δ15N Of Particulate Organic Matter Research Articles

Quantifying the δ15N trophic offset in a cold-water scleractinian coral (CWC): implications for the CWC diet and coral δ15N as a marine N cycle proxy

Abstract. The nitrogen (N) isotope composition (δ15N) of cold-water corals is a promising proxy for reconstructing past ocean N cycling, as a strong correlation was found between the δ15N of the organic nitrogen preserved in coral skeletons and the δ15N of particulate organic matter exported from the surface ocean. However, a large offset of 8 ‰–9 ‰ between the δ15N recorded by the coral and that of exported particulate organic matter remains unexplained. The 8 ‰–9 ‰ offset may signal a higher trophic level of coral dietary sources, an unusually large trophic isotope effect or a biosynthetic δ15N offset between the coral's soft tissue and skeletal organic matter, or some combinations of these factors. To understand the origin of the offset and further validate the proxy, we investigated the trophic ecology of the asymbiotic scleractinian cold-water coral Balanophyllia elegans, both in a laboratory setting and in its natural habitat. A long-term incubation experiment of B. elegans fed on an isotopically controlled diet yielded a canonical trophic isotope effect of 3.0 ± 0.1 ‰ between coral soft tissue and the Artemia prey. The trophic isotope effect was not detectably influenced by sustained food limitation. A long N turnover of coral soft tissue, expressed as an e-folding time, of 291 ± 15 d in the well-fed incubations indicates that coral skeleton δ15N is not likely to track subannual (e.g., seasonal) variability in diet δ15N. Specimens of B. elegans from the subtidal zone near San Juan Channel (WA, USA) revealed a modest difference of 1.2 ± 0.6 ‰ between soft tissue and skeletal δ15N. The δ15N of the coral soft tissue was 12.0 ± 0.6 ‰, which was ∼6 ‰ higher than that of suspended organic material that was comprised dominantly of phytoplankton – suggesting that phytoplankton is not the primary component of B. elegans' diet. An analysis of size-fractionated net tow material suggests that B. elegans fed predominantly on a size class of zooplankton ≥500 µm, implicating a two-level trophic transfer between phytoplankton material and coral tissue. These results point to a feeding strategy that may result in an influence of the regional food web structure on the cold-water coral δ15N. This factor should be taken into consideration when applying the proxy to paleo-oceanographic studies of ocean N cycling.

Interactions between particulate organic matter and dissolved organic matter in a weak dynamic bay revealed by stable isotopes and optical properties

Few studies have incorporated the tools of stable isotopes and optical properties to study the biogeochemical process of organic matter (OM), including particulate organic matter (POM) and dissolved organic matter (DOM), which prevents our comprehension of the interactions between POM and DOM in the marine environment. In this study, the origin, distribution, and fate of POM and DOM in Tieshangang Bay, a weak dynamic bay were investigated by measuring δ13C and δ15N of POM, dissolved organic carbon (DOC), and absorption and fluorescence of DOM (CDOM and FDOM). In the upper bay, POM source was mainly originated from terrestrial discharge, whereas the high fluorescence index (FI) indicated that external aquatic organism activity dominated DOM sources. In contrast, in the outer bay, the slightly increased δ13C and enriched δ15N of POM and enhanced fluorescence intensities of protein-like DOM components indicated the considerable contributions of the in situ biological activities to OM. A net addition of DOC and optical components of DOM occurred in both the upper and outer bays. The decomposition of terrestrial POM was responsible for the addition of DOM due to the weak dynamics in the upper bay, whereas the enhanced primary production and the strong decomposition of freshly produced POM jointly contributed to the addition of DOM in the outer bay. Our study suggests that hydrodynamics regulate the biogeochemistry and interactions of POM and DOM in the weak dynamic bay.

Sources and Distribution of Organic Matter in Coastal Area of Muang Chonburi District, Eastern Thailand: Using Carbon and Nitrogen Stable Isotopes

The sources and spatial distribution of organic matter were investigated in the nearshore area receiving untreated urban wastewater and offshore areas containing bivalve mariculture in Muang Chonburi District. Content, elemental, carbon, and nitrogen isotopic analyses in particulate organic matter (POM) and surface sediments were performed. We found particulate organic carbon (POC), δ13C, and δ15N of POM ranging from 435.0-5,247.7 µg/L, -27.7 to -22.0 ‰, and 1.7-8.6 ‰, respectively, whereas total organic carbon (TOC), total nitrogen (TN), δ13C and δ15N in sediment were 3.5-76.9 µg/g, 0.8-8.3 µg/g, -25.7 to -22.9 ‰ and 2.6-6.0 ‰, respectively. These values show significant differences between nearshore and offshore sampling stations wherein accumulation of organic matter was high in the nearshore and decreased with greater distance offshore, with some retained in the bivalve farming area. The results from a mixing model indicated that organic matter in POM and sediment were initially derived from anthropogenic organic matter. In contrast to the offshore water which had organic matter derived from marine organic matter. This study highlights the distribution of anthropogenic organic waste released to coastal waters with plenty of bivalve farms such as green mussels and oysters. The results can be used to advise national strategies regarding bivalve aquaculture zoning and seafood safety policies going forward.

Sources of nitrogen to stream food webs in tributaries of the Red River Valley, Manitoba

Lake Winnipeg has undergone rapid eutrophication over the last several decades with a large portion of the associated nutrient loads delivered via the Red River. Consequently, subcatchments of the Red River Valley (RRV) are priorities for nutrient management, an aim that requires identification of key nutrient sources to stream food webs. We identified the primary sources of nitrogen to food webs of rural streams of the RRV, as well as variation in nitrogen input between spring and summer. We measured δ15N of particulate organic matter (POM) and collector-gatherer invertebrates collected in spring and summer from 20 subcatchments, which exhibited a range of agricultural intensity and size of municipal wastewater lagoons. δ15N values of POM and collector-gatherer invertebrates were best predicted by the presence of wastewater treatment lagoons, with δ15N values increasing with the number of people served by lagoons in spring and summer. When present, wastewater contributed a greater proportion of nitrogen to stream food webs than agricultural sources. Waste sources also had a greater relative contribution to food webs in summer than spring. Despite wastewater lagoons releasing nitrogen in short-term, pulsed discharges, the influence of wastewater on food web nitrogen persisted from the summer release into the following spring. Based on the observed importance of wastewater as a source of nitrogen to stream food webs, we recommend management agencies consider additional actions to reduce nutrient losses from wastewater treatment facilities as well as agricultural lands to more effectively protect aquatic ecosystems.

Consumer‐diet discrimination of δ13C and δ15N: Source‐ and feeding‐oriented patterns based on gut content analysis in a large subtropical river of China

AbstractUnderstanding the trophic discrimination (∆13C and ∆15N) between consumers and diets in fluvial systems remains difficult because of the variable food sources and complex predator–prey interactions from headwaters to the estuaries. Here, stable carbon (δ13C) and nitrogen (δ15N) isotopes in fish and invertebrates from a large subtropical river in southern China were determined to explore trophic discrimination in conjunction with a gut content analysis. The ∆13C values showed significant differences (p < .05) among functional feeding groups, with fish, shrimp, and insect scrapers presenting higher ∆13C values (1.20 ± 0.23‰ to 1.51 ± 0.31‰) than other groups. The ∆15N values varied significantly between invertebrates (0.64 ± 0.17‰ of insect collector‐gatherers to 1.63 ± 0.36‰ of shrimp predators) and fish (1.98 ± 0.19‰ of detritivores to 2.71 ± 0.43‰ of crustaceavores) and exhibited an increasing tendency from primary to secondary consumers. A linear regression analysis revealed that the longitudinal changes in ∆13C and ∆15N were closely associated with the δ13C of periphyton, the δ15N of particulate organic matter (POM) in water, and the relative contribution (%) of periphyton and organic detritus to the diet composition of consumers. These results indicated that discrimination factors might not only be influenced by the isotope signatures of basal food sources but also downstream shifts in dominant food items utilized by consumers. In particular, trophic discrimination between periphyton– and detritus–based food chains, such as “epilithic diatoms–shrimp scrapers–crustaceavorous fish” and “POM–bivalves–molluscivorous fish,” displayed regionally specific patterns. When back‐calculating to the diet assimilation and trophic position in subtropical streams and rivers, we suggest using the basin‐scale ∆13C value of 0.96 ± 0.26‰ for all consumers and ∆15N values of 1.07 ± 0.32‰ for invertebrates and 2.38 ± 0.37‰ for fish.

Impact of Hydrological Conditions on the Biogeochemical Dynamics of Suspended Particulate Organic Matter in the Upper Mixed Layer of the Southern East China Sea

AbstractWe investigate suspended particles collected from the upper mixed layer in the inner shelf of the southern East China Sea during autumn 2013 for carbon and nitrogen (POC and PN) contents and their isotope compositions (δ13C and δ15N) along with hydrographic parameters to understand the sources and dynamics of particulate organic matter (POM) in the study area. Results indicated that the extensive hydrological processes affect the biogeochemical composition of suspended POM, as revealed by the horizontally mixing POM and the spatial variation of δ15N and molar C/N ratio. Low C/N (2.4–6.5) and a weak correlation between POC and in situ chlorophyll fluorescence suggested that POM is dominated by the recently formed and well‐preserved planktonic OM. By fitting the linear correlation between δ13C and POC data with a photosynthetic fractionation model, we further disentangled dynamic controls of phytoplankton production and species diversity on δ13C variability (−24.3 to −21.3‰), emphasizing the constant effect of productivity‐derived POC on δ13C (0.02‰ per μg/L). The δ15N variability (2.3–7.4‰) is largely controlled by the mixing of isotopically different nitrogen sources, in which the importance of biological nitrogen fixation is unfolded based on the small δ15N and the negative correlation between δ15N of POM and seawater temperature. This implies that Kuroshio‐induced biological N fixation plays an important role in supporting the marine production in the East China Sea. These hydrologically driven δ13C and δ15N changes of marine productivity‐derived POM suggest that internal biophysical dynamics rather than terrestrial versus marine OM mixing largely control the C and N compositional variability in the shelf seas.

Spatiotemporal variation in environmental features and elemental/isotopic composition of organic matter sources and primary producers in the Yundang Lagoon (Xiamen, China).

Aquatic eutrophication is a major problem globally, leading to significant chemical-compositional changes in the first trophic levels of a food web. These will consequently affect the whole food web dynamics in eutrophic coastal ecosystems. In this study, we analyzed the spatiotemporal variation in water chlorophyll-a, total organic carbon (TOC) and TOC/Chl-a ratio, and in the elemental/isotopic (Cat/Nat, δ13C, δ15N) composition of particulate organic matter (POM), phytoplankton, sedimentary organic matter, benthic microalgae, U. lactuca, and decaying leaves of mangrove plants in the eutrophic, urban lagoon of Yundang (Xiamen, China). Investigations were carried out in four different sectors of the lagoon in March (dominated by Ulva lactuca) and September (dominated by phytoplankton) 2009, and the feasible contribution of potential organic matter (OM) sources to POM was assessed in each period. The results showed significant spatiotemporal variation in δ13C and δ15N of POM, owing to changes in its carbon sources. The POM in the diversion canal (POMDC) of the lagoon originated mainly from terrestrial OM both in March and September, as evidenced by a Cat/Nat ratio of 12~17 and a TOC/Chl-a ratio exceeding 400, as well as depleted δ13C (- 27.3~- 23.7‰) and δ15N (- 2.8~0.1‰). The POM in the main canal (POMMC) and the inner (POMIL) and outer (POMOL) sectors of the lagoon were largely composed of lagoon-borne phytoplankton in September. This was revealed by TOC/Chl-a values below 100 and enriched δ13C values (- 22.7~- 17.9‰) which are close to the values typical for fresh phytoplankton. However, these were strongly regulated by exogenous OM in March. The combined contribution of POMMC and POMDC to POM in March reached 64-99% in the inner lagoon and 67-88% in the outer lagoon. Non-living POM, which originates from terrestrial organic detritus, was the main contributor to POM (60.7~85.7%) both in the inner and outer sections of the lagoon. Overall, the present study demonstrates that the temporal changes in the dominance of primary producers were reflected in significant variation in the environmental features and elemental/isotopic composition of OM sources and their contribution to POM in the Yundang Lagoon. To further our understanding of the effects of eutrophication due to different primary producers on the resource partitioning in the Yundang Lagoon, additional studies on the consumers and the whole food web of the lagoon are expected.

Upwelling as the major source of nitrogen for shallow and deep reef‐building corals across an oceanic atoll system

Abstract Oceanographic processes shape coral reefs worldwide by redistributing inorganic nutrients and particulate resources over depth. Deep‐water upwelling occurs frequently in coral reef ecosystems, but its impact on coral nutrition remains unclear. This study investigated the influence of upwelling on the trophic ecology of three common reef‐building corals (Galaxea fascicularis, Pachyseris speciosa and Pocillopora verrucosa) from different reef depths (shallow reef, 10 m, vs. deep reef, 30 m) and reef exposures (oceanic rim vs. Inner Sea) across > 250 km of the Maldives archipelago, Indian Ocean. Carbon and nitrogen stable isotope ratios (δ13C and δ15N) of coral hosts, their symbionts and particulate organic matter (POM) were used to characterize coral trophic strategies. Across the Maldives, consistent mean δ15N values were recorded in hosts (5.5‰) and symbionts (5.2‰) of the three coral species from shallow and deep reefs of oceanic and Inner Sea reef exposures. Coral hosts, symbionts and POM from both depths had δ15N values that were consistent with the isotopic signature of a deep‐water nitrate source transported to surface waters via upwelling. In contrast, a wide range of δ13C values (~10‰) revealed different trophic strategies and isotopic niches among the coral species. Different mean δ13C values of G. fascicularis indicated greater symbiont autotrophy in corals from shallow (‐15.5‰) compared to deep reefs (‐17.6‰). Conversely, the mean δ13C values of P. speciosa (‐15.1‰) and P. verrucosa (‐17.7‰) were not affected by reef depth. These corals maintained consistent trophic strategies over depth, with P. speciosa relying more on autotrophy compared to P. verrucosa. Despite different reef exposure to oceanic waters, coral host and POM δ15N and δ13C values did not differ between oceanic and Inner Sea reef exposures. Nutritional resources appear to be homogenous in the central Maldives due to atoll‐wide water circulation. However, species‐specific trophic strategies resulted in diverse patterns of δ13C values over depth. Because heterotrophic feeding has been linked to coral host survival through coral bleaching events, understanding the trophic ecology of corals within the reef assemblage can provide insight into species resilience under ocean warming conditions. As a member of the typically competitive Pocilloporidae family, the dependence of P. verrucosa on heterotrophy may help this coral be a future “winner” under sustained ocean warming. A plain language summary is available for this article.

Assessing land use and land cover influence on surface water quality using a parametric weighted distance function

Stream water quality is directly influenced by land use and human practices in the surrounding environment. Understanding such effects and the spatial extent of impacts is essential to generate reliable information for ecosystem-based management of water resources. We identified sources of impact on water quality and characterized indicator-specific landscape influence on samples collected during base flow along the Chubut River (43 °S, 69 °W). We modeled Total Nitrogen (TN), Total Phosphorous (TP), Soluble Reactive Phosphorous (SRP) concentrations and δ15N of particulate organic matter along the river, as a function of effective contribution areas (AEC) of Land Use/Land Cover (LULC). AECs were calculated by assuming that landscape influence decays exponentially with the Euclidean distance between a given LULC parcel and the sampling point. We calibrated the model to the observations by estimating an indicator-specific decay rate. Agriculture and barren lands were the main sources of phosphate nutrients whereas urban areas were the main source of TN. Radius of landscape influence for SRP (100–180 km) was larger than for TP (10–25 km), reflecting different patterns of mobilization and delivery in the catchment. δ15N variation was explained by vegetation cover but the influence rapidly decreased (1–4 km) reflecting a mostly autochthonous source of organic matter.

Stable isotopic evidence for the differential contribution of diazotrophs to the epipelagic grazing food chain in the mid‐Pacific Ocean

AbstractAimBiological nitrogen fixation supports primary production in oligotrophic water, but its link to higher trophic levels has not been described fully on a biogeographical basis. Here, we determine the regional patterns of the contribution of the combined nitrogen to biological production within the epipelagic layer of the mid‐Pacific Ocean using the isotopic signatures of nitrogen (δ15N) and carbon (δ13C) in the biological components.LocationThe mid‐Pacific Ocean along 170° W between the southern subtropical front and the Chukchi Sea.Time periodNorthern and austral summer in 2013 and 2014.Major taxa studiedPlanktonic and micronektonic biota in the euphotic layer.MethodsWe measured the geographical variations in δ15N and δ13C of the suspended particulate organic matter (POM), mesozooplankton assemblage and micronektonic fish. We analysed the relationships among these values and the environmental variables of temperature, nitrate concentration and biological nitrogen fixation activity along a 12,000‐km meridional transect.ResultsThe POM δ15N at 0 m was negatively correlated with in situ N2 fixation activity in the subtropical region, whereas that in the equatorial and high‐latitude regions was correlated with the nitrate concentration at 0 m. We found that the ratios of the increase in δ15N to δ13C along the grazing food chain were consistent throughout the equatorial and subtropical regions. Cluster analyses based on the stable isotopic signatures in the biotic components revealed that the food chains in the stations within the subtropical mid‐Pacific Ocean were separated into three groups based on the differential contributions of biological nitrogen fixation.Main conclusionsDistinct food chains from primary to tertiary production sustained by different nitrogen sources, nitrate below the euphotic zone, and diazotrophic nitrogen occur within the same biogeographical provinces in the subtropical mid‐Pacific Ocean. The diazotroph‐dominant community contributes substantially to the apex predators in the central areas of the subtropical gyres.

Relationships between depth and δ15N of Arctic benthos vary among regions and trophic functional groups

Stable isotope ratios of nitrogen (δ15N) of benthic primary consumers are often significantly related to water depth. This relationship is commonly attributed to preferential uptake of 14N from sinking particulate organic matter (POM) by microbes, and suggests that relationships between δ15N and water depth may be affected by local POM sources and flux dynamics. We examined the relationships between δ15N and water depth (20–500 m) for six trophic functional groups using a mixed effects modelling approach, and compared relationships between two contiguous Arctic marine ecosystems with different POM sources and sinking export dynamics: the Canadian Beaufort Sea and Amundsen Gulf. We demonstrate for the first time in the Arctic that δ15N values of mobile epifaunal carnivores increased as a function of depth when considered separately from benthopelagic and infaunal carnivores, which contrarily did not exhibit increasing δ15N with depth. The δ15N of suspension/filter feeders, infaunal deposit feeders and bulk sediment also increased with water depth, and the slopes of the relationships were steeper in the Amundsen Gulf than in the Beaufort Sea. We propose that regional differences in slopes reflect differences in POM sources exported to the benthos. In the Beaufort Sea, terrestrial POM discharged from the Mackenzie River quantitatively dominates the sedimentary organic matter across the continental shelf and slope, dampening change in δ15N of benthic POM with depth. In the Amundsen Gulf, we attribute a faster rate of change in δ15N of POM with increasing depth to larger contributions of marine-derived POM to the benthic sedimentary pool, which had likely undergone extensive biological transformation in the productive offshore pelagic zone. Differences in POM input regimes among regions should be considered when comparing food webs using stable isotopes, as such differences may impact the rate at which consumer δ15N changes with depth.

Possible relation between enrichment of δ15N in the top predator Zalophus californianus and the expansion of the oxygen minimum zone

Stable isotopes of nitrogen were analyzed from the fur of nursing California sea lion (CSL) pups (that depend nutritionally on their mother’s milk for nourishment) collected in the thirteen rookeries from the Gulf of California from 1999 to 2014. Additionally, analyses were obtained from vibrissae of adult females at Los Islotes, a sea lion rookery in Bahia de La Paz with the longest record of CSL pup isotope information. The data show that consistent 15N enrichment occurred at all sea lion rookeries from the Gulf of California. Scat samples covering this time period showed no trend that might explain the δ15N enrichment as a result of shifts in feeding behavior. The enrichment pattern is paralleled by similar shifts in the enrichment of settling particulate organic matter (POM) recovered from sediment traps in the Bahia de La Paz, as well as published changes in POM δ15N in sediment cores from a number of coastal basins along the Pacific coast since 1990. The data are best explained by progressive transfer to surface waters of fractionated 15N nitrogen associated with the expansion of the Oxygen Minimum Zone in the Eastern Tropical Pacific, which is most intense along the coast of Mexico. In the Gulf of California, all levels of the food web are enriched in δ15N values with respect to similar counterparts outside of the region. We propose the nitrogen isotope composition in sea lions as an effective and easy method to monitor the regional expansion of the OMZ in the Eastern Tropical Pacific.

Trophic structure of shrimp-trawl catches in the Pearl River estuary in winter, using stable isotope analyses

The Pearl River estuary is the second largest estuary in the South China Sea. The species composition and trophic structure of shrimp-trawl catches from this estuary were investigated using stable isotope analyses during the winter of 2012. Crabs dominated the catches, constituting 37% of the total biomass in the inner estuary and 45% in the outer estuary. The δ15N and δ13C values for particulate organic matter gradually changed from the inner to the outer estuary. δ15N of particulate organic matter was more enriched in the inner estuary, while δ13C was more enriched in the outer estuary. Higher δ15N values in the inner estuary suggested a stronger influence from anthropogenic eutrophication. The δ15N and δ13C values of shrimp-trawl catches showed the same trends as the particulate organic matter. Trophic structure and faunal composition of shrimp-trawl catches varied greatly between the inner and outer estuary areas. The trophic niches were clearly distinct among fishery groups in the outer estuary. Community-wide metrics of stable isotope values showed that there was greater diversity of food sources and more complex trophic interactions in the inner estuary, whereas trophic levels were more evenly distributed in the outer estuary.

Feeding strategies of tropical and subtropical calanoid copepods throughout the eastern Atlantic Ocean – Latitudinal and bathymetric aspects

The majority of global ocean production and total export production is attributed to oligotrophic oceanic regions due to their vast regional expanse. However, energy transfers, food-web structures and trophic relationships in these areas remain largely unknown. Regional and vertical inter- and intra-specific differences in trophic interactions and dietary preferences of calanoid copepods were investigated in four different regions in the open eastern Atlantic Ocean (38°N to 21°S) in October/November 2012 using a combination of fatty acid (FA) and stable isotope (SI) analyses. Mean carnivory indices (CI) based on FA trophic markers generally agreed with trophic positions (TP) derived from δ15N analysis. Most copepods were classified as omnivorous (CI ∼0.5, TP 1.8 to ∼2.5) or carnivorous (CI⩾0.7, TP⩾2.9). Herbivorous copepods showed typical CIs of ⩽0.3. Geographical differences in δ15N values of epi- (200–0m) to mesopelagic (1000–200m) copepods reflected corresponding spatial differences in baseline δ15N of particulate organic matter from the upper 100m. In contrast, species restricted to lower meso- and bathypelagic (2000–1000m) layers did not show this regional trend. FA compositions were species-specific without distinct intra-specific vertical or spatial variations. Differences were only observed in the southernmost region influenced by the highly productive Benguela Current. Apparently, food availability and dietary composition were widely homogeneous throughout the mesotrophic oceanic regions of the tropical and subtropical Atlantic. Four major species clusters were identified by principal component analysis based on FA compositions. Vertically migrating species clustered with epi- to mesopelagic, non-migrating species, of which only Neocalanus gracilis was moderately enriched in lipids with 16% of dry mass (DM) and stored wax esters (WE) with 37% of total lipid (TL). All other species of this cluster had low lipid contents (<10% DM) without WE. Of these, the tropical epipelagic Undinula vulgaris showed highest portions of bacterial markers. Rhincalanus cornutus, R. nasutus and Calanoides carinatus formed three separate clusters with species-specific lipid profiles, high lipid contents (⩾41% DM), mainly accumulated as WE (⩾79% TL). C. carinatus and R. nasutus were primarily herbivorous with almost no bacterial input. Despite deviating feeding strategies, R. nasutus clustered with deep-dwelling, carnivorous species, which had high amounts of lipids (⩾37% DM) and WE (⩾54% TL). Tropical and subtropical calanoid copepods exhibited a wide variety of life strategies, characterized by specialized feeding. This allows them, together with vertical habitat partitioning, to maintain high abundance and diversity in tropical oligotrophic open oceans, where they play an essential role in the energy flux and carbon cycling.

Elevated 15N/14N in particulate organic matter, zooplankton, and diatom frustule-bound nitrogen in the ice-covered water column of the Bering Sea eastern shelf

We conducted a survey of the natural abundance 15N/14N ratio (δ15N) of particulate organic matter (POM), diatom frustule-bound nitrogen (δ15NDB), and zooplankton from water column material collected with net tows across the eastern Bering Sea shelf in late winter of 2007 and 2008, to investigate the N dynamics of primary and secondary production in relation to the presence of seasonal sea ice. The data reveal a pattern of increasing δ15N northward and eastward (POM: 2.1–14.7‰; frustule-bound N: 4.9–20.7‰; zooplankton: 6.4–18.0‰), with POM δ15N reaching ~9‰ higher than that of water-column nitrate. Higher δ15N in each of these plankton fractions was largely associated with stations covered by sea ice. POM δ15N collected concurrently from within sea ice was not sufficiently 15N-enriched to explain the elevated water-column values. Rather, the δ15N of water-column POM under sea ice appears to derive from the assimilation of ammonium released from shelf sediments. Water-column ammonium δ15N was between 28‰ and 63‰, most likely due to partial nitrification in the sediment and overlying water column. The high δ15N of this ammonium is effectively transmitted to phytoplankton under sea ice because light limitation from the ice cover delays the springtime nitrate assimilation that yields algal biomass with a lower δ15N. Despite this seasonal explanation, published δ15N data from sediment traps, summertime zooplankton, and surface sediment indicate that a shoreward and northward δ15N increase – albeit of a weaker magnitude – is perennial, suggesting that the δ15N of the total annual fixed N supply (including both ammonium and nitrate) also increases shoreward and northward. This requires that the partial nitrification of ammonium underlying the spatial pattern in δ15N is at least partly coupled to denitrification in the sediments that preferentially removes 14N, causing the total fixed N reservoir on the shelf to evolve toward higher δ15N. Shelf geometry and the consequent benthic–pelagic coupling of N cycling thus seem to underlie the spatial pattern in the mean annual δ15N of plankton, while sea-ice cover causes the high δ15N of ammonium on the shelf to be most strongly reflected by the production occurring in the winter and early spring. Our results provide a basis for tracing the geographic and seasonal origins and trophic transfer of N in the Bering shelf ecosystem.

Tracing the isotopic signal of a cyanobacteria bloom through the food web of a Baltic Sea coastal lagoon

This study shows that cyanobacteria blooms support secondary production in a diverse group of benthic and pelagic consumers and illustrate the utility of stable isotopes for tracking the cyanobacteria signal in aquatic food webs. We characterized seasonal patterns in δ13C and δ15N signatures of particulate organic matter (POM) and consumers in a eutrophic coastal lagoon (Curonian Lagoon, SE Baltic Sea) before, during and after a cyanobacteria bloom. We found that during the pre- and post-bloom periods (spring and autumn), POM from the lagoon was isotopically indistinguishable from riverine POM. During the bloom, the increase in phytoplankton biomass and dominance by N2-fixing cyanobacteria resulted in higher δ13C and lower δ15N of POM. These changes in POM were reflected in isotopic signatures of primary consumers with greatest response among fast-growing planktonic and nectobenthic crustaceans and chironomids. Results from end-member mixing analyses suggest that cyanobacteria accounted for 50–80% of production by these consumers during the bloom period. Weaker responses were observed among slow-growing species, particularly long-lived bivalves such as Dreissena. Cyanobacteria-induced shifts in δ13C and δ15N could be tracked to secondary consumers, particularly fast-growing forms such as predatory zooplankton (Leptodora) and juvenile fishes (European perch). We suggest that reconstruction of the food web at the upper trophic levels should incorporate isotopic baselines of both fast- and slow-growing primary consumers to reflect the contribution of blooms events.

Seasonal variability in baseline δ15N and usage as a nutrient indicator in Lake Poyang, China

Seasonal variation in nitrogen isotope ratios (δ15N) of four baseline candidates belonging to the planktonic and benthic food webs of Lake Poyang, China, was investigated. These baseline candidates were particulate organic matter (POM), sediment organic matter (SOM), a filterer mussel (Corbicula flumine), and a grazer snail (Bellamya aeruginosa). δ15N values of POM and SOM varied significantly among the sampling seasons, while no significant seasonal variation was observed among the δ15N values of the mussels and snails sampled in different seasons. POM δ15N was correlated with lake water PO4-P, while SOM δ15N was correlated with NO3-N. Mussel δ15N and lake water TN, NO3-N, and PO4-P were significantly correlated as were snail δ15N and TN, TP, and NO3-N. SOM and snails were better bioindicators of the water column nutrient concentration of Lake Poyang. The significant correlations between mussel δ15N and POM, as well as between snail δ15N and SOM, indicated the potential use of these baselines in trophic evaluations in planktonic and benthic food webs in this water-level fluctuating lake.

δ13C and δ15N of particulate organic matter in the Santa Barbara Channel: drivers and implications for trophic inference

δ13C and δ15N of particulate organic matter in the Santa Barbara Channel: drivers and implications for trophic inference

The isotopic signature of particulate organic C and N in bottom ice: Key influencing factors and applications for tracing the fate of ice‐algae in the Arctic Ocean

AbstractThe isotopic, taxonomic, and nutrient dynamics of protist communities at the bottom of first‐year sea ice were studied for a wide range of ice conditions during the spring‐summer transition in the southeast Beaufort Sea. In bottom ice, total protist abundance ranged from 0.07 to 9.94 × 109 cells m–2 and chlorophyll a biomass ranged from 0.33 to 110 mg m–2. Both variables were positively related to the concentrations of nitrate, silicic acid, and phosphate at the ice‐water interface, with r2 values between 0.63 and 0.79. Results showed that the δ13C of sympagic (ice‐associated) particulate organic matter (POM) (–27.1 to –11.4‰) was mainly influenced by ice protist stocks and dissolved inorganic carbon availability. Flagellated cells maintained a low δ13C relative to diatoms. The δ15N of POM (3.9 to 9.4‰) could not be explained by any of the variables considered and showed considerable overlap with the pelagic δ15N‐POM (0.0 to 10.3‰). Detailed analyses of δ13C‐POM through size fractionation and settling experiments along with a short temporal study of the interaction between sympagic and pelagic communities confirmed or demonstrated that (1) the analysis of stable C isotopes is a valuable tool to trace the flow of ice‐derived matter, (2) sympagic POM contributes substantially to suspended pelagic biomass during release episodes, and (3) ice‐grown autotrophic flagellated cells have a strong potential to interact with the pelagic biota in the coastal Arctic Ocean. This study highlights the need to carefully evaluate stable isotope dynamics before assessing food web interactions or the fate of POM in seasonally ice‐covered waters.

Assessing nonpoint‐source nitrogen loading and nitrogen fixation in lakes using δ15N and nutrient stoichiometry

Runoff from human‐dominated watersheds has greatly altered nitrogen (N) and phosphorus (P) cycling in lakes. Nutrients from human sources are distinct from those from undisturbed ecosystems in several ways including lower N : P ratios, which can drive ecosystems to N‐limited conditions, and enriched stable N isotope ratios. In this study, we used these distinct characteristics to estimate shifts in N sources to 27 lakes across a human density gradient in western Washington. We compared an N stable isotope two‐source mixing model with a mixing model that coupled N stable isotopes to N : P stoichiometry and included N fixation. We found that a two‐source mixing model (human and watershed sources) did not explain observed variation in δ15N of particulate organic matter (POM) and primary consumers (R2 = 0.60) as well as a model that included a third N source (N fixation; R2 = 0.72). When fixed N was facultatively added to the ecosystem below a critical N : P ratio, the more complex mixing model captured the observed patterns in POM and primary‐consumer δ15N among lakes extremely well. In lakes with P concentrations &gt; 20 µg L−1 (N : P mass ratio &lt; 15.3), N fixation became an increasingly important component of the N cycle, accounting for &gt; 50% of lake N budgets. This model provides a novel way to estimate the contribution of nonpoint N sources and N fixation to lakes in watersheds subject to human nutrient inputs.