No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
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
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.
... 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. ...
Submerged macrophytes, as one of the most important primary producers in shallow lakes and streams, have attracted significant research attention, with relevant studies showing dynamic evolution over the past three decades. Here, we investigated the trends and characteristics of 2,836 relevant articles published between 1991 and 2018 based on bibliometric analysis. Our geographic-based results indicated a scarcity of studies on submerged macrophytes in Africa, but strong international collaboration in this field, especially that between Denmark and China. The top two core journal sources (Hydrobiologia and Aquatic Botany) of studies on submerged macrophytes were determined, with another four important sources also identified. In addition, three trends in abstract word dynamics were found over the last three decades: i.e. most frequent word (‘lake’); increased words (those related to restoration, ecosystem state, model, and macrophyte community); and decreased words (those related to fish and specific macrophyte genera). Furthermore, our case study on four macrophyte genera showed a close relationship between Hydrilla and Vallisneria and between Potamogeton and Myriophyllum, as verified via the topic-article relationships determined using the topic model. The thematic evolution map of keywords used during the last three decades showed a clear shift in scientific fields; for example, Myriophyllum spicatum was an important keyword during 1999 and 2011 but not in other periods. We found that studies on submerged macrophytes developed with an increase in applied ecology topics, such as lake restoration, and basic scientific topics, such as freshwater ecosystems and plant physiology.
As we know, the survival of young ramets and stolons is essential for the clonal growth of many aquatic plants. However, few NH4⁺ enrichment experiments on clonal growth of submerged macrophytes have been conducted to provide possible evidences for their declines in eutrophic lakes. Here, the growth and physiological responses of V. natans to the enrichment of NH4⁺-N were examined under six inorganic nitrogen (IN, i.e., the sum of nitrate nitrogen (NO3⁻-N) and ammonium nitrogen (NH4⁺-N)) concentrations (control, 2.5, 4.5, 6.5, 8.5, and 10.5 mg L⁻¹). When NH4⁺-N concentration increased over 0.5 mg L⁻¹, free amino acid (FAA) contents in leaves and stolons increased while soluble carbohydrate (SC) and starch contents decreased, and major growth indices (total biomass of plants, number of ramets, and stolon dry weight (DW)) also showed a degressive tendency. Remarkably, the stolon DW significantly declined with increasing FAA, but significantly positively related to SC and starch. These results indicated that clonal growth of V. natans was inhibited by high NH4⁺-N concentration, and imbalance of C-N metabolism of stolons partly explained the decline of submerged clonal macrophytes. In this study, the leaves of new and small (NS) ramets contained significantly more FAA and less SC than that of mature and mother (MM) plants, indicating that the C-N metabolism of young ramets was easier to be disrupted, consequently inhibiting the clonal growth of aquatic plants. Furthermore, under the condition of high NH4⁺-N concentration, FAA may be a useful indicator of both macrophyte growth and physiological stress of plants.
The ongoing global climate change involves not only increased temperatures but may also produce more frequent extreme events, such as severe rainfall that could trigger a pulse of nutrients to lakes. In shallow lakes, this may affect primary producers through a number of direct and indirect mechanisms. We conducted a six-month mesocosm experiment to elucidate how periphyton (on inert substrata), epiphyton and epipelon biomass responded to a nitrogen (N) pulse, an approximately tenfold enrichment of the NO3-pool, under three contrasting warming scenarios: ambient temperature and ca. +3°C and ca. +4.5°C elevated temperatures (hereafter T1, T2 and T3). After the N pulse, we found a higher periphyton biomass at elevated than at ambient temperatures but no change in epiphyton biomass. Epipelon biomass was lower in T3 than in T1. Both periphyton and epiphyton biomasses correlated negatively with snail biomass, while epiphyton biomass correlated positively with light. Different responses to higher temperatures under short-term extreme nutrient loading conditions may be attributed to differences in the access to nutrient sources and light. Our data suggest that the biomass of periphyton in oligotrophic clear-water lakes will increase significantly under conditions exhibiting short-term extreme nutrient loading in a warmer climate.
The global climate change may lead to more extreme climate events such as severe flooding creating excessive pulse-loading of nutrients, including nitrogen (N), to freshwaters. We conducted a 3-month mesocosm study to investigate the responses of phytoplankton, zooplankton and Vallisneria spinulosa to different N loading patterns using weekly and monthly additions of in total 14 g N m−2 month−1 during the first 2 months. The monthly additions led to higher phytoplankton chlorophyll a and total phytoplankton biomass than at ambient conditions as well as lower leaf biomass and a smaller ramet number of V. spinulosa. Moreover, the biomass of cyanobacteria was higher during summer (August) in the monthly treatments than those with weekly or no additions. However, the biomass of plankton and macrophytes did not differ among the N treatments at the end of the experiment, 1 month after the termination of N addition. We conclude that by stimulating the growth of phytoplankton (cyanobacteria) and reducing the growth of submerged macrophytes, short-term extreme N loading may have significant effects on shallow nutrient-rich lakes and that the lakes may show fast recovery if they are not close to the threshold of a regime shift from a clear to a turbid state.
1. Excess loading of phosphorus (P) and nitrogen (N) triggers a shift in the trophic structure of shallow lakes from a clear-water, macrophyte-dominated state to an algal-dominated turbid state. However , the role of N in the shift is debated, and experimental evidence is, with a few exceptions, based on short-term studies (days to a few months).
2. We studied the effect of N loading on macrophytes (dominated by Potamogeton lucens and Ca-bomba caroliniana), periphyton, filamentous algae and phytoplankton in mesocosms over 10 months (starting in October) in subtropical China (Wuhan). There were three N treatments: controls (CN) without nitrogen addition (mean TN = 1.9 mg L À1), low nitrogen (LN) addition (mean TN = 3.5 mg L À1) and high nitrogen (HN) addition (mean TN = 5.5 mg L À1). Total phosphorus (TP) concentration in the water column remained moderate (0.05–0.07 mg L À1) during the experiment in all treatments.
3. Macrophyte abundance declined in the LN and HN treatments in the first 6 months, but not in controls, followed by a partial recovery in the LN treatments. They disappeared completely in the HN treatments the following summer. Periphyton (biofilm on plastic) and phytoplankton biomass remained unaffected during the first 6 months but increased over the summer by two or three times, compared with controls, in low and high nitrogen treatments, respectively. By contrast, the abundance of filamentous algae increased over winter but declined during the summer with no obvious relationship to the N treatments. There was no difference in the TN or nitrate concentrations or soluble protein, soluble sugar and Chl-a content of P. lucens leaves and stems with increasing N load.
4. Macrophyte populations are partially resilient to abrupt increases in N loading at moderate TP concentrations, but, after prolonged exposure, a complete collapse occurs. Our results further indicate that macrophyte loss is exacerbated by shading by filamentous algae during the winter, and by phy-toplankton and periphyton in the summer, while there was no indication of direct N toxicity.
Acute biochemical responses of Potamogeton crispus L. to high external ammonium were investigated in an aquarium experiment. Shoots of P: crispus were incubated in aquaria for 24 h or 48 h at five treatments of ammonium-0, 1, 5, 10 and 20 mg/L NH4-N. Soluble sugar content of the shoots declined markedly with increasing ammonium levels, whereas soluble amino acid content increased dramatically. Responses of two antioxidant enzymes as well as soluble protein content fit a lognormal distribution with increasing ammonium levels. High ammonium levels (NH4-N≥5 mg/L) caused significant acute biochemical changes in P. crispus, which potentially could lead to significant biochemical damage.
The use of duckweed in domestic wastewater treatment is receiving growing attention over the last few years. Duckweed-based ponds in combination with anaerobic pre-treatment may be a feasible option for organic matter and nutrient removal. The main form of nitrogen in anaerobic effluent is ammonium. This is the preferred nitrogen source of duckweed but at certain levels it may become inhibitory to the plant. Renewal fed batch experiments at laboratory scale were performed to assess the effect of total ammonia (NH3+NH+4) nitrogen and pH on the growth rate of the duckweed Spirodela polyrrhiza. The experiments were performed at different total ammonia nitrogen concentrations, different pH ranges and in three different growth media. The inhibition of duckweed growth by ammonium was found to be due to a combined effect of ammonium ions (NH+4) and ammonia (NH3), the importance of each one depending on the pH.
The submersed macrophyte, Vallisneria natans L., was cultured in laboratory with NH
4+-enriched tap water (1 mg L−1 NH4-N) for 2 months and the stressful effects of high ammonium (NH
4+) concentrations in the water column on this species was evaluated. The plant growth was severely inhibited by the NH
4+ supplement in the water column. The plant carbon and nitrogen metabolisms were disturbed by the NH
4+ supplement as indicated by the accumulation of free amino acids and the depletion of soluble carbohydrates in the plant tissues. The results suggested that high NH
4+ concentrations in the water column may hamper the restoration of submersed vegetation in eutrophic lakes.
Nitrogenous waste excretion in resting dogfish occurred largely (>90 %) as urea-nitrogen (urea-N) efflux across the gills, with a very small urea efflux via the kidney. Ammonia excretion, almost entirely at the gills, accounted for less than 3 % of total nitrogen excretion. Given the extremely high blood urea levels (approximately 640 mmol-N l-1) 'retained' for osmoregulation, and blood ammonia levels (approximately 80 µmol-N l-1) comparable to those of teleosts, the gills of resting dogfish were exceptionally impermeable to both urea and ammonia. Experiments investigated the origins of these low permeabilities and the responses of urea-N and ammonia-N excretion and acidbase status to 6 h infusions with iso-osmotic solutions of NaCl (control), NH4Cl, NaHCO3, urea and its analogues thiourea and acetamide. NaCl had no effects, whereas NH4Cl loading caused intense acidosis and marked elevation of acidic equivalent, ammonia-N and urea-N excretion rates, the latter despite unchanged blood levels of urea-N. Apparent branchial ammonia permeability increased greatly. Acidosis resulted from both stimulated urea production and branchial NH3 loss, the former making the larger contribution. NaHCO3 loading caused intense alkalosis, a marked elevation of basic equivalent excretion and a moderate stimulation of urea-N excretion. Blood urea-N levels were again unchanged. Infusion of urea itself raised blood urea-N levels, but initially reduced branchial urea-N excretion. Acetamide and thiourea infusions both moderately elevated branchial urea-N excretion. We suggest that the low ammonia permeability may arise metabolically from an ammonia scavenging system in the gills, that a 'back-transport' mechanism in the gills may contribute to the low urea permeability, and that the dissociation between blood urea-N levels and excretion rates may reflect urea production at extrahepatic sites. These studies demonstrate that urea synthesis in the dogfish is linked more to nitrogen availability than to acidbase status.
In aquatic environments, the concentration of inorganic carbon is spatially and temporally variable and CO2 can be substantially oversaturated or depleted. Depletion of CO2 plus low rates of diffusion cause inorganic carbon to be more limiting in aquatic than terrestrial environments, and the frequency of species with a CO2-concentrating mechanism (CCM), and their contribution to productivity, is correspondingly greater. Aquatic photoautotrophs may have biochemical or biophysical CCMs and exploit CO2 from the sediment or the atmosphere. Though partly constrained by phylogeny, CCM activity is related to environmental conditions. CCMs are absent or down-regulated when their increased energy costs, lower CO2 affinity, or altered mineral requirements outweigh their benefits. Aquatic CCMs are most widespread in environments with low CO2, high HCO3-, high pH, and high light. Freshwater species are generally less effective at inorganic carbon removal than marine species, but have a greater range of ability to remove carbon, matching the environmental variability in carbon availability. The diversity of CCMs in seagrasses and marine phytoplankton, and detailed mechanistic studies on larger aquatic photoautotrophs are understudied. Strengthening the links between ecology and CCMs will increase our understanding of the mechanisms underlying ecological success and will place mechanistic studies in a clearer ecological context.
Loss of submersed macrophytes is a world-wide phenomenon occurring when shallow lakes become eutrophic due to excess nutrient loading. In addition to the well-known effect of phosphorus, nitrogen as a trigger of macrophyte decline has received increasing attention. The precise impact of high nitrogen concentrations is debated, and the role of different candidate factors may well change over the season. In this study, we conducted experiments with Vallisneria natans during the growing season (June–September) in 10 ponds subjected to substantial differences in nitrogen loading (five targeted total nitrogen concentrations: control, 2, 10, 20, and 100 mg L− 1) and compared the results with those obtained in our earlier published study from the low-growth season (December–April). Like in the low-growth season, growth of V. natans in summer declined with increasing ammonium (NH4) concentrations and particularly with increasing phytoplankton chlorophyll a (ChlaPhyt). Accordingly, we propose that shading by phytoplankton might be of key importance for macrophyte decline, affecting also periphyton growth as periphyton chlorophyll a (ChlaPeri) decreased with increasing ChlaPhyt. Free amino acid contents (FAA) of plants tended to increase with increasing NH4 concentrations, while the relationships between FAA with growth indices were all weak, suggesting that FAA might be a useful indicator of the physiological stress of the plants but not of macrophyte growth. Taken together, the results from the two seasons indicate that although a combination of high nitrogen concentrations (ammonium) and shading by phytoplankton may cause severe stress on macrophytes, active growth in the growing season enabled them to partly overcome the stress.
A number of studies have revealed ammonia to be toxic to aquatic organisms; however, little is known about its effects under natural conditions. To elucidate the role of ammonia, we conducted 96-h acute toxicity tests as well as a whole-ecosystem chronic toxicity test for one year in ten 600-m² ponds. Three common cyprinids, silver carp Hypophthalmichthys molitrix Val. (H.m.), bighead carp Aristichthys nobilis Richardson (A.n.), and gibel carp Carassius auratus gibelio Bloch (C.g.), were used as test organisms. The 96-h LC50 values of un-ionized ammonia (NH3) for H.m., A.n., and C.g. were 0.35, 0.33, and 0.73 mg L− 1, respectively. In the ponds, annual mean NH3 ranged between 0.01 and 0.54 mg L− 1, with 4 ponds having a NH3 higher than the LC50 of A.n. (lowest LC50 in this study). No fish were found dead in the high-nitrogen ponds, but marked histological changes were found in livers and gills. Despite these changes, the specific growth rate of H.m. and A.n. increased significantly with NH3. Our pond results suggest that fish might be more tolerant to high ammonia concentrations in natural aquatic ecosystems than under laboratory conditions. Our finding from field experiments thus suggests that the existing regulatory limits for reactive nitrogen (NH3) established from lab toxicity tests might be somewhat too high at the ecosystem conditions. Field-scale chronic toxicity tests covering full life histories of fish and other aquatic organisms are therefore encouraged in order to optimize determination of the effects of ammonia in natural environments.
To evaluate the relative importance of photosynthetic versus morphological adaptations of submersed macrophytes to low light intensity in lakes, rapid light curves (RLCs), morphological parameters, relative growth rate (RGR), clonal reproduction and abundance of two submersed macrophytes (Potamogeton maackianus and Vallisneria natans) were examined under 2.8%, 7.1%, 17.1% and 39.5% ambient light in a field and outdoor experimental study. The plants increased their initial slope of RLCs (α) and decreased their minimum saturating irradiance (Ek) and maximum relative electron transport rate (ETRm) of RLCs under low light stress, but V. natans was more sensitive in RLCs than P. maackianus. Accordingly, the RGR, plant height and abundance of P. maackianus were higher in the high light regimes (shallow water) but lower in the low light regimes than those of V. natans. At the 2.8% ambient light, V. natans produced ramets and thus fulfilled its population expansion, in contrast to P. maackianus. The results revealed that P. maackianus as a canopy-former mainly elongated its shoot length towards the water surface to compensate for the low light conditions, however, it became limited in severe low light stress conditions. V. natans as a rosette adapted to low light stress mainly through photosynthetic adjustments and superior to severely low light than shoot elongation.
The role of nitrogen (N) in the shift from a macrophyte-dominated state to a phytoplankton-dominated one at high N concentrations in shallow lakes is still debated. To elucidate possible toxic and ecological effects of high N on macrophyte growth, we conducted a short-term (40 day) study of a eutrophication-tolerant macrophyte, Vallisneria spinulosa (Hydrocharitaceae), incubated in pots in a mesocosm system subjected to different N concentrations (1, 3, and 5 mg l-1). Plant leaf and root length as well as growth rate decreased significantly with increased N concentrations, but most N-and P-related physiological parameters, including the soluble protein content, nitrate reductase activity, acid phosphatase activity, and tissue N and P contents, did not differ significantly among the N treatments. Only the alkaline phosphatase activity differed, being lower at high nitrogen loading, likely due to P limitation. Epiphyton and phytoplankton biomasses increased significantly with increasing N loading. Our results including a large number of physiological tests of the macrophytes, therefore, provide supporting evidence that the loss of submerged macrophytes, like V. spinulosa, seen at high N loading in shallow lakes, can be attributed to competition with phytoplankton and epiphyton rather than to toxic effects.
1. Excess loading of phosphorus (P) and nitrogen (N) triggers a shift in the trophic structure of shallow
lakes from a clear-water, macrophyte-dominated state to an algal-dominated turbid state. However,
the role of N in the shift is debated, and experimental evidence is, with a few exceptions, based
on short-term studies (days to a few months).
2. We studied the effect of N loading on macrophytes (dominated by Potamogeton lucens and Cabomba
caroliniana), periphyton, filamentous algae and phytoplankton in mesocosms over 10 months
(starting in October) in subtropical China (Wuhan). There were three N treatments: controls (CN)
without nitrogen addition (mean TN = 1.9 mg L
�1), low nitrogen (LN) addition (mean
TN = 3.5 mg L
�1) and high nitrogen (HN) addition (mean TN = 5.5 mg L
�1). Total phosphorus (TP)
concentration in the water column remained moderate (0.05–0.07 mg L
�1) during the experiment in
all treatments.
3. Macrophyte abundance declined in the LN and HN treatments in the first 6 months, but not in
controls, followed by a partial recovery in the LN treatments. They disappeared completely in the
HN treatments the following summer. Periphyton (biofilm on plastic) and phytoplankton biomass
remained unaffected during the first 6 months but increased over the summer by two or three times,
compared with controls, in low and high nitrogen treatments, respectively. By contrast, the abundance
of filamentous algae increased over winter but declined during the summer with no obvious
relationship to the N treatments. There was no difference in the TN or nitrate concentrations or soluble
protein, soluble sugar and Chl-a content of P. lucens leaves and stems with increasing N load.
4. Macrophyte populations are partially resilient to abrupt increases in N loading at moderate TP
concentrations, but, after prolonged exposure, a complete collapse occurs. Our results further indicate
that macrophyte loss is exacerbated by shading by filamentous algae during the winter, and by phytoplankton
and periphyton in the summer, while there was no indication of direct N toxicity.
In this experiment, carefully controlled pH ranges (7 and 9) were used to distinguish between the effects of unionised NH3 and the NH4+ ion. The objective was to find the effect of different total ammonia nitrogen concentrations and pHs on the carbon metabolism of Ceratophyllum demersum. This study investigated the effects of ammonia on the non-structural carbohydrate (NSC) content in shoots of C. demersum. Ammonia treatment decreased the contents of NSC, soluble sugar, sucrose, fructose and starch in leaves. Meanwhile, increasing the pH value exacerbated the decline of the C. demersum NSC content. Additionally, the activity of invertase was increased during the experiment. These results suggest that ammonia severely inhibits plant growth via disturbing the NSC content. It has been suggested that ammonia has toxic effects on C. demersum, and the higher the pH in water is, the more obvious the physiological responses that C. demersum exhibits. The results of the current study can provide some reference for studying the living conditions of submersed macrophytes under the stress of NH3. This article is protected by copyright. All rights reserved
Ammonium (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.
Global warming may affect snail–periphyton–macrophyte relationships in lakes with implications
also for water clarity. We conducted a 40-day aquaria experiment to elucidate the response of
submerged macrophytes and periphyton on real and artificial plants to elevated temperatures (3�C) under
eutrophic conditions, with and without snails present. With snails, the biomass and length of Vallisneria
spinulosa leaves increased more at the high temperature,and at both temperatures growth was higher than
in absence of snails. The biomass of periphyton on V.spinulosa as well as on artificial plants was higher at
the highest temperature in the absence but not in the presence of snails. The biomass of Potamogeton
crispus (in a decaying state) declined in all treatments and was not affected by temperature or snails. While
total snail biomass did not differ between temperatures, lower abundance of adults (size [1 cm) was
observed at the high temperatures. We conclude that the effect of elevated temperature on the snail–
periphyton–macrophyte relationship in summer differs among macrophyte species in active growth or
senescent species in subtropical lakes and that snails, when abundant, improve the chances of maintaining
actively growing macrophytes under eutrophic conditions,and more so in a warmer future with potentially
denser growth of periphyton.
Laboratory scale batch experiments were performed under controlled conditions at different total ammonia concentrations (10–300 mg N l −1) and controlled pH values of 6.8–8.7 using settled domestic wastewater to measure the effect of the ionised (NH 4 + or ammonium) and un-ionised form (NH 3) on the growth of the duckweed Lemna gibba. Relative growth rates (RGR) varied between 0 and 0.3 per day. The toxicity of total ammonia to duckweed was a result of the effect of both, ionised and un-ionised, forms at low NH 3 concentrations (<1 mg N l −1). At higher NH 3 concentrations, the toxic effect of the ionised form could be disregarded. Relative growth rates of L. gibba decreased linearly with increasing NH 3 concentrations up to a maximum level (8 mg N l −1), above which duckweed died. These data indicate that L. gibba can be used to treat wastewater containing high total ammonia concentrations as long as certain pH levels are not exceeded. Extrapolated relative growth rates resulting from different combinations of pH and total ammonia are given for the examined ranges. Up to a pH of 7.8, a substantial production of 55 kg DW ha −1 per day was achieved. Wastewater treatment using L. gibba becomes impossible at pH levels above approximately 9.8, depending on the temperature.
Ammonium (NH4+) toxicity is a significant ecological and agricultural issue, and an important phenomenon in cell biology. As a result of increasing soil nitrogen input and atmospheric deposition, plants have to deal with unprecedented NH4+ stress from sources below and above ground. This review describes recent advances in elucidating the signaling pathways and in identifying the main physiological targets and genetic loci involved in the effects of NH4+ stress in the roots and shoots of Arabidopsis thaliana. We outline new experimental approaches that are being used to study NH4+ toxicity in Arabidopsis and propose an integrated view of behavior and signaling in response to NH4+ stress in the Arabidopsis system.
1. Strategies of carbon (C) and nitrogen (N) utilisation are among the factors determining plant distri-bution. It has been argued that submersed macrophytes adapted to lower light environments are more efficient in maintaining C metabolic homeostasis due to their conservative C strategy and abil-ity to balance C shortage. We studied how depth distributions of 12 submersed macrophytes in Lake Erhai, China, were linked to their C-N metabolic strategies when facing acute NH þ 4 dosing. 2. NH þ 4 dosing changed C-N metabolism significantly by decreasing the soluble carbohydrate (SC) content and increasing the NH þ 4 -N and free amino acid (FAA) content of plant tissues. 3. The proportional changes in SC contents in the leaves and FAA contents in the stems induced by NH þ 4 dosing were closely correlated (positive for SC and negative for FAA) with the colonising water depths of the plants in Lake Erhai, the plants adapted to lower light regimes being more efficient in maintaining SC and FAA homeostasis. 4. These results indicate that conservative carbohydrate metabolism of submersed macrophytes allowed the plants to colonise greater water depths in eutrophic lakes, where low light availability in the water column diminishes carbohydrate production by the plants.
The annual mean light intensity at the depth limit of the Littorella vegetation was 24–33% of the subsurface light intensity, despite large variations in each attenuation component (lake water, phytoplankton, and epiphytes). In oligotrophic, silicate-poor lakes, the light attenuation above the submerged vegetation was dominated by the water itself, which accounted for 65–72% of the total attenuation. Phytoplankton and epiphytes were equal in importance to each other. In oligotrophic, silicate-rich lakes and lakes receiving a nitrogen supply above background level, the epiphytes were more abundant, accounting for about 50% of the light attenuation. In one lake with a high nutrient supply, the epiphytes were responsible for 86% of the light attenuation.A new method of measuring the effect of shading by the epiphytic community on submerged macrophytes is presented. The light attenuation caused by the phytoplankton and the epiphytes was investigated and related to the depth distribution of the submerged angiosperm, Littorella uniflora.It is shown that the biomass of the epiphytes increased more than the biomass of the phytoplankton in response to an external or internal nutrient loading. Shading by epiphytes is of decisive importance for the depth distribution of Littorella at increasing nutrient supply.
The toxicity of ammonia to fishes has been attributed to the un-ionized ammonia chemical species present in aqueous solution. Because the percent of total ammonia present as un-ionized ammonia (NH3) is so dependent upon pH and temperature, an exact understanding of the aqueous ammonia equilibrium is important for toxicity studies. A critical evaluation of the literature data on the ammonia–water equilibrium system has been carried out. Results of calculations of values of pKa at different temperatures and of percent of NH3 in aqueous ammonia solutions of zero salinity as a function of pH and temperature are presented.
Phosphorus (P) is conventionally thought to limit production in freshwaters and nitrogen (N) that in the sea. Before much human activity, however, co-limitation by N and P was probably normal, with systems developing ratios of N to P tending to the Redfield ratio. Single-factor limitation may be a symptom of human activity in many cases. It is widely believed that N fixation should compensate for N shortage because N fixers are ubiquitous and versatile, but this is not always the case and the issue has hitherto been considered largely with respect to plankton communities. Effects of N on macrophyte communi-ties provide justification for control of both nutrients, at least in shallow lakes and estuaries. Increased N loading reduces plant biodiversity, changes the struc-ture, and is associated with eventual loss, of macro-phyte communities. P control alone may suffice in many deep lakes where denitrification is low and stratified conditions favour cyanobacterial develop-ment. Therein may lie a resolution to current controversies.
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.
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.
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.
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.
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.
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.