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Letter to the Editor Re: Yu Qing, Wang Hai-Jun, Wang Hong-Zhu, Li Yan, Liang Xiao-Min, Xu Chi, Jeppesen Erik Does the responses of Vallisneria natans (Lour.) Hara to high nitrogen loading differ between the summer high-growth season and the low-growth season? Science of the Total Environment 601-602(2017)1513-1521

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Abstract

Yu et al.'s paper showed very interesting effects of high nitrogen (N) on the submerged macrophytes Vallisneria natans: active growth in the growing season enabled the macrophytes partly to overcome the ammonium stress. This result was evident in an experiment using ten pond ecosystems; however, their conclusion that shading induced by high phytoplankton biomass together with the toxicity of high ammonium contributed to the decrease of macrophytes growth was not strongly supported by the data provided in the paper. Three factors influencing how submerged macrophytes respond to high ammonium, not addressed by Yu et al.'s paper, are toxicity of ammonium/ammonia (NH4(+)/NH3), the precise extent of shading in water and species-specific characteristics of macrophytes. In conclusion, a comprehensive consideration of abiotic and biotic factors that involve in the responses of submerged macrophytes to high N is urged in future studies of the role of high N on the growth of submerged macrophytes.

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... We thank for reflecting on our result . Cao et al. (2017) find that our conclusion that shading effects induced by high phytoplankton biomass together with toxicity of high ammonium contributed to the loss of submersed macrophytes is debatable. They argue that species-specific features of macrophytes, pH and light attenuation scenarios should have been included in the study of the influence of high N on submerged macrophytes. ...
... We agree with Cao et al. (2017) that the effects of high ammonium on submersed macrophytes are species specific. Ammonium-related physiological stress on submersed macrophytes depends on light condition, for instance, and is aggravated at low light as suggested by Cao et al. (2011). ...
... With regard to the toxicity of ammonia (NH 3 )/ammonium (NH 4 + ) caused by pH, Cao et al. (2017) argue that toxicity of NH 3 and NH 4 + is pH dependent and that the toxicity of NH 3 is not negligible in our experiment. We agree. ...
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1. While phosphorus (P) is often considered the most important growth limiting factor for plants in lakes, recent studies of shallow lakes indicate that nitrogen (N) may be of greater importance than realized hitherto and that submerged macrophytes may be lost when the N concentration exceeds a certain threshold, as long as the concentration of P is sufficiently high. 2. We studied the effects of different loadings of NH4-N and NO3-N on chlorophyll a and on a macrophyte tolerant of eutrophication, Vallisneria spinulosa (Hydrocharitaceae). In outdoor mesocosms we used water from a pond as control and created four levels of NH4-N and NO3-N (approximately 2.5, 5, 7.5 and 10 mg L−1) by dosing with NH4Cl and NaNO3, respectively. After the experiment, the plants were transferred back to a holding pond to study their recovery. In contrast to previous research, we used a low background concentration of phosphorus (TP 0.024 ± 0.003 mg L−1) so we could judge whether any effects of N were apparent when P is in short supply. 3. Chlorophyll a increased significantly with N dosing for both forms of N, but the increase was highest in the NH4-N dosed mesocosms (maximum 58 μg L−1 versus 42 μg L−1 in the NO3-N mesocosms), probably due to a higher total inorganic N concentration (part of the added ammonia was converted to nitrate during the experiment). 4. Although the number of ramets of V. spinulosa was not affected by the N treatment, the biomass increased up to concentrations of 7.5 mg L−1, while biomass at 10 mg L−1 remained at the control level for both N ions treatments. A similar pattern was apparent for the content of N and soluble sugar of the plant, while there were no differences in the plant P content among treatments. Five months after transplantation back to the pond no difference was found in the number of ramets or in biomass, except that the biomass of plants grown at 10 mg N L−1 during the experiment was greater than that in the control, while the N and P contents of plants were similar to those of the controls. 5. Nitrogen concentration had little influence on the growth of the eutrophication tolerant submerged macrophyte at moderately low concentrations of phosphorus. Moreover, the two N ions showed no toxic effects, suggesting that loss of macrophytes observed in other studies, run at higher phosphorus concentrations, was probably related to enhanced shading by periphyton and/or phytoplankton rather than to any toxic effects of N.
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SUMMARY1Two pH electrodes and a thermistor were used to record conditions in the surface of Esthwaite Water every 15 min over a 12-month period. Combined with approximately weekly measurements of alkalinity they allowed inorganic carbon speciation to be calculated.2Large changes in pH from 7.1 to nearly 10.3, and hence in concentrations of inorganic carbon species, were measured over a year. Carbon speciation and pH varied on a diel, episodic and seasonal basis. Diel variation of up to pH 1.8 was recorded, although typical daily variation was between 0.03 and 1.06 (5 and 95 percentiles). Daily change in concentration of inorganic carbon varied between 4 and 63 mmol m-3 (5 and 95 percentiles).3During lake stratification, episodes of high pH, typically of 1–2 weeks' duration were interspersed with episodes of lower pH. These changes appeared to relate to the weather: e.g. low wind velocity, high pressure, low rainfall and high sunshine hours correlated with periods of high pH.4Seasonal progression of carbon depletion generally followed stratification and the development of high phytoplankton biomass. When the lake was isothermal, the phytoplankton biomass caused relatively small amounts of carbon depletion.5During autumn, winter and spring, the lake had concentrations of CO2* (free CO2) up to 0.12 mol m-3 which is nearly seven times the calculated atmospheric equilibrium concentration so the lake will accordingly be losing carbon to the atmosphere. In contrast, during periods of elevated pH the concentration of CO2* was reduced close to zero and the lake will take up atmospheric CO2. The rates of transfer between water and the atmosphere were estimated using a chemical equilibrium model with three boundary layer thicknesses. The calculations show that over a year the lake loses CO2 to the atmosphere with the current mean atmospheric level of 360 μmol mol-1, at between 0.28 and 2.80 mol m-2 yr-1. During elevated pH, rates of CO2-influx increased up to nearly tenfold as a result of chemical-enhancement by parallel flux of HCO-3. Input of CO2* to the lake from the catchment is suggested to be the main source of the carbon lost to the atmosphere.6The turnover time for CO2 between the air and water was calculated to be 1 year for the gross influx and 3.3 years for the net flux. These values are less than the average water residence time of 0.25 years, which indicates that over a year inflow from streams is a more important source of inorganic carbon than the atmosphere.7Influx of CO2 from the atmosphere was calculated to be roughly equivalent to between 1 and 4% of the rates of production in the water during mid-summer indicating that this source of inorganic carbon is not a major one in this lake.8Influx of CO2 from the hypolimnion was estimated on one occasion to be 6.9 10-9 mol m-2 s-1 using transfer values based on mass eddy-diffusion. These rates are equivalent to 23% of the rate of influx of CO2 from the atmosphere on this occasion which suggests that the hypolimnion is probably a small source of inorganic carbon to the epilimnion. The exception appears to be during windy episodes when pH is depressed. Calculations based on depth-profiles of CO2* and HCO-3 suggest that the measured changes in pH can be accounted for by entrainment of hypolimnetic water into the epilimnion.9The solubility product for calcite was exceeded by up to about sixfold which, although insufficient to allow homogeneous precipitation, may have allowed heterogeneous precipitation around algal particles.
Article
Major efforts have been made world-wide to improve the ecological quality of shallow lakes by reducing external nutrient loading. These have often resulted in lower in-lake total phosphorus (TP) and decreased chlorophyll a levels in surface water, reduced phytoplankton biomass and higher Secchi depth. Internal loading delays recovery, but in north temperate lakes a new equilibrium with respect to TP often is reached after <10–15 years. In comparison, the response time to reduced nitrogen (N) loading is typically <5 years. Also increased top-down control may be important. Fish biomass often declines, and the percentage of piscivores, the zooplankton:phytoplankton biomass ratio, the contribution of Daphnia to zooplankton biomass and the cladoceran size all tend to increase. This holds for both small and relatively large lakes, for example, the largest lake in Denmark (40 km2), shallow Lake Arresø, has responded relatively rapidly to a ca. 76% loading reduction arising from nutrient reduction and top-down control. Some lakes, however, have proven resistant to loading reductions. To accelerate recovery several physico-chemical and biological restoration methods have been developed for north temperate lakes and used with varying degrees of success. Biological measures, such as selective removal of planktivorous fish, stocking of piscivorous fish and implantation or protection of submerged plants, often are cheap versus traditional physico-chemical methods and are therefore attractive. However, their long-term effectiveness is uncertain. It is argued that additional measures beyond loading reduction are less cost-efficient and often not needed in very large lakes. Although fewer data are available on tropical lakes these seem to respond to external loading reductions, an example being Lake Paranoá, Brazil (38 km2). However, differences in biological interactions between cold temperate versus warm temperate-subtropical-tropical lakes make transfer of existing biological restoration methods to warm lakes difficult. Warm lakes often have prolonged growth seasons with a higher risk of long-lasting algal blooms and dense floating plant communities, smaller fish, higher aggregation of fish in vegetation (leading to loss of zooplankton refuge), more annual fish cohorts, more omnivorous feeding by fish and less specialist piscivory. The trophic structures of warm lakes vary markedly, depending on precipitation, continental or coastal regions locations, lake age and temperature. Unfortunately, little is known about trophic dynamics and the role of fish in warm lakes. Since many warm lakes suffer from eutrophication, new insights are needed into trophic interactions and potential lake restoration methods, especially since eutrophication is expected to increase in the future owing to economic development and global warming.
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SummaryAmmonium (NH4+) toxicity is an issue of global ecological and economic importance. In this review, we discuss the major themes of NH4+ toxicity, including the occurrence of NH4+ in the biosphere, response differences to NH4+ nutrition among wild and domesticated species, symptoms and proposed mechanisms underlying toxicity, and means by which it can be alleviated. Where possible, nitrate (NO3−) nutrition is used as point of comparison. Particular emphasis is placed on issues of cellular pH, ionic balance, relationships with carbon biochemistry, and bioenergetics of primary NH4+ transport. Throughout, we attempt to identify areas that are controversial, and areas that are in need of further examination.
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The turbidity of lakes is generally considered to be a smooth function of their nutrient status. However, recent results suggest that over a range of nutrient concentrations, shallow lakes can have two alternative equilibria: a clear state dominated by aquatic vegetation, and a turbid state characterized by high algal biomass. This bi-stability has important implications for the possibilities of restoring eutrophied shallow lakes. Nutrient reduction alone may have little impact on water clarity, but an ecosystem disturbance like foodweb manipulation can bring the lake back to a stable clear state. We discuss the reasons why alternative equilibria are theoretically expected in shallow lakes, review evidence from the field and evaluate recent applications of this insight in lake management.
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1. Submerged plant richness is a key element in determining the ecological quality of freshwater systems; it has often been reduced or completely lost. 2. The submerged and floating-leaved macrophyte communities of 60 shallow lakes in Poland and the U.K. have been surveyed and species richness related to environmental factors by general linearised models. 3. Nitrogen, and more specifically winter nitrate, concentrations were most important in explaining species richness with which they were inversely correlated. Phosphorus was subsidiary. Such an inverse relationship is consistent with findings in terrestrial communities. Polish lakes, with less intensively farmed catchments, had greater richness than the U.K. lakes. 4. The richest U.K. communities were associated with winter nitrate-N concentrations of up to about 1–2 mg L−1 and may correspond with ‘good’ ecological quality under the terms of the European Water Framework Directive. Current concentrations in European lowlands are often much higher.