Stable isotope variability of meso-zooplankton along a gradient of dissolved organic carbon

1.  The δ¹³C and δ¹⁵N signatures of zooplankton vary with dissolved organic carbon (DOC), but inconsistent and limited taxonomic resolution of previous studies have masked differences that may exist among orders, genera or species and are attributable to dietary and/or habitat differences. Here we investigate differences among the isotopic signatures of five zooplankton taxa ( Daphnia , Holopedium , large Calanoida, small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of DOC concentration.
2.  δ¹³C signatures of Daphnia , small calanoids and large calanoids became more depleted with increasing lake DOC, whereas Holopedium and cyclopoid δ¹³C became enriched with increasing DOC concentration.
3.  The variability of δ¹³C and δ¹⁵N isotopic signatures among zooplankton groups was reduced in high-DOC, compared to low-DOC lakes, especially for δ¹³C. Differences in δ¹³C and POM-corrected δ¹⁵N accounted for up to 33.7% and 19.5% of the variance, respectively, among lakes of varying DOC concentration.
4.  The narrow range of signatures found in higher DOC lakes suggests that different taxa have similar food sources and/or habitats. In contrast, the wide range of signatures in low-DOC lakes suggests that different taxa are exploiting different food sources and/or habitats. Together with the variable trends in zooplankton isotopic signatures along our DOC gradient, these results suggest that food web dynamics within the zooplankton community of temperate lakes will change as climate and lake DOC concentrations change.     

Stable carbon and nitrogen isotope ratios in muscle and epidermis of arctic whales

Collection of minimally invasive biopsy samples has become an important method to establish normal stable isotopes reference ranges in various wildlife species. Baseline data enhance the understanding of feeding ecology, habitat use, and potential food limitation in apparently healthy, free‐ranging cetaceans. Epidermis and muscle were collected from subsistence‐hunted northern Alaskan bowhead (n= 133 epidermis/134 muscle) and beluga whales (n= 42/49) and subsistence‐hunted Russian gray whales (n= 25/17). Additional samples were obtained from gray whales stranded in California (n= 18/11) during mortality events (1999, 2000). Both δ¹⁵N and δ¹³C are trophic position and benthic/pelagic feeding indicators, respectively, in muscle and epidermis. Epidermis is generally enriched in ¹⁵N over muscle, while epidermal ¹³C is more depleted. Lipid extraction does not alter δ¹⁵N in either tissue, but affects epidermal δ¹³C. Nitrogen‐15 is enriched in muscle, but not epidermis of stranded compared to subsistence‐hunted gray whales, indicating probable protein catabolism and nutritional stress in stranded whales. Similarly, epidermal δ¹³C of harvested whales is lower than in stranded whales, suggesting depleted lipid stores and/or food limitation in stranded animals. Epidermal isotope signatures are similar in both present‐day bowheads and in an ancient sample from the Northern Bering Sea region. Although only one specimen, this suggests trophic level of the ancient whale compares to modern bowheads after a millennium.     

Bacteria and algae in stream periphyton along a nutrient gradient

1. Stream riffles in southern Ontario and western Quèbec were sampled for biomass (58 stations from 51 streams) and production (22 stations from 21 streams) of algae and bacteria in periphyton to test the hypothesis that bacteria in benthic biofilms compete with algae for nutrients. 2. Algal and bacterial biomass were positively correlated, as were algal and bacterial production. Bacterial production was also positively correlated to algal and bacterial biomass, but the relationship was not significant. The ratio of algal to bacterial biomass did not vary with nutrients whereas algal production tended to increase with nutrients more rapidly than bacterial production. 3. Instream nitrogen concentrations explained 38–58% of the variability in algal biomass and production. Bacterial abundance explained an additional 9–29% of the residual variance in algal production and biomass. However, the relationship between bacterial abundance and algal production and biomass, once nutrients were taken into account, was positive, in contrast to the predicted effect of competition. 4. Hence, we reject our original hypothesis that bacteria in biofilms compete with algae for nutrients and instead suggest that bacteria and algae in biofilms coexist in an association that offers space and resources to sustain production of both groups of organisms.     

Alteration of trophic interactions between periphyton and invertebrates in an acidified stream: a stable carbon isotope study

The hypothesis according to which proliferation of periphytic algae under acid conditions results from a release of grazing pressure is tested. Stable carbon isotope analysis is used to investigate the autochthonous/allochthonous balance of invertebrate feeding in streamside artificial channels that were experimentally acidified. We find that the relative contribution of autochthonous food sources (epilithon) to total invertebrate biomass was slightly lower (after 1 mo of acidification) or not altered (after 2 mo) under acidified conditions when compared with a control. Feeding shifts were exhibited by some invertebrate taxa and provided evidence that acidification modifies trophic interactions between attached algae and primary consumers. Cross-treatment calculations showed that reduction of grazing pressure after the first month of acidification was an effect rather than the cause of periphyton proliferation. Our approach using stable carbon isotope analysis and biomass measurements of macroinvertebrates allows the quantification of the trophic base of lotic secondary producer communities under both experimental and natural conditions.     

Nutrient limitation of algal periphyton in streams along an acid mine drainage gradient

Metal oxyhydroxide precipitates that form from acid mine drainage (AMD) may indirectly limit periphyton by sorbing nutrients, particularly P. We examined effects of nutrient addition on periphytic algal biomass (chl a), community structure, and carbon and nitrogen content along an AMD gradient. Nutrient diffusing substrata with treatments of +P, +NP and control were placed at seven stream sites. Conductivity and SO₄ concentration ranged over an order of magnitude among sites and were used to define the AMD gradient, as they best indicate mine discharge sources of metals that create oxyhydroxide precipitates. Aqueous total phosphorous (TP) ranged from 2 to 23 μg · L^?1 and significantly decreased with increasing SO₄. Mean chl a concentrations at sites ranged from 0.2 to 8.1 μg · cm^?2. Across all sites, algal biomass was significantly higher on +NP than control treatments (Co), and significantly increased with +NP. The degree of nutrient limitation was determined by the increase in chl a concentration on +NP relative to Co (response ratio), which ranged from 0.6 to 9.7. Response to nutrient addition significantly declined with increasing aqueous TP, and significantly increased with increasing SO₄. Thus, nutrient limitation of algal biomass increased with AMD impact, indicating metal oxyhydroxides associated with AMD likely decreased P availability. Algal species composition was significantly affected by site but not nutrient treatment. Percent carbon content of periphyton on the Co significantly increased with AMD impact and corresponded to an increase in the relative abundance of Chlorophytes. Changes in periphyton biomass and cellular nutrient content associated with nutrient limitation in AMD streams may affect higher trophic levels.     

Invertebrate feeding and emergence timing vary among streams along a gradient of riparian forest composition

1. The quality of allochthonous organic matter influences the transfer of energy and nutrients through recipient food webs. We investigated the effects of variation in the composition of riparian forests (deciduous, mixed, coniferous) on the elemental imbalance between basal resources and consumers in streams, on consumer feeding and on potential feedbacks to riparian systems via emergent aquatic insects. 2. We tested for differences in elemental stoichiometry (carbon/nitrogen/phosphorus; C/N/P) and stable isotopes (?¹³C and ?¹⁵N) between deciduous (red alder, Alnus rubra) and coniferous litter (western hemlock, Tsuga heterophylla) and among abundant stream invertebrates from streams draining different riparian forests (deciduous, mixed, coniferous). We then assessed shredder feeding preferences (of the trichopteran, Lepidostoma unicolor) for litter incubated in streams with these different forest types and quantified differences in emergence of aquatic and semiaquatic insects among streams. 3. Both initial (non‐incubated) and stream‐incubated A. rubra litter had lower C/N and C/P and were more depleted in ?¹³C and more enriched in ?¹⁵N, than T. heterophylla litter. The stoichiometry of invertebrate tissue did not vary significantly among taxa or with riparian forest composition. A predator (the plecopteran Chloroperlidae) and a collector‐gatherer (the ephemeropteran Paraleptophlebia gregalis) from mixed and coniferous forest streams were more enriched in ?¹³C and ?¹⁵N isotopes than those from deciduous streams, suggesting that low availability of palatable, N‐rich A. rubra litter may constrain energy flow and nutrient fluxes up through the food web in systems with little or no A. rubra. 4. Consumption of A. rubra litter by L. unicolor was most rapid when the litter had been incubated in streams draining deciduous forests, whereas consumption of T. heterophylla litter was not influenced by the composition of the riparian forest. 5. Peak insect emergence from coniferous forest streams occurred 1 month earlier and at 2–3× higher density than from mixed and deciduous‐forest streams, but total biomass of emerging insects throughout the study period was not different between forest types. Assemblages of emerging insects were different between deciduous and coniferous forest streams, and taxon richness and diversity were nearly 2× greater from deciduous than from coniferous forest streams. 6. Forest composition influences stream invertebrate feeding and could have reciprocal feedbacks onto riparian systems via altered insect emergence.     

Spatial variability of stable carbon and nitrogen isotope ratios in a Mediterranean coastal lagoon

Exploring the trophic pathway of organic matter within the Mauguio lagoon (southern France, western Mediterranean), we found spatial differences in the isotopic composition (both δ¹³C and δ¹⁵N values) of organic matter sources (primary producers, particulate and sedimentary organic matter), which were mirrored in the upper trophic levels (invertebrates and fish). On average, δ¹³C was heavier by about 1.5–2‰ in the location under marine influence than in the sites influenced by freshwater discharge. The opposite trend was found for δ¹⁵N, which attained maximum values in the north-central zone influenced by freshwater delivery. For both C and N stable isotope ratios, the highest spatial variability was found in organic matter sources (2–3‰), while invertebrates and fish exhibited less variability (\~1–2‰). The differences observed may be related to both anthropogenic (wastewater input) and natural (marine vs. terrestrial inputs) factors. Discharge of wastewater, which affects the innermost location, generally determines an increase in the relative abundance of ¹⁵N. In addition, terrestrially derived nutrients and organic matter, which also affect the innermost location, are known to determine a shift towards ¹³C-depleted values. Our results substantiate the finding that the analysis of carbon and nitrogen stable isotopes can help in elucidating origin and fate of organic matter in coastal lagoons, which are characterised by a great spatial variability and complexity.     

Stable carbon and nitrogen isotope ratios of Eucalyptus and Acacia species along a seasonal rainfall gradient in Western Australia

Key message

Eucalyptus and Acacia species were surprisingly similar with respect to variations in δ ¹³ C, δ ¹⁵ N. Both genera respond with speciation and associated changes in leaf structure to drought.

Abstract

Stable carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) in leaves of eucalypts (Corymbia and Eucalyptus) and Acacia (and some additional Fabaceae) species were investigated together with specific leaf area (SLA), leaf nitrogen (N) and leaf phosphorous (P) concentration along a north–south transect through Western Australia covering winter- and summer-dominated rainfall between 100 and 1,200 mm annually. We investigated 62 eucalypts and 78 woody Fabaceae species, mainly of the genus Acacia. Leaf δ¹³C values of Eucalyptus and Acacia species generally increased linearly with latitude from ?29.5 ± 1.3 ‰ in the summer-dominated rainfall zone (15°S–18°S) to about ?25.7 ± 1.1 ‰ in the winter-dominated rainfall zone (29°S–31°S). δ¹⁵N increased initially with southern latitudes (0.5 ± 1.6 ‰ at 15°S; 5.8 ± 3.3 ‰ at 24–29°S) but decreased again further South (4.6 ± 3.5 ‰ at 31°S). The variation in δ¹³C and δ¹⁵N was probably due to speciation of Eucalyptus and Acacia into very local populations. There were no species that were distributed over the whole sampling area. The variation in leaf traits was larger between species than within species. Average nitrogen concentrations were 11.9 ± 1.05 mg g^?1 in Eucalyptus, and were 18.7 ± 4.1 mg g^?1 in Acacia. Even though the average nitrogen concentration was higher in Acacia than Eucalyptus, δ¹⁵N gave no clear indication for N₂ fixation in Acacia. In a multiple regression, latitude (as a surrogate for rainfall seasonality), mean rainfall, leaf nitrogen concentration, specific leaf area and nitrogen fixation were significant and explained 69 % of the variation of δ¹³C, but only 36 % of the variation of δ¹⁵N. Higher nitrogen and phosphorus concentration could give Acacia an advantage over Eucalyptus in arid regions of undefined rainfall seasonality.     

The fate of assimilated nitrogen in streams: an in situ benthic chamber study

1. Nitrogen (N) processing in streams has been investigated using whole‐stream ¹⁵N addition experiments that, in general, have found that a large proportion of added nitrate removed from the water column appears to be assimilated by the stream benthos. The long‐term fate of this retained N is unknown, and of particular interest is the possibility that it becomes denitrified through coupled mineralisation–nitrification–denitrification processes (indirect denitrification). 2. We used in situ chambers to produce highly ¹⁵N‐enriched benthic biofilms and removed the chambers to allow biofilms to interact with ambient stream conditions. Nitrogen assimilation and direct denitrification were estimated from the first chamber deployment. Chambers were periodically reinstalled over 4 weeks to measure tracer ¹⁵N in ammonium (), nitrate () and dinitrogen (N₂), from which we estimated subsequent rates of biotic N transformations, including N mineralisation (ammonification), nitrification and indirect denitrification. We also estimated rates of depuration of ¹⁵N tracer from benthic biomass compartments. 3. Nitrate uptake was roughly equivalent in the sand and cobble habitats that dominated the stream. Direct denitrification (denitrification of from the water column) was an order of magnitude higher in cobble habitats than in sand habitats, accounting for c. 26 and 2% of total nitrate uptake in cobble and sand, respectively. 4. Mean residence times of actively cycling organic N in stream benthos (algae and microbes) were 16 days in cobble habitats and 9 days in sand habitats. The difference between habitat types was driven by the influence of N residence time in epilithic biofilms (18 days) on cobbles. 5. Release of enriched ¹⁵ was the primary flux of remineralised N, while release of enriched ¹⁵ was an order of magnitude less. We detected slight ¹⁵N enrichment in dissolved nitrogen gas (N₂) in post‐enrichment sampling, indicating that indirect denitrification was taking place. However, indirect denitrification accounted for <0.1% of the assimilated N. 6. These experiments agree with results of whole‐stream ¹⁵N additions, in that most added N was assimilated rather than directly denitrified. Assimilation was primarily a short‐term N retention mechanism in this stream, and indirect denitrification of assimilated N accounted for only a minor proportion of the observed ¹⁵N loss over time. 7. Remaining possible fates include export of N as particulate organic matter, which may lead to additional storage of assimilated N in downstream habitats, and consumption by grazers.     

Factors influencing periphyton growth in agricultural streams of central Illinois

Factors limiting periphyton accrual in east-central Illinois agricultural streams were investigated. Nutrient-diffusing substrata were used to examine periphyton macronutrient limitation in streams in two agricultural watersheds. Substrata consisted of sand-agar mixtures with one of six experimental treatments. Macronutrients included carbon, nitrate, phosphate and combinations of the three. Substrata were collected after a 5 and 9 day period and analyzed for chlorophyll a. None of the treatments were significantly greater than the controls at any of the seven stations, thus we conclude that periphyton in these streams was not nutrient limited. Highest periphyton colonization/growth rates were associated with the smaller upstream reaches, while lower rates occurred in the larger downstream reaches. Multiple regression showed that most of the variance in the rate of chlorophyll a accrual after five days was explained through water temperature and turbidity (r² = 0.91); whereas, stream nitrate and phosphate concentrations accounted for no significant portion of the variance. We conclude that instream primary production in agricultural streams of central Illinois is limited by temperature and light.     

Stable isotope ratios of carbon and nitrogen in pollen grains in order to characterize plant functional groups and photosynthetic pathway types

Measurements of delta(13)C, delta(15)N and C : N ratios on modern pollen grains from temperate plants, including whole grains as well as extracted sporopollenin, were analysed in order to characterize physiological plant types at the pollen level and to determine the variation of these parameters in modern pollen grains of the same climatic area. Measurements are presented for 95 batches of whole modern pollen from 58 temperate species and on the stable fraction of modern pollen grains, chemically extracted sporopollenin, for two modern species. Fourier transform infrared (FTIR) and cross-polarization and magic-angle spinning (CP/MAS) sporopollenin spectra were conducted in parallel. C(3) and C(4) plants can be separated by delta(13)C measurements based on pollen. Probabilistic assignments to plant functional groups (herbaceous, deciduous woody, evergreen woody) of C(3) plants by the means of a discriminant analysis can be made for C : N ratios and for delta(13)C. The results are related to other studies on sporopollenin in order to use this method in future work on fossil samples. Stable isotope measurements on pollen allow improved pollen diagrams, including forms that cannot be differentiated at species level, increasing the accuracy and resolution of plant physiological type distribution in quaternary and older fossil sediments.     

Seasonality and distribution of epilithic diatoms,macroalgae and macrophytes in a spring-fed stream system in Ontario,Canada

A study of the epilithic diatom, macroalgal and macrophyte communities from a spring-fed stream in Ontario, Canada was undertaken from September 1996 to July 1997. The relative abundance of the epilithic diatom flora, percent cover of macroalgal and macrophyte taxa, and several physical and chemical stream conditions were monitored along a 20-m stretch at each of four sites, approximately every 2 months. Several stream conditions were relatively constant over the sampling period (pH, maximum width and maximum depth), while others exhibited a distinct seasonal pattern (water temperature, specific conductance and daylength) and some fluctuated strongly with no discernable seasonal pattern (turbidity, current velocity). A total of 124 taxa were identified from the four sites, including 79 epilithic diatoms, three macroalgal diatom species (large gelatinous masses), one cyanobacterium, two red algae, eight green algae, one chrysophyte alga, one tribophyte alga, three mosses, three horsetails and 23 angiosperm taxa. Species richness was positively correlated to stream channel maximum width and depth, indicating that the total number of species tends to increase in a downstream direction. Distribution of several diatom and macroalgal species was significantly correlated to stream conditions (e.g. Gomphonema parvulum and Phormidium subfuscum with current velocity); however, the vast majority of species did not display seasonal variation in abundance that could be explained by changes in stream conditions. Many of the taxa identified from Blue Springs Creek are common elsewhere in North America.     

汉江上游金水河悬浮物及水体碳氮稳定同位素组成特征

金水河位于南水北调中线工程水源地的汉江上游,研究其污染物来源及分布规律对水源地水资源保护尤为重要。研究了不同水文季节金水河中悬浮颗粒物C和N稳定同位素值、水体硝酸盐与铵盐含量及其N稳定同位素特征。结果表明:金水河流域可溶性氮素与悬浮颗粒物的来源具有明显的空间性和季节性差异,并且流域内叶绿素浓度、水体浊度、悬浮物浓度都会对河流碳氮素稳定同位素值造成影响,主要体现在环境因子的变化制约着水体中硝化和反硝化生物对氮素的可利用性。结果显示:1)水体中悬浮颗粒物的碳稳定同位素为-8.03‰-14.57‰,平均值为2.59‰;氮稳定同位素范围为-7.50‰-7.34‰,平均值为:4.33‰,表明悬浮颗粒物的来源主要为外源性土壤有机质与内源性水生植物残体的混合;2)河流水体中铵盐与硝酸盐N-稳定同位素范围分别为-5.86‰-17.20‰,平均值为5.02‰及-1.48‰-15.86‰,平均值为5.75‰;水体可溶性氮素主要来源为大气沉降、河流水生生物以及地表径流所带入的化肥农药等;3)悬浮颗粒物含量不仅随着河流径流量的季节性变化而变化,还随着人为干扰强度的加强而呈递增的趋势,水体悬浮颗粒物含量最高达到(9.883±3.45)mg/L。而NH_4~+及NO_3~-的浓度也呈现出相同的趋势,含量分别为0.07-0.45 mg/L,平均值为0.25 mg/L;0.08-0.44 mg/L,平均值为0.37 mg/L。稳定同位素测定为河流生态系统提供了一个整合时空氮素来源和转移循环过程的综合指标,揭示了环境因子对河流生态系统氮循环的影响过程与机制。     

Effects of water stress on oxygen, hydrogen and carbon isotope ratios in two species of cotton plants

Abstract. Two cotton species ( Gossypium hirsutum L. cv. SJ-2 and Gossypium barbadence cv. S-5) were grown under irrigated (wet) and non-irrigated (dry) conditions in the same field. Leaf water was enriched in ¹⁸O and deuterium in the dry treatment relative to the wet treatment for both species. Only in plants of S-5 was a similar enrichment observed in leaf cellulose. In both species, the isotopic composition of leaf cellulose must reflect the isotopic composition of the actual water pool involved in cellulose synthesis. Therefore, our observations indicate that one species (SJ-2) can maintain a relative isolation of this water pool from direct evapotranspirational effects. Such plant species will more faithfully record, in the isotopic composition of organic matter, the isotopic composition of ground water. In contrast, the isotopic composition of organic matter in plants such as S-5 could be used as an integrated signal reflecting humidity conditions during growth. Water use efficiency, based on seed-cotton yield and total water applied, correlated linearly with differences in carbon isotopic ratios between species in both the wet and dry treatments and between treatments in each species.     

Stable carbon and nitrogen isotope signatures of decomposing tropical macrophytes

The Pantanal of Mato Grosso, Brazil, is a large, seasonal wetland, which exhibits high macrophyte productivity at the beginning of the rainy season, when the floodplain becomes flooded. During inundation, from December through May, there is rapid turnover of decomposing macrophyte litter, which is subsequently colonized and consumed by various organisms. In this paper, the variation in the carbon and nitrogen isotope signatures of decomposing macrophytes and detritus was determined to provide an isotopic baseline for the elucidation of higher trophic levels. Seven abundant macrophyte species, Cyperaceae sp., Pontederia lanceolata, Cabomba furcata, Salvinia auriculata, Eichhornia crassipes, Nymphaea amazonum and Paspalum repens, were exposed in mesocosm decomposition experiments lasting 21 or 100 days. Stable isotope ratios of carbon and nitrogen and the atomic C/N ratios were determined for decomposing plant material, particulate organic matter (POM), the microbial film, and aquatic invertebrate larvae. The ¹³C values for the macrophytes did not change during decomposition. However, the variability of ¹⁵N was high (range of ± 6 ) due to microbial activity. There was no consistent difference in the isotopic signatures of macrophytes and POM. C/N ratios decreased from 17 to 50 in macrophytes, to 7 to 12 in POM. The isotopic signatures and C/N ratios of the microbial film were the same as those of POM. We concluded that heterotrophic processes did not fractionate stable carbon isotopes but caused an increase in the variability of stable nitrogen ratios and a change in the C/N ratios in our experimental system. Therefore, it was not possible to distinguish fresh and senescent material or even POM when used as a food source. The ¹³C values of the aquatic larvae were closely coupled to those of the carbon source provided.     

Intrapopulation variation in carbon and nitrogen stable isotope ratios in the earthworm Aporrectodea longa

The natural abundance variations in carbon and nitrogen stable isotope ratios in a population of the earthworm Aporrectodea longa, a species known to feed on both soil and plant litter, is reported in this paper. Worms were collected from a small land area of an old white clover field and body tissue and mucus were analyzed separately. The range of isotopic values was small, but patterns of variation were not random. Tissue carbon and nitrogen isotope ratios were significantly higher in adult than in juvenile A. longa and tissue nitrogen isotope ratios tended to increase with increasing biomass of individuals. Further, carbon and nitrogen isotope ratios were positively correlated in both tissue and mucus. Possible causes of the observed patterns, including physiological effects, body composition and assimilation of C and N from different plant, soil and microbial sources are discussed. It is concluded that the causes of natural variability in isotopic composition must be understood and validated experimentally before natural abundance stable isotope methods can be used for the analysis of trophic relations among detritivorous soil invertebrates.     

Effects of nutrients and light on periphyton biomass and nitrogen uptake in Mediterranean streams with contrasting land uses

1. Nutrient diffusing substrata (NDS) were used to determine the relative importance of nutrients and light as potential limiting factors of periphyton biomass and nitrogen (N) uptake in Mediterranean streams subjected to different human impacts. The nutrients examined were phosphorus (P) and N, and we also further differentiated between the response of periphyton communities to N species (i.e. NO₃‐N and NH₄‐N). To examine the effect of light and nutrients on periphyton biomass, chlorophyll a accrual rates on NDS located at open and closed canopy sites were compared. The effect of nutrient availability on periphyton uptake was measured by ¹⁵N changes on the NDS after NO₃‐¹⁵N short‐term nutrient additions. 2. Results show that light was the main factor affecting algal biomass in the study streams. Algal biomass was in general higher at open than at closed canopy sites. Nutrient availability, as simulated with the NDS experiments, did not enhance algal biomass accrual in either of the 2 light conditions. 3. In the control treatments (i.e. ambient concentrations), periphyton NO₃‐N uptake rates increased and C : N molar ratios decreased consistently with increases in N availability across streams. NO₃‐N uptake rates were altered when ambient N concentrations were increased artificially in the N amended NDS. Periphyton assemblages growing on N enriched substrata seemed to preferentially take up N diffusing from the substratum rather than N from the water column. This response differed among streams, and depended on ambient N availability. 4. Periphyton biomass was not significantly different between substrata exposed to the two forms of available N sources. Nonetheless, we found differences in the effects of both N sources on the uptake of N from the water column. NH₄‐N seemed to be the preferred source of N for periphyton growing on NDS. 5. Results suggest that the effect of riparian zones on light availability, although seldom considered by water managers, may be more important than nutrients in controlling eutrophication effects derived from human activities. Finally, our results confirm that not only increases in concentration, but also stoichiometric imbalances should be considered when examining N retention in human altered streams.