Effects of irradiance on diel and seasonal patterns of nutrient uptake by stream periphyton

1. Irradiance strongly affects the abundance of stream periphyton communities that in turn influence patterns of instream nutrient uptake. We examined relationships between irradiance and periphyton nutrient uptake taking into account diel and seasonal variation in ambient irradiance. 2. Uptake of dissolved N, P and C by periphyton as areal uptake (U) and demand (V_f) was determined under 11 irradiance levels (0–100% of ambient conditions) using shallow stream‐side experimental channels. Experiments were conducted once per season over one annual cycle with both day and night uptake rates assessed, together with periphyton biomass and autotrophic production rates. 3. No consistent diel variation in areal uptake or demand was detected for the predominant inorganic or total dissolved nutrients even at the highest irradiances. Lack of variation may indicate nutrient limitation, with photosynthetic sequestration and storage of C during the day for subsequent utilisation at night. Alternatively, oxygen consumption by photoautotrophs at night may stimulate compensatory heterotrophic uptake (e.g. denitrification). 4. In all seasons, release of dissolved organic N was detected during the day but to a lesser extent at night. This was not directly related to irradiance levels, indicating that heterotrophic metabolism (e.g. microbial decomposition) contributes to this phenomenon. 5. Areal uptake and demand for the predominant inorganic and total dissolved nutrients increased in response to increasing irradiance in some or all seasons, but rates were typically higher during the spring and summer. Saturation of areal uptake and demand at elevated irradiances was evident during the spring. demand was also saturated at higher irradiances in the summer and autumn. Maximum demand was comparable during spring and summer, but saturation occurred at lower irradiance in summer (24 h average 135–145 μmol m^?2 s^?1) relative to spring (312–424 μmol m^?2 s^?1), indicating more efficient nutrient uptake in summer. Higher total periphyton biomass in summer, but comparable autotrophic biomass (chlorophyll a), implies that heterotrophic metabolism may contribute to this greater efficiency. In spring, autotrophic biomass peaked at an irradiance level of 225 μmol m^?2 s^?1, also suggesting a role for heterotrophic metabolism in demand at higher irradiances. 6. The results of this study show that irradiance levels exert a strong influence on the nature and quantity of instream nutrient uptake with N demand saturated at elevated irradiance levels during the spring, summer and autumn. Our results also suggest that heterotrophic metabolism makes a measurable contribution to instream nutrient uptake even under higher irradiances that favour autotrophic activity.     

抚仙湖不同类型岸带沉水植物分布及水体氮磷特征

2005年6-7月,通过野外断面采样并结合水质分析,对抚仙湖不同类型岸带沉水植物分布及水体氮磷特征进行了调查.结果表明:抚仙湖岸带分为4个类型,岩石陡峭岸带、冲积平缓岸带、河口岸带以及湖湾岸带;沉水植物种类分布最多出现在河口型岸带和湖湾岸带,均为9种;最低出现在岩石陡峭岸带,为5种;沉水植物生物量最高出现在冲积平缓岸带,为8 300 g·m-2,最低出现在岩石陡峭岸带,为2416 g·m-2;沉水植物分布最深为A1岩石陡峭岸带,平均水深8.5 m,最深达到11.0 m;最浅为B2冲积平缓岸带,平均水深1.9 m,最深为6 m;岸带水体综合氮磷指标,以D2湖湾岸带最高,总氮、总磷分别达到5.34和0.145 mg·L-;最低为A2岩石陡峭岸带,总氮、总磷分别为0.87和0.015 mg·L-1抚仙湖沉水植物对水体氮磷的吸收固定总量约为总氮为5.28×104 kg,总磷为7 500 kg.     

Nutrient uptake in streams draining agricultural catchments of the midwestern United States

1. Agriculture is a major contributor of non‐point source pollution to surface waters in the midwestern United States, resulting in eutrophication of freshwater aquatic ecosystems and development of hypoxia in the Gulf of Mexico. Agriculturally influenced streams are diverse in morphology and have variable nutrient concentrations. Understanding how nutrients are transformed and retained within agricultural streams may aid in mitigating increased nutrient export to downstream ecosystems. 2. We studied six agriculturally influenced streams in Indiana and Michigan to develop a more comprehensive understanding of the factors controlling nutrient retention and export in agricultural streams using nutrient addition and isotopic tracer studies. 3. Metrics of nutrient uptake indicated that nitrate uptake was saturated in these streams whereas ammonium and phosphorus uptake increased with higher concentrations. Phosphorus uptake was likely approaching saturation as evidenced by decreasing uptake velocities with concentration; ammonium uptake velocity also declined with concentration, though not significantly. 4. Higher whole‐stream uptake rates of phosphorus and ammonium were associated with the observed presence of stream autotrophs (e.g. algae and macrophytes). However, there was no significant relationship between measures of nutrient uptake and stream metabolism. Water‐column nutrient concentrations were positively correlated with gross primary production but not community respiration. 5. Overall, nutrient uptake and metabolism were affected by nutrient concentrations in these agriculturally influenced streams. Biological uptake of ammonium and phosphorus was not saturated, although nitrate uptake did appear to be saturated in these ecosystems. Biological activity in agriculturally influenced streams is higher relative to more pristine streams and this increased biological activity likely influences nutrient retention and transport to downstream ecosystems.     

Growth and nutrient uptake by two species of Elodea in experimental conditions and their role in nutrient accumulation in a macrophyte-dominated lake

The capacity of Elodea nuttallii (Planch.) St. John and Elodea canadensis Michx. to remove nitrogen from water was evaluated in laboratory experiment. The growth rate of plants and their effect on the nitrogen level of hypertrophic Lake Zwemlust (the Netherlands) as well as on lake water enriched with nitrogen were investigated. The plants grew best in water enriched with up to 2 mg NH₄-Nl^–1 and 2 mg NH₄-Nl^–1 plus 2 mg NO₃ Nl^–1. During a 14 day experiment, plants absorbed from 75% to 90% of nitrogen. Higher nitrogen concentration than 4 mg l^–1 had a negative effect on growth of both species. Elodea nuttallii and E. canadensis prefer NOinf4/^p+ over NOinf4/^p– when both ions were present in water in equal concentrations.     

Modelling of Nutrient Emissions in River Systems – MONERIS – Methods and Background

MONERIS is a semi‐empirical, conceptual model, which has gained international acceptance as a robust meso‐ to macro scale model for nutrient emissions. MONERIS is used to calculate nitrogen (N) and phosphorus (P) emissions into surface waters, in‐stream retention, and resulting loads, on a river catchment scale. This paper provides the first (i) comprehensive overview of the model structure (both the original elements and the new additions), (ii) depiction of the algorithms used for all pathways, and for retention in surface waters, and (iii) illustration of the monthly disaggregation of emissions and the implementation of measures. The model can be used for different climatic conditions, long term historical studies, and for future development scenarios. The minimum validated spatial resolution is 50 km², with a temporal resolution of yearly or monthly time steps. The model considers seven emission pathways (atmospheric deposition on surface waters, overland flow, erosion, tile drainage, groundwater, emissions from sealed urban areas, and point sources), and six emission sources (natural background, fertilizer application, nitrogen atmospheric deposition on arable land and other areas, urban sources, and point sources); and these are calculated separately for different land‐uses. The pathway and source‐related approach is a prerequisite for the implementation of measures to reduce non‐point and point‐source emissions. Therefore, we have modified MONERIS by the addition of a “management alternative” tool which can identify the potential effectiveness of nutrient reduction measures. MONERIS is an appropriate tool for addressing the scientific and political aspects of river basin management in support of a good surface water quality. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nitrogen stable isotope ratios in surface sediments, epilithon and macrophytes from upland lakes with differing nutrient status

1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were each visited once during June and July 2000. From each lake, samples of surface sediment epilithon, macrophytes and total dissolved nitrogen (TDN) were collected for nitrogen stable isotope analysis. As part of a wider programme, samples were also collected for chemical analysis and bioassays. 2. Considerable variation was found in δ¹⁵N values in all measured nitrogen compartments. Some regional variation was evident but was generally weak. Sediment and epilithon δ¹⁵N were positively correlated with δ¹⁵N of TDN, suggesting that baseline nitrogen isotope ratios influence those in some organic matter compartments in the lakes. 3. Sediment δ¹⁵N was higher when inorganic nitrogen concentration in the water was low, possibly reflecting reduced isotope fractionation under these conditions. However, this was not the case for epilithon or macrophytes. Sediment δ¹⁵N values were also negatively related to annual nitrogen deposition. 4. Sediment, epilithon and macrophyte δ¹⁵N values all showed significant relations to nutrient limitation in the lakes as determined by algal bioassays. We suggest that sediment δ¹⁵N might be developed as a simple integrating measure of the degree of nitrogen limitation in lakes.     

The effects of varying culture nitrogen and phosphorus levels on nutrient uptake and storage by the waterhyacinth Eichhornia crassipes (Mart) Solms

The uptake of nitrogen (N) by waterhyacinth (Eichhornia crassipes) was maximal when the culture solution contained a combination of 36 ppm N and 6,53 ppm phosphorus (P). N uptake was inhibited by increasing P or decreasing N in the culture medium. Uptake of P was stimulated by the elevation of either N or P or both. An increase of P above O ppm inhibited the accumulation of N. Similarly, the accumulation of P was enhanced by increasing levels of P but was reduced with increasing levels of N. Both N and P levels were always greatest in leaves; next highest in floats and lowest in roots. High tissue levels of these two nutrients and the capacity for nutrient removal from water by waterhyacinth indicates a possible role for this plant in the reduction of eutrophication in nutrient polluted waters as well as the use of harvested, dried tissue for fertilizer.     

The impact of urbanization on seasonal hydrologic and nutrient budgets of a small North American watershed

Present day hydrologic and nutrient budgets have been measured and presettlement budgets estimated for a small urban watershed in Madison, Wisconsin. The importance of different seasons and sources to the total loading were compared for past and present conditions. The seasonal budget revealed that while spring contributes the greatest loadings in both scenarios, summer exhibits the greatest change in loadings from past to present. As for sources this seepage lake receives most of its water and N from groundwater; however, most P comes from surface runoff. The greatest impact on the watershed since settlement appears to be the large increase in runoff matched by a decrease in evapotranspiration and subsurface flow. This shift in hydrology is largely responsible for the substantial increase in P loading and the decrease in the N : P ratio of total loading. The significance of the lake's modified morphometry to areal and volumetric loading is also explored.     

Factors affecting ammonium uptake in streams – an inter-biome perspective

1. The Lotic Intersite Nitrogen eXperiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter‐biome variability of ammonium uptake to physical, chemical and biological processes. 2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole‐stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of ¹⁵N‐ammonium and downstream measurements of ¹⁵N‐ammonium concentration. 3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in‐stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake. 4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa). 5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake.     

Global pattern and controls of biological nitrogen fixation under nutrient enrichment: A meta‐analysis

Biological nitrogen (N) fixation (BNF), an important source of N in terrestrial ecosystems, plays a critical role in terrestrial nutrient cycling and net primary productivity. Currently, large uncertainty exists regarding how nutrient availability regulates terrestrial BNF and the drivers responsible for this process. We conducted a global meta‐analysis of terrestrial BNF in response to N, phosphorus (P), and micronutrient (Micro) addition across different biomes (i.e, tropical/subtropical forest, savanna, temperate forest, grassland, boreal forest, and tundra) and explored whether the BNF responses were affected by fertilization regimes (nutrient‐addition rates, duration, and total load) and environmental factors (mean annual temperature [MAT], mean annual precipitation [MAP], and N deposition). The results showed that N addition inhibited terrestrial BNF (by 19.0% (95% confidence interval [CI]: 17.7%?20.3%); hereafter), Micro addition stimulated terrestrial BNF (30.4% [25.7%?35.3%]), and P addition had an inconsistent effect on terrestrial BNF, i.e., inhibiting free‐living N fixation (7.5% [4.4%?10.6%]) and stimulating symbiotic N fixation (85.5% [25.8%?158.7%]). Furthermore, the response ratios (i.e., effect sizes) of BNF to nutrient addition were smaller in low‐latitude (<30°) biomes (8.5%?36.9%) than in mid‐/high‐latitude (≥30°) biomes (32.9%?61.3%), and the sensitivity (defined as the absolute value of response ratios) of BNF to nutrients in mid‐/high‐latitude biomes decreased with decreasing latitude (p ≤ 0.009; linear/logarithmic regression models). Fertilization regimes did not affect this phenomenon (p > 0.05), but environmental factors did affect it (p < 0.001) because MAT, MAP, and N deposition accounted for 5%?14%, 10%?32%, and 7%?18% of the variance in the BNF response ratios in cold (MAT < 15°C), low‐rainfall (MAP < 2,500 mm), and low‐N‐deposition (<7 kg ha^?1 year^?1) biomes, respectively. Overall, our meta‐analysis depicts a global pattern of nutrient impacts on terrestrial BNF and indicates that certain types of global change (i.e., warming, elevated precipitation and N deposition) may reduce the sensitivity of BNF in response to nutrient enrichment in mid‐/high‐latitude biomes.     

Nutrient resorption in wetland macrophytes: comparison across several regions of different nutrient status

This study explored patterns of nutrient resorption in wetland macrophytes to test the prediction that plants from regions with a strong nutrient limitation will show higher resorption of the limiting nutrient. Nitrogen and phosphorus resorption was assessed in macrophytes from marshes of different nutrient status in tropical and temperate regions, and expressed as resorption efficiency (NRE, PRE) and proficiency (NRP, PRP). Macrophytes were grouped into three categories: Typha, graminoids and broadleaved plants. Nitrogen was less limiting than P, consequently N availability varied less than P availability, NRP and NRE were lower, and N resorption was mostly incomplete. NRP was determined more by growth form than by local conditions. The large range of soil P concentrations allowed an exploration of relationships between P availability and resorption along a wide gradient. P-limited macrophytes (N : P > 16) had significantly higher PRP and PRE. Resorption proficiency was found to be a more sensitive indicator of changes in nutrient availability than resorption efficiency. The results confirmed that resorption in wetland macrophytes depends on nutrient availability, and is higher at nutrient-limited sites. A particularly strong relationship was found between resorption indicators and P limitation expressed either as live tissue N : P or soil P.     

Stochastic modelling of nutrient loading and lake ecosystem response in relation to submerged macrophytes and benthivorous fish

SUMMARY 1. The strong stabilising effect of increased submerged macrophytes (charophytes) and benthivorous fish reduction on the clear water state was shown for shallow Lake Veluwe and Lake Wolderwijd. 2. The first two links in the chain of relationships from external phosphorus (P) loading to in‐lake total‐P concentrations to chlorophyll a concentrations to water transparency, showed a significant correlation with the areal fraction of coverage with charophytes. Higher coverages lead to (i) lower ratios of the in‐lake total‐P concentration compared with the volume weighted average concentration in the inlet water, indicating a higher retention of P in the presence of charophytes (ii) lower chlorophyll a to total‐P ratios, indicating a positive effect of charophytes on top‐down control of algae, and (iii) higher water transparency because of lower algal turbidity. Transparency further improved as a result of benthivorous fish reduction and a significant positive correlation between non‐algal turbidity and benthivorous fish biomass. 3. A model was developed taking into account the inherent variability in precipitation and uncertainties in the empirical relationships determining phosphorus export from stream catchments and other sources and eutrophication variables in the receiving lakes. The model was used to compute (i) probability distributions for in‐lake total‐P, chlorophyll a and Secchi Disc transparency in relation to the coverage with charophytes and benthivorous fish biomass, and (ii) exceedence probabilities with respect to critical values for in‐lake total‐P and water transparency for several management scenarios. 4. The effects of an expected rise in external nutrient loading on the in‐lake total‐P and chlorophyll a concentrations and on water transparency can be compensated for by two proposed control measures: (i) extended treatment at a waste water treatment plant directly discharging into Lake Veluwe, and (ii) diverting the outlet of a stream draining a catchment with high fertilisation. The minimal internal charophyte coverage needed to sufficiently stabilise the clear water state and to meet with the objective of a summer mean water transparency of at least 1 m was estimated at well over 30% of the lake area, while the benthivorous fish stock should be maintained at the present level of c. 20 kg ha^?1.     

Annual changes of nitrogen and phosphorus in two aquatic macrophytes (Nymphaea Tuberosa and Ceratophyllum Demersum)

The nitrogen and phosphorus content of Nymphaea tuberosa and Ceratophyllum demersum, in Lake Onalaska, Wisconsin, was studied for a year. On a yearly basis, N. tuberosa exhibited nitrogen and phosphorus differences among seasons and among plant parts. Variation among plant parts was also evident in C. demersum. However, within individual plant structures, no seasonal differences were observed.Funds were provided by the River Studies Center, University of Wisconsin-La Crosse, Thomas O. Claflin, Director.Funds were provided by the River Studies Center, University of Wisconsin-La Crosse, Thomas O. Claflin, Director.     

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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Light and nutrient control phytoplankton biomass responses to global change in northern lakes

Global change affects terrestrial loadings of colored dissolved organic carbon (DOC) and nutrients to northern lakes. Still, little is known about how phytoplankton respond to changes in light and nutrient availability across gradients in lake DOC. In this study, we used results from whole‐lake studies in northern Sweden to show that annual mean phytoplankton biomass expressed unimodal curved relationships across lake DOC gradients, peaking at threshold DOC levels of around 11 mg/L. Whole‐lake single nutrient enrichment in selected lakes caused elevated biomass, with most pronounced effect at the threshold DOC level. These patterns give support to the suggested dual control by DOC on phytoplankton via nutrient (positively) and light (negatively) availability and imply that the lakes' location along the DOC axis is critical in determining to what extent phytoplankton respond to changes in DOC and/or nutrient loadings. By using data from the large Swedish Lake Monitoring Survey, we further estimated that 80% of northern Swedish lakes are below the DOC threshold, potentially experiencing increased phytoplankton biomass with browning alone, and/or combined with nutrient enrichment. The results support the previous model results on effects of browning and eutrophication on lake phytoplankton, and provide important understanding of how northern lakes may respond to future global changes.     

Phosphorus availability and elevated CO2 affect biological nitrogen fixation and nutrient fluxes in a clover-dominated sward

The response of biological nitrogen fixation (BNF) to elevated CO(2) was examined in white clover (Trifolium repens)-dominated swards under both high and low phosphorus availability. Mixed swards of clover and buffalo grass (Stenotaphrum secundatum) were grown for 15 months in 0.2 m2 sand-filled mesocosms under two CO2 treatments (ambient and twice ambient) and three nutrient treatments [no N, and either low or high P (5 or 134 kg P ha(-1)); the third nutrient treatment was supplied with high P and N (240 kg N ha(-1))]. Under ambient CO2, high P increased BNF from 410 to 900 kg ha(-1). Elevated CO2 further increased BNF to 1180 kg ha(-1) with high P, but there was no effect of CO2 on BNF with low P. Allocation of N belowground increased by approx. 50% under elevated CO2 irrespective of supplied P. The results suggest that where soil P availability is low, elevated CO2 will not increase BNF, and pasture quality could decrease because of a reduction in aboveground N.     

Effects of nutrient additions on litter production and nutrient return in a nutrient-poor Cape fynbos ecosystem

Litter production and N and P return were determined at bimonthly intervals for two years in 10×5 m plots, amended with a complete factorial fertilizer addition of N as NH₄NO₃(N_a), P as Ca₃(PO₄)₂(P_a) and a mixture of all essential nutrients excluding N and P (M_a) in a 4–7-year-old post-fire sand-plain lowland fynbos ecosystem, South Africa. Litter production increased with vegetation age, was highly seasonal and peaked from late spring to mid-summer (November to January). No significant differences in annual litter production and N return were found in response to the nutrient treatments, although both tended to increase during the second year in response to N_a and M_a. Phosphorus return increased significantly with P_a, and to a lesser extend, N₃, during the first year, whereas it increased in response to N_a and M_a and decreased in the P_a amended plots during the second year. The nutrient treatments did not result in a change in the timing of the annual peak litter production period or in the plant growth form composition of the litter. The litter layer dry mass and N and P contents increased in response to N_a and M_a, while P_a resulted in an increased P content. The evidence from this study indicates that the vegetative growth of the evergreen sclerophyllous shrubs and hemicryptophytes of sand-plain lowland fynbos is not only limited by N, as shown by other studies on shoot growth and vegetation cover, but also by one or more other nutrients excluding P.     

Effects of nutrient loading,temperature regime and grazing pressure on nutrient limitation of periphyton in experimental ponds

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