Use of stable isotopes to understand food webs in Macao wetlands

In this study, components of the food-web in Macao wetlands were quantified using stable isotope ratio techniques based on carbon and nitrogen values. The δ¹³C and δ¹⁵N values of particulate organic matter (δ¹³C_POM and δ¹⁵N_POM, respectively) ranged from ?30.64 ± 1.0 to ?28.1 ± 0.7 ‰, and from ?1.11 ± 0.8 to 3.98 ± 0.7 ‰, respectively. The δ¹³C values of consumer species ranged from ?33.94 to ?16.92 ‰, showing a wide range from lower values in a freshwater lake and inner bay to higher values in a mangrove forest. The distinct dietary habits of consumer species and the location-specific food source composition were the main factors affecting the δ¹³C values. The consumer ¹⁵N-isotope enrichment values suggested that there were three trophic levels; primary, secondary, and tertiary. The primary consumer trophic level was represented by freshwater herbivorous gastropods, filter-feeding bivalves, and plankton-feeding fish, with a mean δ¹⁵N value of 5.052 ‰. The secondary consumer level included four deposit-feeding fish species distributed in Fai Chi Kei Bay and deposit-feeding gastropods in the Lotus Flower Bridge flat, with a mean δ¹⁵N value of 6.794 ‰. The tertiary consumers group consisted of four crab species, one shrimp species, and four fish species in the Lotus Flower Bridge Flat, with a mean δ¹⁵N value of 13.473 ‰. Their diet mainly comprised organic debris, bottom fauna, and rotten animal tissues. This study confirms the applicability of the isotopic approach in food web studies.     

Trophic level delineation and resource partitioning in a South African afromontane forest bird community using carbon and nitrogen stable isotopes

Southern African forests are naturally fragmented yet hold a disproportionately high number of bird species. Carbon and nitrogen stable isotopes were measured in feathers from birds captured at Woodbush (n = 27 species), a large afromontane forest in the eastern escarpment of Limpopo province, South Africa. The δ¹³C signatures of a range of forest plants were measured to categorise the food base. Most plants sampled, including two of five grass species, had δ¹³C signatures typical of a C₃ photosynthetic pathway (?29.5 ± 1.9‰). Three grass species had a C₄ signature (?12.0 ± 0.6‰). Most bird species had δ¹³C values representing a predominantly C₃‐based diet (?24.8‰ to ?20.7‰). δ¹⁵N values were as expected, with higher levels of enrichment associated with a greater proportion of dietary animal matter. The cohesive isotopic niche defining most species (n = 22), where the ranges for δ¹³C and δ¹⁵N were 2.4‰ and 3.4‰, respectively, highlight the difficulties in understanding diets of birds in a predominantly C₃‐based ecosystem using carbon and nitrogen stable isotopes. However, variation in isotopic values between and within species provides insight into possible niche width and the use of resources by different birds within a forest environment.     

Contrasting δ<Superscript>15</Superscript>N Values of Atmospheric Deposition and <Emphasis Type="Italic">Sphagnum</Emphasis> Peat Bogs: N Fixation as a Possible Cause

Nitrogen (N) isotope systematics were investigated at two high-elevation ombrotrophic peat bogs polluted by farming and heavy industry. Our objective was to identify N sources and sinks for isotope mass balance considerations. For the first time, we present a time-series of δ¹⁵Ν values of atmospheric input at the same locations as δ¹⁵Ν values of living Sphagnum and peat. The mean δ¹⁵Ν values systematically increased in the order: input NH₄ ⁺ (?10.0‰) < input NO₃ ^? (?7.9‰) < peat porewater (?5.6‰) < Sphagnum (?5.0‰) < shallow peat (?4.2‰) < deep peat (?2.2‰) < runoff (?1.4‰) < porewater N₂O (1.4‰). Surprisingly, N of Sphagnum was isotopically heavier than N of the atmospheric input (P < 0.001). If partial incorporation of reactive N from the atmosphere into Sphagnum was isotopically selective, the residual N would have to be isotopically extremely light. Such N, however, was not identified anywhere in the ecosystem. Alternatively, Sphagnum may have contained an admixture of isotopically heavier N. Ambient air contains such N in the form of N₂ (δ¹⁵Ν_N2 = 0‰). Because high energy is required to break the triple bond, microbial N fixation is likely to proceed only under limited availability of pollutant N. Also for the first time, a δ¹⁵Ν comparison is presented between anoxic deeper peat and porewater N₂O. Isotopically light N is removed from anoxic substrate by denitrification, whose final product, N₂, escapes into the atmosphere. Porewater N₂O is an isotopically heavy residuum following partial N₂O reduction to N₂.     

Stable isotopes as indicators for seasonally dominant nitrogen cycling processes in a subarctic lake

Nitrogen stable isotopes (δ¹⁵N) of dissolved inorganic nitrogen (DIN = NH₄⁺ and NO₃^–), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) were measured in Smith Lake, Alaska to assess their usefulness as proxies for the biological nitrogen cycling processes, nutrient concentration, and lake productivity. Large seasonal variations in δ¹⁵NH₄⁺, δ¹⁵NO₃^– and δ¹⁵N_PON occurred in response to different processes of nitrogen transformation that dominated a specific time period of the annual production cycle. In spring, ¹⁵N depletion in all three pools was closely related to the occurrences of a N₂‐fixing cyanobacterial bloom (Anabaena flos‐aquae). In summer, δ¹⁵N_PON increased as phytoplankton community shifted to use NH₄⁺ and decreased as a brief N₂‐fixing bloom (Aphanizomenon flos‐aquae) occurred in August. In early and mid‐winter, microbial nitrogen processes were dominated by nitrification that resulted in the largest isotope fractionation between NO₃^– and NH₄⁺ in the annual cycle. This was followed by denitrification that led to the highest ¹⁵N enrichment in NO₃^–. A peak of NH₄⁺ assimilation by phytoplankton along with the elevated δ¹⁵N_PON and Chl a concentration occurred just before the ice break due to increased light penetration. The δ¹⁵N_DON displayed little temporal and spatial variations. This suggests that the DON pool was not altered by biological transformations of nitrogen as the results of its large size and possibly refractory nature. There was a positive correlation between Chl a concentration and δ¹⁵N_PON, and a negative correlation between NH₄⁺ and δ¹⁵N_PON, suggesting that δ¹⁵N_PON is a useful proxy for nitrogen productivity and ammonium concentration. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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

Key message

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

Abstract

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

Waterlogging and fire impacts on nitrogen availability and utilization in a subtropical wet heathland (wallum)

Protein, amino acids and ammonium were the main forms of soluble soil nitrogen in the soil solution of a subtropical heathland (wallum). After fire, soil ammonium and nitrate increased 90- and 60-fold, respectively. Despite this increase in nitrate availability after fire, wallum species exhibited uniformly low nitrate reductase activities and low leaf and xylem nitrate. During waterlogging soil amino acids increased, particularly γ-aminobutyric acid (GABA) which accounted for over 50% of amino nitrogen. Non-mycorrhizal wallum species were significantly (P < 0.05) ¹⁵N-enriched (0.3–4.3‰) compared to species with mycorrhizal associations (ericoid-type, ecto-, va-mycorrhizal) which were strongly depleted in ¹⁵N (-6.3 to -1.8‰). Lignotubers and roots had δ¹⁵N signatures similar to that of the leaves of respective species. The exceptions were fine roots of ecto-, ecto/va-, and ericoid type mycorrhizal species which were enriched in ¹⁵N (0.1–2.4‰). The 5¹⁵N signatures of δ¹⁵N_{total soil N} and δ¹⁵N_{soil NH4+} were in the range 3.7–4.5‰, whereas δ¹⁵N_{soil NO3?} was significantly (P < 0.05) more enriched in ¹⁵N (9.2–9.8‰). It is proposed that there is discrimination against ¹⁵N during transfer of nitrogen from fungal to plant partner. Roots of selected species incorporated nitrogen sources in the order of preference: ammonium > glycine > nitrate. The exception were proteoid roots of Hakea (Proteaceae) which incorporated equal amounts of glycine and ammonium.     

Anthropogenic nutrient enrichment of seagrass and coral reef communities in the Lower Florida Keys: discrimination of local versus regional nitrogen sources

Land-based nutrient pollution represents a significant human threat to coral reefs globally. We examined this phenomenon in shallow seagrass and coral reef communities between the Content Keys (southern Florida Bay) and Looe Key (south of Big Pine Key) in the Lower Florida Keys by quantifying the role of physical forcing (rainfall, wind, tides) and water management on mainland South Florida to nutrient enrichment and blooms of phytoplankton, macroalgae, and seagrass epiphytes. Initial studies (Phase I) in 1996 involved daily water quality sampling (prior to, during, and following physical forcing events) at three stations (AJ, an inshore area directly impacted by sewage discharges; PR, a nearshore patch reef located inshore of Hawk Channel; and LK, an offshore bank reef at Looe Key) to assess the spatial and temporal patterns in advection of land-based nutrients to the offshore reefs. Concentrations of dissolved inorganic nitrogen (DIN=NH₄⁺+NO₃⁻+NO₂⁻), soluble reactive phosphorus (SRP), and chlorophyll a increased at PR and LK following a wind event (∼15 knots, northeast) in mid-February. The highest DIN (mostly NH₄⁺) and SRP concentrations of the entire study occurred at the inshore AJ during an extreme low tide in March. Following the onset of the wet season in May, mean NH₄⁺ and chlorophyll a concentrations increased significantly to maximum seasonal values at PR and LK during summer; relatively low concentrations of NO₃⁻ and a low f-ratio (NO₃⁻/NH₄⁺+NO₃⁻) at all stations during summer do not support the hypothesis that the seasonal phytoplankton blooms resulted from upwelling of NO₃⁻. A bloom of the seagrass epiphyte Cladosiphon occidentalis (phaeophyta) followed the onset of the rainy season and increased NH₄⁺ concentrations at LK, resulting in very high epiphyte:blade ratios (∼3:1) on Thalassia testudinum. Biomass of macroalgae increased at all three stations from relatively low values (<50 g dry wt m⁻²) in winter and early spring to higher values (∼100-300 g dry wt m⁻²) typical of eutrophic seagrass meadows and coral reefs following the onset of the rainy season. The mean δ¹⁵N value of Laurencia intricata (rhodophyta) during 1996 at AJ (+4.7‰) was within the range reported for macroalgae growing on sewage nitrogen; lower values at the more offshore PR (+3.1‰) and LK (+2.9‰) were at the low end of the sewage range, indicating an offshore dilution of the sewage signal during the 1996 study. However, transient increases in δ¹⁵N of Cladophora catanata (chlorophtyta) from ~+2% to +5% at LK concurrent with elevated NH₄⁺ concentrations following rain and/or wind events in May and July suggest episodic advection of sewage nitrogen to the offshore LK station. The Phase II study involved sampling of macroalgae for δ¹⁵N along a gradient from the Content Keys through Big Pine Key and offshore to LK in the summer wet season of 2000 and again in the drought of spring 2001. During the July 2000 sampling, macroalgae in nearshore waters around Big Pine Key had elevated δ¹⁵N values (~+4‰) characteristic of sewage enrichment; lower values (~+2‰) at LK were similar to values reported for macroalgae in upstream waters of western Florida Bay influenced by nitrogen-rich Everglades runoff. That pattern contrasted with the drought sampling in March 2001, when δ¹⁵N values of macroalgae were elevated (+6‰) to levels characteristic of sewage enrichment over a broad spatial scale from the Content Keys to LK. These results suggest that regional-scale agricultural runoff from the mainland Everglades watersheds as well as local sewage discharges from the Florida Keys are both significant nitrogen sources supporting eutrophication and algal blooms in seagrass and coral reef communities in the Lower Florida Keys. Hydrological and physical forcing mechanisms, including rainfall, water management on the South Florida mainland, wind, and tides, regulate the relative importance and variability of these anthropogenic nitrogen inputs over gradients extending to the offshore waters of the Florida Reef Tract.     

Identification of nitrogen sources and transformations within karst springs using isotope tracers of nitrogen

Isotope analyses of nitrate and algae were used to gain better understanding of sources of nitrate to Florida’s karst springs and processes affecting nitrate in the Floridan aquifer at multiple scales. In wet years, δ¹⁵N and δ¹⁸O of nitrate ranged from +3 to +9‰ in headwater springs in north Florida, indicating nitrification of soil ammonium as the dominant source. With below normal rainfall, the δ¹⁵N and δ¹⁸O of nitrate were higher in almost all springs (reaching +20.2 and +15.3‰, respectively) and were negatively correlated with dissolved oxygen. In springs with values of δ¹⁵N-NO₃ and δ¹⁸O-NO₃ greater than +10‰, nitrate concentrations declined 40–50% in dry years and variations in the δ¹⁵N and δ¹⁸O of nitrate were consistent with the effects of denitrification. Modeling of the aquifer as a closed system yielded in situ fractionation caused by denitrification of 9 and 18‰ for Δ¹⁸O and Δ¹⁵N, respectively. We observed no strong evidence for local sources of nitrate along spring runs; concentrations declined downstream (0.42–3.3?μmol-NO₃ L^?1 per km) and the isotopic dynamics of algae and nitrate indicated a closed system. Correlation between the δ¹⁵N composition of nitrate and algae was observed at regional and spring-run scales, but the relationship was complicated by varying isotopic fractionation factors associated with nitrate uptake (Δ ranged from 2 to 13‰). Our study demonstrates that nitrate inputs to Florida’s springs are derived predominantly from non-point sources and that denitrification is detectable in aquifer waters with relatively long residence time (i.e., matrix flow).     

A δ<Superscript>15</Superscript>N assessment of nitrogen deposition for the endangered epiphytic orchid <Emphasis Type="Italic">Laelia speciosa</Emphasis> from a city and an oak forest in Mexico

Atmospheric nitrogen deposition poses a major threat to global biodiversity. Tropical epiphytic plants are especially at risk given their reliance on atmospheric sources of nutrients. The leaf, pseudobulb, and root carbon and nitrogen content, C:N ratio, as well as the nitrogen isotopic composition were studied for individuals of Laelia speciosa from a city and from an oak forest in Mexico. The nitrogen content of leaves was similar between the city and the oak forest, reaching 1.3 ± 0.2 % (dry mass). The δ¹⁵N of leaves, pseudobulbs, and roots reached 5.6 ± 0.2 ‰ in the city, values found in sites exposed to industrial and vehicular activities. The δ¹⁵N for plant from the oak forest amounted to –3.1 ± 0.3 ‰, which is similar to values measured from sites with low industrial activities. Some orchids such as Laelia speciosa produce a single pseudobulb per year, i.e., a water and nutrient storage organ, so the interannual nitrogen deposition was studied by considering the ten most recent pseudobulbs for plants from either site formed between 2003 and 2012. The C:N ratio of the ten most recent pseudobulbs from the oak forest, as well as that of the pseudobulbs formed before 2010 for plants in the city were indistinguishable from each other, averaging 132.4 ± 6.5, while it was lower for the two most recent pseudobulbs in the city. The δ¹⁵N values of pseudobulbs from the oak forest averaged ?4.4 ± 0.1 ‰ for the entire series. The δ¹⁵N ranged from 0.1 ± 1.6 ‰ for the oldest pseudobulb to 4.7 ± 0.2 ‰ for the pseudobulb formed in the city from 2008 onwards. Isotopic analysis and the C:N ratio for L. speciosa revealed that rates of nitrogen deposition were higher in the city than in the forest. The δ¹⁵N values of series of pseudobulbs showed that it is possible to track nitrogen deposition over multiple years.     

Tracing estuarine organic matter sources into the southern North Sea using C and N isotopic signatures

Sources and distribution of particulate organic matter in surface waters of the Humber and Thames estuaries and in the East Anglian plume in the southern North Sea were investigated in winter 2006/2007. Carbon (C) and nitrogen (N) stable isotopes provided evidence for the presence of three particulate organic matter sources; riverine plankton (δ¹³C ?30 ‰ and δ¹⁵N 7.9 ‰) identified in the Thames estuary only, marine plankton (average δ¹³C ?21.4 ‰ and δ¹⁵N 4.5 ‰) and a third source with an enriched ¹³C signature (>?16.7 ‰) and elevated C:N ratio (>12.7). Particulate organic matter with enriched ¹³C values were observed throughout the Humber estuary and at the marine end-member of the Thames estuary. While bacterial cycling of organic carbon undoubtedly takes place within these estuaries, these processes on their own are unlikely to account for the isotopic signatures seen. The ¹³C enriched organic matter source is suggested to be due to particulate organic matter input from marsh plants and seagrasses such as Spartina spp. and Zostera on the adjacent salt marshes and mudflats and/or macroalgae along the banks of the estuaries. This ¹³C enriched signal was also identified approximately 50 km offshore within the southern North Sea, in the East Anglian plume, which transports UK riverine water off-shore in a discrete plume. This plume therefore provides a mechanism to transport this estuarine derived organic matter pool offshore out of the estuaries. These results indicate that estuarine derived organic matter from marsh plants, seagrasses and/or macroalgae contributes to the southern North Sea organic matter pool and is therefore likely to contribute to winter-time shelf sea carbon and nitrogen cycles.     

Variations of the nitrate isotopic composition in the St. Lawrence River caused by seasonal changes in atmospheric nitrogen inputs

We present 42 dual-isotope nitrate analyses of fresh water samples collected in the St. Lawrence River between June 2006 and July 2008. Measured δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? values correlate negatively, while δ¹⁸O–NO₃ ^? displays no negative correlation with nitrate concentration. This suggests that nitrate uptake and/or elimination by denitrification is not the main driver of observed variations in nitrate concentration and isotopic signature in the St. Lawrence River. In addition, δ¹⁸O–NO₃ ^? is negatively correlated with the seasonally variable δ¹⁸O of ambient water, indicating that the variation in the isotopic signature of nitrate is barely modulated by in-stream nitrate regeneration (nitrification). It rather is constrained by along-river changes in the external sources of nitrate. Given the distinct nitrogen (N) and oxygen (O) isotopic signature of atmospheric nitrate, we argue that observed seasonal variations of δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? in the St. Lawrence River are due to variable contributions of snowmelt-derived water. Based on a N and O isotope mass balance, we show that total nitrate loading in the St. Lawrence River is dominated by a N input from the Great Lakes (47 ± 28 %) and from nitrate regeneration of both internal and external N (48 ± 22 %). While temporal nitrate N and O isotope dynamics in the St. Lawrence River are mainly influenced by the atmospheric N input fluctuations, with an increase in atmospheric loading during spring, atmospheric N plays overall a rather insignificant role with regards to the N budget (5 ± 4 %).     

Tourism’s nitrogen footprint on a Mesoamerican coral reef

Globally, the eutrophication of coastal marine environments is a worsening problem that is accelerating the loss of biodiversity and ecosystem services. Coral reefs are among the most sensitive to this change, as chronic inputs of agricultural and wastewater effluents and atmospheric deposition disrupt their naturally oligotrophic state. Often, anthropogenic alteration of the coastal nitrogen pool can proceed undetected as rapid mixing with ocean waters can mask chronic and ephemeral nitrogen inputs. Monitoring nitrogen stable isotope values (δ ¹⁵N) of benthic organisms provides a useful solution to this problem. Through a 7-yr monitoring effort in Quintana Roo, Mexico, we show that δ ¹⁵N values of the common sea fan Gorgonia ventalina were more variable near a developed (Akumal) site than at an undeveloped (Mahahual) site. Beginning in 2007, the global recession decreased tourist visitations to Akumal, which corresponded with a pronounced 1.6 ‰ decline in sea fan δ ¹⁵N through 2009, at which time δ ¹⁵N values were similar to those from Mahahual. With the recovery of tourism, δ ¹⁵N values increased to previous levels. Overall, 84 % of the observed variation in δ ¹⁵N was explained by tourist visitations in the preceding year alone, indicating that variable nitrogen source contributions are correlated with sea fan δ ¹⁵N values. We also found that annual precipitation accounted for some variation in δ ¹⁵N, likely due to its role in groundwater flushing into the sea. Together, these factors accounted for 96 % of the variation in δ ¹⁵N. Using a mixing model, we estimate that sewage can account for up to 42 % of nitrogen in sea fan biomass. These findings illustrate the high connectivity between land-based activities and coral reef productivity and the measurable impact of the tourism industry on the ecosystem it relies on.     

An Assessment of Contemporary and Historic Nitrogen Availability in Contrasting Coastal Douglas-Fir Forests Through δ15N of Tree Rings

Wood nitrogen isotope composition (δ¹⁵N) provides a potential retrospective evaluation of ecosystem N status but refinement of this index is needed. We calibrated current wood δ¹⁵N of Douglas-fir (Pseudotsuga menziesii), an ectomycorrhizal tree species, against a productivity gradient of contrasting coastal forests of southern Vancouver Island (Canada). We then examined historical δ¹⁵N via increment cores, and tested whether wood δ¹⁵N corresponded with climatic fluctuations. Extractable soil N ranged from 11 to 43 kg N ha^?1 along the productivity gradient, and was characterized by a progressive replacement of N forms (amino acids, NH₄ ⁺ and NO₃ ^?). Current wood δ¹⁵N was significantly less depleted (?5.0 to ?2.6 ‰) with increasing productivity, although linear correlations were stronger with Δδ¹⁵N (the difference between wood and soil δ¹⁵N) to standardize the extent of isotopic fractionation by ectomycorrhizal fungi. An overall decline in wood δ¹⁵N of 0.9 ‰ over the years 1900–2009 was detected, but trends diverged widely among plots, including positive, negative and no trend with time. We did not detect significant correlations in detrended wood δ¹⁵N with mean annual temperature or precipitation. The contemporary patterns in stand productivity, soil N supply and wood δ¹⁵N were moderately strong, but interpreting historical patterns in δ¹⁵N was challenging because of potential variations in N uptake related to stand dynamics. The lack of wood δ¹⁵N correlations with climate may be partly due to methodological limitations, but might also reflect the relative stability in N supply due to the overriding constraints of soil organic matter quantity and quality.     

Assimilation and nitrification in pelagic waters: insights using dual nitrate stable isotopes (δ<Superscript>15</Superscript>N, δ<Superscript>18</Superscript>O) in a shallow lake

Nitrate dual stable isotopes (δ¹⁵N and δ¹⁸O of NO₃ ^?) have proven to be a powerful technique to elucidate nitrogen (N) cycling pathways in aquatic systems. We applied this technique for the first time in the pelagic zone of a small temperate meso-eutrophic lake to identify the dominant N cycling pathways, and their spatial and temporal variability. We measured the lake NO₃ ^? δ¹⁵N and δ¹⁸O signatures over an annual cycle and compared them to that of the watershed. Both δ¹⁵N and δ¹⁸O of NO₃ ^? in the lake increased during summer relative to the inputs. Relationships between lake NO₃ ^? isotopic composition and concentrations were different across thermal strata with an apparent isotope effect in the epilimnion of ¹⁵ε_epi = 4.6‰ and ¹⁸ε_epi = 10.9‰. We found a strong deviation of the lake NO₃ ^? δ¹⁸O and δ¹⁵N from the expected 1:1 line for assimilation (slope = 1.73) suggesting that nitrification was co-occurring. We estimated that nitrification could support between 5 and 30% of nitrate-based production during the growing season, but was negligible in early spring and fall, and probably more dominant under ice. We showed that the technique is promising to study N processes at the ecosystem scale in shallow lakes, particularly during winter. Our results suggest that recycled NO₃ ^? could support primary productivity and influence phytoplankton composition in the surface waters of small lakes.     

Characteristics of stable isotope signature of diet in tissues of captive Japanese macaques as revealed by controlled feeding

We determined the magnitude of isotopic fractionation of carbon and nitrogen stable isotope ratios (as enrichment factors, Δδ¹³C and Δδ¹⁵N, respectively) between the tissues and diets of captive Japanese macaques (Macaca fuscata) using a controlled feeding experiment, to provide basic data for reconstructing their feeding habits. The Δδ¹³C and Δδ¹⁵N values, respectively, were 0.9 ± 0.2 ‰ (mean ± standard deviation, SD) and 3.0 ± 0.3 ‰ for whole blood, 1.3 ± 0.2 ‰ and 4.3 ± 0.3 ‰ for plasma, and 0.8 ± 0.2 ‰ and 3.0 ± 0.2 ‰ for red blood cells. However, the Δδ¹³C and Δδ¹⁵N values for hair were 2.8 ± 0.3 ‰ and 3.4 ± 0.2 ‰, respectively. No difference was detected in the δ¹³C and δ¹⁵N values of hair sampled from different parts of the body. We investigated the effects of diet on δ¹³C in growing hair by alternating the diet of the macaques each month between two diets that differed markedly in δ¹³C. Hair regrown after shaving repeatedly recorded the δ¹³C of the diet consumed during the time of hair growth. On the other hand, hair naturally grown during the diet-change experiment did not show a clear pattern. One possible reason is that the hair had grown abnormally under unnatural indoor conditions and showed complicated isotope signatures. To reconstruct the long-term feeding history of Japanese macaques, we need to further clarify the relationships between the stable isotope signature of diet and various body tissues.     

Exploring cycad foliage as an archive of the isotopic composition of atmospheric nitrogen

Molecular nitrogen (N₂) constitutes the majority of Earth's modern atmosphere, contributing ~0.79 bar of partial pressure (pN₂). However, fluctuations in pN₂ may have occurred on 10⁷–10⁹ year timescales in Earth's past, perhaps altering the isotopic composition of atmospheric nitrogen. Here, we explore an archive that may record the isotopic composition of atmospheric N₂ in deep time: the foliage of cycads. Cycads are ancient gymnosperms that host symbiotic N₂‐fixing cyanobacteria in modified root structures known as coralloid roots. All extant species of cycads are known to host symbionts, suggesting that this N₂‐fixing capacity is perhaps ancestral, reaching back to the early history of cycads in the late Paleozoic. Therefore, if the process of microbial N₂ fixation records the δ¹⁵N value of atmospheric N₂ in cycad foliage, the fossil record of cycads may provide an archive of atmospheric δ¹⁵N values. To explore this potential proxy, we conducted a survey of wild cycads growing in a range of modern environments to determine whether cycad foliage reliably records the isotopic composition of atmospheric N₂. We find that neither biological nor environmental factors significantly influence the δ¹⁵N values of cycad foliage, suggesting that they provide a reasonably robust record of the δ¹⁵N of atmospheric N₂. Application of this proxy to the record of carbonaceous cycad fossils may not only help to constrain changes in atmospheric nitrogen isotope ratios since the late Paleozoic, but also could shed light on the antiquity of the N₂‐fixing symbiosis between cycads and cyanobacteria.     

Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef

Biological dinitrogen (N₂) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, and N is therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m^?2 reef d^?1, with hard corals making the largest contribution (41–76%). Diazotrophy was omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m^?2 reef d^?1. Planktonic GPP and BNF rates were respectively approximately 60- and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle.     

Effects of nutrient enrichment on the release of dissolved organic carbon and nitrogen by the scleractinian coral Montipora digitata

The effects of nutrient enrichment on the release of dissolved organic carbon and nitrogen (DOC and DON, respectively) from the coral Montipora digitata were investigated in the laboratory. Nitrate (NO₃ ⁻) and phosphate (PO₄ ³⁻) were supplied to the aquarium to get the final concentrations of 10 and 0.5 μmol l⁻¹, respectively, and the corals were incubated for 8 days. The release rate of DON per unit coral surface area significantly decreased after the nutrient enrichment, while the release rate of DOC was constant. Because the chlorophyll a (chl a) content of zooxanthellae per unit surface area increased, the release rate of DOC significantly decreased when normalized to unit chl a. These results suggested that the incorporation of NO₃ ⁻ and PO₄ ³⁻ stimulated the synthesis of new cellular components in the coral colonies and consequently, reduced extracellular release of DOC and DON. Actually, significant increase in N and P contents relative to C content was observed in the coral’s tissue after the nutrient enrichment. The present study has concluded that inorganic nutrient enrichment not only affects coral-algal metabolism inside the colony but also affects a microbial community around the coral because the organic matter released from corals functions as energy carrier in the coral reef ecosystem.     

VARIATION IN INTERNAL δ15N AND δ13C DISTRIBUTIONS AND THEIR BULK VALUES IN THE BROWN MACROALGA PADINA AUSTRALIS GROWING IN SUBTROPICAL OLIGOTROPHIC WATERS1

The utility of δ¹⁵N measurements in Padina australis Hauck as a probe for its external nitrogen (N) sources was tested by monitoring the bulk values of chemical components [δ¹⁵N, δ¹³C, and N and carbon (C) contents] and their internal distributions during a 12 d incubation in a controlled environment. Under the saturated conditions of isotopically heavier nitrate than that of original algal tissue, the bulk δ¹⁵N in P. australis was enriched, but less than what was predicted from a simple mixing model, signaling possible isotopic discrimination during N assimilation and subsequent N efflux from the cells. The enhanced N content (%), which occurred simultaneously with this δ¹⁵N shift, was a useful signal indicating this phenomenon. Bulk δ¹⁵N was enriched, especially around the meristem, in tissues growing under conditions of higher irradiance and temperature, probably due in part to dissolved organic nitrogen (DON) excretion. The δ¹³C enhancement in bulk algal tissues, also associated with high photosynthetic activity, may be an additional signal indicating this unbalanced internal δ¹⁵N distribution. However, in summer and winter environmental conditions with periodic nitrate supplies simulating typical fringing reef waters, the difference in measured algal bulk δ¹⁵N from theoretical predictions was within ±1.0‰. This difference is very small compared with the variation in δ¹⁵N in possible N sources in coastal areas. In the field, therefore, δ¹⁵N in Padina can be used effectively to trace N sources in both space and time after determining algal N content and δ¹³C to determine whether large alterations occur in algal δ¹⁵N.     

Estimating tissue‐specific discrimination factors and turnover rates of stable isotopes of nitrogen and carbon in the smallnose fanskate Sympterygia bonapartii (Rajidae)

This study aimed to estimate trophic discrimination factors (TDFs) and metabolic turnover rates of nitrogen and carbon stable isotopes in blood and muscle of the smallnose fanskate Sympterygia bonapartii by feeding six adult individuals, maintained in captivity, with a constant diet for 365 days. TDFs were estimated as the difference between δ¹³C or δ¹⁵N values of the food and the tissues of S. bonapartii after they had reached equilibrium with their diet. The duration of the experiment was enough to reach the equilibrium condition in blood for both elements (estimated time to reach 95% of turnover: C t95%_blood = 150 days, N t95%_blood = 290 days), whilst turnover rates could not be estimated for muscle because of variation among samples. Estimates of Δ¹³C and Δ¹⁵N values in blood and muscle using all individuals were Δ¹³C_blood = 1·7‰, Δ¹³C_muscle = 1·3‰, Δ¹⁵N_blood = 2·5‰ and Δ¹⁵N_muscle = 1·5‰, but there was evidence of differences of c.0·4‰ in the Δ¹³C values between sexes. The present values for TDFs and turnover rates constitute the first evidence for dietary switching in batoids based on long‐term controlled feeding experiments. Overall, the results showed that S. bonapartii has relatively low turnover rates and isotopic measurements would not track seasonal movements adequately. The estimated Δ¹³C values in S. bonapartii blood and muscle were similar to previous estimations for elasmobranchs and to generally accepted values in bony fishes (Δ¹³C = 1·5‰). For Δ¹⁵N, the results were similar to published reports for blood but smaller than reports for muscle and notably smaller than the typical values used to estimate trophic position (Δ¹⁵N c. 3·4‰). Thus, trophic position estimations for elasmobranchs based on typical Δ¹⁵N values could lead to underestimates of actual trophic positions. Finally, the evidence of differences in TDFs between sexes reveals a need for more targeted research.