Nutrient stoichiometry of linked catchment‐lake systems along a gradient of land use

1. Catchments export nutrients to aquatic ecosystems at rates and ratios that are strongly influenced by land use practices, and within aquatic ecosystems nutrients can be processed, retained, lost to the atmosphere, or exported downstream. The stoichiometry of carbon and nutrients can influence ecosystem services such as water quality, nutrient limitation, biodiversity, eutrophication and the sequestration of nutrients and carbon in sediments. However, we know little about how nutrient stoichiometry varies along the pathway from terrestrial landscapes through aquatic systems. 2. We studied the stoichiometry of nitrogen and phosphorus exported by three catchments of contrasting land use (forest versus agriculture) and in the water column and sediments of downstream reservoirs. We also related stoichiometry to phytoplankton nutrient limitation and the abundance of heterocystous cyanobacteria. 3. The total N : P of stream exports varied greatly among catchments and was 18, 54 and 140 (molar) in the forested, mixed‐use and agricultural catchment, respectively. Total N : P in the mixed layers of the lakes was less variable but ordered similarly: 35, 52 132 in the forested, mixed‐use and agricultural lake, respectively. In contrast, there was little variation among systems in the C : N and C : P ratios of catchment exports or in reservoir seston. 4. Phytoplankton in the forested lake were consistently N limited, those in the agricultural lake were consistently P limited, and those in the mixed‐use lake shifted seasonally from P‐ to N limitation, reflecting N : P supply ratios. Total phytoplankton and cyanobacteria biomass were highest in the agricultural lake, but heterocystous (potentially N fixing) cyanobacteria were most abundant in the forested lake, corresponding to low N : P ratios. 5. Despite large differences in catchment export and water column N : P ratios, the N : P of sediment burial (integrated over several decades) was very low and remarkably similar (4.3–7.3) across reservoirs. N and P budgets constructed for the agricultural reservoir suggested that denitrification could be a major loss of N, and may help explain the relatively low N : P of buried sediment. 6. Our results show congruence between the catchment export N : P, reservoir N : P, phytoplankton N versus P limitation and the dominance of heterocystous cyanobacteria. However, the N : P stoichiometry of sediments retained in the lakes was relatively insensitive to catchment stoichiometry, suggesting that a common set of biogeochemical processes constrains sediment N : P across lakes of contrasting catchment land use.     

Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia

Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast, stream water DOC concentrations did not exhibit seasonal variation. Higher DOC and NO₃ ⁻ concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there was an additional NO₃ ⁻ source or reduction of a NO₃ ⁻ sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated area.     

Nutrient export to an Eastern Atlantic coastal zone: first modeling and nitrogen mass balance

We have studied 15 catchments supplying freshwater to a French Atlantic coastal lagoon, where increase in nitrogen loads due to agriculture is supposed to have destabilized the ecosystem in the last decades. The catchment is a lowland composed of Pleistocene sands with an average slope of 0.25%. To study the nutrient export in relation to land-use surface waters were sampled bi-weekly between October 2006 and January 2009 and land-use was established by plane photographs and Geographic Information System (GIS). Cultivated pine forests represent more than 80% of the total surface and 7% of the catchment area has been deforested recently. Significant areas of some catchments are used for maize crop. Housing is confined to the coastal zone. Maize and forest crop give a robust signature in terms of nitrate export. In view of modeling the nutrient fluxes, we have established the mean export rate for every land-use: forested parcels, deforested parcels, cultivated surfaces, and housing areas export 45, 93, 2850, and 61 kg N-nitrate km^?2 year^?1, respectively. Exports of ammonium, dissolved organic N (DON), and dissolved inorganic P (DIP) could not be related to land use. The mean export is 13, 100, and 0.57 kg km^?2 year^?1 for N-ammonium, DON, and DIP, respectively. The modeling of nitrogen flux is in good agreement with our measures for the largest catchment, which supplies about 90% of the total continental DIN flux. However, small catchments are more dynamic due to hydrological conditions and the model is less accurate. This work has permitted to complete and unify scattered studies about nutrient cycling in this area. Thus we have established and compared the nitrogen budget of cornfields and cultivated pine forest. We have emphasized that (i) fertilizer use should be reduced in cornfields because they stock between 200 and 6400 kg DIN km^?2 year^?1, and (ii) the nitrogen budget in pine forest mostly depends on tree harvesting and symbiotic N-fixation, which is poorly constrained. Export of N by rivers represents a small contribution to the N budget of soils.     

Export of DOM from Boreal Catchments: Impacts of Land Use Cover and Climate

Dissolved organic matter (DOM) is an important fraction in carbon (C) and nutrient budgets for aquatic ecosystems and can have broad effects on food webs and nutrient cycling. To look at the role land use cover and climate might play in DOM transport from the boreal region, the export of total organic carbon (TOC), total organic nitrogen (TON) and dissolved organic phosphorus (DOP) was estimated for Finnish main rivers and their sub-catchments, altogether 86 catchments, situated between latitudes 60° N and 69° N and covering 297,322 km², 88% of the total area of Finland. On an average, 94% of the TOC, 90% of the total nitrogen (TN) and 40% of the total phosphorus (TP) in Finnish rivers was in a dissolved form. The majority of the DOM export from Finnish catchments consists of organic C. The TOC export increased with increasing peatland proportion (r = 0.39, p = 0.003), while TON export increased with the increasing percentage of agricultural land (r = 0.60, p <0.001). Although upstream lakes covered only on average 9% of the catchment area, they were the most important predictor for TOC, TON and DOP export (r = −0.83, r = −0.82 and r = −0.61, respectively). The higher the upstream lake percentage, the lower the export indicating organic matter retention in lakes.     

The prediction of nutrients into estuaries and their subsequent behaviour: application to the Tamar and comparison with the Tweed,U.K.

Some of the techniques used to model nitrogen (N) and phosphorus (P) discharges from a terrestrial catchment to an estuary are discussed and applied to the River Tamar and Tamar Estuary system in Southwest England, U.K. Data are presented for dissolved inorganic nutrient concentrations in the Tamar Estuary and compared with those from the contrasting, low turbidity and rapidly flushed Tweed Estuary in Northeast England. In the Tamar catchment, simulations showed that effluent nitrate loads for typical freshwater flows contributed less than 1% of the total N load. The effect of effluent inputs on ammonium loads was more significant (10%). Cattle, sheep and permanent grassland dominated the N catchment export, with diffuse-source N export greatly dominating that due to point sources. Cattle, sheep, permanent grassland and cereal crops generated the greatest rates of diffuse-source P export. This reflected the higher rates of P fertiliser applications to arable land and the susceptibility of bare, arable land to P export in wetter winter months. N and P export to the Tamar Estuary from human sewage was insignificant. Non-conservative behaviour of phosphate was particularly marked in the Tamar Estuary. Silicate concentrations were slightly less than conservative levels, whereas nitrate was essentially conservative. The coastal sea acted as a sink for these terrestrially derived nutrients. A pronounced sag in dissolved oxygen that was associated with strong nitrite and ammonium peaks occurred in the turbidity maximum region of the Tamar Estuary. Nutrient behaviour within the Tweed was very different. The low turbidity and rapid flushing ensured that nutrients there were essentially conservative, so that flushing of nutrients to the coastal zone from the river occurred with little estuarine modification.     

Climate variability masks the impacts of land use change on nutrient export in a suburbanizing watershed

Suburbanization negatively impacts aquatic systems by altering hydrology and nutrient loading. These changes interact with climate and aquatic ecosystem processes to alter nutrient flux dynamics. We used a long term data set (1993–2009) to investigate the influence of suburbanization, climate, and in-stream processes on nitrogen and phosphorus export in rivers draining the Ipswich and Parker River watersheds in northeastern MA, USA. During this timeframe population density increased in these watersheds by 14 % while precipitation varied by 46 %. We compared nutrient export patterns from the two larger watersheds with those from two nested headwater catchments collected over a nine year period (2001–2009). The headwater catchments were of contrasting, but stable, land uses that dominate the larger watersheds (suburban and forested). Despite ongoing land use change and an increase in population density in the mainstem watersheds, we did not detect an increase in dissolved inorganic nitrogen (DIN) or PO₄ concentration or export over the 16-year time period. Inter-annual climate and associated runoff variability was the major control. Annual DIN and PO₄ export increased with greater annual precipitation in the Ipswich and the Parker River watersheds, as well as the forested headwater catchment. In contrast, annual DIN export fluxes from the suburban headwater catchment were less affected by precipitation variability, with inter-annual export fluxes negatively correlated with mean annual temperature. The larger watershed exports diverged from headwater exports, particularly during summer, low-flow periods, suggesting retention of DIN and PO₄. Our study shows suburban headwater exports respond to inter-annual variation in runoff and climate differently than forested headwater exports, but the impacts from headwater streams could be buffered by the river network. The net effect is that inter-annual variation and network buffering can mitigate higher nutrient exports from larger suburbanizing watersheds over decadal time periods.     

植被覆盖度和综合治理对纸坊沟流域土壤氮素流失的影响

同小流域土壤侵蚀一样,小流域土壤氮素随洪流流失也受到植被覆盖度的影响,通常经过调整小流域内土地利用结构以达到控制水土流失.该研究以8.27 km2纸坊沟流域和1:400比例流域模型为研究对象,研究植被覆盖度和综合治理对纸坊沟流域土壤氮素流失的影响.结果表明:在模拟降雨下,当流域植被覆盖度分别为60%、40%、20%和0时,流域模型铵态氮流失量分别为87.08、44.31、25.16和13.71 kg/km2,硝态氮为85.50、74.05、63.95和56.23 kg/km2,全氮为0.81、1.18、1.98和7.51 t/km2;在自然降雨下,1998年与1992年相比,全流域年土壤侵蚀量为1 086 t/km2和1 119 t/km2,氮素流失量为8 758.5和7 562.2 kg,减少了15.8%,其中农地减少了52.0%.流域对降水中的矿质氮具有过滤作用,硝态氮的过滤作用明显高于铵态氮.洪流泥沙中＜20 μm微团聚体富集造成了泥沙有机质和全氮的富集.植被覆盖虽能有效地减少流域土壤侵蚀和全氮的流失,却能增加土壤矿质氮的流失.坡地退耕还林草可显著减少流域土壤氮素流失.     

Influence of land use on stream ecosystem function in a Mediterranean catchment

1. Due to the hierarchical organization of stream networks, land use changes occurring at larger spatial scales (i.e. the catchment) can affect physical, chemical and biological characteristics at lower spatial scales, ultimately altering stream structure and function. Anthropogenic effects on streams have primarily been documented using structural metrics such as water chemistry, channel alteration and algal biomass. Functional parameters, including metrics of nutrient retention and metabolism, are now being widely used as indicators of stream condition. 2. Within this hierarchical context, we used a multivariate approach to examine how structural and functional (i.e. nutrient retention and metabolism) attributes of streams are related to catchment variables, including land use. The study was done in 13 streams located within a single Mediterranean catchment, but draining sub‐catchments with contrasting land use. 3. At the catchment scale, results showed two contrasting land use gradients: (i) from forested‐ to urban‐dominated catchments and (ii) from low to moderate agricultural‐dominated catchments. Variation in structural and functional parameters was strongly related to these land use gradients. Specifically, NH₄⁺ demand (measured as the uptake velocity, V_f) decreased along the gradient from forested‐ to urban‐dominated catchments primarily in response to increases in stream nutrient concentrations [NH₄⁺, dissolved organic nitrogen (DON) and carbon (DOC)]. Both primary production and respiration increased along the gradient of agricultural development in response to increases in algal biomass (chlorophyll a). Soluble reactive phosphorus demand was not related to any of the land use gradients. 4. Our results illustrate the connections among factors operating at different spatial scales (i.e. from catchments to streams) and their distinct influence on stream ecosystem function. Managers should take into consideration these connections when designing stream management and restoration plans. Because ecologically successful stream management and restoration is expected to restore function as well as structure to streams, the use of appropriate measures of functional processes is required. Nutrient retention and metabolism parameters are good candidates to fill this gap.     

Nitrogen and phosphorus budgets for a tropical watershed impacted by agricultural land use: Guayas, Ecuador

Large-scale changes in land use are occurring in many tropical regions, with significant impacts on nitrogen and phosphorus biogeochemistry. In this study we examine the relationships between land use, anthropogenic nutrient inputs, and riverine nutrient exports in a major agricultural watershed of the Pacific coast of South America, the Guayas River basin of Ecuador. We present comprehensive nutrient budgets for nitrogen (N) and phosphorous (P) for the Guayas River basin and 10 sub-watersheds. We quantify the four major anthropogenic nutrient fluxes into and out of the region: N and P fertilizer application, N fixation by leguminous crops, net import/export of N and P in agricultural products (food and feed), and atmospheric deposition. We also estimate inputs of N from biological N fixation in forests and of P from weathering sources in soils and bedrock. The sum of these sources represents net inputs of N and P to each watershed region. Overall, synthetic fertilizers are the largest input to the Guayas Basin for N (53%) and P (57%), and the largest outputs are N and P in crops. Losses of N and P in river export account for 14–38% of total N and P inputs, and there is significant accumulation of N and P, or unmeasured forms of N and P export, in most of the sub-basins. Nutrient balances are indicative of the sustainability of land use practices in a region, where a negative balance of N or P indicates nutrient depletion and subsequent loss of soil fertility, yield, and economic viability. Although the nutrient balance of the entire Guayas Basin is positive, there are negative or near zero balances in two sub-watersheds with extensive banana, coffee and permanent crops. In these basins, degradation of soil quality may be occurring due to these net nutrient losses. Our data show that nutrients are leaving the basin primarily as export crops, with riverine losses of nutrients smaller than crop exports. Nonetheless, there is a direct relationship between nutrient inputs and river outputs, suggesting that agricultural management practices in the basin may have direct impacts on N and P delivery to the highly productive Guayas estuary.     

植被覆盖度和综合治理对纸坊沟流域土壤氮素流失的影响

同小流域土壤侵蚀一样,小流域土壤氮素随洪流流失也受到植被覆盖度的影响,通常经过调整小流域内土地利用结构以达到控制水土流失。该研究以8.27 km2纸坊沟流域和1:400比例流域模型为研究对象,研究植被覆盖度和综合治理对纸坊沟流域土壤氮素流失的影响。结果表明:在模拟降雨下,当流域植被覆盖度分别为60%、40%、20%和0时,流域模型铵态氮流失量分别为87.08、44.31、25.16和13.71 kg/km2,硝态氮为85.50、74.05、63.95和56.23 kg/km2,全氮为0.81、1.18、1.98和7.51 t/km2;在自然降雨下,1998年与1992年相比,全流域年土壤侵蚀量为1 086 t/km2和1 119 t/km2,氮素流失量为8 758.5和7 562.2 kg,减少了15.8%,其中农地减少了52.0%。流域对降水中的矿质氮具有过滤作用,硝态氮的过滤作用明显高于铵态氮。洪流泥沙中<20 mm微团聚体富集造成了泥沙有机质和全氮的富集。植被覆盖虽能有效地减少流域土壤侵蚀和全氮的流失,却能增加土壤矿质氮的流失。坡地退耕还林草可显著减少流域土壤氮素流失。     

A worldwide view of organic carbon export from catchments

Growing interest in the effects of global change on the metabolism, stoichiometry and cycling of carbon in aquatic ecosystems has motivated research on the export of organic carbon (OCE) from catchments. In this article, quantitative and functional features of the annual export rates of total, particulate and dissolved organic carbon (TOC, POC and DOC) were reviewed, and the stoichiometry of export (OC:N, OC:P and N:P) from 550 catchments worldwide was reported. TOC export ranged 2.1–92,474?kg?C?km^?2?year^?1, POC export ranged 0.4–73,979?kg?C?km^?2?year^?1 and DOC export ranged 1.2–56,946?kg?C?km^?2?year^?1. Exports of TOC and DOC were strongly linked, but POC export was unrelated to DOC. The DOC fraction comprised on average 73?±?21% of TOC export. The export rates of organic carbon were poorly related to those of total nitrogen and total phosphorus. Discrete and continuous environmental variables failed to predict TOC export, but DOC export was influenced by discharge and catchment area worldwide. Models of OCE in different catchment types were controlled by different environmental variables; hydrological variables were generally better predictors of OCE than anthropogenic and soil variables. Elemental ratios of carbon export in most catchments were above the Redfield ratio, suggesting that phosphorus may become the limiting nutrient for downstream plant growth. These ratios were marginally related to environmental data. More detailed hydrological data, consideration of in-stream processes and the use of quasi-empirical dynamical models are advocated to improve our knowledge of OCE rates and those of other nutrients.     

The influence of impoundments on nutrient budgets in two catchments of Southeastern Michigan

N and P budgets quantify inputs and outputs of nutrients at the catchment scale to allow evaluation of inputs and outputs as well as inferences about transport and processing based on unaccounted-for nutrients. N and P budgets were constructed for two catchments in southeastern Michigan with markedly different numbers of impoundments, over two years, to evaluate the influence of impoundments on nutrient fluxes from each catchment. The Huron, with 88 impoundments >10 ha, stored 156 kg P km⁻² y⁻¹, while the Raisin (with 14 impoundments) had a net export of 102 kg P km⁻² y⁻¹. The Huron catchment also stored and denitrified more N than the Raisin catchment – 2,418 kg N km⁻² y⁻¹ compared to 1,538 kg N km⁻² y⁻¹. Riverine export of N and P also varied markedly between the catchments, with the Huron River exporting 288 kg N and 7 kg P km⁻² y⁻¹ and the Raisin River exporting 1,268 kg N and 34 kg P km⁻² y⁻¹. We then re-calculated budget results from previous studies using the approach of the present study, altering input and outputs fluxes as well as system boundaries to obtain comparable budgets. For these comparable budgets, annual P outputs on average accounted for 77% of inputs whereas N outputs accounted for only 39% of N inputs. Across catchments, the percent of inputs exported by the river averaged 16% for N and 5% for P, indicating more effective retention of P than N.     

Hydrological and biological processes modulate carbon,nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec,Canada)

Changes in atmospheric deposition, stream water chemistry, and solute fluxes were assessed across 15 small forested catchments. Dramatic changes in atmospheric deposition have occurred over the last three decades, including a 70% reduction in sulphur (S) deposition. These changes in atmospheric inputs have been associated with expected changes in levels of acidity, sulphate and base cations in streams. Soil retention of S appeared to partially explain rates of chemical recovery. In addition to these changes in acid–base chemistry we also observed unexpected changes in nitrogen (N) biogeochemistry and nutrient stoichiometry of stream water, including decreased stream N concentrations. Among all catchments the average flux of dissolved inorganic nitrogen (DIN) was best predicted by average runoff, soil chemistry (forest floor C/N) and levels of acid deposition (both S and N). The rate of change in stream DIN flux, however, was much more closely correlated with reductions in rates of S deposition rather than those of DIN. Unlike DIN fluxes, the average concentrations as well as the rates of decline in streamwater nitrate (NO₃) concentration over time were tightly linked to stream dissolved organic carbon/dissolved organic nitrogen ratios DOC/DON and DON/TP rather than catchment characteristics. Declines in phosphorus adsorption with increasing soil pH appear to contribute to the relationship between C, N, and P in our study catchments. Our observations suggest that catchment P availability and its alteration due to environmental changes (e.g. acidification) might have profound effects on N cycling and catchment N retention that have been largely unrecognized.     

Biogeochemical Budgets in a Mediterranean Catchment with High Rates of Atmospheric N Deposition – Importance of Scale and Temporal Asynchrony

In this study biogeochemical export in a set of catchments that vary from 6 ha to almost 1500 ha is investigated. Studying catchments across this large range of scales enables us to investigate the scale dependence and fundamental processes controlling catchment biogeochemical export that would not have been possible with a more limited data set. The Devil Canyon catchment, in the San Bernardino Mountains, California, has some of the highest atmospheric N deposition rates in the world (40–90 kg ha⁻¹year⁻¹ at the crest of the catchment). These high rates of deposition have translated into consistently high levels of NO⁻in 3 some streams of the San Bernardino Mountains. However, the streams of the Devil Canyon catchment have widely varying dissolved inorganic nitrogen (DIN) concentrations and export. These differences are also, to a more limited extent, present for dissolved organic carbon (DOC) but not in other dissolved species (Cl⁻, SO²⁻₄,Ca²⁺ and other weathering products). As catchment size increases DIN and DOC concentrations first increase until catchment area is ∼150 ha but then decrease as catchment scale increases beyond that size. The scale dependence of DIN export implies that catchments at different spatial scales are at different degrees of N saturation. The reason for this scale effect appears to be the dominance of flushing of DIN out of soil at small scales due to the temporal asynchrony between nutrient availability and biological N demand, the groundwater exfiltration of this flushed DIN at intermediate scales and the removal of this DIN from streamflow through in-stream processes and groundwater–surface water interaction at larger scales. While the particular scale effect observed here may not occur over the same range in catchment area in other ecosystems, it is likely that other ecosystems have similar scale dependant export for DIN and DOC.     

The contribution of flood disturbance, catchment geology and land use to the habitat template of periphyton in stream ecosystems

1. Periphyton chlorophyll a (chl a), ash-free dry mass, taxonomic composition, and cellular and water-column nutrients were analysed every 4 weeks for a year at sixteen stream sites in New Zealand. The hypothesis was investigated that broad-scale differences in mean monthly periphyton development are defined primarily by the frequency of flood disturbances and the periphyton's interaction with the nutrients. it us of the streams as determined by catchment geology and land use. 2. Overall, mean monthly chl a concentration declined with increasing flood frequency (r= -0.711, P < 0.001), and seasonality in chl a was better defined at sites with a low frequency of floods. Chlorophyll a concentration was generally low throughout the year at sites with frequent floods (> 15 yr^?1). 3. No relationship existed between inorganic nutrient concentrations and catchment geology or land development. However, conductivity declined significantly as a function of the percentage of the catchment underlain by nutrient-poor, hard rocks (plutonic and fine-grained metamorphic rocks) (r= -0.515, P < 0.05), but increased significantly with the percentage of the catchment in intensive agricultural land use (r= 0.799, P < 0.001). 4. Cellular nutrient concentrations suggested that nitrogen was the nutrient most commonly limiting periphyton production. In turn, cellular N concentrations declined significantly with increasing percentage of the catchment in hard rock (r= -0.5M, P < 0.05) and increased with percentage of the catchments in intensive agricultlural land use (r= 0.948, P < 0.001). 5.The sites were classified into three enrichment groups (high, moderate and low) based on their land use and underlying geology. Cellular N concentrations varied significantly among these enrichment groups (ANOVA F= 14.661, P < 0.001). 6. Log chl a decreased significantly with increases in the annual 80th percentile velocity. However, the relationship was significantly different among the enrichment groups. 7. A stepwise multiple regression on the full dataset identified that the frequency of floods, proportion of the catchment in high-intensity agricultural land use and proportion in alkaline rocks were the most significant factors explaining variation in mean monthly chl a among the sites (r²= 89%). 8. Overall, the results showed that flood disturbance and catchment enrichment regimes are probably the principal axes of the habitat template of periphyton among the study streams, and could be used to explain and predict broad-scale differences in periphyton development among other temperate stream ecosystems.     

Chronic catchment nitrogen enrichment and stoichiometric constraints on the bioavailability of dissolved organic matter from leaf leachate

1. Chronic nitrogen (N) deposition may alter the bioavailability of dissolved organic matter (DOM) in streams by multiple pathways. Elevated N deposition may alter the nutrient stoichiometry of DOM as well as nutrient availability in stream water. 2. We evaluated the influence of a decadal‐scale experimental N enrichment on the relative importance of DOM nutrient content and inorganic nutrient availability on the bioavailability of DOM. We measured the consumption of dissolved organic carbon (DOC) and changes in nutrient concentration, DOM components and enzyme activity in a bottle incubation assay with different DOM and nutrient treatments. To evaluate the effect of DOM stoichiometry, we used leaf leachates of different carbon/N/phosphorus (C : N :P) ratio, made from leaf litter sourced in the reference and N‐enriched catchments at the Bear Brook Watershed in Maine (BBWM). We also manipulated the concentration of inorganic N and P to compare the effect of nutrient enrichment with DOM stoichiometry. 3. DOC from the N‐enriched catchment was consumed 14% faster than that from the reference catchment. However, mean DOC consumption for both leachates was more than doubled by the simultaneous addition of N and P, compared to controls, while the addition of N or P alone increased consumption by 42 and 23%, respectively. The effect of N and/or P enrichment consistently had a greater effect than DOM source for all response variables considered. 4. We subsequently conducted DOC uptake measurements using leaf leachate addition under ambient and elevated N and P in the streams draining the reference and N‐enriched catchments at BBWM. In both streams, DOC uptake lengths were shorter when N and P were elevated. 5. Although both DOM stoichiometry and inorganic nutrient availability affect DOM bioavailability, N and P co‐limitation appears to be the dominant driver of reach‐scale processing of DOM.     

Aggregating land use quantity and intensity to link water quality in upper catchment of Miyun Reservoir

Eutrophication resulting from nutrient enrichment decreases water quality and harms ecosystem structure and function, and its degree is significantly affected by land use in the catchment. Quantifying the relationship between eutrophication and land use can help effectively manage land use to improve water quality. Previous studies principally utilized land use quantity as an indicator to link water quality parameters, but these studies lacked insight into the impact of land use intensity. Taking the upper catchment of Miyun Reservoir as a case study, we developed a method of aggregating land use quantity and intensity to build a new land use indicator and tested its explanatory power on water quality. Six nutrient concentrations from 52 sub-watersheds covering the whole catchment were used to characterize the spatial distribution of eutrophication. Based on spatial techniques, empirical conversion coefficients, remote sensing data, and socio-economic statistical data, land use intensity was measured and mapped visually. The new land use indicator was calculated and linked to nutrient concentrations by Pearson correlation coefficients. Results demonstrated that our new indicator incorporating intensity information can quantify the different nutrient-exporting abilities of different land use areas. Compared to traditional indicators that only incorporate land use quantity, most Pearson correlation coefficients between the new indicator and water nutrient concentrations increased. This new information enhanced the explanatory power of land use on water nutrient concentrations, and so will be able to help us understand the impact of land use on water quality and guide decision making for better land use management.     

N Retention in Urbanizing Headwater Catchments

Urbanization can potentially alter watershed nitrogen (N) retention via combined changes in N loading, water runoff, and N processing potential. We examined N export and retention for two headwater catchments (∼4 km²) of contrasting land use (16% vs. 79% urban) in the Plum Island Ecosystem (PIE-LTER) watershed, MA. The study period included a dry year (2001–2002 water year) and a wet year (2002–2003 water year). We generalized results by comparing dissolved inorganic nitrogen (DIN) concentrations from 16 additional headwater catchments (0.6–4.2 km²) across a range of urbanization (6–90%). Water runoff was 25–40% higher in the urban compared to the forested catchment, corresponding with an increased proportion of impervious surfaces (25% vs. 8%). Estimated N loading was 45% higher and N flux 6.5 times higher in the urban than in the forested catchment. N retention (1 − measured stream export / estimated loading) was 65–85% in the urban site and 93–97% in the forested site, with lower retention rates during the wetter year. The mechanisms by which N retention stays relatively high in urban systems are poorly known. We show that N retention is related to the amount of impervious surface in a catchment because of associated changes in N loading (maximized at moderate levels of imperviousness), runoff (which continues to increase with imperviousness), and biological processes that retain N. Continued declines in N retention due to urbanization have important negative implications for downstream aquatic systems including the coastal zone.     

Riverine nitrogen export in Swedish catchments dominated by atmospheric inputs

We present the first estimates of net anthropogenic nitrogen input (NANI) in European boreal catchments. In Swedish catchments, nitrogen (N) deposition is a major N input (31–94%). Hence, we used two different N deposition inputs to calculate NANI for 36 major Swedish catchments. The relationship between riverine N export and NANI was strongest when using only oxidized deposition (NO_y) as atmospheric input (r² = 0.70) rather than total deposition (i.e., both oxidized and reduced nitrogen, NO_y + NH_x deposition, r² = 0.62). The y-intercept (NANI = 0) for the NANI calculated with NO_y is significantly different from zero (p = 0.0042*) and indicates a background flux from the catchment of some 100 kg N km^?2 year^?1 in addition to anthropogenic inputs. This agrees with similar results from North American boreal catchments. The slope of the linear regressions was 0.25 for both N deposition inputs (NO_y and NO_y + NH_x), suggesting that on average, 25% of the anthropogenic N inputs is exported by rivers to the Baltic Sea. Agricultural catchments in central and southern Sweden have increased their riverine N export up to tenfold compared to the inferred background flux. Although the relatively unperturbed northernmost catchments receive significant N loads from atmospheric deposition, these catchments do not show significantly elevated riverine N export. The fact that nitrogen export in Swedish catchments appears to be higher in proportion to NANI at higher loads suggests that N retention may be saturating as loading rates increase. In northern and western Sweden the export of nitrogen is largely controlled by the hydraulic load, i.e., the riverine discharge normalized by water surface area, which has units of distance time^?1. Besides hydraulic load the percent total forest cover also affects the nitrogen export primarily in the northern and western catchments.     

Impact of catchment land use on bacterial communities within stream biofilms

The bacterial community structure in epilithic biofilms within 18 different streams was characterised using a community DNA fingerprinting technique (automated ribosomal intergenic spacer analysis—ARISA). Each stream has previously been described in terms of the dominant catchment land use, relative level of human disturbance and using a broad suite of water quality variables. Combination of ARISA with multivariate statistical analysis and ordination revealed that bacterial communities in streams located within rural catchments were significantly different to those within urban catchments. Broad-scale catchment land use described the largest component of the observed variation with no single water quality variable found to be a dominant determinant of the observed bacterial community variability, assessed using distance based redundancy analysis (dbRDA) of the ARISA data. This study highlights the potential of bacterial ARISA to provide a rapid and cost-effective approach to monitor the impact of catchment land use on aquatic ecosystems, such as the influence of encroaching urban development on the ecological health of rural streams.