Nitrogen dynamics and microbial food web structure during a summer cyanobacterial bloom in a subtropical,shallow, well-mixed,eutrophic lake (Lake Taihu,China)

Nitrogen dynamics and microbial food web structure were characterized in subtropical, eutrophic, large (2,338 km²), shallow (1.9 m mean depth), and polymictic Lake Taihu (China) in Sept–Oct 2002 during a cyanobacterial bloom. Population growth and industrialization are factors in trophic status deterioration in Lake Taihu. Sites for investigation were selected along a transect from the Liangxihe River discharge into Meiliang Bay to the main lake. Water column nitrogen and microbial food web measurements were combined with sediment–water interface incubations to characterize and identify important processes related to system nitrogen dynamics. Results indicate a gradient from strong phosphorus limitation at the river discharge to nitrogen limitation or co-limitation in the main lake. Denitrification in Meiliang Bay may drive main lake nitrogen limitation by removing excess nitrogen before physical transport to the main lake. Five times higher nutrient mineralization rates in the water column versus sediments indicate that sediment nutrient transformations were not as important as water column processes for fueling primary production. However, sediments provide a site for denitrification, which, along with nitrogen fixation and other processes, can determine available nutrient ratios. Dissimilatory nitrate reduction to ammonium (DNRA) was important, relative to denitrification, only at the river discharge site, and nitrogen fixation was observed only in the main lake. Reflecting nitrogen cycling patterns, microbial food web structure shifted from autotrophic (phytoplankton dominated) at the river discharge to heterotrophic (bacteria dominated) in and near the main lake.     

Seasonal pattern in nutrient limitation and grazing control of the phytoplankton community in a non-stratified lake

1. The relative importance of zooplankton grazing and nutrient limitation in regulating the phytoplankton community in the non-stratified Lake Kvie, Denmark, were measured nine times during the growing season.
2. Natural phytoplankton assemblage bioassays showed increasing importance of nutrient limitation during summer. Growth rates at ambient nutrient concentrations were continually below 0.12 per day, while co-enrichment with nitrogen (N) and phosphorus (P) to above concentration-saturated conditions enhanced growth rates from May to the end of July.
3. Stoichiometric ratios of important elements in seston (C : N, C : P, N : P), in lake water (TN : TP), in external loading (TN : TP) and in internal loading (DIN : DIP) were measured to determine whether N or P could be the limiting nutrient. TN : TP molar ratio of both lake water, benthic fluxes and external loading suggested P limitation throughout the growing season. However, seston molar ratios suggested moderate P-deficiency only during mid-summer.
4. Abundance and community structure of the zooplankton varied considerably through the season and proved to be important in determining the responses of algal assemblages to grazing. High abundance of cladocerans and rotifers resulted in significant grazing impact, while cyclopoid copepods had no significant effect on the phytoplankton biomass.
5. Regeneration of ammonium and phosphate by zooplankton were periodically important for phytoplankton growth. A comparison of nutrient regeneration by zooplankton with nutrient inputs from sediment and external sources indicated that zooplankton may contribute significantly in supplying N and P for the growth of phytoplankton.     

Effects of sediment resuspension on phytoplankton production: teasing apart the influences of light, nutrients and algal entrainment

1. Wind‐induced sediment resuspension can affect planktonic primary productivity by influencing light penetration and nutrient availability, and by contributing meroplankton (algae resuspended from the lake bed) to the water column. We established relationships between sediment resuspension, light and nutrient availability to phytoplankton in a shallow lake on four occasions. 2. The effects of additions of surficial sediments and nutrients on the productivity of phytoplankton communities were measured in 300 mL gas‐tight bottles attached to rotating plankton wheels and exposed to a light gradient, in 24 h incubations at in situ temperatures. 3. While sediment resuspension always increased primary productivity, resuspension released phytoplankton from nutrient limitation in only two of the four experiments because the amount of available nitrogen and phosphorus entrained from the sediments was small compared with typical baseline levels in the water column. In contrast, chlorophyll a entrainment was substantial compared with baseline water column concentrations and the contribution of meroplankton to primary production was important at times, especially when seasonal irradiance in the lake was high. 4. Comparison of the in situ light climate with the threshold of light‐limitation of the phytoplankton indicated that phytoplankton in the lake were only likely to be light‐limited at times of extreme turbidity (e.g. >200 nephelometric turbidity units), particularly when these occur in winter. Therefore, resuspension influenced phytoplankton production mainly via effects on available nutrients and by entraining algae. The importance of each of these varied in time. 5. The partitioning of primary productivity between the water column and sediments in shallow lakes greatly influences the outcome of resuspension events for water column primary productivity.     

The influence of water quality and sediment geochemistry on the horizontal and vertical distribution of phosphorus and nitrogen in sediments of a large,shallow lake

Distinct horizontal water column concentration gradients of nutrients and chlorophyll a (Chl a) occur within large, shallow, eutrophic Lake Taihu, China. Concentrations are high in the north, where some of the major polluted tributaries enter the lake, and relatively low in the south, where macrophytes generally are abundant. It is not clear, however, whether these water column concentration gradients are similarly reflected in spatial heterogeneity of nutrient concentrations within the bottom sediments. The main objective of this study was therefore to test if horizontal and vertical variations in the phosphorus and nitrogen content in bottom sediments of Lake Taihu are significantly related to (1) horizontal variations in overlying water column nutrient concentrations and (2) other sediment geochemical constituents. We measured the concentration of total phosphorus (TP) and total nitrogen (TN) in surficial sediments (0–2 cm) and TP, TN and Chl a concentrations in water column samples, collected from 32 sites in 2005. In 2006 sediment, TP, TN, carbon, iron and manganese concentrations were measured vertically at 2 cm intervals, extending to a depth of approximately 20 cm, at an additional eight sites. Linear correlation analysis revealed that surficial sediment TP concentrations across the 32 stations were related significantly, though weakly, to annual mean water column concentrations of TP, TN as well as Chl a. Correlations of surficial sediment TN with water column variables were, however, not significant (P > 0.05). Amongst the geochemical variables tested, the vertical variability of sediment TP concentrations was most strongly related to sediment manganese and carbon concentrations. A multiple stepwise linear regression revealed that the combination of sediment manganese and carbon concentrations explained 91% of the horizontal variability in sediment TP concentrations and 65% of the vertical variability. Handling editor: Luigi Naselli-Flores     

Nitrogen and phosphorus accumulation and cycling by Nymphoides peltata (Gmel.) O. Kuntze (Menyanthaceae)

In 1980, the seasonal changes in nitrogen and phosphorus concentration of various plant parts of Nymphoides peltata (Gmel.) O. Kuntze, together with aspects of nitrogen and phosphorus cycling by this species were studied in an oxbow lake of the river Waal (The Netherlands). The nitrogen and phosphorus stores of the water, seston, sediment and macrophyte compartments were assessed each month.The underground Nymphoides structures had high nitrogen and phosphorus concentrations before and after the main growing season, while during summer the aboveground plant parts had high nutrient contents. Nymphoides peltata accumulated maximum amounts of nitrogen (334 mmol m⁻²) and phosphorus (56.6 mmol m⁻²) in July. The upper layers of the bottom appeared to be an enormous nutrient reservoir (94–99% of total) of which the largest part was not directly available to Nymphoides. Nutrient uptake from the sediments by N. peltata is suggested by the fact that the bottom and/or interstitial water of the sample station devoid of rooted macrophytes, contained higher concentrations of nitrogen and phosphorus than that of the Nymphoides stands. The annual flux of nutrients from Nymphoides to the detritus compartment was estimated to be ca. 1200 mmol nitrogen and 164 mmol phosphorus per m² of littoral. During breakdown of the detritus there was a relatively fast net conversion of organically bound nitrogen and phosphorus to inorganic forms, especially at higher temperatures.Nymphoides has the potential to function as an important nitrogen and phosphorus pump, which regenerates sediment nutrients.     

Nutrient ratios, differential retention, and the effect on nutrient limitation in a deep oligotrophic lake

Nutrient ratios have been related to nutrient limitation of algal growth in lakes. Retention of nutrients in lakes, by sedimentation and by denitrification, reduces the nutrient concentrations in the water column, thereby enhancing nutrient limitation. Differential retention of nitrogen and phosphorus alters their ratios in lakes and thereby contributes to determine whether nitrogen or phosphorus limits algal growth. We examined the relationships between differential nutrient retention, nutrient ratios, and nutrient limitation in Lake Brunner, a deep oligotrophic lake. The observed retention of nitrogen (20%) and phosphorus (47%) agreed with predictions by empirical equations from literature. As a result of differential retention with a much larger proportion of phosphorus retained than that of nitrogen, the nitrogen:phosphorus ratio was higher in the lake (69) than in the inflows (46). While the mean ratio in the inflows suggested no or only moderate phosphorus limitation, the lake appeared to be severely phosphorus limited. Combining empirical equations from literature that predict nitrogen and phosphorus retention suggests that the nitrogen:phosphorus ratio is enhanced by greater retention of phosphorus compared to nitrogen only in deep lakes with relatively short residence times, such as Lake Brunner. In contrast, in most lakes differential retention is expected to result in lower nitrogen:phosphorus ratios.     

The influence of Procambarus clarkii (Cambaridae,Decapoda) on water quality and sediment characteristics in a Spanish floodplain wetland

This study describes the effects of the American red swamp crayfish, Procambarus clarkii Girard, on water quality and sediment characteristics in the Spanish floodplain wetland, Las Tablas de Daimiel National Park. Our short term enclosure study during a summer drawdown revealed that crayfish acted as a nutrient pump that transformed and translocated sediment bound nutrients to the water column. Water quality impoverishment was mainly due to the increase of dissolved inorganic nutrients (soluble reactive phosphorus and ammonia), and a significant increase of total suspended solids occurred likely as a result of crayfish associated bioturbation. At the same time, crayfish reduced the content of organic matter in the sediment and we observed a slight increase of total sediment phosphorus and nitrogen content as a result of crayfish benthic activity. P. clarkii effects, in terms of internal nutrient loading (229.91 mg TP m^–2 d^–1), were shown to be important on a local scale, indicating the significance of internal nutrient supply to water column primary producers particularly under low external supply (summer). Extrapolations to the whole ecosystem, however, revealed a negligible crayfish contribution (0.06%) to total internal nutrient loading (0.035 mg TP m^–2 d^–1). Hence, crayfish spatial heterogeneity patterns are important in global and local matter fluxes and nutrient cycles in wetlands.     

Iron Constraints on Planktonic Primary Production in Oligotrophic Lakes

Phototrophic primary production is a fundamental ecosystem process, and it is ultimately constrained by access to limiting nutrients. Whereas most research on nutrient limitation of lacustrine phytoplankton has focused on phosphorus (P) and nitrogen (N) limitation, there is growing evidence that iron (Fe) limitation may be more common than previously acknowledged. Here we show that P was the nutrient that stimulated phytoplankton primary production most strongly in seven out of nine bioassay experiments with natural lake water from oligotrophic clearwater lakes. However, Fe put constraints on phytoplankton production in eight lakes. In one of these lakes, Fe was the nutrient that stimulated primary production most, and concurrent P and Fe limitation was observed in seven lakes. The effect of Fe addition increased with decreasing lake water concentrations of total phosphorus and dissolved organic matter. Possible mechanisms are low import rates and low bioavailability of Fe in the absence of organic chelators. The experimental results were used to predict the relative strength of Fe, N, and P limitation in 659 oligotrophic clearwater lakes (with total phosphorus ≤ 0.2 μM P and total organic carbon < 6 mg C l⁻¹) from a national lake survey. Fe was predicted to have a positive effect in 88% of these lakes, and to be the nutrient with the strongest effect in 30% of the lakes. In conclusion, Fe, along with P and N, is an important factor constraining primary production in oligotrophic clearwater lakes, which is a common lake-type throughout the northern biomes. This paper is dedicated to the memory of Prof. Peter Blomqvist (deceased 2004).     

Nutrient limitation of phytoplankton communities in Subarctic lakes and ponds in Wapusk National Park, Canada

We determined the limiting nutrient of phytoplankton in 21 lakes and ponds in Wapusk National Park, Canada, using nutrient enrichment bioassays to assess the response of natural phytoplankton communities to nitrogen and phosphorus additions. The goal was to determine whether these Subarctic lakes and ponds were nutrient (N or P) limited, and to improve the ability to predict future impacts of increased nutrient loading associated with climate change. We found that 38% of lakes were not limited by nitrogen or phosphorus, 26% were co-limited by N and P, 26% were P-limited and 13% were N-limited. TN/TP, DIN/TP and NO₃ ⁻/TP ratios from each lake were compared to the Redfield ratio to predict the limiting nutrient; however, these predictors only agreed with 29% of the bioassay results, suggesting that nutrient ratios do not provide a true measure of nutrient limitation within this region. The N-limited lakes had significantly different phytoplankton community composition with more chrysophytes and Anabaena sp. compared to all other lakes. N and P limitation of phytoplankton communities within Wapusk National Park lakes and ponds suggests that increased phytoplankton biomass may result in response to increased nutrient loading associated with environmental change.     

Wind-induced increases in trophic state characteristics of a large (27 km 2), shallow (1.5 m mean depth) Florida lake

Nutrient and chlorophyll concentrations in Lake Newnan (27 km², mean depth 1.5 m), Florida showed dramatic increases from 1991 to 1998. Historical data showed Lake Newnan never had sufficient aquatic macrophyte abundance for a shift in alternate stable states to account for increases in trophic state characteristics. External phosphorus and nitrogen loads from incoming streams were monitored from August 1997 to July 1998 to determine if external supplies of nutrients were responsible for increases in lake nutrient and chlorophyll concentrations. During the study period, external nutrient loading rates were not correlated to lake nutrient concentrations. Phosphorus and nitrogen models based on the external loading estimates predicted the lake total phosphorus and total nitrogen concentrations to be 370% and 680% less, respectively, than the observed lake total phosphorus and total nitrogen mean concentrations. Consequently, phosphorus and nitrogen exports were 280% and 540% greater, respectively, than stream input loading. Data during the study period revealed strong inverse relations between lake stage and total phosphorus (r=–0.78), total nitrogen (r=–0.71), and chlorophyll (r=–0.90) concentrations. Long-term data (1965–1998) also revealed inverse correlations (r=–0.48 to –0.52) between lake stage and total phosphorus, total nitrogen, and chlorophyll concentrations. Applying fundamental wave theory and using a bathymetric map, it is probable that as much as 70% of the lake bottom sediment could be subjected to resuspension 50% of the time when the lake stage falls below 19.9 m mean sea level (msl). Above a lake stage of 19.90 m msl, less than 20% of the lake bottom sediment can potentially be resuspended 50% of the time. A percent frequency distribution from 1991 to 1998 showed that over 30% of the lake stages fell below 19.9 m msl. However, from 1967 to 1990, only 8% of the lake stage values fell below 19.9 m msl. Increases in total phosphorus, total nitrogen and chlorophyll concentrations in Lake Newnan were likely caused by an increased probability of internal loading due to decreased lake levels, and not to external loading of phosphorus and nitrogen.     

Determination of the Limiting Plant Nutrient in the Water of Lake Balaton by Algal Assay Procedure

Algal assays were performed on water samples taken from different sites in Lake Balaton. Selenastrum capricornutum was used in the test to determine the primarily limiting plant nutrient. The results of supplementary nutrient additions to the bottle tests were evaluated by cell counts. The tests have indicated phosphorus as growth rate limiting, and in two cases as biomass limiting plant nutrient, but periodic occurrences of nitrogen limitation cannot be excluded.     

Dynamics of Nutrient Contents (Phosphorus,Nitrogen) in Water,Sediment and Plants After Restoration of Connectivity in Side‐Channels of the River Rhine

During the last century, canalization of the Rhine river led to disconnection of side‐arms, over‐sedimentation of these channels, loss of the fluvial dynamics, and aquatic vegetation change or disappearance. Recent restoration projects aim to reconnect disconnected arms to the main channel. The objective of this study was to assess the nutrient dynamics in restored channels during the vegetation colonization process. In spring, summer, and autumn 2009, the phosphorus and nitrogen contents were measured in water, sediment, and plants, sampled in six channels, two reference sites and four restored ones at different dates. Aquatic vegetation was monitored during the same period. Sites were mesotrophic related to the water nutrient concentrations. However, vegetation communities indicated a eutrophic level, as they were dominated by species like Elodea nuttallii, Myriophyllum spicatum, and Potamogeton perfoliatus. Sites were discriminated by P content and mineral nitrogen in the sediment. We showed an effect of species and season on the plant nutrient content, but there was no relationship between plant nutrient content and nutrients in water and sediment. A negative correlation between mean N plant content and the cover of each species was found. Vegetation characteristics (species richness and cover) and bioavailable phosphorus in the sediment were also correlated. In the restored side‐arms of the river Rhine, phosphorus‐rich sediment seems to be important in the recolonization dynamics, as it was linked to higher species richness, whereas nitrogen played a role in the colonization patterns as a growth limiting factor.     

Bioassays with Stigeoclonium Kütz. (Chlorophyceae) to identify nitrogen and phosphorus limitations

Eleven strains of the filamentous algae genus Stigeoclonium Kütz. (Chlorophyceae) were tested as bioassay organisms to identify nitrogen and phosphorus limitations to growth in a variety of waters. The assay results were related to the nutrient status of the waters during the year. Optimal growth was obtained at an inorganic phosphorus concentration of 0.65 mg P 1^?1 and an inorganic nitrogen concentration of 3.75 mg N 1^?1. The algal growth potentials were generally higher in November and February than in August. In summer, nitrogen became important as a limiting nutrient. When ratios of inorganic nitrogen to inorganic phosphorus (N/P) were above 6.2, phosphorus was primarily limiting and at lower N/P ratios nitrogen became the important limiting factor. Stigeoclonium proved to be a suitable bioassay organism to identify nutrient limitation in freshwaters.     

Lake‐type‐specific seasonal patterns of nutrient limitation in German lakes,with target nitrogen and phosphorus concentrations for good ecological status

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杭州西湖北里湖沉积物氮磷内源静态释放的季节变化及通量估算

通过采集北里湖不同季节的柱状芯样,在实验室静态模拟沉积物氨氮(NH+4-N)和可溶解性磷酸盐(PO3-4-P)的释放,同时研究了沉积物间隙水中NH+4-N及PO3-4-P的垂直分布特征.结果表明,沉积物间隙水NH+4-N随深度的增加有上升的趋势,PO3-4-P随深度的增加呈先升后降的趋势.氮、磷营养盐在沉积物—水界面均存在浓度梯度,表明存在自间隙水向上覆水扩散的趋势.沉积物NH+4-N在春季、夏季、秋季、冬季的释放速率分别为0.074 mg·m-2· d-1、0.340mg· m-2· d-1、0.087 mg· m-2· d-1、0.0004 mg·m-2·d-1,pO3-4-P的释放速率则分别为0.340 mg·m-2·d-1、0.518 mg·m-2·d-1、0.094 mg·m-2·d-1、-0.037 mg· m-2·d-1.不同采样点表现出明显的季节和空间差异性,释放速率表现为夏季＞春季、秋季＞冬季.根据静态模拟出的不同季节下内源氮、磷释放速率计算,全湖内源氮、磷营养盐的贡献分别为0.0037、0.0057t/a.该研究可为北里湖富营养化及内源污染的治理提供基础数据.     

The application of algal growth potential tests (AGP) to the canals and lakes of western Netherlands

Four hundred and forty bioassays with Scenedesmus quadricauda (Turp.) Bréb. as a test organism have been carried out with samples from canals and lakes in the western part of the Netherlands. The results are used to assess the algal growth potential (AGP) and to determine the limiting nutrient(s) for maximum biomass production. Special attention has been paid to the effects of deep-freezing and autoclaving as pretreatment of water samples on pH and nutrient concentrations.The AGP ranged from very low in the relatively isolated polder lakes to very high in canals and lakes, which form part of the basin system of Rijnland. The lowest yields are observed in nitrogen and phosphorus co-limited waters, while the highest are found in waters limited by nitrogen alone. AGP proved to be primarily determined by the amount of nitrogen, especially nitrate, in the samples and only secondarily by the amount of phosphorus.The observed ranges indicating phosphorus limitation, > 50 for inorganic and > 30 for total N/P ratios, lie considerably higher than reported so far. It is concluded that, once the relations between AGP and nutrients are established, AGP tests do not have to be carried out routinely, but still can be very useful in special studies, e.g. in lake restoration projects.     

The trophic history of Enäjärvi,SW Finland,with special reference to its restoration problems

The eutrophicated Enäjärvi was studied by paleolimnological analyses and sediment mapping. The sedimentary record indicates that the lake nutrient balance had deteriorated due to lowering of the lake water level in the year 1928. From that event onwards Chironomus plumosus and Cyclotella astraea characterize the chironomid and diatom communities. The concentrations of sedimentary total and mobile phosphorus show that since then the internal load of phosphorus has controlled the nutrient cycle of the lake. The areal distribution of mobile phosphorus can be explained by dominant wind directions and wind resuspension of the sediment is the key factor in the nutrient cycle. Restoration of Enäjärvi must be based on actions which stabilize the surface sediment and improve its natural phosphorus-binding capacity. They include the regulation of lake water to as high a level as possible and the removal of the majority of roach.     

Relative influences of submersed macrophytes and bioturbating fauna on biogeochemical processes and microbial activities in freshwater sediments

1. Invertebrates and aquatic plants often play a key role in biogeochemical processes occurring at the water–sediment interface of aquatic ecosystems. However, few studies have investigated the respective influences of plants and bioturbating animals on ecological processes (nutrient fluxes, benthic oxygen uptake, microbial activities) occurring in freshwater sediments. 2. We developed a laboratory experiment in aquaria to quantify the effects of (i) one invertebrate acting as a bioturbator (Tubifex tubifex); (ii) one submersed plant with a high sediment‐oxygenating potential (Myriophyllum spicatum) and (iii) one submersed plant with a low sediment‐oxygenating potential (Elodea canadensis). 3. The tubificid worms significantly increased the fluxes of nitrogen at the water–sediment interface (influx of nitrate, efflux of ammonium), whereas the two plant species did not have significant influences on these nitrogen fluxes. The differences in nitrogen fluxes between tubificid worms and plants were probably due to the bioirrigation process caused by T. tubifex, which increased water exchanges at the water–sediment interface. Tubifex tubifex and M. spicatum produced comparable reductions of nutrient concentrations in pore water and comparable stimulations of benthic oxygen uptake and microbial communities (percentages of active eubacteria and hydrolytic activity) whereas E. canadensis had a very weak influence on these variables. These differences between the two plants were due to their contrasting abilities to increase oxygen in sediments by radial oxygen losses (release of oxygen from roots). 4. Our study suggests that the bioirrigation process and radial oxygen loss are major functional traits affecting biogeochemical functioning at the water–sediment interface of wetlands.     

Effects of nitrogen,phosphorus and carbon enrichment on planktonic and periphytic algae in a softwater,oligotrophic lake in Florida,USA

The objective of this study was to investigate nutrient limitation of algal abundance in Anderson-Cue Lake, a softwater clear oligotrophic lake in north-central Florida. Nutrient diffusing clay pots and cylindrical enclosures were used in the field to test effects of different combinations of nitrogen, phosphorus, silica, and carbon on algal standing crop and composition of periphytic and planktonic algae, respectively. Effects of nutrient enrichment on periphytic algae were examined in two studies conducted 31 May – 8 July and 10 June – 15 July 1991. Nutrient effects on planktonic algae were examined in one study from 13 June – 1 July 1991. Planktonic and periphytic algal biovolume was significantly higher (p<0.05) when nitrogen and carbon were added in combination than with treatments without nitrogen, carbon, or nitrogen and carbon. Treatments with nitrogen and carbon combined resulted in lower algal diversity and dominance by coccoid green algae andScenedesmus. Results indicate that carbon and nitrogen can be limiting factors to algal growth in Anderson-Cue Lake and possibly other lakes of similar water quality.     

Changes in N and P limitation induced by water level fluctuations in Nature Park Kopački Rit (Croatia): nutrient enrichment bioassay

The importance of nutrients as limiting factors might vary in environments with different limnological characteristics. In this article we evaluate the effect of frequency and duration of flooding on nutrient limitation in a riparian floodplain. Variations in N and P limitations were studied in the period of low (2003) and high (2004) water level in two different floodplain habitat types in the Nature Park Kopački Rit (Croatia), a floodplain area of the Danube River. In 2003 and 2004, the limnological characteristics of floodplain lake (Lake Sakadaš) and the channel (Stara Drava) differed due to their hydrological regimes. Potential for nutrient limitation was determined by DIN:TP and TN:TP ratios, while the actual nutrient limitation was assessed by nutrient enrichment bioassay. A change from non-limited to N-limited conditions in the channel, and consistent actual N limitation in the lake was determined by the nutrient enrichment experiment. Of the two ratios, DIN:TP matched better with the bioassay data. Phosphorus limitation was only occasionally evident. Changes in trophic conditions from hypereutrophy to eutrophy (between low and high water levels) reflected the importance of the hydrologic regime as a factor which can modify the trophic state of Lake Sakadaš. Compared with 2003, the increase of total nitrogen concentrations in 2004 calls attention to the importance of nitrogen inputs from the Danube to the system with excessive phosphorus concentrations.