Nitrogen Loss and Denitrification as Studied in Relation to Reductions in Nitrogen Loading in a Shallow,Hypertrophic Lake (Lake Søbygård,Denmark)

A detailed mass balance on nitrogen was carried out in shallow and hypertrophic Lake Søbygård during 4.5 years before through 2.5 years after a 36 % reduction in nitrogen loading. Annual mean loss rate of nitrogen was 159–229 mg N m⁻² d⁻¹ before the loading reduction and 125 mg N m⁻² d⁻¹ after. In spite of a short hydraulic retention time (18–27 days) the proportion of nitrogen loading lost in the lake was high (38–53 %) and not affected by changes in loading. Calculated denitrification accounted for 86–93% of the loss rate, while 7–14% was permanently buried. Marked seasonal variations in the loss percentage were found during the season, ranging from 23 % in first quarter to 65 % in third quarter. The seasonal variation in the loss percentage of nitrogen showed a hysteresis like relationship to temperature, with a high percentage in fourth quarter. This suggests that the amount of available substrate, which mainly consists of sedimentated phytoplankton, accumulated during summer, is an important regulating factor. The ability of various published input-output models to predict the observed changes in in-lake nitrogen concentration in Lake Søbygård was tested. This study has further confirmed that small lakes with short retention and high nitrogen loading may significantly reduce the nitrogen loading of downstream aquatic environments.     

Factors controlling hydrochemical and trophic state variables in 86 shallow lakes in Europe

The total nitrogen mass balance was studied over a two-year period in two reservoirs located in south-eastern Poland. Theobserved nitrogen retention (N_ret) inthe reservoirs ranged from 0.18 to 2.12g m^-2d^-1 (mean 0.83g m^-2d^-1) in case of theRzeszów Reservoir and from -0.02 to0.41 g m^-2d^-1 (mean 0.13g m^-2d^-1) in case of the SolinaReservoir, while the percentage retention(N_ret) ranged from 7 to 42% (mean22%) and -15 to 73% (mean 35%),respectively. A good relationship betweenN_ret and nitrogen loads was observedfor the Solina Reservoir. The data obtainedby the author and collected from theresearches of 6 other reservoirs located insouthern Poland were used to develop anempirical model suitable to predict thenitrogen seasonal dynamics in a reservoir.The model allows for the determination ofboth the annual mean N_ret value andits seasonal variation in a reservoir.     

Nitrogen input–output budgets for lake-containing watersheds in the Adirondack region of New York

The Adirondack region of New York is characterized by soils and surface waters that are sensitive to inputs of strong acids, receiving among the highest rates of atmospheric nitrogen (N) deposition in the United States. Atmospheric N deposition to Adirondack ecosystems may contribute to the acidification of soils through losses of exchangeable basic cations and the acidification of surface waters in part due to increased mobility of nitrate (NO₃^–). This response is particularly evident in watersheds that exhibit nitrogen saturation. To evaluate the contribution of atmospheric N deposition to the N export and the capacity of lake-containing watersheds to remove, store, or release N, annual N input–output budgets were estimated for 52 lake-containing watersheds in the Adirondack region from 1998 to 2000. Wet N deposition was used as the N input and the lake N discharge loss was used as the N output based on modeled hydrology and measured monthly solute concentrations. Annual outputs were also estimated for dissolved organic carbon (DOC). Wet N deposition increased from the northeast to the southwest across the region. Lake N drainage losses, which exhibited a wider range of values than wet N deposition, did not show any distinctive spatial pattern, although there was some evidence of a relationship between wet N deposition and the lake N drainage loss. Wet N deposition was also related to the fraction of N removed or retained within the watersheds (i.e., the fraction of net N hydrologic flux relative to wet N deposition, calculated as [(wet N deposition minus lake N drainage loss)/wet N deposition]). In addition to wet N deposition, watershed attributes also had effects on the exports of NO₃^–, ammonium (NH₄⁺), dissolved organic nitrogen (DON), and DOC, the DOC/DON export ratio, and the N flux removed or retained within the watersheds (i.e., net N hydrologic flux, calculated as [wet N deposition less lake N drainage loss]). Elevation was strongly related with the lake drainage losses of NO₃^–, NH₄⁺, and DON, net NO₃^– hydrologic flux (i.e., NO₃^– deposition less NO₃^– drainage loss), and the fraction of net NO₃^– hydrologic flux, but not with the DOC drainage loss. Both DON and DOC drainage losses from the lakes increased with the proportion of watershed area occupied by wetlands, with a stronger relationship for DOC. The effects of wetlands and forest type on NO₃^– flux were evident for the estimated NO₃^– fluxes flowing from the watershed drainage area into the lakes, but were masked in the drainage losses flowing out of the lakes. The DOC/DON export ratios from the lake-containing watersheds were in general lower than those from forest floor leachates or streams in New England and were intermediate between the values of autochthonous and allochthonous dissolved organic matter (DOM) reported for various lakes. The DOC/DON ratios for seepage lakes were lower than those for drainage lakes. In-lake processes regulating N exports may include denitrification, planktonic depletion, degradation of DOM, and the contribution of autochthonous DOM and the influences of in-lake processes were also reflected in the relationships with hydraulic retention time. The N fluxes removed or stored within the lakes substantially varied among the lakes. Our analysis demonstrates that for these northern temperate lake-containing watershed ecosystems, many factors, including atmospheric N deposition, landscape features, hydrologic flowpaths, and retention in ponded waters, regulated the spatial patterns of net N hydrologic flux within the lake-containing watersheds and the loss of N solutes through drainage waters.     

Predicting concentration of total phosphorus and chlorophyll a in a lake with short hydraulic residence time

The relationship between total phosphorus and chlorophyll a concentration was determined for Skinner Lake, Indiana over an annual cycle in 1978–79. Total nitrogen:total phosphorus ratios in the epilimnion ranged from 19 to 220 suggesting a phosphorus-dependent algal yield in the epilimnion. Approximately 90% of annual TP loading reached the lake via streamflow, and 93% of this entered during snowmelt and spring-overturn periods. At that time incoming water flushed the lake 2.4 times. Atmospheric loading accounted for 1.4% of annual TP load. Internal hypolimnetic TP loading occurred during summer stratification. Mean [chl a] for the ice-free period was 15.15 mg m^–3, within the range expected for eutrophic lakes.The 1978–79 data were used in conjuction with the Vollenweider & Kerekes (1980) model to produce a model specific for the Skinner Lake system. The model predicted mean epilimnetic total phosphorus and chlorophyll a concentrations from mean total phosphorus concentration in inlet streams and from lake water residence time during the period of spring overturn and summer stratification. The Skinner-specific model was tested in 1982 and it closely predicted observed mean epilimnetic [TP] and [chl a] during the ice-free period. This study shows that variability in lake models which average data over an annual period can be reduced by considering lake-specific seasonal variation in hydrology and external TP loading.     

Chemical limnology of two artificial lakes used for both stormwater management and recreation

The aims of this study were to document the mainly chemical behaviour of two linked artificial lakes used for both stormwater management and recreation in the new town of Craigavon. Further, the understanding of their behaviour should help in their management and the design of other similar lakes.The lake mean total phosphorus (73 µg P l^–1), nitrate (0.50 mg N l^–1) and chlorophyll a (25 µg l^–1) concentrations, Secchi depth (1.2 m) and the estimated total phosphorus loading (1.98 g m^–2 a^–1) all classify the main lake as eutrophic. An important source of the phosphorus load on the lakes is the urban area of Craigavon (52% of the total load). The interrelationships between total phosphorus, chlorophyll a and Secchi depth in the main lake are similar to those in natural ones. In addition, the lake follows the total phosphorus load — trophic state relationships (lake total phosphorus and chlorophyll a concentrations and Secchi depth) found to apply elsewhere. These two points indicate that the artificial lakes in Craigavon behave similarly to natural ones.     

Nutrient additions by waterfowl to lakes and reservoirs: predicting their effects on productivity and water quality

Lakes and reservoirs provide water for human needs and habitat for aquatic birds. Managers of such waters may ask whether nutrients added by waterfowl degrade water quality. For lakes and reservoirs where primary productivity is limited by phosphorus (P), we developed a procedure that integrates annual P loads from waterfowl and other external sources, applies a nutrient load-response model, and determines whether waterfowl that used the lake or reservoir degraded water quality. Annual P loading by waterfowl can be derived from a figure in this report, using the days per year that each kind spent on any lake or reservoir. In our example, over 6500 Canada geese (Branta canadensis) and 4200 ducks (mostly mallards, Anas platyrhynchos) added 4462 kg of carbon (C), 280 kg of nitrogen (N), and 88 kg of P y^–1 to Wintergreen Lake in southwestern Michigan, mostly during their migration. These amounts were 69% of all C, 27% of all N, and 70% of all P that entered the lake from external sources. Loads from all external sources totaled 840 mg P m^–2 y^–1. Application of a nutrient load-response model to this concentration, the hydraulic load (0.25 m y^–1), and the water residence time (9.7 y) of Wintergreen Lake yielded an average annual concentration of total P in the lake of 818 mg m^–3 that classified the lake as hypertrophic. This trophic classification agreed with independent measures of primary productivity, chlorophyll-a, total P, total N, and Secchi disk transparency made in Wintergreen Lake. Our procedure showed that waterfowl caused low water quality in Wintergreen Lake.Contribution 824 of the National Fisheries Research Center-Great Lakes, 1451 Green Road, Ann Arbor, Michigan 48105, U.S.A. and 722 of the Kellogg Biological Station, Michigan State University.     

Phosphorus retention as a function of external loading,hydraulic turnover time,area and relative depth in 54 lakes and reservoirs

We analysed phosphorus retention as a function of external loading, hydraulic turnover time, area and relative depth on the basis of published data from 54 lakes and reservoirs in different climate regions around the world. Our analysis demonstrated that reservoirs and lakes that received higher areal loading of phosphorus (TP_in) also retained more P per m² but the proportion of the external P loading retained in the waterbody (retention coefficient, R _P) remained generally independent of TP_in. The waterbodies with longer hydraulic residence times (T _R) retained larger proportions of external P and the correlation between R _P and T _R was much stronger in lakes with areas larger than 25 km² than in the whole data set. TP_in and T _R together determined 78% of the variation in R _P in large lakes. We also partially confirmed our hypothesis that waterbodies with bigger relative depths (Z _R) retain more of the external phosphorus than larger and shallower waterbodies with lower Z _R. The hypothesis was, however, validated only for lakes larger than 25 km² and for those with T _R <0.3 year, where R _P increased significantly with increasing Z _R. In stratified lakes, increasing relative depth correlated with reduced P retention capacity, demonstrating the complex nature of phosphorus biogeochemistry in lake ecosystems.     

Applicability of phosphorus budget models to southern African man-made lakes

An investigation of the phosphorus loading characteristics of 31 southern African man-made was lakes made. The lakes were characterized by low water retention times, with most of the lakes having retention times of less than one year. Catchment phosphorus export rates showed wide variation (1–162 mg P m^-2 y^-1) with those lakes experiencing excessive municipal wastewater inputs having export rates in excess of 53 mg m^-2 y^-1. The phosphorus data were tested against the Vollenweider (1976) and Dillon & Rigler (1974) phosphorus budget models which predict in-lake steady state concentrations of phosphorus. It was found that both models displayed good potential for the prediction of steady state concentrations of phosphorus, with better results being obtained from the Dillon & Rigler (1974) model. However, because phosphorus concentrations within these lakes may not necessarily be related to trophic status the use of these models as a predictive tool for eutrophication control still requires further development.     

Retention of nutrients in river basins

In Denmark, as in many other European countries, the diffuse losses of nitrogen (N) and phosphorus (P) from the rural landscape are the major causes of surface water eutrophication and groundwater pollution. The export of total N and total P from the Gjern river basin amounted to 18.2 kg ha^–1 and 0.63 kg P ha^–1 during June 1994 to May 1995. Diffuse losses of N and P from agricultural areas were the main nutrient source in the river basin contributing 76% and 51%, respectively, of the total export.Investigations of nutrient cycling in the Gjern river basin have revealed the importance of permanent nutrient sinks (denitrification and overbank sedimentation) and temporary nutrient storage in watercourses. Temporary retention of N and P in the watercourses thus amounted to 7.2–16.1 g N m^–2 yr^–1 and 3.7–8.3 g P m^–2 yr–1 during low-flow periods. Deposition of P on temporarily flooded riparian areas amounted from 0.16 to 6.50 g P m^–2 during single irrigation and overbank flood events, whereas denitrification of nitrate amounted on average to 7.96 kg N yr^–1 per running metre watercourse in a minerotrophic fen and 1.53 kg N yr^–1 per linear metre watercourse in a wet meadow. On average, annual retention of N and P in 18 Danish shallow lakes amounted to 32.5 g N m^–2 yr^–1 and 0.30 g P m^–2 yr^–1, respectively, during the period 1989–1995.The results indicate that permanent nutrient sinks and temporary nutrient storage in river systems represent an important component of river basin nutrient budgets. Model estimates of the natural retention potential of the Gjern river basin revealed an increase from 38.8 to 81.4 tonnes yr^–1 and that P-retention increased from –0.80 to 0.90 tonnes yr^–1 following restoration of the water courses, riparian areas and a shallow lake. Catchment management measures such as nature restoration at the river basin scale can thus help to combat diffuse nutrient pollution.     

The role of ammonium and nitrate retention in the acidification of lakes and forested catchments

The relative contribution of HN0₃ to precipitation acidity in eastern Canada has increased in recent years leading to some concern that the relative importance of NO^– ₃ deposition in acidification of terrestrial and aquatic ecosystems may increase. To gauge the extent of this impact, annual mass balances for N0^– ₃ and NH^{+ 4} were calculated for several forested catchments and lakes in Ontario. Retention of NH⁺ ₄ (R _NH4) by forested catchments was consistently high compared to retention of NO₃ ^– (R _NO3) which was highly variable. Retention of inorganic nitrogen was influenced by catchment grade and areal water discharge. In lakes, the reciprocals of retention of N0^– ₃ and NH⁺ ₄ were linearly related to the ratio of lake mean depth to water residence time (z/; equal to areal water discharge), and retention did not appear to be a function of degree of acidification of the lakes. Net N consumption-based acidification of lakes, defined as the ratio of annual NH; mass to N0^– ₃ mass consumption, was negatively correlated with / and N consumption-related acidification was most likely to occur when – was < 1.5 m yr^–1.If retention mechanisms are unaffected by changes in deposition, changes in deposition will still result in changes in surface water concentrations although the changes will be of similar proportions. Therefore, NO^– ₃ saturation should not be defined by concentrations alone, but should be defined as decreasing long-term, average NO^– ₃ retention in streams and lakes in response to long-term increases in NO^– ₃ deposition. Analysis o f survey data will be facilitated by grouping lakes and catchments according to similar characteristics.     

Optimum growth conditions and light utilization efficiency of Spirulina platensis (= Arthrospira fusiformis) (Cyanophyta) from Lake Chitu, Ethiopia

Spirulina platensis (= Arthrospira fusiformis) was isolated from Lake Chitu, a saline, alkaline lake in Ethiopia, where it forms an almost unialgal population. Optimum growth conditions were studied in a turbidostat. Cultures grown in modified Zarrouk's medium and exposed to a range of light intensities (20–500 µmol photons m^–2s^–1) showed a maximum specific growth rate (µ_max) of 1.78 d^–1. Quantum yield for growth (µ) was 3.8% at the optimum light for growth of 330 µmol photons m^–2s^–1, and ranged from 2.8 to 9.4%. With increase in irradiance, the chlorophyll a concentration decreased, and the carotenoids/chlorophyll a ratio increased by a factor of 2.4. The phosphorus to carbon ratio (P/C) showed some variation, while the nitrogen to carbon ratio (N/C) remained relatively constant, thus causing fluctuations in the N:P ratio (7–11) of cells. An optimum N:P ratio of about 7 was attained in cells growing at the optimum light for growth. Results from the continuous culture experiments agreed well with maximum values of photosynthetic efficiency given in the literature for natural populations of S. platensis in the soda lakes of East Africa, Lake Arenguade (Ethiopia), and Lake Simbi (Kenya).     

Influence of internal loading on phosphorus concentration in shallow lakes before and after reduction of the external loading

An analysis of data from 49 shallow lakes showed, that the parameters of empirical models between phosphorus loading and concentration in the lake (e.g. Vollenweider type of relations) differ significantly for lakes without or with a reduced external loading. For lakes without a reduction of the external loading the summer phosphorus concentration is determined by the external phosphorus loading and the hydraulic loading. For these lakes the classical models suffice; deviations between calculations and measurements are partly due to errors made in the determination of the loading.In contrast, for lakes where the external loading was reduced, the measured internal loading explains most of the variation in the summer lake concentration. The external loading is of minor importance and the classical models cannot be applied. The internal loading measured before reduction of the external loading is not useful in predicting the concentration afterwards. Instead of the internal loading, the sediment composition can be used. The advantage of using sediment composition is that these variables are easier to determine and vary less in time. The most promising variable is the ratio between total P and total Fe in the sediment.Abbreviations: Q_s hydraulic loading (m y^-1) - hydraulic retention time (y) - L_ext external phosphorus loading (gP m^-2 y^-1) - L_int internal phosphorus loading (gP m^-2 y^-1) - P_lake phosphorus concentration in the lake (gP m^-3) - P_inlet phosphorus concentration in the inlet water (gP m^-3) - P_sed phosphorus content on the sediment (gP kg^-1 d.w.) - Fe_sed iron content of the sediment (gFe kg^-1 d.w.) - Y dependent variable multiple regression calculations - X1, X2 independent variables multiple regressions calculations - a, a1 constants - a2, b constants     

Chemical composition of interstitial water and diffusive fluxes within the diatomaceous sediment in Lake Myvatn, Iceland

The sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice-cover of 190 days per year. This is due to relatively high solar radiation, nutrient rich inflow waters, N₂ fixation and internal nutrient loading. In order to define direction and magnitude of diffusive fluxes, soil water samplers were used to collect interstitial water from 25–150 cm depth, from within the diatomaceous sediment at the bottom of Lake Myvatn. Water depth at the sampling site was 225 cm. The pH of the interstitial water ranged from 7.16 to 7.30, while the pH of the lake water was 9.80–10.00. The concentrations of most solutes were similar 16 cm above the bottom of the lake at the sampling site and at the lake outlet. The concentrations of NO₃, S, F, O₂, Al, Cr, Mo, V, U, Sn and Sb were higher in the lake water than in the interstitial water. They will therefore diffuse from the lake water into the interstitial water. The concentrations of orthophosphates, PO₄, and total dissolved P were highest at 25 cm depth, but Co and NH₄ concentrations were highest at 50 to 100 cm depth. Thus they diffuse both up towards the lake bottom and down deeper into the sediments. The concentrations of Na, K, Ca, Mg, Sr, Mn, Li and alkalinity were greater within the sediments than in the lake water and increased continuously with depth. The Si concentration of the interstitial water was higher than in the lake water, it was highest at 25 cm depth and decreased slightly down into the sediments. The concentration gradient was greatest for bicarbonate, HCO₃ ^–, 1.5×10^–7 mol cm^–3 cm^–1, and then in declining order for the solutes with the highest gradient; NH₄, Si, Na, Ca, Mg, -S (diffusion into the sediments), K, PO₄, Cl, Fe and Mn. The estimated annual diffusive flux of PO₄ for Lake Myvatn was 0.1 g P m^–2 yr^–1, about 10% of the total PO₄ input to Lake Myvatn. The H₄SiO₄° flux was 1.3 g Si m^–2 yr^–1, <1% of both the input and the annual net Si fixation by diatoms within the lake and the diffusive flux of dissolved inorganic carbon was 1% of the annual net C fixation by diatoms. Annual diffusive flux of NH₄ ⁺ was 1.9 g N m^–2 yr^–1 similar to the input of fixed N to the lake and 24% of the net N fixation within Lake Myvatn. Thus it is important for the nitrogen budget of Lake Myvatn and the primary production in the lake since fixed nitrogen is the rate determining nutrient for primary production.     

The effects of reduced phosphorus and nitrogen loading on phytoplankton in Mondsee,Austria

Following restoration measures (ring canalization, treatment plant with phosphorus precipitation), phosphorus loading declined step-wise from 26.21 t year^–1 to 9.18 t year^–1 during the period 1979–1984 while P-retention increased from 48% to 78%. Phosphorus loading was poorely correlated with precipitation. Inorganic nitrogen load, largely NO₃– N, did not decline but was significantly correlated with precipitation (r = 0.95) throughout the investigation period (1978–1989).Total phosphorus loading reached acceptable levels in 1984 when compared to critical loading calculated according to Vollenweider (1976). Phosphorus input to the lake has remained at these levels in recent years.Average annual chlorophyll-a concentrations and biovolume of phytoplankton in the top 20 m layer of the lake decreased, in correspondence with the respective phosphorus concentrations, from eutrophic to mesotrophic levels. The decline was accompanied by a drastic reduction of blue-green algal populations, and especially of Oscillatoria rubescens D.C..     

Spatial and temporal variation in nitrogen fixation and its importance to phytoplankton in phosphorus‐rich lakes

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Ecological Significance of Nitrogen Cycling by Tubificid Communities in Shallow Eutrophic Lakes of the Danube Delta

Importance of tubificid populations on nitrogen cycle in two categories of shallow eutrophic lakes in the Danube Delta was quantitatively assessed for the 1992-1993 period. The structure of the primary producers in the studied lakes was used to discriminate between the two categories:(i) lakes dominated by macrophytes (A₁) and (ii) lakes dominated by phytoplankton (A₂). In both categories tubificid worms represented important fraction of the entire benthic community (35 and 32%, respectively, as number of individuals). They influence the sediment-water exchange of nutrients. The main processes involved are excretion of nutrients and their continuous release from sediments by molecular diffusion or through channels created by bioturbation. Inorganic nitrogen released from bottom sediments may regulate nitrogen load in the water body and thus, phytoplankton production. In 1992-1993, nitrogen stocks in tubificid biomass accounted for 5.3% in A₁ lakes and 15.6% in A₂ lakes of the amount stocked in phytoplankton, and only for 1.2 and 2.9% respectively, of the nitrogen load in water body. Nitrogen excretion rates ranged between 60.52 and 153.74 mg N m^–2 year^–1, and release rates from sediments between 378.26 and 960.87 mg N m^–2 year^–1, the lowest values being recorded for A₂ category. Differences are related to tubificid biomass, structure and abundance of primary producers and to nutrient load in different ecosystems. Ratios between release rate of inorganic nitrogen by tubificid worms and sedimentation rate of organic nitrogen in the two categories of lakes were 8.3 and 6.4% respectively. Contribution of nitrogen released daily from sediments to the dissolved inorganic nitrogen load in the water column was less than 0.5%. However, in A₁ and A₂ lakes, the released nitrogen had a potential to sustain 24.74 and 8.01%, respectively, of the annual phytoplankton production. These values suggest the significance of tubificids in keeping the eutrophication process at a high level, especially during the periods when nitrogen is the main limiting factor for phytoplankton production.     

Nitrogen fixation by white clover in pastures grazed by dairy cows: Temporal variation and effects of nitrogen fertilization

Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N₂ fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha^–1 and received urea at 0, 200 or 400 kg N ha^–1 yr^–1 in 8–10 split applications. A fourth farmlet received 400 kg N ha^–1 yr^–1 and had 4.4 cows ha^–1.There was large variation in annual clover production and total N₂ fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha^–1 yr^–1. Despite this variation, total pasture production in the 0 N treatment remained at 75–85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years.Fertilizer N application decreased the average proportion of clover N derived from N₂ fixation (P_N; estimated by ¹⁵N dilution) from 77% in the 0 N treatment to 43–48% in the 400 N treatments. The corresponding average total N₂ fixation decreased from 154 kg N ha^–1 yr^–1 to 39–53 kg N ha^–1 yr^–1. This includes N₂ fixation in clover tissue below grazing height estimated at 70% of N₂ fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N₂ fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N₂ fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons.In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease P_N from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N₂ fixation because lower pasture mass reduced grass competition, particularly during spring.     

Increased nutrient loading and rapid changes in phytoplankton expected with climate change in stratified South European lakes: sensitivity of lakes with different trophic state and catchment properties

We hypothesised that increasing winter affluence and summer temperatures, anticipated in southern Europe with climate change, will deteriorate the ecological status of lakes, especially in those with shorter retention time. We tested these hypotheses analysing weekly phytoplankton and chemistry data collected over 2 years of contrasting weather from two adjacent stratified lakes in North Italy, differing from each other by trophic state and water retention time. Dissolved oxygen concentrations were higher in colder hypolimnia of both lakes in the second year following the cold winter, despite the second summer was warmer and the lakes more strongly stratified. Higher loading during the rainy winter and spring increased nutrient (N, P, Si) concentrations, and a phytoplankton based trophic state index, whilst the N/P ratio decreased in both lakes. The weakened Si limitation in the second year enabled an increase of diatom biovolumes in spring in both lakes. Chlorophyll a concentration increased in the oligo-mesotrophic lake, but dropped markedly in the eutrophic lake where the series of commonly occurring cyanobacteria blooms was interrupted. The projected increase of winter precipitation in southern Europe is likely to increase the nutrient loadings to lakes and contribute to their eutrophication. The impact is proportional to the runoff/in-lake concentration ratio of nutrients rather than to the retention time, and is more pronounced in lakes with lower trophy.     

Controlling Cyanobacterial Blooms in Hypertrophic Lake Taihu,China: Will Nitrogen Reductions Cause Replacement of Non-N2 Fixing by N2 Fixing Taxa?

Excessive anthropogenic nitrogen (N) and phosphorus (P) inputs have caused an alarming increase in harmful cyanobacterial blooms, threatening sustainability of lakes and reservoirs worldwide. Hypertrophic Lake Taihu, China’s third largest freshwater lake, typifies this predicament, with toxic blooms of the non-N₂ fixing cyanobacteria Microcystis spp. dominating from spring through fall. Previous studies indicate N and P reductions are needed to reduce bloom magnitude and duration. However, N reductions may encourage replacement of non-N₂ fixing with N₂ fixing cyanobacteria. This potentially counterproductive scenario was evaluated using replicate, large (1000 L), in-lake mesocosms during summer bloom periods. N+P additions led to maximum phytoplankton production. Phosphorus enrichment, which promoted N limitation, resulted in increases in N₂ fixing taxa (Anabaena spp.), but it did not lead to significant replacement of non-N₂ fixing with N₂ fixing cyanobacteria, and N₂ fixation rates remained ecologically insignificant. Furthermore, P enrichment failed to increase phytoplankton production relative to controls, indicating that N was the most limiting nutrient throughout this period. We propose that Microcystis spp. and other non-N₂ fixing genera can maintain dominance in this shallow, highly turbid, nutrient-enriched lake by outcompeting N₂ fixing taxa for existing sources of N and P stored and cycled in the lake. To bring Taihu and other hypertrophic systems below the bloom threshold, both N and P reductions will be needed until the legacy of high N and P loading and sediment nutrient storage in these systems is depleted. At that point, a more exclusive focus on P reductions may be feasible.     

Study of operational conditions of simultaneous nitrification and denitrification in a Carrousel oxidation ditch for domestic wastewater treatment

The study on the operational conditions of simultaneous nitrification and denitrification (SND) in the channel of oxidation ditch (OD) without the need for a special anoxic tank was carried out based on lab-scale and pilot-scale experiments using real domestic wastewater. The influence of sludge loading and component proportion in influent, temperature, hydraulic retention time (HRT), dissolved oxygen (DO) and operational mode on SND was investigated. The result indicated that the optimal DO (ODO) of SND occurrence was confirmed majorly by the sludge loading of influent and temperature, the high TCOD/NH₃–N and short HRT can enhance the occurrence of SND. A new operational mode was proposed that achieved a higher removal efficiency of 60–70% for total nitrogen by SND with HRT of 4–6 h, and the concentrations of NH₃–N and TN in effluent are less than 5 and 15 mg/L, respectively.