The role of bacteria in the nutrient exchange between sediment and water in a flow-through system

The contribution of bacteria to phosphorus (P) and nitrogen (N ) release from, or retention in, sediment was studied in a flow-through system. Live and formaldehyde-killed sediment communities were incubated in 25-liter bottles with a continuous flow of P- or P + N-enriched water. Sediment bacteria in the killed communities were inhibited by adding formaldehyde (final concentration 0.04% v/v) to the sediment before the start of the experiment. Bacterial activity in the live sediments measured with [³H]thymidine and [¹⁴C]leucine incorporation techniques did not change essentially during the experiment period (7–8 days). Chemical mechanisms were found to be of principal importance in PO₄-P retention in the sediment. In the live samples, the net retention of PO₄-P was lower than in the killed samples, which was likely due to the reduced O₂ conditions in the sediment as a consequence of bacterial mineralization. In total P exchange, however, bacteria increased the retention rate by recycling dissolved organic P in the sediment. In the live communities the retention of N was very efficient, and all the introduced NH₄ -N and NO₃-N was immobilized by sediment bacteria. Nitrogen enrichment, however, did not alter the P exchange rates. The gradual emergence of bacterial activity (and grazing) in the killed communities, subsequent to the dilution of formaldehyde concentration, enhanced the release of PO₄-P and NH₄-N from sediment.     

Horizontal and vertical gradients in sediment nutrients on mudflats in the Shannon estuary,Ireland

Spatial variability in nutrient concentrations in the intertidal sediments and pore waters has been studied along downshore transects and in core profiles at three locations in the Shannon estuary on the west coast of Ireland. The parameters measured were N, as Kjeldahl N, NO₃, NO₂, and NH₃, and P as total P and PO₄ along with a range of other environmental variables such as salinity and sediment organic content. The concentrations of all nutrients varied with season, but winter values were generally low in comparison with polluted mainland european estuaries. There was a great deal of variation in nutrient concentrations along the transects, and coefficients of variability of up to 153% (NH₃), 173% (NO₃), 129% (NO₂) and 117% (PO₄) were found. Overall, there was little evidence of any trends in concentration in any of the nutrients from the top to the bottom of the transects, although it was occasionally possible to link particular instances to local conditions such as the presence of the channel or a stream. Sediment core profiles showed typical patterns, with NO₃ concentrations for example being highest in surface sediments, while NH₃ and PO₄ concentrations increased with depth. Rather surprisingly perhaps, NO₃ could still be detected on occasion at depths of up to 20 cm, well below the Redox Potential Discontinuity (RPD) and the limit of oxygen penetration which oxygen microelectrodes had measured as being within a few mm of the surface. This was ascribed to the activities of the macrofauna, in that the oxidised sediment which lined the burrows could clearly be seen in some cores. This study not only shows that nutrient distributions along estuarine gradients are linked to physicochemical factors such as oxygenation and freshwater/marine influence, but also that sediment instability, through random physical events such as storms, and macrofaunal activity play an important role and that these latter factors deserve closer attention.     

The seasonal dynamics of amino acids and other nutrients in Alaskan Arctic tundra soils

Past research strongly indicates the importance of amino acids in the N economy of the Arctic tundra, but little is known about the seasonal dynamics of amino acids in tundra soils. We repeatedly sampled soils from tussock, shrub, and wet sedge tundra communities in the summers of 2000 and 2001 and extracted them with water (H₂O) and potassium sulfate (K₂SO₄) to determine the seasonal dynamics of soil amino acids, ammonium (NH₄⁺), nitrate (NO₃^–), dissolved organic nitrogen (DON), dissolved organic carbon (DOC), and phosphate (PO₄^2–). In the H₂O extractions mean concentrations of total free amino acids (TFAA) were higher than NH₄⁺ in all soils but shrub. TFAA and NH₄⁺ were highest in wet sedge and tussock soils and lowest in shrub soil. The most predominant amino acids were alanine, arginine, glycine, serine, and threonine. None of the highest amino acids were significantly different than NH₄⁺ in any soil but shrub, in which NH₄⁺ was significantly higher than all of the highest individual amino acids. Mean NO₃^– concentrations were not significantly different from mean TFAA and NH₄⁺ concentrations in any soil but tussock, where NO₃^– was significantly higher than NH₄⁺. In all soils amino acid and NH₄⁺ concentrations dropped to barely detectable levels in the middle of July, suggesting intense competition for N at the height of the growing season. In all soils but tussock, amino acid and NH₄⁺ concentrations rebounded in August as the end of the Arctic growing season approached and plant N demand decreased. This pattern suggests that low N concentrations in tundra soils at the height of the growing season are likely the result of an increase in soil N uptake associated with the peak in plant growth, either directly by roots or indirectly by microbes fueled by increased root C inputs in mid-July. As N availability decreased in July, PO₄^2– concentrations in the K₂SO₄ extractions increased dramatically in all soils but shrub, where there was a comparable increase in PO₄^2– later in the growing season. Previous research suggests that these increases in PO₄^2– concentrations are due to the mineralization of organic phosphorus by phosphatase enzymes associated with soil microbes and plant roots, and that they may have been caused by an increase in organic P availability.     

The effects of drying on sediment nitrogen content in a Mediterranean intermittent stream: a microcosms study

Mediterranean climates predispose aquatic systems to both flood and drought periods, therefore, stream sediments may be exposed to desiccation periods. Changes in oxygen concentrations and sediment water content influence the biotic processes implicated in nitrogen dynamics. The objectives of this study were to identify (1) the changes of inorganic nitrogen in stream sediments during the transition from wet to dry conditions, and (2) the underlying processes in N dynamics and its regulation. Extractable sediment NO₃ ⁻-N and NH₄ ⁺-N, organic matter and extractable organic carbon content were assessed during natural desiccation in microcosms with sediments from an intermittent Mediterranean stream. In agreement with our initial hypothesis, our results showed how the NO₃ ⁻-N content of the sediment was enhanced during the first 10 days of sediment drying, whereas NH₄ ⁺-N was lost by 14 days post-drying. During the first 10 days, sediment desiccation seemed to stimulate the net N-mineralization and net nitrification from sediments. Afterwards, the extractable NO₃ ⁻-N concentration sharply dropped, which may be attributed to lower ammonium-oxidation rates as ammonium and organic matter are depleted, and to an increase in NO₃ ⁻-N consumption by microbial populations. Denitrification was inhibited, with a significant decrease as % water-filled pore space lowered. We hypothesize that the sediment inorganic N content enhanced during sediment desiccation could be released as part of the N pulse observed after sediment rewetting. However, the stream N availability after rewetting dried sediments would differ depending on desiccation period duration.     

九龙江河口区养虾塘沉积物-水界面营养盐交换通量特征

通过对九龙江河口区陆基养虾塘水样和沉积物样品采集分析及结合室内模拟实验,探讨了虾塘在不同养殖阶段沉积物-水界面营养盐通量时间变化特征及其主要影响因素。虾塘沉积物向上覆水体释放NO_x~--N(NO_2~--N和NO_3~--N)、NH_4~+-N和PO_4~(3-)-P能力均呈现随养殖时间推移而降低的特征。沉积物在养殖中期和后期分别呈现对上覆水体NO_x~--N和PO_4~(3-)-P的吸收现象,但总体表现为释放(平均通量分别为(1.87±1.15)、(1.58±0.52)mg m~(-2)h~(-1)和(1.22±0.62)mg m~(-2)h~(-1))。沉积物-水界面溶解无机氮交换以NH_4~+-N为主(沉积物平均释放通量为(46.18±13.82)mg m~(-2)h~(-1))。沉积物间隙水与上覆水间的营养盐浓度差(梯度)及温度对上述交换通量的时间动态特征具有重要调控作用。研究结果表明养殖初期或中期沉积物较高的无机氮(尤其是NO_2~--N和NH_4~+-N)释放是养殖塘水质恶化的一个极具潜力的污染内源,可能会对虾的健康生长产生负面效应,控制沉积物无机氮释放是养虾塘养殖初期和中期重要的日常管理活动之一。     

Interactions between C and N metabolism in Dunaliella salina cells cultured at elevated CO2 and high N concentrations

The green algaDunaliella salina UTEX 200 was cultured at high (5 percnt;) [CO₂] in a medium containing 10 mmol/L of either NO₃⁻ or NH₄⁺ as the sole N source. Specific growth rate was 50 percnt; higher for NH₄⁺-grown cells than for their counterparts cultured in the presence of NO₃⁻. Cell size, protein content, Rubisco protein, phosphoenolpyruvate carboxylase (PEPC) activity, and light independent carbon fixation were enhanced by growth in the presence of NH₄⁺. On the other hand, maximal photosynthetic rate and cell glycerol concentration were lower when N was supplied as NH₄⁺. The activity of glutamine synthetase was affected very little by the N-source.D. salina UTEX 200 showed some peculiarities in its mechanism of adaptation to high [N] in comparison to other strains previously used for similar studies. This allowed dissection of the underlying mechanism of the growth response to high [N], highlighting the potential role of PEPC, the main anaplerotic enzyme, as a pivotal player in the adaptation of cells to these conditions.     

Vulnerability of macronutrients to the concurrent effects of enhanced temperature and atmospheric pCO<Subscript>2</Subscript> in representative shelf sea sediment habitats

Fundamental changes in seawater carbonate chemistry and sea surface temperatures associated with the ocean uptake of anthropogenic CO₂ are accelerating, but investigations of the susceptibility of biogeochemical processes to the simultaneous occurrence of multiple components of climate change are uncommon. Here, we quantify how concurrent changes in enhanced temperature and atmospheric pCO₂, coupled with an associated shift in macrofaunal community structure and behavior (sediment particle reworking and bioirrigation), modify net carbon and nutrient concentrations (NH₄-N, NO_x-N, PO₄-P) in representative shelf sea sediment habitats (mud, sandy-mud, muddy-sand and sand) of the Celtic Sea. We show that net concentrations of organic carbon, nitrogen and phosphate are, irrespective of sediment type, largely unaffected by a simultaneous increase in temperature and atmospheric pCO₂. However, our analyses also reveal that a reduction in macrofaunal species richness and total abundance occurs under future environmental conditions, varies across a gradient of cohesive to non-cohesive sediments, and negatively moderates biogeochemical processes, in particular nitrification. Our findings indicate that future environmental conditions are unlikely to have strong direct effects on biogeochemical processes but, particularly in muddy sands, the abundance, activity, composition and functional role of invertebrate communities are likely to be altered in ways that will be sufficient to regulate the function of the microbial community and the availability of nutrients in shelf sea waters.     

MACROALGAL RESPONSES TO NITROGEN SOURCE AND AVAILABILITY: AMINO ACID METABOLIC PROFILING AS A BIOINDICATOR USING GRACILARIA EDULIS (RHODOPHYTA)1

The use of macroalgae as biological indicators of dissolved nutrient source and availability in the water column was investigated. Total tissue nitrogen (N) content, pigments, and amino acids of the red alga Gracilaria edulis (Gmelin) Silva were compared to N source and availability in laboratory and field incubations to identify responses that would serve as bioindicators of N. Field-collected algae were preincubated (6–8 wk) in low-nutrient seawater to deplete their luxury reserves ofN. Incubations were then conducted for periods of 3 d in laboratory aquaria (N-spiked seawater) and in the field using macroalgal incubation chambers. After incubation in different N sources (NH₄⁺, NO₃^?, and urea) in laboratory aquaria, photosynthetic pigments (phycoerythrin and chlorophyll a) and total tissue N increased, in response to increasing [NH₄⁺] but not to [NO₃^?] or [urea]. Incubation in two ranges of [NH₄⁺], one from 0 to 80 μM and the other from 0 to 800 μM, in laboratory aquaria increased the total amino acid pool. Citrulline concentrations were the most responsive to [NH₄⁺] (r²= 0. 84). NH₄⁺ source treatments produced increases in citrulline, phenylalanine, serine, and free NH₄⁺ and decreases in alanine; NO₃^? treatments produced increases in glutamic acid, citrulline, and alanine; and urea treatments produced increases in free NH₄⁺ and decreases in phenylalanine and serine. The observed variations in amino acid content facilitated the development of an index for each N source based on relative concentrations of various amino acids (i. e. metabolic profiling). Gracilaria edulis was incubated along a field N gradient in the Brisbane River (three sites) and Moreton Bay (four sites), Queensland, Australia. Both phycoerythrin and tissue N appeared to respond equally to NH₄⁺ and NO₃^? availability in the field. N source indices, based on amino acid concentration, were effective predictors of both [NH₄⁺] and [NO₃^?] over a wide range of concentrations along the field gradient. Macroalgal physiological responses, particularly amino acid content, to changes in source and availability of N appear to be useful as sensitive bioindicators of N.     

An exploration of how litter controls drainage water DIN, DON and DOC dynamics in freely draining acid grassland soils

Surface and subsurface litter fulfil many functions in the biogeochemical cycling of C and N in terrestrial ecosystems. These were explored using a microcosm study by monitoring dissolved inorganic nitrogen (DIN) (NH₄ ⁺–N?+?NO₃ ^?–N), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations and fluxes in drainage water under ambient outdoor temperatures. Subsurface litter remarkably reduced the DIN concentrations in winter, probably by microbial N uptake associated with higher C:N ratio of added litter compared with soil at 10–25?cm depth. Fluxes of DIN were generally dominated by NO₃ ^?–N; but NH₄ ⁺–N strongly dominated DIN fluxes during freeze–thaw events. Appreciable concentrations of NH₄ ⁺–N were observed in the drainage from the acid grassland soils throughout the experiment, indicating NH₄ ⁺–N mobility and export in drainage water especially during freeze–thaw. Litter contributed substantially to DOC and DON production and they were correlated positively (p?<?0.01) for all treatments. DOC and DON concentrations correlated with temperature for the control (p?<?0.01) and surface litter (p?<?0.001) treatments and they were higher in late summer. The subsurface litter treatment, however, moderated the effect of temperature on DOC and DON dynamics. Cumulative N species fluxes confirmed the dominance of litter as the source of DON and DOC in the drainage water. DON constituted 42, 46 and 62% of cumulative TDN flux for control, surface litter and subsurface litter treatments respectively.     

The influence of NO3 - and NH4 + nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) plants

The carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) grown hydroponically at a constant pH on either 4 mM or 12 mM NO₃ ^- or NH₄ ⁺ nutrition were investigated using either ¹⁴C or ¹⁵N techniques. Greater allocation of ¹⁴C to amino-N fractions occurred at the expense of allocation of ¹⁴C to carbohydrate fractions in NH₄ ⁺-compared to NO₃ ^--fed plants. The [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 mM NO₃ ^--compared to 12 mM NH₄ ⁺-fed wheat. In both 4 mM and 12 mM N-fed maize the [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO₃ ^--compared to NH₄ ⁺-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture with one root-half in NO₃ ^--and the other in NH₄ ⁺-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of ¹⁴N and ¹⁵N within separate sets of plants confirmed that NH₄ ^--fed plants accumulated more amino-N compounds than NO₃ ^--fed plants. Wheat roots supplied with ¹⁵NH₄ ⁺ for 8 h were found to accumulate ¹⁵NH₄ ⁺ (8.5 g ¹⁵N g^-1 h^-1) whereas in maize roots very little ¹⁵NH₄ ⁺ accumulated (1.5 g ¹⁵N g^-1 h^-1)It is proposed that the observed accumulation of ¹⁵NH₄ ⁺ in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH₄ ⁺, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C₄ maize plants ensure greater carbon availability to the root than in the C₃ wheat plants. These differences in carbon and nitrogen partitioning between NO₃ ^--and NH₄ ⁺-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO₃ ^- and NH₄ ⁺ nutrition.     

Nitrogen transformations revealed by isotope dilution in an organically fertilized hybrid poplar plantation

Measuring nitrogen (N) transformations from organic fertilizers can help in selecting applications rates that provide sufficient soluble N to promote tree growth in short-rotation plantations. The objective of this study was to determine how organic fertilizers (papermill biosolids, liquid pig slurry) affected microbially-mediated N transformations in soils. Soil samples were collected from a hybrid poplar plantation before fertilization, 1 month after fertilizer application and at the end of the growing season. Net N mineralization and nitrification were evaluated during a 28 d laboratory incubation, while gross N transformations were assessed using a ¹⁵N isotope dilution technique. Pig slurry application increased soil ammonium (NH₄-N) and nitrate (NO₃-N) concentrations within 1 month, while papermill biosolids increased soil NH₄-N and NO₃-N concentrations at the end of the growing season. Gross N consumption rates were greater than gross N production rates. The NH₄-N and NO₃-N consumption rates were positively correlated with labile carbon and microbial biomass. The gross nitrification rate was 18 to 67% of the gross mineralization rate but 30% or less of the gross NH₄-N consumption rate, indicating that NH₄ consumption was overestimated by the isotope dilution technique. We conclude that N cycling in this hybrid poplar plantation was characterized by rapid consumption of plant-available N following N mineralization and nitrification.     

How Physical Disturbance and Nitrogen Addition Affect the Soil Carbon Decomposition?

The decomposition of soil organic carbon (SOC) plays a critical role in regulating atmospheric CO₂ concentrations and climate dynamics. However, the mechanisms and factors controlling SOC decomposition are still not fully understood. Here, we conducted a 60 days incubation experiment to test the effects of physical disturbance and nitrogen (N) addition on SOC decomposition. N addition increased the concentration of NO₃^- by 51% in the soil, but had little effect on the concentration of NH₄⁺. N addition inhibited SOC decomposition, but such an effect differed between disturbed and undisturbed soils. In disturbed and undisturbed soils, application of N decreased SOC decomposition by 37% and 15%, respectively. One possible explanation is that extra N input suppressed microbial N mining and/or increased the stability of soil organic matter by promoting the formation of soil aggregates and incorporating part of the inorganic N into organic matter, and consequently decreased microbial mineralization of soil organic matter. Physical disturbance intensified the inhibition of N on SOC decomposition, likely because physical disturbance allowed the added N to be better exposed to soil microbes and consequently increased the availability of added N. We conclude that physical disturbance and N play important roles in modulating the stability of SOC.     

Tapir (Tapirus) enteroliths

Enterolith fragments from two tapir species and horses were subjected to x-ray diffraction analysis. Tapir enteroliths were formed as layers of mineral deposited around a foreign nidus. The structure was similar to that of equine enteroliths except that tapir enteroliths lacked a central region of radially symmetrical coarse crystals. The enteroliths from tapirs were composed primarily of vivianite [Fe₃(PO₄)₂ · 8H₂O] and newberyite [MgH(PO₄) · 3H₂O], instead of the struvite [Mg(NH₄)(PO₄) · 6H₂O] of enteroliths from horses. The reason for this difference is not known. Based on the chemistry of these mineral precipitates and information from other species, it was concluded that dietary manipulation to maximize carbohydrate fermentation and minimize protein fermentation in the large intestine may help prevent enterolithiasis in tapirs. Zoo Biol 16:427–433, 1997. © 1997 Wiley-Liss, Inc.     

海水中藻菌共培养体系对碳氮磷的吸收转化

海洋环境中,细菌和微藻之间的物质交换是生源要素在自然界中迁移转化的重要方式。为进一步了解生源要素的生物地球化学循环,在实验室模拟条件下,研究了共培养体系中营养盐和有机物在细菌和微藻之间的转换。通过纯培养中肋骨条藻(Skeletonema costatum)、东海原甲藻(Prorocentrum donghaiense)、天然海水中的细菌以及藻菌混合培养,分析了营养盐和有机物随藻菌生物量的变化情况,并计算了溶解有机碳(DOC)和溶解有机氮(DON)的浓度比值[(DOC/DON)a]。结果发现,在共培养体系中,细菌对中肋骨条藻的生长有抑制作用,对东海原甲藻影响不明显;中肋骨条藻有利于细菌生长,东海原甲藻抑制细菌生长,这种不同可能与微藻的粒径有关。海洋细菌在2种藻的指数生长均期均会促进微藻吸收氨氮(NH_4-N),但在生长末期NH_4-N以释放为主。硝氮(NO_3-N)的浓度与藻的生长呈负相关,但在衰亡期NO_3-N略有增加,表明NO_3-N再生所需时间较长。细菌对硝氮的吸收量较少,但对其再生有贡献。细菌和中肋骨条藻对磷酸盐(PO_4-P)的吸收存在竞争,但与东海原甲藻的竞争关系不明显。不同培养体系中DOC浓度变化不同,在藻菌共培养体系中增加较快,纯藻培养体系中增加缓慢,在纯菌培养体系中缓慢减少。通过对DOC与DON浓度比值的分析,发现用判断颗粒有机碳(POC)来源的方法可以分析DOC的来源。     

Priming Effects of the Endophytic Fungus Phomopsis liquidambari on Soil Mineral N Transformations

Nitrogen (N) is a crucial nutrient for soil biota, and its cycling is determined by the organic carbon decomposing process. Some endophytic fungi are latent saprotrophs that trigger their saprotrophic metabolism to promote litter organic matter cycling as soon as the host tissue senesces or dies. However, the effects of endophytic fungi on litter and soil N dynamics in vitro have rarely been investigated. In this study, we investigated N dynamics (total and mineral N) in both litter and soil in incubations of a pure culture of an endophytic fungus Phomopsis liquidambari with litter and following soil burial of the litter. Soil enzymes and microbial communities participating in the N transformations were also investigated. A pure culture of P. liquidambari released litter NH ₄ ⁺ –N in the initial stages (10 days) of the incubation. However, following soil burial, the presence of both P. liquidambari and soil ammonia-oxidizing bacteria (AOB) resulted in an increase in soil NO ₃ ^? –N. These results indicate that the endophytic fungus P. liquidambari in vitro stimulates organic mineralization and promote NH ₄ ⁺ –N release. Such effects triggered soil AOB-driven nitrification process.     

不同近地表土壤水文条件下雨滴打击对黑土坡面养分流失的影响

目前,土壤水分饱和和壤中流条件下,雨滴打击对养分流失的影响尚不清楚.通过3个近地表土壤水文条件(自由入渗、土壤水分饱和与壤中流)下,土槽上方架设与不架设尼龙纱网模拟降雨对比试验,研究雨滴打击对黑土坡面侵蚀过程及NO3-N、NH4-N与PO4-P随径流和侵蚀泥沙迁移的影响.结果表明,纱网覆盖消除雨滴打击后坡面侵蚀量和泥沙浓度分别减少59.4％-71.6％和57.3％ -73.0％,不同水文条件下减少量的排序为:自由入渗＞壤中流＞土壤水分饱和.消除雨滴打击后养分随径流流失的减少仅在自由入渗条件下体现较明显,该水文条件下NO3-N、NH4-N与PO4-P流失分别减少33.3％、23.1％和40.7％;3种水文条件下,消除雨滴打击均明显减少养分随泥沙的流失,其中自由人渗条件下减少效果最明显,该水文条件下,NO3-N和NH4-N流失分别减少20.9％ -54.9％和25.0％ -62.3％,而PO4-P流失减少在74.6％以上.雨滴打击增大了NO3-N的淋失,但对NH4-N与PO4-P的淋失几乎无影响.消除雨滴打击后,自由人渗条件下养分的等效径流迁移深度减少26.7％-42.6％,而土壤水分饱和与壤中流条件下基本无变化.以上研究结果为有效防治坡面土壤侵蚀和农业非点源污染提供科学理论依据,尤其是在壤中流出现的地方.     

Seasonal variation in nutrient limitation of microbial biofilms colonizing organic and inorganic substrata in streams

Humans have increased the availability of nutrients including nitrogen and phosphorus worldwide; therefore, understanding how microbes process nutrients is critical for environmental conservation. We examined nutrient limitation of biofilms colonizing inorganic (fritted glass) and organic (cellulose sponge) substrata in spring, summer, and autumn in three streams in Michigan, USA. Biofilms were enriched with nitrate (NO₃ ⁻), phosphate (PO₄ ³⁻), ammonium (NH₄ ⁺), NO₃ ⁻ + PO₄ ³⁻, NH₄ ⁺ + PO₄ ³⁻, or none (control). We quantified biofilm structure and function as chlorophyll a (i.e., primary producer biomass) and community respiration on all substrata. In one stream, we characterized bacterial and fungal communities on cellulose in autumn using clone library sequencing and denaturing gradient gel electrophoresis to determine if community structure was linked to nutrient limitation status. Despite oligotrophic conditions, primary producer biomass was infrequently nutrient limited. In contrast, respiration on organic substrata was frequently limited by N + P combinations. We found no difference between biofilm response to NH₄ ⁺ versus NO₃ ⁻ enrichment, although the response to both N-species was positively related to water column PO₄ ³⁻ concentrations and temperature. Molecular analysis for fungal community composition suggested no relationship to nutrient limitation, but the dominant members of the bacterial community on cellulose were different on NO₃ ⁻, PO₄³, and NO₃ ⁻ + PO₄ ³⁻ treatments relative to control, NH₄ ⁺, and NH₄ ⁺ + PO₄ ³⁻ treatments, which matched patterns for biofilm respiration rates from each treatment. Our results show discrete patterns of nutrient limitation dependent upon substratum type and season, and imply changes in bacterial community structure and function may be linked following nutrient enrichment in streams.