Experimental and field studies were conducted to evaluate the effects of NH4+ enrichment on growth and distribution of the submersed macrophyte, Vallisneria natans L, in lakes of the Yangtze River in China, based on the balance between free amino acids (FAA) and soluble carbohydrates (SC) in the plant tissue. Increase of NH4+ rather than NO3– concentrations in the water column caused FAA accumulation and SC depletion of the plant. The plant showed a unimodal pattern of biomass distribution along both FAA/SC ratios and external NH4+ concentrations, indicating that a moderate NH4-N concentration (<0.3 mg L–1) benefited the plant, whereas the high NH4-N concentration (>0.56 mg L–1) eliminated the plant completely. Therefore, 0.56 mg NH4-N mg L–1 in the water column was taken as the upper limit for V. natans in lakes of the Yangtze River basin. The mesocosm experiment showed that at a high external NH4-N (0.81 mg L–1), V. natans failed to propagate with a loss of half SC content (5 mg g–1 DW) in the rhizomes, indicating that the consumption of carbohydrates for detoxification of excess NH4+ into non-toxic FAA significantly diminished carbohydrate supply to the rhizomes. This might consequently inhibit the vegetative reproduction of the plant, and also might be an important cause for the decline and disappearance of the plant with eutrophication. The present study for the first time reports substantial ecophysiological evidences for NH4+ stress to submersed macrophytes, and indicates that NH4+ toxicity arising from eutrophication probably plays a key role in the deterioration of submersed macrophytes like V. natans.

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In sheltered, eutrophicated estuaries, reduced nitrogen (NH x ), and pH levels in the water layer can be greatly enhanced. In laboratory experiments, we studied the interactive eVects of NH x , pH, and shoot density on the physiology and survival of eelgrass (Zostera marina). We tested long-term tolerance to NH x at pH 8 in a 5-week experiment. Short-term tolerance was tested for two shoot densities at both pH 8 and 9 in a 5-day experiment. At pH 8, eelgrass accumulated nitrogen as free amino acids when exposed to high loads of NH x , but showed no signs of necrosis. Low shoot density treatments became necrotic within days when exposed to NH x at pH 9. Increased NH 3 intrusion and carbon limitation seemed to be the cause of this, as intracellular NH x could no longer be assimilated.Remarkably, experiments with high shoot densities at pH 9 showed hardly any necrosis, as the plants seemed to be able to alleviate the toxic eVects of high NH x loads through joint NH x uptake. Our results suggest that NH x toxicity can be important in worldwide observed seagrass mass mortalities. We argue that the mitigating eVect of high seagrass biomass on NH x toxicity is a positive feedback mechanism, potentially leading to alternative stable states in Weld conditions.

Radial oxygen losses (ROL) from the roots of three Rumex species, that occur in the river ecosystem in The Netherlands and show a differential response towards flooding, were compared. Oxygen loss from whole root systems was demonstrated and the ROL of single roots was quantified. Radial oxygen losses were higher in the floodtolerant R. maritimus and R. crispus than in the intolerant R. thyrsiflorus. In all species oxygen loss occurred over the whole root surface between the base and the apex, but the rates differed as well as root wall permeabilities to oxygen. High oxygen losses in R. maritimus and R. crispus were correlated with high internal oxygen pressures near the root apex, consistent with prolonged root growth under anaerobic conditions in these species. On a flooded clay soil, the more tolerant species showed soil penetration and iron oxidation to greater depths, but all species developed an iron plaque on the roots. Shoot iron content was highest in the flood-tolerant R. maritimus. Upon flooding of the flood-intolerant R. thyrsiflorus, however, there was a substantial decrease in shoot dry weight and tissue nutrient levels. This was attributed to restricted root development rather than to iron toxicity. toxicity.

The relationship between the nitrogen and the free amino acid content in Stratiotes aloides L. was investigated for 11 healthy stands in the Netherlands. In the shoots, the mean free asparagine and free arginine levels were strongly correlated with the mean total N concentrations and the ammonium concentrations in the water layer. The percentage of total N present as free amino acids varied from 5.4 % to 27.5 % between the stands. In the stands with the highest total N levels, up to 92 % of the nitrogen in the free-amino acid pool consisted of asparagine and arginine. The results reveal that the accumulation of these "N-efficient" free amino acids was caused by increased ammonium availability in the environment. The accumulation of specific free amino acids is regarded as an early indication for N-stress in still healthy S. aloides stands.

1. Softwater lakes are generally dominated by slow growing, small, isoetid plant species that are adapted to the carbon-and nutrient-limited conditions in these lakes. We investigated the strategy of a fast growing species, Sparganium angustifolium, for occupying softwater lakes. A field survey was carried out in Norwegian carbon-limited Isoëteto-Lobelietum softwater lakes to compare abiotic conditions at locations with and without S. angustifolium. In addition, long term abiotic changes (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) related to the sudden establishment of the species on experimentally limed plots were studied. Based on the results, the carbon acquisition mechanism of S. angustifolium was tested in eco-physiological laboratory experiments. 2. The redox potential was significantly lower at locations with S. angustifolium (220 ± 2.3) compared to locations without S. angustifolium (338.1 ± 13.9). The lower redox potential was accompanied by significantly higher concentrations of HCO 3 ) , CO 2 and Fe 2+ in the sediment pore water, indicating in-lake alkalinity generation due to higher iron reduction rates in the generally iron-rich sediments. In addition, the lower redox potential was accompanied by a higher nutrient availability (NH 4 + and PO 4 3) ) in the sediment pore water. Since there were no differences in water quality between the lakes, the ability of S. angustifolium to grow in softwater lakes very likely depends upon the higher dissolved inorganic carbon (DIC) and nutrient concentrations present in the sediment pore water. 3. Results from the liming experiment revealed that appearance of S. angustifolium on limed plots was related to the dissolution of Ca and Mg carbonates and development of a lower redox potential in the sediment. These processes were accompanied by a sustained increase in the availability of DIC in the sediment pore water. 4. The eco-physiological experiments indicated that S. angustifolium can increase in biomass and produce floating leaves at a relatively high DIC availability in the root medium. In addition, it appeared that S. angustifolium can take up CO 2 by the roots. As far as we know, the ability to use sediment CO 2 has only been described as an adaptation typical for isoetid plant species. Use of the relatively large sediment CO 2 pools present in these sediment types (>1000 lmol L )1 ) to enable development of long

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