and organic solutes between roots and leaves. Isoetids should rely on AMF for P uptake on nutrient‐poor mineral sediments but are capable of growing without AMF on organic sediments.