Anthropogenic Nitrogen Sources Research Articles

Meta-analysis: Global patterns and drivers of denitrification, anammox and DNRA rates in wetland and marine ecosystems

Nitrogen cycling is one of the most important biogeochemical processes on Earth, and denitrification, anammox and DNRA processes are important nitrogen cycling processes in estuarine ecosystems. However, due to the large input of anthropogenic nitrogen sources, a large number of environmental problems have now occurred in the estuary. But the global patterns and controlling factors of denitrification, anammox and DNRA rates in wetland marine ecosystems are not yet known. We reached our conclusions through a global synthesis of 546 observation sites from 78 peer-reviewed papers: The three rates were generally higher in areas near wetlands than in coastal areas. The rate of denitrification was highest in the subtropical region the seasonal variability was not significant; and TOC was the main factor controlling denitrification. The rate of anammox was significantly higher in the subtropical region than in the tropical and boreal zones, and the seasonal variability was significant; and at the same time, TN was the main driver of the anammox rate of the wetland ocean. DNRA rates were significantly higher in the tropics than in the subtropics and temperate zones; and the main driver of DNRA rates was temperature. Nitrogen cycle functional genes also had an indirect effect on their rates. With NH4 + -N significantly affecting nirK abundance and TN significantly affecting the gene abundance of nirS; TOC and TN had a greater effect on hzo abundance, which indirectly affected anammox rates; for DNRA, C/N significantly affects the gene abundance of nrfA, which indirectly affects the DNRA rate. Therefore, the findings of this study indicate that physicochemical indicators about N and climatic characteristics have a profound effect on the nitrogen cycling process, which provides a good feedback for studying the role of denitrification and provides a positive impact on global climate and environmental governance.

Spatio-temporal patterns in the biomass, species composition and nitrogen content of drift macroalgae in an urbanised coastal embayment

Understanding spatio-temporal variability in the production of drift macroalgae is useful for monitoring and assessing the influence of local anthropogenic nutrient sources on coastal ecosystems. In this study we: (1) assessed how the drift macroalgal community composition and biomass have changed over a 22-year period in Port Phillip Bay (a large coastal embayment in Victoria, Australia); and (2) measured the amount of accumulated nitrogen in drift macroalgae and the contribution of anthropogenic nitrogen sources through isotopic analysis. The surveys revealed little overlap in species composition (<13%) and a significant decrease in standing stock biomass (from 631 to 49 g m-2) between surveys (1995/1996 and 2017/2018). This was associated with a high degree of turnover (β-diversity) at the genus level between surveys (mean – 0.53), resulting in distinct communities at all sites, particularly among red algal genera. In addition, the occurrence of ephemeral macroalgal blooms and a semi-perennial mat of the red macroalga Botryocladia sonderi at 10-15 m depth at locations close to nutrient sources indicate drift macroalgae are important nutrient accumulators in the bay. Isotopic analysis revealed a high contribution of anthropogenic-derived nitrogen (δ15N > 17‰ and ~ 3% N content) at depths and locations close to nutrient sources, particularly at times of the year when large wastewater discharges occur. Harvesting of drift macroalgae could be a cost-effective solution for offsetting nitrogen inputs from anthropogenic sources, but it will be important to first investigate whether harvesting results in negative impacts to associated fauna or nutrient cycling.

Impact of typhoons on anthropogenic nitrogen sources in Lake Sihwa, South Korea

This study investigated the nitrate dual isotopic compositions (δ15NNO3 and δ18ONO3) of water samples to trace nitrate sources in Lake Sihwa, which encompasses various land-use types (e.g., urban, industry, wetland, and agriculture). The biogeochemical interactions of anthropogenic nitrogen sources (e.g., soil, road dust, and septic water) were also evaluated through multiple pathways from terrestrial boundaries to the water column. Based on increased concentrations of dissolved total nitrogen (DTN; 3.1 ± 1.6 mg/L) after typhoon, the variation of element stoichiometry (N:P:Si) in this system shifted to the relatively N-rich conditions (DIN/DIP; 14.1 ± 8.1, DIN/DSi; 1.4 ± 1.8), potentially triggering the occurrence of harmful algal blooms. Furthermore, discriminative isotopic compositions (δ15NNO3; 4.0 ± 2.1 ‰, δ18ONO3; 6.1 ± 4.3 ‰) after the typhoon suggested the increased DTN input of anthropogenic origins within Lake Sihwa would be mainly transported from urban sources (76 ± 9 %). Consequently, the isotopic-based approach may be useful for effective water quality management under increased anthropogenic activities near aquatic systems.

A Quantitative Approach for Identifying Nitrogen Sources in Complex Yeongsan River Watershed, Republic of Korea, Based on Dual Nitrogen Isotope Ratios and Hydrological Model

Effective management of nitrate loading in complex river systems requires quantitative estimation to trace different nitrogen sources. This study aims to validate an integrated framework using soluble nitrogen isotope ratios (δ15N–NH4 and δ15N–NO3) and hydrological modeling (hydrological simulation program SPARROW) of the main stream and tributaries in the Yeongsan River to determine anthropogenic nitrogen fluxes among different land-use types in the complex river watershed. The δ15N–NH4 and δ15N–NO3 isotopic compositions varied across different land-use types (4.9 to 15.5‰ for δ15N–NH4 and −4.9 to 12.1‰ for δ15N–NO3), reflecting the different sources of nitrogen in the watershed (soil N including synthetic fertilizer N, manure N, and sewage treatment plant effluent N). We compared the soluble nitrogen isotopic compositions (δ15N–NH4 and δ15N–NO3) of the river water with various nitrogen sources (soil N, manure N, and sewage N) to assess their contribution, revealing that N from sewage treatment plant effluent as a point source was dominant during the dry season and N from forest- and soil-derived non-point sources was dominant due to intensive rainfall during the wet season. The coefficient of determination (R2) between the measured pollution load and the predicted pollution load calculated by the SPARROW model was 0.95, indicating a high correlation. In addition, the EMMA-based nitrogen contributions compared to the SPARROW-based nitrogen fluxes were similar to each other, indicating that large amounts of forest- and soil-derived N may be transported to the Yeongsan River watershed as non-point sources, along with the effect of sewage treatment plant effluent N as a point source. This study provides valuable insights for the formulation of management policies to control nitrogen inputs from point and non-point sources across different land-use types for the restoration of water quality and aquatic ecosystems in complex river systems. Given the recent escalation in human activity near aquatic environments, this framework is effective in estimating the quantitative contribution of individual anthropogenic nitrogen sources transported along riverine systems.

Understanding the Impact of a Major Hydro-Agricultural Project in Low Mondego Area (Portugal)

The Low Mondego (Baixo Mondego, in Portuguese) river basin has been considerably modified by human interventions to prevent floods and to improve agricultural conditions over recent decades. This work analyzes the main impacts arising from those interventions and the socio-economic context in which they occurred. The development and application of a framework to compute the variation of the nitrogen surplus between 1986 and 2018 in the Low Mondego watershed in the central part Portugal is presented. The nitrogen mass balances take into consideration different land use types, inputs and outputs, thereby making it possible to understand how human interventions have impacted the variation of the surplus. It is noticeable that the major nitrogen sources are related to the agricultural sector. However, factors such as the implementation of the Code of Good Agricultural Practices, as well as social conditions, and regulations to reduce nitrogen emissions to the atmosphere helped to cut the nitrogen surplus. This work shows how improving the agricultural conditions has served to increase the crop productivity in improved areas. Very particular social circumstances led to the reduction in anthropogenic nitrogen sources, which has been accompanied by a decline in the nitrogen that is exported at the river outlet.

Characterizing the impact of Three Gorges Dam on the Changjiang (Yangtze River): A story of nitrogen biogeochemical cycling through the lens of nitrogen stable isotopes

The alterations of nitrogen sources and cycling within the Three Gorges Reservoir (TGR) and downstream the Changjiang were investigated to understand the impacts of the construction of the Three Gorges Dam (TGD) and anthropogenic inputs from the associated watershed. Water samples collected in October 2016 were analyzed for hydrologic parameters, nutrient concentrations, and stable isotopes of nitrate (NO3−), ammonium (NH4+) and particulate matter. Nitrate dual stable isotope values ranged from +5.8‰ to +7.1‰ and −1.9‰ to +0.4‰ for δ15N and δ18O, respectively. δ15N values in particulate nitrogen (PN) ranged from +0.5‰ to +8.5‰, with slightly lower values before the dam. δ15N-NH4+ values ranged between +10.5‰ and +19.4‰, likely reflecting the presence of ammonium assimilation throughout the TGR. The contribution of different nitrogen sources was calculated using a Bayesian mixing model. These sources, including soil organic nitrogen, ammonium fertilizer, and sewage effluent, contributed to elevated DIN concentrations within the TGR (83.2 μM–178.5 μM). The construction of the dam has also likely induced changes in the river environment such as ammonium assimilation in the surface waters and nitrification and/or remineralization within the deep waters of the TGR. Overall, during this investigation period, the TGR acted as a sink of PN (retaining 29%), yet negligibly influenced levels of TDN with ~96.5% of TDN exported to the downstream Changjiang and estuary. It is important to understand the long-term impacts of the TGD on the ecological environment of the Changjiang. This study highlights the influence that anthropogenic nitrogen sources have on the natural biogeochemical cycling within the TGR, showing the urgent need to reduce anthropogenic nitrogen pollution.

Monitoring the Return of Marine-Derived Nitrogen to Riparian Areas in Response to Dam Removal on the Elwha River, Washington

Two sources of nitrogen to Pacific Northwest riparian areas are marine-derived nitrogen (MDN) via anadromous Pacific salmon (Oncorhynchus spp.) and atmospheric nitrogen fixation via red alder (Alnus rubra). The recent removal of two large dams on the Elwha River, WA, opened up about 60 km of previously inaccessible river habitat for anadromous salmon. We used naturally abundant stable nitrogen isotopes to establish baseline data to monitor the influx of MDN to riparian zones of Elwha River tributaries post dam removal. We sampled riparian soil, overstory, and understory vegetation in sites with nitrogen-fixing A. rubra and sites with bigleaf maple (Acer macrophyllum) at an undammed reference site, and along three tributaries, one between the former dams (accessible to anadromous salmon since 2012) and the others upstream of the former dams (no anadromous salmon). Based on δ15N measurements of soil and vegetation, we did not detect MDN at any of the tributaries, including the reference tributary. However, the understory riparian vegetation between the former dams had a higher δ15N than the other tributaries, which may be due to upstream anthropogenic nitrogen sources. Although A. rubra foliage was isotopically distinct from A. macrophyllum, and A. rubra litter had higher total nitrogen, soil and understory vegetation in A. rubra and A. macrophyllum sites did not differ isotopically. Monitoring of these areas and those further upstream on the Elwha River will allow us to trace the return of MDN to the watershed, and help to clarify the role that anadromous fish play in riparian ecosystems.

The effects of incubation time, temperature and nitrogen concentration on the isotopic signature (δ15N) of the macroalga Chondrus crispus

Macroalgae can incorporate the isotopic signature (δ15N) of their external environment and be used as ‘bioindicators’ to map and identify between multiple sources of anthropogenic nitrogen. However, evidence suggests that the isotopic signature of their tissues can also be influenced by a range of other factors, which could confound their use as a monitoring tool. Hence, this study aimed to better understand the factors affecting macroalgae isotopic signatures, using Chondrus crispus as a model species. This was achieved by exposing C. crispus to 15N-enriched NO3− and/or NH4+ at varying concentrations, temperatures and incubation times under controlled laboratory conditions. Longer incubation times promoted macroalgae δ15N values closer to that of the external nitrogen source. However, even after 28 days incubation, macroalgae δ15N was still >30‰ lower than the source. This suggests that the δ15N of C. crispus is unlikely to reach isotopic equilibrium with its external environment within a time frame that is feasible for most field studies. However, C. crispus can be used to highlight relative isotopic differences between sources even at very low nitrogen concentrations (< 4 μM). Macroalgae incorporated the isotopic signature of external sources faster at higher nitrogen concentrations. This was fastest for macroalgae exposed to a combination of NH4+ and NO3−, followed by just NH4+, and slowest for NO3−. These results could have implications for bioindicator studies where proportions and concentrations of NO3− and NH4+ vary in time and space. Likewise, future field studies may have to account for temperature in their sampling design if temperatures vary spatially and/or seasonally, as the δ15N of C. crispus more closely resembled that of the external nitrogen source at 7 °C than 14 °C. Overall, this study found that the δ15N of C. crispus was not just a function of the source, as it was also influenced by incubation time, temperature and by the concentration and types of nitrogen present. As C. crispus can tolerate high concentrations of NH4+ and a wide range of temperatures, it could be a suitable bioindicator species for monitoring nitrogen rich effluents in many different environments providing the results are interpreted cautiously.

Stable isotopes of algae and macroinvertebrates in streams respond to watershed urbanization, inform management goals, and indicate food web relationships

Watershed development and anthropogenic sources of nitrogen are among leading causes of negative impacts to aquatic ecosystems around the world. The δ15N of aquatic biota can be used as indicators of anthropogenic sources of nitrogen enriched in 15N, but this mostly has been done at small spatial extents or to document effects of point sources. In this study, we sampled 77 sites along a forest to urban land cover gradient to examine food webs and the use of δ15N of periphyton and macroinvertebrate functional feeding groups (FFGs) as indicators of watershed development and nitrogen effects on streams. Functional feeding groups had low δ15N variability among taxa within sites. Mean absolute differences between individual taxa and their respective site FFG means were < 0.55‰, whereas site means of δ15N of FFGs had ranges of approximately 7–12‰ among sites. The δ15N of periphyton and macroinvertebrate FFGs distinguished least disturbed streams from those with greater watershed urbanization, and they were strongly correlated with increasing nitrogen concentrations and watershed impervious cover. Nonmetric multidimensional scaling, using δ15N of taxa, showed that changes in macroinvertebrate assemblages as a whole were associated with forest-to-urban and increasing nitrogen gradients. Assuming an average +3.4‰ per trophic level increase, δ15N of biota indicated that detrital pathways likely were important to food web structure, even in streams with highly developed watersheds. We used periphyton and macroinvertebrate FFG δ15N to identify possible management goals that can inform decisions affecting nutrients and watershed land use. Overall, the δ15N of periphyton and macroinvertebrates were strong indicators of watershed urban development effects on stream ecosystems, and thus, also could make them useful for quantifying the effectiveness of nitrogen, stream, and watershed management efforts.

Tracing nitrate sources with dual isotopes and long term monitoring of nitrogen species in the Yellow River, China

A heavy load of nitrogenous compounds reflects nutrient loss and influences water quality in large rivers. Nitrogenous concentrations and dual isotopes of nitrate were measured to ascertain the spatial and temporal distributions of nitrate transformation in the Yellow River, the second-longest river in China. Assessment of the long-term record indicates that [NO3−–N] has increased by two-fold over the past three decades. Weekly observation of ammonium over a twelve-year period revealed high concentrations and suggests impairment of water quality, particularly since 2011. The estimated total dissolved nitrogen flux was 7.2 times higher in middle reaches than that at head waters. Anthropogenic nitrogen sources become more important in lower section of the upper reaches and middle reaches because of intensive agricultural activities and urban input. Nitrate in the lower reaches was mainly derived from transportation of upstream nitrate and point sources from cities. The spatial variation of ammonium and nitrate isotopes show that nitrification is a key process governing nitrogen transformation. Riverine biological processes could potentially be responsible for the shift of nitrate isotope signature. The first step to reducing nitrogen load and improving water quality will be containment and careful management of sources from urban input, sewage waste and irrigation runoff.

Stable isotope analyses of precipitation nitrogen sources in Guiyang, southwestern China

To constrain sources of anthropogenic nitrogen (N) deposition is critical for effective reduction of reactive N emissions and better evaluation of N deposition effects. This study measured δ15N signatures of nitrate (NO3−), ammonium (NH4+) and total dissolved N (TDN) in precipitation at Guiyang, southwestern China and estimated contributions of dominant N sources using a Bayesian isotope mixing model. For NO3−, the contribution of non-fossil N oxides (NOx, mainly from biomass burning (24 ± 12%) and microbial N cycle (26 ± 5%)) equals that of fossil NOx, to which vehicle exhausts (31 ± 19%) contributed more than coal combustion (19 ± 9%). For NH4+, ammonia (NH3) from volatilization sources (mainly animal wastes (22 ± 12%) and fertilizers (22 ± 10%)) contributed less than NH3 from combustion sources (mainly biomass burning (17 ± 8%), vehicle exhausts (19 ± 11%) and coal combustions (19 ± 12%)). Dissolved organic N (DON) accounted for 41% in precipitation TDN deposition during the study period. Precipitation DON had higher δ15N values in cooler months (13.1‰) than in warmer months (−7.0‰), indicating the dominance of primary and secondary ON sources, respectively. These results newly underscored the importance of non-fossil NOx, fossil NH3 and organic N in precipitation N inputs of urban environments.

Can created tidal marshes reduce nitrate export to downstream estuaries?

Constructed marshes have the potential to serve as a buffer between anthropogenic sources of nitrogen and nutrient sensitive estuaries. Yet, there is little information available on the reduction of nitrogen loads that created tidal marshes might provide. A 5.6ha brackish marsh was constructed between row crop agricultural production and an estuary in North Carolina. Nitrate-N fluxes in and out of the marsh were monitored multiple times per hour for over a year to capture tidal and storm dynamics. Out of the five storms when most of the fluxes occurred, nitrate was retained in four (up to 42%), and released in the fifth, such that over the monitoring period the marsh retained 9% (40kg) of the nitrate that entered. Nutrient addition tracer studies confirmed the potential nitrate retention (up to 45%) of the marsh. The residence time of water in the constructed brackish marsh was identified as the primary factor limiting nitrate retention through an examination of the factors that were related to percent nitrate retention. A retention model based on a mass transfer coefficient was applied during the storm events and tracer studies to further evaluate the marsh N retention capacity. The model adequately simulated the nitrate retention with calibrated mass transfer coefficients (0.1–0.56m day−1) that fell within the range of those reported in literature for other systems. These results indicate that constructed tidal marshes can serve as substantial nitrate sinks and can be an integral part of overall plans to reduce the nutrient export to estuaries.

Groundwater – the disregarded component in lake water and nutrient budgets. Part 2: effects of groundwater on nutrients

AbstractLacustrine groundwater discharge (LGD) transports nutrients from a catchment to a lake, which may fuel eutrophication, one of the major threats to our fresh waters. Unfortunately, LGD has often been disregarded in lake nutrient studies. Most measurement techniques are based on separate determinations of volume and nutrient concentration of LGD: Loads are calculated by multiplying seepage volumes by concentrations of exfiltrating water. Typically low phosphorus (P) concentrations of pristine groundwater often are increased due to anthropogenic sources such as fertilizer, manure or sewage. Mineralization of naturally present organic matter might also increase groundwater P. Reducing redox conditions favour P transport through the aquifer to the reactive aquifer‐lake interface. In some cases, large decreases of P concentrations may occur at the interface, for example, due to increased oxygen availability, while in other cases, there is nearly no decrease in P. The high reactivity of the interface complicates quantification of groundwater‐borne P loads to the lake, making difficult clear differentiation of internal and external P loads to surface water. Anthropogenic sources of nitrogen (N) in groundwater are similar to those of phosphate. However, the environmental fate of N differs fundamentally from P because N occurs in several different redox states, each with different mobility. While nitrate behaves essentially conservatively in most oxic aquifers, ammonium's mobility is similar to that of phosphate. Nitrate may be transformed to gaseous N2 in reducing conditions and permanently removed from the system. Biogeochemical turnover of N is common at the reactive aquifer‐lake interface. Nutrient loads from LGD were compiled from the literature. Groundwater‐borne P loads vary from 0.74 to 2900 mg PO4‐P m−2 year−1; for N, these loads vary from 0.001 to 640 g m−2 year−1. Even small amounts of seepage can carry large nutrient loads due to often high nutrient concentrations in groundwater. Large spatial heterogeneity, uncertain areal extent of the interface and difficult accessibility make every determination of LGD a challenge. However, determinations of LGD are essential to effective lake management. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Variability in δ15N of intertidal brown algae along a salinity gradient: Differential impact of nitrogen sources

While it is generally agreed that δ15N of brown macroalgae can discriminate between anthropogenic and natural sources of nitrogen, this study provides new insights on net fractionation processes occurring in some of these species. The contribution of continental and marine sources of nitrogen to benthic macroalgae in the estuary-ria system of A Coruña (NW Spain) was investigated by analyzing the temporal (at a monthly and annual basis) and spatial (up to 10km) variability of δ15N in the macroalgae Ascophyllum nodosum and three species of the genus Fucus (F. serratus, F. spiralis and F. vesiculosus). Total nitrate and ammonium concentrations and δ15N-DIN, along with salinity and temperature in seawater were also studied to address the sources of such variability. Macroalgal δ15N and nutrient concentrations decreased from estuarine to marine waters, suggesting larger dominance of anthropogenic nitrogen sources in the estuary. However, δ15N values of macroalgae were generally higher than those of ambient nitrogen at all temporal and spatial scales considered. This suggests that the isotopic composition of these macroalgae is strongly affected by fractionation during uptake, assimilation or release of nitrogen. The absence of correlation between macroalgal and water samples suggests that the δ15N of the species considered cannot be used for monitoring short-term changes. But their long lifespan and slow turnover rates make them suitable to determine the impact of the different nitrogen sources integrated over long-time periods.

Simulations of Nitrate Leaching from Sugarcane Farm in Metahara, Ethiopia, Using the LEACHN Model

Metahara is the largest sugarcane farm in Ethiopia. It produces around 120,000 tons of sugar per year. The farm has been facing some problems such as salinity and sodicity, which has been studied by different experts for several decades. However, the other universal problem of agricultural farms is nitrate leaching loss, which has never been studied in the site, owing to lack of resources and expertise. The amount of nitrate leaching from agricultural farms can be measured directly from drainage rates or estimated by using numerical models. Measurements of drainage flow can be done by using lysimeters, but normally it can be estimated from water balance calculations or from field measurements of hydraulic gradients and hydraulic conductivities. However, in reality, hydraulic conductivity is highly variable and measurements in the field can be very laborious. Moreover, predicting nitrate leaching losses by using numerical models from such data- poor study area is also another problem. Nevertheless, groundwater nitrate concentration of the farm is measured by using the UV screening and distillation methods. Using the experimental results as an input for the model calibration, the amount of nitrate leaching from the farm is predicted for a 47 years of simulation period using the LEACHN model. In this case, both the measured and predicted values of nitrate leaching losses show that there is no nitrate problem in the site. However, even though the likelihood of detecting nitrate contamination in the study area is low, potential anthropogenic nitrogen sources must be carefully managed, for it is better to be safe than regretful.

Carbon stable isotopes as indicators of coastal eutrophication.

Coastal ecologists and managers have frequently used nitrogen stable isotopes (delta15N) to trace and monitor sources of anthropogenic nitrogen (N) in coastal ecosystems. However, the interpretation of delta15N data can often be challenging, as the isotope values fractionate substantially due to preferential retention and uptake by biota. There is a growing body of evidence that carbon isotopes may be a useful alternative indicator for eutrophication, as they may be sensitive to changes in primary production that result from anthropogenic nutrient inputs. We provide three examples of systems where delta13C values sensitively track phytoplankton production. First, earlier (1980s) mesocosm work established positive relationships between delta13C and dissolved inorganic nitrogen and dissolved silica concentrations. Consistent with these findings, a contemporary mesocosm experiment designed to replicate a temperate intertidal salt marsh environment also demonstrated that the system receiving supplementary nutrient additions had higher nutrient concentrations, higher chlorophyll concentrations, and higher delta13C values. This trend was particularly pronounced during the growing season, with differences less evident during senescence. And finally, these results were replicated in the open waters of Narragansett Bay, Rhode Island, USA, during a spring phytoplankton bloom. These three examples, taken together with the pre-existing body of literature, suggest that, at least in autotrophic, phytoplankton-dominated systems, delta13C values can be a useful and sensitive indicator of eutrophication.

Stable Isotopes as Tracers of Anthropogenic Nitrogen Sources, Deposition, and Impacts

The global nitrogen cycle has been perturbed by human activities, including agriculture, land-use change, and fossil fuel burning. This perturbation ranges from the local to global scale, as anthropogenic reactive nitrogen can be transported over long distances in the atmosphere, in groundwater, and in stream networks and can even impact the open ocean. Stable isotope signatures characteristic of reactive nitrogen can be used to trace its deposition in the present day, as well as in the past. Here we focus on the use of stable isotopes to trace the sources, transport, and impacts of anthropogenic nitrogen in the modern nitrogen cycle.

Stable isotope composition of suspended particulate organic matter in twenty reservoirs from Guangdong, southern China: Implications for pelagic carbon and nitrogen cycling

We studied seasonal variations in the carbon and nitrogen stable isotopes (δ13C and δ15N) of particulate organic matter (POM) in the surface water of 20 man-made reservoirs in southern China during March, August and December 2010. These reservoirs are located from subtropical to tropical region, varied in trophic states and were influenced by several types of human activities. The geomorphometric and biogeochemical gradients in tropical/subtropical regions are complicated and poorly understood because of low variation in temperature and high variation in hydrological processes. The POM samples were collected from all the reservoirs to assess the seasonal variation patterns of δ13CPOM and δ15NPOM. Variation in δ13CPOM followed seasonal thermal and hydrological cycles. By contrast, δ15NPOM did not seasonally differ, which may have been complicated by phytoplankton assimilation of N originating in agricultural waste entering the reservoirs and of atmospheric nitrogen during the wet season as well as an increased relative contribution of animal waste in the reservoirs in the dry season. Within each sampling period, nutrient availability is more important than thermal and solar radiation inputs in the summer, and these physical drivers are more important during the dry and cold winter month in controlling δ13CPOM and δ15NPOM. On an annual basis, trophic states (total P, total N and chlorophyll a) are the primary drivers for the changes in both δ13CPOM and δ15NPOM across reservoirs. When the seasonal effect is removed using annual averages from each reservoir, we found that latitude, trophic states, pH, rainfall, water temperature, reservoir age, catchment area to reservoir area (CA:RA) ratio and together explained about 80% of the variance in both δ13CPOM and δ15NPOM. Our findings also suggest that the trend of δ15NPOM is less predictable than δ13CPOM. The consistent 15N depletion and enrichment of POM in different reservoirs point to the importance of different anthropogenic sources of nitrogen in the reservoirs.

Stable nitrogen isotopes in coastal macroalgae: Geographic and anthropogenic variability

Growing human population adds to the natural nitrogen loads to coastal waters. Both anthropogenic and natural nitrogen is readily incorporated in new biomass, and these different nitrogen sources may be traced by the measurement of the ratio of stable nitrogen isotopes (δ15N). In this study δ15N was determined in two species of macroalgae (Ascophyllum nodosum and Fucus vesiculosus), and in nitrate and ammonium to determine the relative importance of anthropogenic versus natural sources of nitrogen along the coast of NW Spain. Both algal species and nitrogen sources showed similar isotopic enrichment for a given site, but algal δ15N was not related to either inorganic nitrogen concentrations or δ15N in the water samples. The latter suggests that inorganic nitrogen inputs are variable and do not always leave an isotopic trace in macroalgae. However, a significant linear decrease in macroalgal δ15N along the coast is consistent with the differential effect of upwelling. Besides this geographic variability, the influence of anthropogenic nitrogen sources is evidenced by higher δ15N in macroalgae from rias and estuaries compared to those from open coastal areas and in areas with more than 15×103 inhabitants in the watershed. These results indicate that, in contrast with other studies, macroalgal δ15N is not simply related to either inorganic nitrogen concentrations or human population size but depends on other factors as the upwelling or the efficiency of local waste treatment systems.

Using δ15N in Fish Larvae as an Indicator of Watershed Sources of Anthropogenic Nitrogen: Response at Multiple Spatial Scales

There is growing interest in applying δ15N in biota as an indicator of anthropogenic nutrient inputs to coastal environments because changes in δ15N correlate to inputs of land-based nutrients. In complex coastal receiving waters, however, land-use effects on biota δ15N may be masked by local hydrologic processes, especially exchange with coastal waters of different geochemical character. We examined δ15N differences among larval fish, a novel biotic indicator, in coastal receiving waters at both among and within watershed scales. Our goal was to characterize how hydrologic processes within coastal river mouths and embayments mediate the effect of land-based N sources on larval fish δ15N. We sampled three Lake Superior river-embayment systems from watersheds that span a large population density gradient. Over all stations, mean fish δ15N ranged from 2.7 ‰ to 10.8 ‰. Within each system, we found a different pattern in δ15N across the river–lake transition zone. Correlations between fish δ15N and water quality, particularly NH 4 + and total nitrogen, were highly significant and corresponded to known differences in sewage waste water inputs. A multivariate model that included both watershed-based population density and NH 4 + was found to provide the best fit to the δ15N data among a series of multi- and univariate candidate models. These results demonstrate that: (1) fish larvae δ15N responded at within watershed scales, and (2) within coastal receiving waters, fish larvae δ15N was related to waste water inputs at the watershed scale; however, expression at specific locations within a coastal system was strongly influenced by local hydrologic processes.