Aquatic angiosperms are derived from terrestrial ancestors and appear to have re-invaded water on many occasions. While removing problems of water supply and reducing the need for supporting tissue, freshwaters have a potentially low and fluctuating supply of CO2 for photosynthesis, as well as generally low light. This paper reviews the structural, morphological, physiological, and biochemical features of freshwater macrophytes in the context of maximising net carbon uptake underwater, and discusses how inorganic carbon may influence macrophyte ecology. Submerged leaves tend to have a low photosynthetic capacity on an area basis, matching the low rates of supply of CO2 and light. Morphological and structural strategies to overcome potential carbon limitation include possession of aerial or floating leaves, and lacunal connexions to high concentrations of sedimentary CO2 via the roots. Physiological and biochemical strategies include crassulacean acid metabolism, C4-like metabolism in Hydrilla and Egeria, and the ability to use HCO3–. The activity of all these can be regulated by environmental conditions to maximize growth rate. Use of HCO3–. is the most widespread carbon acquisition strategy, present in about half of the tested submerged angiosperms. It is more common in lakes of high alkalinity and in the elodeid growth form.

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1. The relative contribution of roots and leaves to nutrient uptake by submerged stream macrophytes was tested in experiments where plants were grown in an outdoor flow‐channel system. Water was supplied from a nutrient‐rich stream with inorganic nitrogen and phosphorus concentrations typical of Danish streams. 2. Four submerged macrophyte species were tested, Elodea canadensis, Callitriche cophocarpa, Ranunculus aquatilis and Potamogeton crispus, and all species were able to satisfy their demand for mineral nutrients by leaf nutrient uptake alone. This was evident from manipulative experiments showing that removal of the roots had no negative impact on the relative growth rate of the plants. Further, the organic N and P concentrations of the plant tissue was constant with time for the de‐rooted plants. 3. Enrichment of water and/or sediment had no effect on the relative growth rate of two species, E. canadensis and C. cophocarpa, indicating that in situ nutrient availability was sufficient to cover the needs for growth. Despite the lack of a response in growth rate, a reduced root/shoot biomass ratio was observed with nutrient enrichment of water and/or sediment, and an increased tissue‐P concentration in response to open‐water enrichment. 4. The open‐water nutrient concentrations of the stream in which the experiments were performed are in the upper part of the range found for Danish farmland streams (the majority of Danish streams). Still, however, the negligible effect of nutrient enrichment on the growth of submerged macrophytes observed suggests that mineral nutrient availability might play a minor role in controlling macrophyte growth in most Danish streams.

Summary• Both roots and leaves of free-floating plants can potentially take up nutrients. In this study, the ability and relative contribution of roots and fronds for N uptake by the floating macrophyte Lemna minor was investigated.• The NH 4 + and NO 3 -uptake kinetics of roots and fronds were measured on plants acclimated to three different NH 4 NO 3 concentrations.• Lemna had the capacity to take up NH 4 + and NO 3 -through both roots and fronds; uptake kinetics for the two tissue types were comparable on an area basis. The overall contribution of root and frond to whole-plant uptake, estimated from measured kinetic characteristics, varied depending on plant N status (the root contribution increased from 32 to 73% for N-satiated and N-depleted plants, respectively).• The shift in the balance between root and frond contribution to whole-plant uptake resulted from a 1.5 -38 times greater increase in the area-specific uptake capacity and affinity of roots relative to fronds, combined with a larger decrease in the minimum concentration for uptake (C min ) for roots than fronds. At the morphological level, root-frond surface area increased with declining N supply, which might be beneficial to the plants since the area return per unit biomass invested was nine times greater for roots than for fronds.

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