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.     

Contribution of potential organic matter sources to the macrobenthos nutrition in the Natori estuarine tidal flats,Japan

In this investigation, we used stable isotope and fatty acid biomarker analyses to estimate and compare the potential food sources that support macrobenthos (Nuttallia olivacea, Corbicula japonica, and Hediste sp.) in the Natori estuarine tidal flats of Japan. The δ¹³C and δ¹⁵N mean values for the sediment organic matter (SOM) were ?23.6‰ and 6.1‰, respectively, which were due to the contribution of terrestrial and/or aquatic vascular plant particulate organic matter (POM) from upper stream river or surrounding areas. Furthermore, from the results of the IsoSource mixing model, the contributions of estuarine POM to the diets of Hediste sp., C. japonica, and N. olivacea were 85.1%, 74.9%, and 48.9%, respectively. Moreover, essential fatty acids such as 20:5ω3, 18:2ω6 and 18:3ω3 highly contributed to the diets of macrobenthos from benthic diatoms, terrestrial and/or aquatic vascular plants. The contents of fatty acid markers of terrestrial OM (e.g., long chain fatty acids [LCFAs]) in the 3 species of macrobenthos were low in comparison to those of other food sources. Overall, the marine POM dietary contribution was minimal, while terrestrial OM, bacteria, and benthic diatoms constituted a significant portion of the macrobenthos diet, although the contribution varied among the 3 species of macrobenthos.     

Effects of land use on sources and ages of inorganic and organic carbon in temperate headwater streams

The amounts, sources and relative ages of inorganic and organic carbon pools were assessed in eight headwater streams draining watersheds dominated by either forest, pasture, cropland or urban development in the lower Chesapeake Bay region (Virginia, USA). Streams were sampled at baseflow conditions six different times over 1 year. The sources and ages of the carbon pools were characterized by isotopic (δ¹³C and ?¹⁴C) analyses and excitation emission matrix fluorescence with parallel factor analysis (EEM–PARAFAC). The findings from this study showed that human land use may alter aquatic carbon cycling in three primary ways. First, human land use affects the sources and ages of DIC by controlling different rates of weathering and erosion. Relative to dissolved inorganic carbon (DIC) in forested streams which originated primarily from respiration of young, ¹⁴C-enriched organic matter (OM; δ¹³C = ?22.2 ± 3 ‰; ?¹⁴C = 69 ± 14 ‰), DIC in urbanized streams was influenced more by sedimentary carbonate weathering (δ¹³C = ?12.4 ± 1 ‰; ?¹⁴C = ?270 ± 37 ‰) and one of pasture streams showed a greater influence from young soil carbonates (δ¹³C = ?5.7 ± 2.5 ‰; ?¹⁴C = 69 ‰). Second, human land use alters the proportions of terrestrial versus autochthonous/microbial sources of stream water OM. Fluorescence properties of dissolved OM (DOM) and the C:N of particulate OM (POM) suggested that streams draining human-altered watersheds contained greater relative contributions of DOM and POM from autochthonous/microbial sources than forested streams. Third, human land uses can mobilize geologically aged inorganic carbon and enable its participation in contemporary carbon cycling. Aged DOM (?¹⁴C = ?248 to ?202 ‰, equivalent¹⁴C ages of 1,811–2,284 years BP) and POM (?¹⁴C = ?90 to ?88 ‰, ¹⁴C ages of 669–887 years BP) were observed exclusively in urbanized streams, presumably a result of autotrophic fixation of aged DIC (?297 to ?244 ‰, ¹⁴C age = 2,251–2,833 years BP) from sedimentary shell dissolution and perhaps also watershed export of fossil fuel carbon. This study demonstrates that human land use may have significant impacts on the amounts, sources, ages and cycling of carbon in headwater streams and their associated watersheds.     

Comparison of the exploitation of methane-derived carbon by tubicolous and non-tubicolous chironomid larvae in a temperate eutrophic lake

Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) in three sympatric species of larval chironomids were analyzed in a temperate eutrophic shallow lake in Japan. Markedly lower δ¹³C values were reported in Chironomus plumosus (?51.2 ‰) and Tanypus sp. (?43.5 ‰) than those in photoautotrophic carbon sources [particulate organic matter (POM) and sediment]. There were positive correlations between δ¹³C and δ¹⁵N in the two chironomid species. These results indicated that they assimilated carbon derived from biogenic methane by exploiting methane-oxidizing bacteria (MOB). In contrast, Propsilocerus akamusi exhibited similar δ¹³C values to those of POM or sediment. A δ¹³C-based isotope mixing model was used to estimate the dietary contributions of MOB to each chironomid species. The mean contributions ranged from 11 to 15 % in C. plumosus, 13 to 19 % in Tanypus sp., but only up to 5 % in P. akamusi. In an aquarium, we observed that individuals of C. plumosus and Tanypus sp., which exhibited low δ¹³C values, built U-shaped larval tubes in the sediment, and an oxidized layer developed around these tubes. Propsilocerus akamusi did not exhibit this behavior. These results suggest that tube building may provide larval chironomids with greater access to methane-derived carbon through increased opportunities to feed on MOB.     

Assessing nature and dynamics of POM in transitional environment (the Curonian Lagoon,SE Baltic Sea) using a stable isotope approach

The nature of the particulate organic matter (POM) as well as its temporal and spatial distribution and dynamics in the Curonian Lagoon (south-eastern part of the Baltic Sea) were investigated. The organic matter was characterized by the organic carbon and nitrogen content, δ¹³C and δ¹⁵N signatures as well as POC/Chl-a and C/N ratios. Additionally, data on hydrological, chemical and biological parameters were used for better understanding the POM distribution and dynamics. The sampling was performed at 13 stations in the Curonian Lagoon and its outflow in the Baltic Sea during the 2012–2013 period. Samples were also collected at the Nemunas River mouth in order to test the riverine impact. Obtained results showed that isotopic values of carbon and nitrogen ranged from −36.1‰ to −25.2‰ and from −0.9‰ to 15.5‰, respectively. The isotopic composition, together with the low C/N molar (∼7) and POC/Chl-a ratios (<100) of the POM, suggested the dominance of living phytoplankton in POM throughout the year with the higher input of detrital material (C/N >10, POC/Chl-a ratios >100) in late autumn − winter.The results of multivariate analysis evidenced a spatial distinction of POM distribution in the northern-transitional and central confined areas and allowed us to distinguish the main driving factors. The seasonal variation of the δ¹³C and δ¹⁵N values in POM (towards higher δ¹³C and lower δ¹⁵N values in the summer − early autumn period and lower δ¹³C and higher δ¹⁵N values in the late autumn − spring period) was determined by combination of factors such as availability of inorganic carbon and nitrogen, the riverine discharge, seasonal phytoplankton succession and by the short-term saline water intrusion to the northern-transitional part of the lagoon.     

Fine-scale movement and assimilation of carbon in saltmarsh and mangrove habitat by resident animals

Despite theories of large-scale movement and assimilation of carbon in estuaries, recent evidence suggests that in some estuaries much more limited exchange occurs. We measured the fine-scale movement and assimilation of carbon by resident macroinvertebrates between adjacent saltmarsh and mangrove habitats in an Australian estuary using δ¹³C analysis of animals at different distances into adjacent patches of habitat. δ¹³C values of crabs (Parasesarma erythrodactyla –15.7 ± 0.1‰, Australoplax tridentata –14.7 ± 0.1‰) and slugs (Onchidina australis –16.2 ± 0.3‰) in saltmarsh closely matched that of the salt couch grass Sporobolus virginicus (–15.5 ± 0.1‰). In mangroves, δ¹³C values of crabs (P. erythrodactyla –22.0 ± 0.2‰, A. tridentata –19.2 ± 0.3‰) and slugs (–19.7 ± 0.3‰) were enriched relative to those of mangroves (–27.9 ± 0.2‰) but were more similar to those of microphytobenthos (–23.7 ± 0.3‰). The δ¹³C values of animals across the saltmarsh-mangrove interface fitted a sigmoidal curve, with a transition zone of rapidly changing values at the saltmarsh-mangrove boundary. The width of this transition indicated that the movement and assimilation of carbon is limited to between 5 and 7 m. The δ¹³C values of crabs and slugs, especially those in saltmarsh habitat, clearly indicate that the movement and assimilation of carbon between adjacent saltmarsh and mangrove habitat is restricted to just a few metres, although some contribution from unmeasured sources elsewhere in the estuary is possible. Such evidence demonstrating the extent of carbon movement and assimilation by animals in estuarine habitats is useful in determining the spatial arrangement of habitats needed in marine protected areas to capture food web processes.     

Estimating stable carbon isotope values of microphytobenthos in the Arctic for application to food web studies

Most studies on Arctic food webs have neglected microphytobenthos as a potential food source because we currently lack robust measurements of δ¹³C values for microphytobenthos from this environment. As a result, the role of microphytobenthos in high latitude marine food webs is not well understood. We combined field measurements of the concentration of aqueous carbon dioxide and the stable carbon isotopic composition of dissolved inorganic carbon (δ¹³C_DIC) from bottom water in the Beaufort and Chukchi seas with a set of stable carbon isotopic fractionation factors reflecting differences in algal taxonomy and physiology to estimate the stable carbon isotope composition of microphytobenthos-derived total organic carbon (δ¹³C_p). The δ¹³C_p for Phaeodactylum tricornutum, a pennate diatom likely to be a dominant microphytobenthos taxon, was estimated to be ?23.9 ± 0.4 ‰ as compared to a centric diatom (Porosira glacialis, δ¹³C_p = ?20.0 ± 1.6 ‰) and a marine haptophyte (Emiliana huxleyi, δ¹³C_p = ?22.7 ± 0.5 ‰) at a growth rate (µ) of 0.1 divisions per day (d^?1). δ¹³C_p values increased by ~2.5 ‰ when µ increased from 0.1 to a maximum growth rate of 1.4 d^?1. We compared our estimates of δ¹³C_p values for microphytobenthos with published measurements for other carbon sources in the Arctic and sub-Arctic. We found that microphytobenthos values overlapped with pelagic sources, yet differed from riverine and ice-derived carbon sources. These model results provide valuable insight into the range of possible isotopic values for microphytobenthos from this region, but we remain cautious in regard to the conclusiveness of these findings given the paucity of field measurements currently available for model validation.     

Reconstructing long‐term changes (150 years) in the carbon cycle of a clear‐water lake based on the stable carbon isotope composition (δ13C) of chironomid and cladoceran subfossil remains

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Isotope alteration caused by changes in biochemical composition of sedimentary organic matter

Stable carbon (C) and nitrogen (N) isotope ratios of sedimentary organic matter (OM) can reflect the biogeochemical history of aquatic ecosystems. However, diagenetic processes in sediments may alter isotope records of OM via microbial activity and preferential degradation of isotopically distinct organic components. This study investigated the isotope alteration caused by preferential degradation in surface sediments sampled from a eutrophic reservoir in Germany. Sediments were treated sequentially with hot water extraction, hydrochloric acid hydrolysis, hydrogen peroxide oxidation and di-sodium peroxodisulfate oxidation to chemically simulate preferential degradation pathways of sedimentary OM. Residue and extracts from each extraction step were analyzed using elemental analyzer-isotope ratio mass spectrometry and solid-state ¹³C nuclear magnetic resonance spectroscopy. Our results show that stable C and N isotope ratios reacted differently to changes in the biochemical composition of sedimentary OM. Preferential degradation of proteins and carbohydrates resulted in a 1.2‰ depletion of ¹³C, while the isotope composition of ¹⁵N remained nearly the same. Sedimentary δ¹⁵N values were notably altered when lignins and lipids were oxidized from residual sediments. Throughout the sequential fractionation procedure, δ¹³C was linearly correlated with the C:N of residual sediments. This finding demonstrates that changes in biochemical composition caused by preferential degradation altered δ¹³C values of sedimentary OM, while this trend was not observed for δ¹⁵N values. Our study identifies the influence of preferential degradation on stable C isotope ratios and provide additional insight into the isotope alteration caused by post-depositional processes.

     

Past climate changes and ecophysiological responses recorded in the isotope ratios of saguaro cactus spines

The stable isotope composition of spines produced serially from the apex of columnar cacti has the potential to be used as a record of changes in climate and physiology. To investigate this potential, we measured the δ¹⁸O, δ¹³C and F¹⁴C values of spines from a long-lived columnar cactus, saguaro (Carnegiea gigantea). To determine plant age, we collected spines at 11 different heights along one rib from the stem apex (3.77 m height) to the base of a naturally occurring saguaro. Fractions of modern carbon (F¹⁴C) ranged from 0.9679 to 1.5537, which is consistent with ages between 1950 and 2004. We observed a very strong positive correlation (r = 0.997) between the F¹⁴C age of spines and the age of spines determined from direct and repeated height measurements taken on this individual over the past 37 years. A series of 96 spines collected from this individual had δ¹⁸O values ranging from 38‰ to 50‰ [Vienna standard mean ocean water (VSMOW)] and δ¹³C values from ?11.5‰ to ?8.5‰ [Vienna Peedee belemnite (VPDB)]. The δ¹⁸O and δ¹³C values of spines were positively correlated (r = 0.45, P < 0.0001) and showed near-annual oscillations over the ～15-year record. This pattern suggests that seasonal periods of reduced evaporative demand or greater precipitation input may correspond to increased daytime CO₂ uptake. The lowest δ¹⁸O and δ¹³C values of spines observed occurred during the 1983 and 1993 El Niño years, suggesting that the stable isotope composition recorded in spine tissue may serve as a proxy for these climate events. We compared empirical models and data from potted experimental cacti to validate these observations and test our hypotheses. The isotopic records presented here are the first ever reported from a chronosequence of cactus spines and demonstrate that tissues of columnar cacti, and potentially other long-lived succulents, may contain a record of past physiological and climatic variation.     

Terrestrial and marine trophic pathways support young-of-year growth in a nearshore Arctic fish

River discharge supplies nearshore communities with a terrestrial carbon source that is often reflected in invertebrate and fish consumers. Recent studies in the Beaufort Sea have documented widespread terrestrial carbon use among invertebrates, but only limited use among nearshore fish consumers. Here, we examine the carbon source and diet of rapidly growing young-of-year Arctic cisco (Coregonus autumnalis) using stable isotope values (δ¹³C and δ¹⁵N) from muscle and diet analysis (stomach contents) during a critical and previously unsampled life stage. Stable isotope values (δ¹⁵N and δ¹³C) may differentiate between terrestrial and marine sources and integrate over longer time frames (weeks). Diet analysis provides species-specific information, but only from recent foraging (days). Average δ¹³C for all individuals was ?25.7 ‰, with the smallest individuals possessing significantly depleted δ¹³C values indicative of a stronger reliance of terrestrial carbon sources as compared to larger individuals. Average δ¹⁵N for all individuals was 10.4 ‰, with little variation among individuals. As fish length increased, the proportion of offshore Calanus prey and neritic Mysis prey increased. Rapid young-of-year growth in Arctic cisco appears to use terrestrial carbon sources obtained by consuming a mixture of neritic and offshore zooplankton. Shifts in the magnitude or phenology of river discharge and the delivery of terrestrial carbon may alter the ecology of nearshore fish consumers.     

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.     

Isotopic indicators of environmental change in a subtropical wetland

The δ¹⁵N and δ¹³C signatures of major organic matter (OM) pools were measured across chemical and hydrologic gradients in a large (58,800 ha) subtropical wetland to evaluate whether stable isotopes were useful indicators of environmental change. Once a rainfall-driven wetland, the Loxahatchee National Wildlife Refuge in the Florida Everglades now receives agricultural and urban drainage that has increased phosphorus (P) and mineral loads around the wetland perimeter. Additionally, water impoundment at the southern end has produced a latitudinal hydrologic gradient, with extended hydroperiods in the south and overdrained conditions in the north.Detritus (?4.8‰ to 8.6‰), floc (?1.4‰ to 3.6‰), and metaphyton (?6.6‰ to +7.4‰) δ¹⁵N declined southward with changes in hydrology as indicated by water depth. This pattern was attributed to higher mineralization rates under shorter hydroperiods. These signatures were also strongly correlated with increased nutrient and mineral loading. Rooted macrophyte δ¹⁵N, by contrast, appeared more responsive to soil nutrient pools. Cattail (?8.9‰ to +7.7‰) was restricted to the wetland perimeter and had the widest δ¹⁵N range, which was positively correlated with soil P. Sawgrass (?5.3‰ to +7.7‰) occurred across most of the wetland, but its δ¹⁵N was not strongly correlated to any gradient. Patterns for δ¹³C were more strongly related to chemical gradients caused by canal intrusion than to latitude or hydrology. Again, metaphyton and detrital signatures were more sensitive to water chemistry changes than macrophytes. This pattern is consistent with their locations at the soil–water (detritus-floc), and air–water (metaphyton) interface. Metaphyton δ¹³C (?36.1‰ to ?21.5‰) which had the broadest range, was affected by DIC source and pool size. In contrast, cattail δ¹³C (?28.7‰ to ?26.4‰) was more closely related to soil P and sawgrass δ¹³C (?30.1‰ to ?24.5‰) was not related to any environmental gradient except latitude. There was no correlation between the two isotopes for any OM pool except cattail.These results indicate that isotopic signatures of microbial (metaphyton and detrital) pools are more responsive to changes in wetland hydrology and water chemistry while those of rooted macrophytes respond only to the extent that soil chemistry is altered. Rooted macrophytes also differ in the sensitivity of their isotopic signatures to environmental change. The selection of OM pools for isotopic analysis will, therefore, affect the sensitivity of the analysis and the resulting patterns. Furthermore, δ¹⁵N may be more robust and interpretable than δ¹³C as an indicator of ecosystem change in wetlands exposed to multiple or complex anthropogenic gradients.     

Organic matter sources for fish larvae and juveniles in a marine-estuarine interface (Mar Chiquita lagoon,Argentina)

The aim of this study was to analyze the organic matter (OM) origin in the nearshore and the surf zones adjacent to Mar Chiquita lagoon, in order to determine the importance of allochthonous estuarine detritus and in situ marine phytoplankton as carbon (C) sources for young fishes prior to entering the lagoon. Water samples from both the nearshore and the surf zones were collected for estimation of Chlorophyll a concentration, and δ¹³C and δ¹⁵N values of the particulate organic matter (POM). Isotopic composition of the zooplankton and fish larvae from both zones and fish juveniles from the surf zone were also estimated. The contribution of potential OM sources was quantified by Bayesian stable isotope mixing models (SIAR). SIAR models revealed that the POM composition in the coastal area close to Mar Chiquita lagoon comprised a mix of primary producers (terrestrial and marine). The cordgrass Spartina densiflora that develops in the saltmarshes bounding the lagoon, contributed most to the POM of the surf zone, being important as a C source for zooplankton from the same zone. Towards deeper waters (10–12 m, ~2.5 km offshore) with higher chlorophyll a concentrations, phytoplankton contributed most to the POM. Spartina densiflora was not relevant as a C source for larval and juvenile fishes. However, it was of importance to the group of primary consumers, which apparently sustain other zooplankton organisms that fish fed on. In this sense, the OM derived from S. densiflora and exported to the coastal area contributes to sustaining the prey for young fishes outside Mar Chiquita.     

Stable isotopic (δ13C and δ18O) signatures of biogenic calcretes marking discontinuity surfaces: a case study from Upper Cretaceous carbonates of central Dalmatia and eastern Istria,Croatia

Biogenic calcretes associated with a regional Cretaceous to Paleogene subaerial unconformity and an intraformational composite (polygenic) surface in Upper Cretaceous intra-platform peritidal successions in central Dalmatia and eastern Istria, Croatia (Adriatic-Dinaridic Carbonate Platform), were analyzed for their δ¹³C and δ¹⁸O signatures in order to provide insight into the conditions of subaerial exposure and calcrete development. The distinctly negative δ¹³C signatures of biogenic calcretes marking the regional subaerial unconformity differ considerably from the δ¹³C values of the host marine limestones. This indicates carbon isotope exchange of primary marine CaCO₃ with CO₂ released by root and rhizomicrobial respiration and subsequent precipitation of pedogenic calcrete. The range of δ¹³C (from ?13.1 to ?8.2 ‰ Vienna PeeDee Belemnite standard, VPDB) and δ¹⁸O (from ?10.1 to ?6.1 ‰ VPDB) values of calcretes are similar to those reported from calcretes elsewhere, and the δ¹³C values of biogenic calcretes with typical Microcodium aggregates (?13.1 to ?12.3 ‰ VPDB) at the ?ibenik locality are very close to, or at the lower limit of, values for soil carbonates formed in isotopic equilibrium with soil CO₂. These values are expected for authigenic pedogenic carbonates formed under the influence of C₃ plant communities, without influence from heavier carbon from pre-existing carbonate and lack of input of atmospheric CO₂. Such low δ¹³C values support the interpretation of Microcodium aggregates as being precipitated under a direct biological control within the soil, although the relationship between formation mechanisms and stable isotope signatures of Microcodium needs further investigation. The δ¹³C values (?4.4 to ?3.6 ‰ VPDB) of rhizogenic calcretes formed inside firmground Thalassinoides burrows of the composite surface at the ?ibenik locality are more negative than the δ¹³C values of the host marine limestones, which confirms that the composite surface went through a phase of meteoric pedo(dia)genesis. However, the overall δ¹³C values of calcretes are less negative than expected, which might reflect contamination from associated primary marine carbonate. This study represents the first detailed stable isotope investigation of calcretes from carbonate successions of the External Dinarides, and the results may be applied to discontinuities present in other shallow-water carbonate rock successions.     

Evaluation of commercial and natural food in experimental cultures of white shrimp based on stable carbon isotopes

The farming of shrimp is developing quickly worldwide, and recently, ingredients such as seaweeds in low proportion (25 to 4 %), incorporated into the commercial food, have been shown to improve the shrimp productive variables. The change of commercial foods to commercial feed with a proportion of natural food, and finally, to natural food has been little and simultaneously evaluated. The aim of our study was to determine the relative contribution of dietary carbon to the growth of Litopenaeus vannamei fed with a proportion of 4 % Sargassum (δ¹³C = ?20.9?±?0.05?‰), 4 % Ulva (δ¹³C = ?20.6?±?0.6?‰) meal, and a control diet (δ¹³C?=??22.6?±?0.2?‰) in 60-L tanks for 30 days, and finally, with the green seaweed Ulva spp. (δ¹³C = ?13.2?±?0.25?‰) and Ulva meal (δ¹³C = ?14.5?±?0.6?‰) in open-air ponds for 120 days, by measuring δ¹³C for each of the foods and in the muscle of shrimp. After 15 days, the rates of metabolic turnover (Δ¹³C = δ¹³C_shrimp ? δ¹³C_food) were constant until the end of the experiment in the tanks. The muscle of shrimp assimilated carbon from all diets, which demonstrated the potential use of combined diets and the optimization of their use in diets containing seaweed. Our data will be useful in future interpretations of field and laboratory isotopic values for this species.     

Kangaroo tooth enamel oxygen and carbon isotope variation on a latitudinal transect in southern Australia: implications for palaeoenvironmental reconstruction

Tooth enamel apatite carbonate carbon and oxygen isotope ratios of modern kangaroos (Macropus spp.) collected on a 900-km latitudinal transect spanning a C₃–C₄ transition zone were analysed to create a reference set for palaeoenvironmental reconstruction in southern Australia. The carbon isotope composition of enamel carbonate reflects the proportional intake of C₃ and C₄ vegetation, and its oxygen isotope composition reflects that of ingested water. Tooth enamel forms incrementally, recording dietary and environmental changes during mineralisation. Analyses show only weak correlations between climate records and latitudinal changes in δ¹³C and δ¹⁸O. No species achieved the δ¹³C values (~?1.0 ‰) expected for 100 % C₄ grazing diets; kangaroos at low latitudes that are classified as feeding primarily on C₄ grasses (grazers) have δ¹³C of up to ?3.5 ‰. In these areas, δ¹³C below ?12 ‰ suggests a 100 % C₃ grass and/or leafy plant (browse) diet while animals from higher latitude have lower δ¹³C. Animals from semi-arid areas have δ¹⁸O of 34–40 ‰, while grazers from temperate areas have lower values (~28–30 ‰). Three patterns with implications for palaeoenvironmental reconstruction emerge: (1) all species in semi-arid areas regularly browse to supplement limited grass resources; (2) all species within an environmental zone have similar carbon and oxygen isotope compositions, meaning data from different kangaroo species can be pooled for palaeoenvironmental investigations; (3) relatively small regional environmental differences can be distinguished when δ¹³C and δ¹⁸O data are used together. These data demonstrate that diet–isotope and climate–isotope relationships should be evaluated in modern ecosystems before application to the regional fossil record.     

Spatial distribution of rhizodeposit carbon of maize (Zea mays L.) in soil aggregates assessed by multiple pulse 13C labeling in the field

Background and aims

Rhizosphere effect is controlled by spatial distribution of rhizodeposits, which may be influenced by soil aggregation and soil moisture regime in relation to water uptake by roots. The objectives of this study were to measure soil organic carbon (SOC) concentration and its δ¹³C abundance by aggregate size in the rooted bulk soil and by distance in the root-free soil vertically and horizontally away from roots, and to measure DOC concentration and its δ¹³C abundance in pore water in the rooted bulk soil after a seasonal pulse labelings of ¹³CO₂ to maize (Zea mays L.).

Methods

Pulse labeling was conducted in the field once a week for 11 weeks. Soil cells (50 mm in diameter and 100 mm long) mimicking root-free soils were imbedded vertically and horizontally 25–50 mm away from the main root of a maize crop. The rooted bulk soils were sampled to extract soil pore water at different suctions and to fractionate aggregates by wet sieving. The root-free soil cells were sliced by 1 mm intervals from the root end to 20 mm away. All the sampling was 12 days after the last labeling after the crop was harvested.

Results and discussion

The δ¹³C abundance before and after the continuous labeling was ?24.20?±?0.05?‰ and ?23.80?±?0.05?‰ in the rooted bulk soil. The labeling caused increases in δ¹³C abundance in all the aggregates in the rooted bulk soil and down to 14 mm away from the roots in both the root-free cells. The δ¹³C abundance was enriched in the >2 mm and 1–2 mm aggregates (?23.17?±?0.12?‰ and ?23.26?±?0.05?‰) though the SOC concentration was not different among the >0.25 mm aggregates, indicating that rhizodeposits or their metabolites were protected and distributed widely in whole soil through soil aggregation. The δ¹³C abundance in pore water (?24.0?±?0.01?‰) was much lower than those soil aggregates and greatest from the >2 μm soil pores though the DOC concentration was greater from the <20 μm soil pores. The δ¹³C abundance was in general greater in the horizontal cell than in the vertical cell. The δ¹³C abundance decreased with the increasing distance to the roots in the vertical cell and peaked at the 5 and 6 mm distance to the roots in the horizontal cell (?23.66?±?0.11?‰ and ?23.5?±?0.10?‰), possibly due to the drier condition unfavorable to microbial decomposition in the horizontal cell. The higher δ¹³C abundance in the horizontal cell than in the vertical cell was accompanied by a lower SOC concentration and a lower C: N ratio within 3 mm away from the roots, suggesting a stronger priming effect due to the longer residence time of rhizodeposits in the horizontal cell than in the vertical cell.

Conclusions

Rhizodeposits or their metabolites were protected during soil aggregation and distributed to 14 mm beyond the rhizosphere in the natural soil-plant system. This extension is of significance in regulating the formation of soil structure and the priming of soil organic matter during the whole life cycle of plants, which needs further study.     

Underwater sinkhole sediments sequester Lake Huron’s carbon

Lake Huron’s submerged sinkhole habitats are impacted by high-conductivity groundwater that allows photosynthetic cyanobacterial mats to form over thick, carbon-rich sediments. To better understand nutrient cycling in these habitats, we measured the stable isotopic content of carbon and nitrogen in organic and inorganic carbon pools in Middle Island sinkhole, a ~23 m deep feature influenced by both groundwater and overlying lake water. Two distinct sources of dissolved CO₂ (DIC) were available to primary producers. Lake water DIC (δ ¹³C = ?0.1 ‰) differed by +5.9 ‰ from groundwater DIC (δ ¹³C = ?6.0 ‰). Organic carbon fixed by primary producers reflected the two DIC sources. Phytoplankton utilizing lake water DIC were more enriched in ¹³C (δ ¹³C = ?22.2 to ?23.2 ‰) than mat cyanobacteria utilizing groundwater DIC (δ ¹³C = ?26.3 to ?30.0 ‰). Sinkhole sediments displayed an isotopic signature (δ ¹³C = ?23.1 ‰) more similar to sedimenting phytoplankton than the cyanobacterial mat. Corroborated by sediment C/N ratios, these data suggest that the carbon deposited in sinkhole sediments originates primarily from planktonic rather than benthic sources. ²¹⁰Pb/¹³⁷Cs radiodating suggests rapid sediment accumulation and sub-bottom imaging indicated a massive deposit of organic carbon beneath the sediment surface. We conclude that submerged sinkholes may therefore act as nutrient sinks within the larger lake ecosystem.     

Role of body size and temporal hydrology in the dietary shifts of shortjaw tapertail anchovy Coilia brachygnathus (Actinopterygii, Engraulidae) in a large floodplain lake

Seasonal water-level changes in floodplain lakes can induce variations in primary and secondary production, thus affecting trophic interactions. In this study, we tested the latter by studying size- and temporal hydrology-related shifts in the diet of shortjaw tapertail anchovy Coilia brachygnathus (Actinopterygii, Engraulidae) from Lake Poyang. During the wet season, δ¹³C values ranged from ?28.2‰ for small anchovies to ?24.6‰ for larger individuals, but δ¹⁵N ranged from 18.9‰ for smaller fish to 12.4‰ for larger fish. Significant ¹³C-enrichment and ¹⁵N-depletion occurred with increasing size, revealing that different carbon sources were used as the fish grew. Given the high hydrologic fluctuation levels, significant differences in δ¹³C values were observed among larger anchovies between seasons, indicating a temporal dietary shift. Anchovies fed primarily on shrimp and fish during the low-water season despite the predominance of zooplankton during the two seasons studied, which indicated increased piscivorous reliance. C. brachygnathus exhibited higher δ¹⁵N values during the wet season because the food items were ¹⁵N-enriched. Human waste brought by floods could be another possible interpretation. Considering C. brachygnathus is an important link between plankton production and higher piscivorous trophic levels, changes in the species are expected to affect the functioning of lake food webs along the trophic pathway.