Sediment δ15N Values Research Articles

The indirect response of an aquatic ecosystem to long‐term climate‐driven terrestrial vegetation in a subalpine temperate lake

AbstractAimTo assess whether climate directly influences aquatic ecosystem dynamics in the temperate landscape of Tasmania or whether the effects of long‐term climatic change are mediated through the terrestrial environment (indirect climate influence).LocationPaddy's Lake is located at 1065 m a.s.l. in temperate north‐west Tasmania, a continental island south‐east of mainland Australia (41°15–43°25′ S; 145°00–148°15′ E).MethodsWe developed a new 13,400 year (13.4 kyr) palaeoecological dataset of lake sediment subfossil cladocerans (aquatic grazers), bulk organic sediment carbon (C%) and nitrogen (N%) and δ13C and δ15N stable isotopes. Comparison of this new data was made with a recently published pollen, geochemistry and charcoal records from Paddy's Lake.ResultsLow cladoceran diversity at Paddy's Lake is consistent with other temperate Southern Hemisphere lakes. The bulk sediment δ15N values demonstrate a significant lagged negative response to pollen accumulation rate (pollen AR). Compositional shifts of dominant cladoceran taxa (Bosmina meridionalis and Alona guttata) occur following changes in both pollen AR and pollen (vegetation) composition throughout the 13.4 kyr record at Paddy's Lake. The δ15N values demonstrate a significant positive lagged relationship to the oligotrophic:eutrophic cladoceran ratio.Main conclusionsLong‐term changes in cladoceran composition lag changes in both pollen AR and terrestrial vegetation composition. We interpret pollen AR as reflecting climate‐driven changes in terrestrial vegetation productivity and conclude that climate‐driven shifts in vegetation are the principal driver of the cladoceran community during the last ca. 13.4 kyr. The significant negative lagged relationship between pollen AR and δ15N reflects the primary control of vegetation productivity over within‐lake nutrient status. Thus, we conclude that the effects of long‐term climate change on aquatic ecosystem dynamics at our site are indirect and mediated by the terrestrial environment. Vegetation productivity controls organic soil development and has a direct influence over lake trophic status via changes in the delivery of terrestrial organic matter into the lake.

Palaeoecological evidence for sustained change in a shallow Murray River (Australia) floodplain lake: regime shift or press response?

Paleolimnological techniques can reveal long-term perturbations and associated stable state transitions of lake ecosystems. However, such transitions are difficult to predict since changes to lake ecosystems can be abrupt or gradual. This study examined whether there were past transitions in the ecological regime of Kings Billabong, a shallow River Murray wetland in southeast Australia. A 94-cm-long core, covering c. 90 years of age, was analysed at 1 cm resolution for subfossil cladocerans, diatoms and other proxies. Prior to river regulation (c. 1930), the littoral to planktonic ratios of cladocerans and diatoms, and bulk sediment δ13C values were high, while the period from c. 1930 to c. 1970 experienced considerable changes to the wetland ecosystem. The abrupt nature of changes of planktonic cladocerans and diatoms, particularly after the onset of river regulation (1930s), was triggered by inundation, high rates of sedimentation and shifts in bulk sediment δ15N values. However, the transition of a once littoral-dominated community, to one favouring an increasingly turbid, plankton-dominated trophic condition following river regulation was relatively slow and lasted for decades. The progression to a new regime was likely delayed by the partial recovery of submerged plant communities and related internal dynamics.

Extreme 15N Depletion in Seagrasses

Seagrass beds form an important part of the coastal ecosystem in many parts of the world but are very sensitive to anthropogenic nutrient increases. In the last decades, stable isotopes have been used as tracers of anthropogenic nutrient sources and to distinguish these impacts from natural environmental change, as well as in the identification of food sources in isotopic food web reconstruction. Thus, it is important to establish the extent of natural variations on the stable isotope composition of seagrass, validating their ability to act as both tracers of nutrients and food sources. Around the world, depending on the seagrass species and ecosystem, values of seagrass N normally vary from 0 to 8 ‰ δ15N. In this study, highly unusual seagrass N isotope values were observed on the east coast of Qatar, with significant spatial variation over a scale of a few metres, and with δ15N values ranging from +2.95 to −12.39 ‰ within a single bay during March 2012. This pattern of variation was consistent over a period of a year although there was a seasonal effect on the seagrass δ15N values. Seagrass, water column and sediment nutrient profiles were not correlated with seagrass δ15N values and neither were longer-term indicators of nutrient limitation such as seagrass biomass and height. Sediment δ15N values were correlated with Halodule uninervis δ15N values and this, together with the small spatial scale of variation, suggest that localised sediment processes may be responsible for the extreme isotopic values. Consistent differences in sediment to plant 15N discrimination between seagrass species also suggest that species-specific nutrient uptake mechanisms contribute to the observed δ15N values. This study reports some of the most extreme, negative δ15N values ever noted for seagrass (as low as −12.4 ‰) and some of the most highly spatially variable (values varied over 15.4 ‰ in a relatively small area of only 655 ha). These results are widely relevant, as they demonstrate the need for adequate spatial and temporal sampling when working with N stable isotopes to identify food sources in food web studies or as tracers of anthropogenic nutrients.

Different responses of sedimentary δ15N to climatic changes and anthropogenic impacts in lakes across the Eastern margin of the Tibetan Plateau

Knowledge of historical variations in sedimentary δ15N values and their relation to modern ones in lakes is critical in understanding N cycling in inland aquatic systems. Here we present late Holocene sedimentary δ15N variations in four lakes across the Eastern margin of the Tibetan Plateau (ETP), namely Lake Qinghai and Lake Chaonaqiu over north ETP (N-ETP) area, and Lake Yihai and Lake Lugu over south ETP (S-ETP) area. The results show that: (1) during historical periods when anthropogenic forcing was weak, the centennial/multi-decadal changes in δ15N values were dominated by climatic changes. Higher precipitation brought greater nutrients supply and led to higher Lacustrine Primary Productivity (LPP) and higher δ15N values in lake sediments, and vice versa. (2) During the recent/modern epoch with enhanced anthropogenic impacts, sedimentary δ15N values in Lake Qinghai and Lake Chaonaqiu (mid-high latitude lakes with low LPP) showed decreasing trends, possibly related to the increased global atmospheric reactive nitrogen (Nr) deposition. On the contrary, sedimentary δ15N values in Lake Lugu and Lake Yihai (subtropical lakes with high LPP) showed increasing trends, most likely because more dissolved inorganic nitrogen in lake water was used during LPP processes. This study highlights that LPP plays an important role in modifying sedimentary δ15N values both for historical and modern times, whereas atmospheric Nr deposition seems to influence the modern/recent sedimentary δ15N trends in lakes with low LPP.

A 400‐year isotopic record of seabird response to eastern tropical Pacific productivity

Seabirds play an important role in coastal environments, serving as key indicators of marine ecosystem variability as well as biovectors that influence terrestrial productivity and carbon storage. Long‐term estimates of seabird populations remain rare, but lakes that support large seabird populations in their watersheds can archive a history of seabird activity in their sediment records. Here we present a seabird guano‐influenced sediment record from Genovesa Crater Lake, Galápagos Islands, home to the world's largest reported colony of red‐footed boobies (Sula sula) and smaller populations of other species. Influx of seabird guano into Genovesa Crater Lake produces high sedimentary δ15N values, and temporal variability in sediment δ15N primarily reflects changes in guano influx through time. Two abrupt increases in sedimentary δ15N occurred at 1835 AD and 1965 AD, and variance increased following the 1965 AD shift. The largest of these abrupt shifts at 1835 AD coincided, within age model error, with an abrupt increase in marine productivity indicators in sediment records off the coast of Perú and Chile. In the latter part of the twentieth century, δ15N values increased during periods of higher landings of Peruvian anchoveta and sardines. We hypothesise that seabird presence and activity on Genovesa increased during periods of higher regional marine productivity. Enhanced variance in Genovesa δ15N following the 1965 AD shift may reflect a modern population more susceptible to climate and environmental variability than at any other time in the last 400 years.

Nitrogen isotope ratios of nitrate and N* anomalies in the subtropical South Pacific

AbstractNitrogen isotopic ratios of nitrate (δ15N– ) were analyzed above 1000 m water depth along 17°S in the subtropical South Pacific during the revisit WOCE P21 cruise in 2009. The δ15N– and N* values were as high as 17‰ and as low as −18 μmol N L−1, respectively, at depths around 250 m east of 115°W, but were as low as 5‰ and as high as +1 μmol N L−1, respectively, in subsurface waters west of 170°W. The relationships among concentrations, N* values, δ15N– values, and oxygen and nitrite concentrations suggest that a few samples east of 90°W were from suboxic and nitrite‐accumulated conditions and were possibly affected by in situ water column denitrification. Most of the high‐δ15N– and negative‐N* waters were probably generated by mixing between Subantarctic Mode Water from the Southern Ocean and Oxygen Deficit Zone Water from the eastern tropical South Pacific, with remineralization of organic matter occurring during transportation. Moreover, the relationship between δ15N– and N* values, as well as Trichodesmium abundances and size‐specific nitrogen fixation rates at the surface, suggest that the low‐δ15N– and positive‐N* subsurface waters between 160°E and 170°W were generated by the input of remineralized particles created by in situ nitrogen fixation, mainly by Trichodesmium spp. Therefore, the δ15N values of sediments in this region are expected to reveal past changes in nitrogen fixation or denitrification rates in the subtropical South Pacific.

Late Pliocene to early Pleistocene (2.4–1.25 Ma) paleoproductivity changes in the Bering Sea: IODP expedition 323 Hole U1343E

Late Pliocene to early Pleistocene paleoproductivity changes in the Bering Sea were reconstructed using geochemical concentrations and mass accumulation rates (MARs) of CaCO3, biogenic opal, and total organic carbon (TOC), and sedimentary nitrogen isotope ratios (δ15N) at IODP Expedition 323 Hole U1343E, drilled in the northern slope area (1956m deep) of the Bering Sea. CaCO3 concentration is generally low, but prominent CaCO3 peaks occur intermittently due to subseafloor authigenic carbonate formation rather than biogenic accumulation, regardless of glacial–interglacial variations. Biogenic opal concentrations reflect orbital-scale glacial–interglacial variations. However, TOC concentration did not show clear glacial–interglacial variation, probably due to poor preservation. The sedimentary δ15N values vary synchronously with biogenic opal concentration on orbital timescales. The co-varying pattern of opal productivity and δ15N values at Hole U1343E is a result of nutrient utilization controlled by diatom productivity in the Bering slope area where Fe is not a limiting factor. Biogenic opal and TOC MARs showed a temporal shift at around 1.9Ma from a high productivity period under nutrient-enriched conditions to a low productivity period under relatively nutrient-depleted conditions. High diatom productivity with low δ15N values before 1.9Ma is associated with abundant nutrient supply by upwelling in relation to strong surface current system. This productivity decrease at about 1.9Ma was also found in the southern Bering Sea (Site U1341) and may be related to global opal reorganization.

The trophic and metabolic pathways of foraminifera in the Arabian Sea: evidence from cellular stable isotopes

Abstract. The Arabian Sea is a region of elevated productivity with the highest globally recorded fluxes of particulate organic matter (POM) to the deep ocean, providing an abundant food source for fauna at the seafloor. However, benthic communities are also strongly influenced by an intense oxygen minimum zone (OMZ), which impinges on the continental slope from 100 to 1000 m water depth. We compared the trophic ecology of foraminifera on the Oman and Pakistan margins of the Arabian Sea (140–3185 m water depth). These two margins are contrasting both in terms of the abundance of sedimentary organic matter and the intensity of the OMZ. Organic carbon concentrations of surficial sediments were higher on the Oman margin (3.32 ± 1.4%) compared to the Pakistan margin (2.45 ± 1.1%) and sedimentary organic matter (SOM) quality estimated from the Hydrogen Index was also higher on the Oman margin (300–400 mg HC mg TOC−1) compared to the Pakistan margin (< 250 mg HC mg TOC−1). The δ13C and δ15N values of sediments were similar on both margins (−20 and 8‰, respectively). Stable isotope analysis (SIA) showed that foraminiferal cells had a wide range of δ13C values (−25.5 to −11.5‰), implying that they utilise multiple food sources; indeed δ13C values varied between depths, foraminiferal types and between the two margins. Foraminifera had broad ranges in δ15N values (−7.8 to 27.3‰). The enriched values suggest that some species may store nitrate to utilise in respiration; this was most notable on the Pakistan margin. Depleted foraminiferal δ15N values, particularly at the Oman margin, may reflect feeding on chemosynthetic bacteria. We suggest that differences in productivity regimes may be responsible for the differences observed in foraminiferal isotopic composition. In addition, at the time of sampling, whole jellyfish carcasses (Crambionella orsini) and a carpet of jelly detritus were observed across the Oman margin transect. Associated chemosynthetic bacteria may have provided an organic-rich food source for foraminifera at these sites. Our data suggest that foraminifera in OMZ settings can utilise a variety of food sources and metabolic pathways to meet their energetic demands.

Nitrate source identification in the Baltic Sea using its isotopic ratios in combination with a Bayesian isotope mixing model

Abstract. Nitrate (NO3−) is the major nutrient responsible for coastal eutrophication worldwide and its production is related to intensive food production and fossil-fuel combustion. In the Baltic Sea NO3− inputs have increased 4-fold over recent decades and now remain constantly high. NO3− source identification is therefore an important consideration in environmental management strategies. In this study focusing on the Baltic Sea, we used a method to estimate the proportional contributions of NO3− from atmospheric deposition, N2 fixation, and runoff from pristine soils as well as from agricultural land. Our approach combines data on the dual isotopes of NO3− (δ15N-NO3− and δ18O-NO3−) in winter surface waters with a Bayesian isotope mixing model (Stable Isotope Analysis in R, SIAR). Based on data gathered from 47 sampling locations over the entire Baltic Sea, the majority of the NO3− in the southern Baltic was shown to derive from runoff from agricultural land (33–100%), whereas in the northern Baltic, i.e. the Gulf of Bothnia, NO3− originates from nitrification in pristine soils (34–100%). Atmospheric deposition accounts for only a small percentage of NO3− levels in the Baltic Sea, except for contributions from northern rivers, where the levels of atmospheric NO3− are higher. An additional important source in the central Baltic Sea is N2 fixation by diazotrophs, which contributes 49–65% of the overall NO3− pool at this site. The results obtained with this method are in good agreement with source estimates based upon δ15N values in sediments and a three-dimensional ecosystem model, ERGOM. We suggest that this approach can be easily modified to determine NO3− sources in other marginal seas or larger near-coastal areas where NO3− is abundant in winter surface waters when fractionation processes are minor.

Separating the influences of diagenesis, productivity and anthropogenic nitrogen deposition on sedimentary δ15N variations

The stable isotopic composition of nitrogen (δ15N) in organic lake sediments is frequently used to infer changes in the source or cycling of N prior to sedimentation. However, diagenetic processes that occur after sedimentation can systematically alter the primary isotopic signal recorded in sedimentary organic matter and must be accounted for in order to detect changes in the δ15N of nitrogen inputs to the sediment surface. Here we present a null model that estimates the diagenetic effect on sedimentary lake δ15N records. The model was tested using lake sediment cores from relatively pristine alpine lakes in the Wind River and Teton Ranges of Wyoming, USA. Model-inferred increases in productivity were tested against independent productivity proxies (diatom accumulation rates and the planktonic to benthic ratio), and inferred changes in anthropogenic nitrogen contribution were validated with records of atmospheric nitrogen deposition from the nearby Fremont Glacier, Wyoming. Diagenetic overprinting significantly altered sediment δ15N profiles, and the degree of alteration was not constant through time. Of the cores analyzed, ∼30–70% of the variability can be explained by diagenesis alone, with the remainder explained by either a change in productivity or a change in the isotopic composition of the source material. Our null model of isotopic fractionation proved to be successful at separating the diagenetic overprinting from other causes of isotopic shifts, thereby providing environmental scientists with an analytical tool to partition the effects of diagenesis and environmental change on sedimentary δ15N values.

Co-variation of nitrogen isotopes and redox states through glacial–interglacial cycles in the Black Sea

In all aquatic environments, nitrogen cycling within the water column is strongly influenced by oxygen. We hypothesize that the nitrogen isotopic composition (δ15N) of organic matter deposited in the sediments is a proxy for the redox state of the water column at the time of deposition. We tested the hypothesis by measuring the bulk sedimentary δ15N values in a drill core from the Black Sea, a basin that alternates between oxic, less saline conditions and anoxic, marine conditions on glacial–interglacial time scales. We reconstructed these changes in Black Sea redox conditions using sedimentary δ15N, total organic carbon (TOC), total nitrogen (TN), redox-sensitive metals, and micropaleontological data from a deep-sea core (DSDP Site 380). The sedimentary data reveal that during the transitions between oxic and anoxic conditions, δ15N values increased relative to the preceding and succeeding quasi-steady-state oxic and anoxic periods. The results indicate that the reciprocal transitional states from anoxic to oxic conditions were accompanied by intense denitrification; during the quasi-stable oxic and anoxic states (characterized by glacial fresh water and interglacial marine conditions) nitrification and complete nitrate utilization, respectively, dominate the nitrogen cycle. While other factors may influence the δ15N record, our results support the hypothesis that the variations in nitrogen isotopic composition of organic matter are strongly influenced by changes in redox state in the Black Sea subphotic zone on glacial–interglacial time scales, and can be explained by a relatively simple model describing the effects of oxygen on the microbial processes that drive the nitrogen cycle in marine ecosystems. Our model suggests that the nitrogen isotopic composition of marine sediments, on geological time scales, can be used to reconstruct the redox state of the overlying water column.

Late Miocene through early Pleistocene nutrient utilization and export production in the Antarctic Zone of the Southern Ocean

We use bulk sediment δ15N values and opal and carbon mass accumulation rates (MAR) to reconstruct nutrient utilization and export productivity at Ocean Drilling Program Site 745 (Antarctic Zone of the Southern Ocean) spanning the late Miocene through early Pleistocene (~6.5–1.4Ma). We investigate whether early Pliocene climatic warmth and subsequent cooling can be related to changes in high latitude productivity. Results indicate that δ15N values increase to above late Holocene levels from the late Miocene through the late Pliocene (6.5 to 2Ma). Opal and carbon MARs are low during the early Pliocene. Relatively high δ15N together with low export production is consistent with a more southerly position of the Polar Frontal Zone (PFZ) allowing the expansion of nitrate depleted, low nutrient upper waters south toward Site 745. The interpretation is supported by a relatively small δ15N gradient between Site 745 and a site in the Subantarctic Zone of the Southern Ocean (Site 1090). There are no unique changes in the Site 745 δ15N values or export productivity at 2.7Ma. During the late Pliocene to early Pleistocene climate transition (between ~2.1 and at 1.7Ma), δ15N values display large variations approaching those observed during the last glacial to interglacial transition in this latitude band. Opal and carbon MARs also show large fluctuations, but in the opposite sense with maxima corresponding to minima in the δ15N record and vice versa. The pattern of high δ15N values associated with low export production may reflect changes in nutrient utilization in response to changes in water column stratification once the PFZ has moved north of the location of Site 745. Our results provide a mechanism for enhancing early Pliocene CO2 concentrations via reduced uptake of CO2 due to low productivity in the Southern Ocean. Once the PFZ has moved north, the region may have become sensitive to changes in water column stratification, potentially contributing to fluctuations in CO2.

Elucidating trophic pathways in benthic deep-sea assemblages of the Mid-Atlantic Ridge north and south of the Charlie-Gibbs Fracture Zone

The Mid-Atlantic Ridge (MAR) is a topographically complex feature in the North Atlantic Ocean with little exploration of benthic fauna except in association with hydrothermal venting, resulting in the biodiversity and ecosystem functioning of the MAR benthos away from these sites remaining largely unknown. Stable isotope analysis of carbon, nitrogen and sulphur were undertaken on fauna collected north and south of the Charlie-Gibbs Fracture Zone, in areas believed to be devoid of hydrothermal venting, to investigate the trophodynamics of the benthic assemblage. δ13C and δ34S values of the benthic and bentho-pelagic fauna indicated a dependence on photosynthetic primary production, with no influence from an unknown chemosynthetic source. A large trophic discrimination in δ13C between consumers and potential food sources (particulate organic matter and surficial sediments) suggested reworking of organic carbon before assimilation by benthic fauna. Interpretation of sediment δ15N values was difficult as these ranged between −0.74 and 23.14‰, suggesting further work is required to understand nitrogen utilisation by benthic deposit feeders. Differences in trophic guilds (predators, predator–scavenger, surface deposit feeders, subsurface deposit feeders and suspension feeders) were evident for δ13C and δ15N, with the main difference between the mobile predators–scavengers and the benthic deposit feeders. Dividing the assemblage based on trophic guilds into food chains dependent on phytodetritus (deposit and suspension feeders) and predation–scavenging (fishes and crustaceans) resulted in strong positive correlations between δ15N and δ13C at both stations. These reflected the 2 dominant trophic pathways organic matter passed through deposit feeders and predatory and scavenging deep-sea fishes and crustaceans.

Stable isotope (δ13C and δ15N) values of sediment organic matter in subtropical lakes of different trophic status

Lake sediments contain archives of past environmental conditions in and around water bodies and stable isotope analyses (δ13C and δ15N) of sediment cores have been used to infer past environmental changes in aquatic ecosystems. In this study, we analyzed organic matter (OM), carbon (C), nitrogen (N), phosphorus (P), and δ13C and δ15N values in sediment cores from three subtropical lakes that span a broad range of trophic state. Our principal objectives were to: (1) evaluate whether nutrient concentrations and stable isotope values in surface deposits reflect modern trophic state conditions in the lakes, and (2) assess whether stratigraphic changes in the measured variables yield information about shifts in trophic status through time, or alternatively, diagenetic changes in sediment OM. Three Florida (USA) lakes of very different trophic status were selected for this study. Results showed that both δ13C and δ15N values in surface sediments of the oligo-mesotrophic lake were relatively low compared to values in surface sediments of the other lakes, and were progressively lower with depth in the sediment core. Sediments of the eutrophic lake had δ13C values that declined upcore, whereas δ15N values increased toward the sediment surface. The eutrophic lake displayed δ13C values intermediate between those in the oligo-mesotrophic and hypereutrophic lakes. Sediments of the hypereutrophic lake had relatively higher δ13C and δ15N values. In general, we found greater δ13C and δ15N values with increasing lake trophic state.

Carbon and nitrogen stable isotope ratios in surface sediments from lakes of western Ireland: implications for inferring past lake productivity and nitrogen loading

We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface sediment δ13C and δ15N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ13C and δ15N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments) produced δ13C and δ15N measurements similar to values from catchment vegetation. δ13C and δ15N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ13C values from lake sediment samples in agricultural catchments were more negative than δ13C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production had a greater influence on δ13C values than fractionation due to algal productivity. δ15N from lake sediment samples in agricultural catchments were more positive than δ15N in non-impacted bogland catchments. Lower δ15N values from non-impacted lake catchments reflected δ15N values of catchment vegetation, while higher δ15N values in agricultural catchments reflected the high δ15N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation processes were more important than early diagenesis for lake sediment δ15N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk sediment δ15N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when working with clay-rich sediments. This study confirms the usefulness of δ13C and δ15N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process studies and help identify key processes controlling lake sediment δ13C and δ15N in a study area.

Insights into nitrogen cycling in the western Gulf of California from the nitrogen isotopic composition of diatom-bound organic matter

The nitrogen isotopic composition of organic matter contained within diatom frustules of different sediment size fractions from the Guaymas Basin, Gulf of California, suggests that multiple nutrient cycling processes can be examined from a single sediment sample. The diatom-bound δ15N values from the greater and less than 63 μm size fractions were analyzed from six intervals of core MD02–2517, each representing ∼30–40 years. The diatom-bound δ15N values are compared to bulk sedimentary δ15N values, weight percent biogenic opal, and weight percent organic carbon. Bulk sedimentary δ15N values range from 10‰ to 12.5‰ and diatom-bound δ15N values of the 63 μm fraction range from 1‰ to 7‰. The best explanation for the low diatom δ15N values in the >63 μm fraction is that the large diatoms grew at the base of the euphotic zone in excess nitrate and under low light conditions. Large diatom δ15N values correlate significantly with bulk sedimentary δ15N values (r = 0.68, 0.82), indicating that large diatoms, such as Thalassiothrix longissima, are an important component of export production fueled by a nonupwelling source of nitrogen. The >63 μm diatoms are estimated to contribute between 3% and 15% of the total nitrogen in the sediments based upon an assumed Si-to-N ratio of 1.2.

Regulation of the nitrogen biogeochemistry of mountain lakes by subsidies of terrestrial dissolved organic matter and the implications for climate studies

Stable isotopes of nitrogen (N) were analyzed in modern sediments of mountain lakes, dissolved organic matter (DOM), and sediment cores spanning the past 12,000 yr to test the hypothesis that spatial and temporal (100‐ 1000 yr) variation in the N content of mountain lakes is regulated by influx of allochthonous DOM. Analysis of spatial patterns in an elevation gradient of 75 mountain lakes revealed that most N was associated with DOM rather than inorganic N, particularly in subalpine lakes (, 1700 m above sea level). Similarly, analysis of N isotope ratios (δ15N) from 22 lakes showed that whole sediments of subalpine sites were significantly more depleted (0.74‰ ± 1.58‰) than were those of alpine lakes above 2200 m (3.04‰ ± 1.21‰), consistent with the depleted δ15N of isolated DOM (~1.3‰). Sedimentary δ15N values of Crowfoot Lake, presently near tree line, also varied greatly during the past 12,000 yr, with enriched values (~4%) during the alpine phases of the lake's history and depleted values (~1%) during the intervening subalpine phase (ca. 10,050‐4160 14C yr before present) when DOM was abundant. In contrast, sedimentary δ15N values remained constant (~2.5%) at Snowflake Lake, an alpine reference site that never experienced a DOM‐rich subalpine phase. These analyses suggest that climate regulates N influx and lake biogeochemistry by changing the subsidies of terrestrial DOM, and warn that future climate change may initially reduce N influx on a decadal scale by reducing hydrologic transfer before increasing N subsidies on a centennial scale by increasing terrestrial production of DOM.

Landscape-scale effects of urban nitrogen on a chain of freshwater lakes in central North America

We measured nitrogen (N) transport to and storage in nine lakes linked by the Qu'Appelle River, Saskatchewan, Canada, to quantify the unique effects of N on the eutrophication of phosphorus (P)-replete lakes. Stable isotope content (δ15N) was measured for dissolved N, periphyton, particulate organic matter (POM), and sediment samples collected at 10 stations along Wascana Creek and the Qu'Appelle River, lotic ecosystems that receive wastewaters from the City of Regina and that drain into Pasqua Lake. Urban effluent (δ15N ~16 ± 2%) enriched dissolved N isotope ratios of river water by up to 15% but was not stored in lotic sediments. Instead, urban N increased δ15N signatures of lotic periphyton and POM by 10-15% and was transported to Pasqua Lake, where sedimentary δ15N values increased from ~6.5% during the 19th century to 14.0% by the 1990s. This increase was linearly correlated both to the mass of dissolved N released from Regina (r2 = 0.84, p “ 0.0001) and to a 300% increase in the production of Pasqua Lake (as fossil pigments) since ca. 1880 (r2 = 0.69, p “ 0.0001). Similar isotopic enrichment was recorded in five downstream lakes, but not three reference ecosystems, although the degree of downstream enrichment declined rapidly, mainly as a result of sequestration of urban N in lake sediments. Together, these patterns demonstrate that urban N can directly degrade surface waters of P-sufficient lakes, but that these ecosystems can eliminate urban effects through permanent storage of wastewater N in their sediments.

Nitrogen cycle of the Baltic Sea from an isotopic perspective

Stable nitrogen isotope ratios (15N/14N; δ15N) were determined in sediments, suspended matter, and water at selected sites in the Baltic Sea area in order to set up a source budget and trace the fate of anthropogenic N sources. Sediments of the shallow near‐coastal area of the southern and eastern Baltic Sea have an average δ15N of 7.3 ± 2.1‰, interpreted as a characteristic trace of residual anthropogenic nitrogen delivered by rivers and diffuse runoff. In contrast, δ15N values in sediments deposited in the basins of the central Baltic Sea are depleted (average δ15N of 3.5 ± 0.6‰), indicating a significant contribution by diazotrophic cyanobacteria. Statistical analysis of long‐term nutrient data (1969–2001) indicates no increase in nitrate concentrations in the central Baltic Proper, where only phosphate concentrations have increased. The physical circulation pattern and the enthalpy, as derived from a circulation model, show a closed circulation cell in the Baltic Proper with limited transport of riverine material into the basins and elevated temperatures in the Baltic Proper. Together, the isotope and nutrient data suggest that eutrophication by riverine nitrogen is pronounced in the coastal rim of the Baltic Sea, and that coastal sediments appear to be very efficient in removing riverborne nitrogen by denitrification. A nitrogen isotope mass balance model suggests that N loss by sediment denitrification and N input by N fixation can be as high as 855 ktons N yr−1.

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

Summary1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were each visited once during June and July 2000. From each lake, samples of surface sediment epilithon, macrophytes and total dissolved nitrogen (TDN) were collected for nitrogen stable isotope analysis. As part of a wider programme, samples were also collected for chemical analysis and bioassays.2. Considerable variation was found in δ15N values in all measured nitrogen compartments. Some regional variation was evident but was generally weak. Sediment and epilithon δ15N were positively correlated with δ15N of TDN, suggesting that baseline nitrogen isotope ratios influence those in some organic matter compartments in the lakes.3. Sediment δ15N was higher when inorganic nitrogen concentration in the water was low, possibly reflecting reduced isotope fractionation under these conditions. However, this was not the case for epilithon or macrophytes. Sediment δ15N values were also negatively related to annual nitrogen deposition.4. Sediment, epilithon and macrophyte δ15N values all showed significant relations to nutrient limitation in the lakes as determined by algal bioassays. We suggest that sediment δ15N might be developed as a simple integrating measure of the degree of nitrogen limitation in lakes.