Assessment of nutritional subsidies to freshwater mussels using a multiple natural abundance isotope approach

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Seasonal variation in stable C and N isotope ratios of the Rhone River inputs to the Mediterranean Sea (2004–2005)

The temporal variation in the stable carbon and nitrogen isotope ratios of particulate organic matter (POM) in the Rhone River was investigated on a monthly basis during a 2-year period (2004–2005). In spite of high month-to-month variation, interannually consistent seasonal trends appeared, with significantly lower δ¹³C (<?28.2‰) in spring than in the other seasons. In contrast, δ¹⁵N did not exhibit significant temporal variation. In spring and summer, high chlorophyll a and b concentrations were associated with low C/N values (<8) and a high percentage of organic carbon (%C) and organic nitrogen (%N), testifying to high development of autochthonous riverine phytoplankton (mainly diatoms and chlorophytes). In fall and winter, higher δ¹³C (>?27.2‰) and C/N (>8) values, and lower %C, %N, and chlorophylls concentrations indicated the predominance of allochthonous terrestrial detritus material in the river POM. The lower δ¹³C values recorded in spring–summer, when the phytoplankton biomass was high, were related to the lower carbon isotopic signatures of freshwater diatoms and chlorophytes compared to those of terrestrial plants. Overall, Rhone River POM was mainly composed of terrestrially derived material (90%), with autochthonous phytoplankton representing only 10% as a mean, in spite of a higher mean contribution of phytoplankton (27%) to river POM in summer.     

Bacterial Growth on Allochthonous Carbon in Humic and Nutrient-enriched Lakes: Results from Whole-Lake 13C Addition Experiments

Organic carbon (C) in lakes originates from two distinct sources—primary production from within the lake itself (autochthonous supply) and importation of organic matter from the terrestrial watershed (allochthonous supply). By manipulating the ¹³C of dissolved inorganic C, thereby labeling within-lake primary production, we examined the relative importance of autochthonous and allochthonous C in supporting bacterial production. For 35 days, NaH¹³CO₃ was added daily to two small, forested lakes. One of the lakes (Peter) was fertilized so that primary production exceeded total respiration in the epilimnion. The other lake (Tuesday), in contrast, was low in productivity and had high levels of colored dissolved organic C (DOC). To obtain bacterial C isotopes, bacteria were regrown in situ in particle-free lake water in dialysis tubes. The contribution of allochthonous C to bacterial biomass was calculated by applying a two-member mixing model. In the absence of a direct measurement, the isotopic signature of the autochthonous end-member was estimated indirectly by three different approaches. Although there was excess primary production in Peter Lake, bacterial biomass consisted of 43–46% allochthonous C. In Tuesday Lake more than 75% of bacterial growth was supported by allochthonous C. Although bacteria used autochthonous C preferentially over allochthonous C, DOC from the watershed contributed significantly to bacterial production. In combination with results from similar experiments in different lakes, our findings suggest that the contribution of allochthonous C to bacterial production can be predicted from ratios of chromophoric dissolved organic matter (a surrogate for allochthonous supply) and chlorophyll a (a surrogate for autochthonous supply).     

Sediment organic matter in mountain lakes of north-western Slovenia and its stable isotopic signatures: records of natural and anthropogenic impacts

Sediment organic matter (OM) and its stable carbon and nitrogen isotopes were studied in 12 Slovenian mountain lakes in the Julian Alps. The lakes have different catchment areas and display a range of trophic states. Surface sediment atomic C/N ratios ranged from 8.4 to 13.2. Based on these C/N ratios, we concluded that autochthonous OM dominates in these lakes and constitutes approximately 65–92% of the total OM. Higher contributions of autochthonous OM sources were observed in lakes above the tree line. Relatively constant C/N ratios in the deeper sediments suggest that degradation processes are most intense in the upper few centimetres of the sediments and/or that remaining OM is relatively resistant to further degradation. Surface sediment δ¹³C and δ¹⁵N values ranged from −36.1 to −14.1‰ and from −5.2 to +1.1‰, respectively. In sediment cores from seven lakes, higher δ¹³C and lower δ¹⁵N values characterize oligotrophic lakes situated above the tree line, whereas the reverse is true for eutrophic lakes below the tree line that are also exposed to more anthropogenic impact. Carbon and nitrogen biogeochemical cycling differs considerably among the lakes. Stratigraphic shifts in carbon, total nitrogen, C/N ratios and stable C and N isotopes in cores record changes in inputs, and hence water column processes, as well as alterations in loading to the lakes. The stratigraphic variations are also the result of post-depositional diagenetic changes in the upper few centimetres of sediment. All the lakes show impacts from recent increases in atmospheric deposition of dissolved inorganic nitrogen. Application of sediment OM analysis thus proved to be useful to reconstruct paleoecological changes in sensitive mountain lake ecosystems that are either natural and/or anthropogenically derived.     

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 %).     

Environmental controls on benthic food web functions and carbon resource use in subarctic lakes

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Shifts in the trophic base of intermittent stream food webs

Understanding spatial and temporal variation in the trophic base of stream food webs is critical for predicting population and community stability, and ecosystem function. We used stable isotope ratios (¹³C/¹²C, and ¹⁵N/¹⁴N) to characterize the trophic base of two streams in the Ozark Mountains of northwest Arkansas, U.S.A. We predicted that autochthonous resources would be more important during the spring and summer and allochthonous resources would be more important in the winter due to increased detritus inputs from the riparian zone during autumn leaf drop. We predicted that stream communities would demonstrate increased reliance on autochthonous resources at sites with larger watersheds and greater canopy openness. The study was conducted at three low-order sites in the Mulberry River Drainage (watershed area range: 81–232 km²) seasonally in 2006 and 2007. We used circular statistics to examine community-wide shifts in isotope space among fish and invertebrate consumers in relation to basal resources, including detritus and periphyton. Mixing models were used to quantify the relative contribution of autochthonous and allochthonous energy sources to individual invertebrate consumers. Significant isotopic shifts occurred but results varied by season and site indicating substantial variation in the trophic base of stream food webs. In terms of temporal variation, consumers shifted toward periphyton in the summer during periods of low discharge, but results varied during the interval between summer and winter. Our results did not demonstrate increased reliance on periphyton with increasing watershed area or canopy openness, and detritus was important at all the sites. In our study, riffle–pool geomorphology likely disrupted the expected spatial pattern and stream drying likely impacted the availability and distribution of basal resources.     

流溪河水库颗粒有机物及浮游动物碳、氮稳定同位素特征

为了解影响流溪河水库颗粒有机物(POM)碳和氮稳定同位素(δ¹³C和δ¹⁵N)变化的主要因素,及其与浮游动物δ¹³C和δ¹⁵N之间的关系,于2008年5月至12月份对POM及浮游动物的δ¹³C和δ¹⁵N进行了研究。颗粒有机物碳稳定同位素(δ¹³C_POM)和氮稳定同位素(δ¹⁵N_POM)的季节性变化幅度分别为5.1‰和2.2‰,5月和7月份δ¹³C_POM较高,而在10月和12月份降低,这主要与降雨将大量外源有机物带入水库而引起的外源及内源有机物在POM组成上发生变化有关。δ¹⁵N_POM总体呈上升趋势,可能是由降雨引起的外源负荷、初级生产力、生物固氮等因素共同作用的结果。浮游动物的δ¹³C及δ¹⁵N总的变化趋势与POM的相似,也具有明显的季节性变化,食物来源的季节变化可能是造成其变化的主要原因。在5月份,浮游动物的食物来源为POM中δ¹³C较高的部分,也就是外源有机物,而在10月及12月份,其食物则可能主要为浮游植物。     

The carbon cycle of Quebec boreal reservoirs investigated by elemental compositions and isotopic values

Dissolved and particulate organic matter (POM) of three Quebec boreal reservoirs of different ages (Laforge-1, 7 years; Robert-Bourassa, 25 years and Cabonga, 70 years at the time of sampling) and sets of lakes from the same watersheds was analyzed using organic carbon concentrations, C/N and C/P elemental composition, δ¹³C and δ¹⁵N isotopic values. The reservoirs are characterized by lower dissolved organic carbon concentrations with lower C/N ratios and by lower δ¹³C and higher δ¹⁵N in POM. They contain more autochthonous dissolved organic matter and less terrigenous organic matter than the lakes. Some of those characteristics are more pronounced in the younger than in the older reservoirs. The differences can be attributed to two causes: (1) more extended degradation of terrigenous organic matter, caused by an increase in residence time; and (2) differences in food web structure resulting from the phenomenon known as trophic upsurge, in newly flooded reservoirs. The results indicate that some effects of reservoir creation on the carbon cycle are short term perturbations, others however long term features of those reservoirs. The implications of these findings for CO₂ emissions from reservoirs are discussed.     

Sources and export fluxes of inorganic and organic carbon and nutrient species from the seasonally ice-covered Yukon River

Climate and environmental changes are having profound impacts on Arctic river basins, but the biogeochemical response remains poorly understood. To examine the effect of ice formation on temporal variations in composition and fluxes of carbon and nutrient species, monthly water and particulate samples collected from the lower Yukon River between July 2004 and September 2005 were measured for concentrations of organic and inorganic C, N, and P, dissolved silicate (Si(OH)₄), and stable isotope composition (δD and δ¹⁸O). All organic carbon and nutrient species had the highest concentration during spring freshet and the lowest during the winter season under the ice, indicating dominant sources from snowmelt and flushing of soils in the drainage basin. In contrast, inorganic species such as dissolved inorganic carbon (DIC) and Si(OH)₄ had the highest concentrations in winter and the lowest during spring freshet, suggesting dilution during snowmelt and sources from groundwater and leaching/weathering of mineral layer. The contrasting relation with discharge between organic, such as dissolved organic carbon (DOC), and inorganic, such as DIC and Si(OH)₄, indicates hydrological control of solute concentration but different sources and transport mechanisms for organic and inorganic carbon and nutrient species. Concentration of DOC also shows an inter-annual variability with higher DOC in 2005 (higher stream flow) than 2004 (lower stream flow). Average inorganic N/P molar ratio was 110?±?124, with up to 442 under the ice and 38–70 during the ice-open season. While dissolved organic matter had a higher C/N ratio under the ice (45–62), the particulate C/N ratio was lower during winter (21–26) and spring freshet (19). Apparent fractionation factors of C, N, P, Si and δD and δ¹⁸O between ice and river water varied considerably, with high values for inorganic species such as DIC and Si(OH)₄ (45 and 9550, respectively) but lower values for DOC (4.7). River ice formation may result in fractionation of inorganic and organic solutes and the repartitioning of seasonal flux of carbon and nutrient species. Annual export flux from the Yukon River basin was 1.6?×?10¹² g-DOC, 4.4?×?10¹² g-DIC, and 0.89?×?10¹² g-POC during 2004–2005. Flux estimation without spring freshet sampling results in considerable underestimation for organic species but significant overestimation for inorganic species regardless of the flux estimation methods used. Without time-series sampling that includes frozen season, an over- or under-estimation in carbon and nutrient fluxes will occur depending on chemical species. Large differences in carbon export fluxes between studies and sampling years indicate that intensive sampling together with long-term observations are needed to determine the response of the Yukon River to a changing climate.     

Detection of organic pollution of streams in southern Sweden using benthic macroinvertebrates

A ²¹⁰Pb-dated sediment core from a small bay in the southern basin of Lake Petén Itzá, Guatemala documents recent cultural eutrophication. Increased sediment accumulation beginning ～1930 A.D. coincided with catchment population growth and was a consequence of watershed deforestation and increased surface run-off. At the same time, geochemical records from the Lake Petén Itzá sediment core indicate increased phosphorus loading and organic matter accumulation. High nutrient concentrations after 1965 A.D. coincided with lower sediment C/N ratios, suggesting an increase in the relative contribution of phytoplankton to the organic matter pool. This inference is confirmed by the dominance of eutrophic and hypereutrophic diatom species. Organic matter δ¹³C values decreased after 1965 A.D., seemingly contradicting other indicators of recent eutrophication in the southern basin of Lake Petén Itzá. Relatively depleted δ¹³C values in recent sediments, however, may reflect a contribution from ¹³C-depleted sewage effluent. Increased δ¹⁵N of organic matter after 1965 A.D. indicates changes in the dissolved inorganic nitrogen delivered to the lake. The relatively small increase in δ¹⁵N (～0.6‰ ) is less than might be expected with nitrate loading from sewage and soils, and might be offset by the presence of nitrogen-fixing cyanobacteria with low δ¹⁵N values.     

Organic matter distribution and retention along transects from hilltop to kettle hole within an agricultural landscape

In agricultural landscapes, the spatio-temporal distribution of organic matter (OM) varies greatly across landscape structures and soil types. We investigated patterns of organic carbon (OC) content, polyvalent cations, and isotopic values for specific OM fractions along transects spanning topographic positions from erosional to depositional areas, including aquatic sediments within a single kettle hole. We hypothesized different drivers exist at different scales. At the transect scale, we hypothesized (1) landscape form and land management to explain patterns of isotopic and OC content from different OM fractions. At the aggregate scale, (2) we expected different OM-mineral associations to explain stabilized OM. We also hypothesized, (3) that shallow sediment δ¹³C and δ¹⁵N of the kettle hole reflected different terrestrial sources. We found that distinct differences in the OM turnover rates existed between the fractions suggesting that different processes are affecting the transformation rates that are recorded in the isotopic composition patterns. Erosion along with plant productivity drive mineral-associated fractions over the transect, while microbial decomposition and slurry influence freely available and aggregated OM fractions. The type and magnitude of OM-mineral associations changed along the transect while binding OM of different decomposition status. OM in mineral-associated fractions in kettle hole sediments were derived from clay- and silt-sized particles from the field, whereas OM in freely available and aggregated fractions potentially originated from macrophytes. We conclude that kettle holes constitute important sinks for terrestrial OM across the landscape.     

Pigment carbon and nitrogen isotopic signatures in euxinic basins

The carbon and nitrogen isotopic signatures of chloropigments and porphyrins from the sediments of redox‐stratified lakes and marine basins reveal details of past biogeochemical nutrient cycling. Such interpretations are strengthened by modern calibration studies, and here, we report on the C and N isotopic composition of pigments and nutrients in the water column and surface sediment of redox‐stratified Fayetteville Green Lake (FGL; New York). We also report δ¹³C and δ¹⁵N values for pyropheophytin a (Pphe a) and bacteriochlorophyll e (Bchl e) deposited in the Black Sea during its transition to a redox‐stratified basin ca. 7.8 ka. We propose a model for evolving nutrient cycling in the Black Sea from 7.8 to 6.4 ka, informed by the new pigment data from FGL. The seasonal study of water column nutrients and pigments at FGL revealed population dynamics in surface and deep waters that were also captured in the sediments. Biomass was greatest near the chemocline, where cyanobacteria, purple sulfur bacteria (PSB), and green sulfur bacteria (GSB) had seasonally variable populations. Bulk organic matter in the surface sediment, however, was derived mainly from the oxygenated surface waters. Surface sediment pigment δ¹³C and δ¹⁵N values indicate intact chlorophyll a (Chl a) was derived from near the chemocline, but its degradation product pheophytin a (Phe a) was derived primarily from surface waters. Bacteriopheophytin a (Bphe a) and Bchl e in the sediments came from chemocline populations of PSB and GSB, respectively. The distinctive δ¹³C and δ¹⁵N values for Chl a, Phe a, and Bphe a in the surface sediment are inputs to an isotopic mixing model that shows their decomposition to a common porphyrin derivative can produce non‐specific sedimentary isotope signatures. This model serves as a caveat for paleobiogeochemical interpretations in basins that had diverse populations near a shallow chemocline.     

Fingerprinting environmental conditions and related stress using stable isotopic composition of rice (Oryza sativa L.) grain organic matter

Sea level rise (SLR) is a primary factor responsible for inundation of low-lying coastal regions across the world, which in turn governs the agricultural productivity. In this study, rice (Oryza sativa L.) cultivated seasonally in the Kuttanad Wetland, a SLR prone region on the southwest coast of India, were analysed for oxygen, hydrogen and carbon isotopic ratios (δ¹⁸O, δ²H and δ¹³C) to distinguish the seasonal environmental conditions prevalent during rice cultivation. The region receives high rainfall during the wet season which promotes large supply of fresh water to the local water bodies via the rivers. In contrast, during the dry season reduced river discharge favours sea water incursion which adversely affects the rice cultivation. The water for rice cultivation is derived from regional water bodies that are characterised by seasonal salinity variation which co-varies with the δ¹⁸O and δ²H values. Rice cultivated during the wet and the dry season bears the isotopic imprints of this water. We explored the utility of a mechanistic model to quantify the contribution of two prominent factors, namely relative humidity and source water composition in governing the seasonal variation in oxygen isotopic composition of rice grain OM. δ¹³C values of rice grain OM were used to deduce the stress level by estimating the intrinsic water use efficiency (WUE_i) of the crop during the two seasons. 1.3 times higher WUE_i was exhibited by the same genotype during the dry season. The approach can be extended to other low lying coastal agro-ecosystems to infer the growth conditions of cultivated crops and can further be utilised for retrieving paleo-environmental information from well preserved archaeological plant remains.     

Seasonal and El Niño Southern Oscillation-driven variations in isotopic and elemental patterns among estuarine primary producers: implications for ecological studies

Estuaries are complex systems where environmental fluctuations occur over distinct timescales due to local meteorological and large-scale climatic factors. Consequently, studies with low temporal resolution and taxonomic coverage may fail to detect isotopic variations in basal sources, providing biased interpretations of isotope mixing models. We investigated the seasonal and El Niño Southern Oscillation (ENSO)-driven interannual variations in δ¹³C, δ¹⁵N and C:N values among distinct basal sources and their implications for mixing models interpretation in a subtropical estuary. δ¹³C variations among sources differed in their magnitude and timescales, being large enough to confound source-specific values. Macroalgae and POM δ¹³C varied seasonally, whereas ENSO effects prevailed for C₃ and C₄ salt marsh plants, highlighting the contrasting influence of local versus remote environmental drivers on short- and long-lived primary producers, respectively. Peaks of δ¹⁵N were detected for all sources during short-term anthropogenic nutrient inputs. Isotope mixing model comparisons showed that overlooking isotopic variations in basal sources under distinct ENSO conditions can cause misinterpretation of local trophic interactions and nutrient cycling. The present study contributes to design appropriate sampling delineations in highly variable aquatic environments, emphasizing the importance of comprehensive, long-term monitoring of estuarine primary producers to encompass environmental drivers of stable isotopic variations.

     

Horizontal distribution of carbon and nitrogen and their isotopic compositions in the surface sediment of Lake Biwa

The horizontal distribution of the abundance and isotopic composition of carbon and nitrogen was studied on surface sediment samples (0–15 cm) collected from the entire area of Lake Biwa, the largest freshwater lake in Japan. As water depth increased, a marked increase in organic matter content was observed at the sampling sites, especially in the western North Basin, characterized by a steep slope. In the northwestern North Basin, which has no major inflowing streams, the sediments contained large amounts of organic matter, suggesting the possibility of lateral transportation of sedimented matter from other places by lake currents. The total amounts of carbon and nitrogen in the top-2 cm of sediment of the entire area of Lake Biwa were estimated to be 9.2 × 10⁴ tC and 1.0 × 10⁴ tN. The δ¹⁵N values in the littoral sediment were low and close to those in the inflowing river sediment, suggesting selective sedimentation of allochthonous organic matter onto the littoral area. In the North Basin, vertical profiles of organic matter content and δ¹³C values of the sediments in the littoral area showed a smaller downward decrease than in the profundal area, whereas δ¹⁵N values decreased with sediment depth in both areas. It was suggested that the littoral sediments contained abundant amounts of allochthonous and relatively refractory organic matter. Further, it was suggested that the autochthonous organic matter originated from primary production deposited mainly on the profundal zone and was easily decomposed in early diagenesis after sedimentation. Received: July 30, 1999 / Accepted: December 10, 1999     

Spatial heterogeneity of trophic pathways in the invertebrate community of a temperate bog

Summary 1. To examine spatial heterogeneity of trophic pathways on a small scale (<5 m diameter), we conducted dual stable isotope (δ¹³C and δ¹⁵N) analyses of invertebrate communities and their potential food sources in three patchy habitats [sphagnum lawn (SL), vascular‐plant carpet (VC) and sphagnum carpet] within a temperate bog (Mizorogaike Pond, Kyoto, Japan). 2. In total, 19 invertebrate taxa were collected from the three habitats, most of which were stenotopic, i.e. collected from a single habitat. Amongst the habitats, significant variation was observed in the isotopic signatures of dominant plant tissues and their detrital matter [benthic particulate organic matter (BPOM)], both of which were potential organic food sources for invertebrates. Site‐specific isotopic variation amongst detritivores was found in δ¹³C but not in δ¹⁵N, reflecting site‐specificity in the isotopic signatures of basal foods. The eurytopic hydrophilid beetle Helochares striatus was found in all habitats, but showed clear site variation in its isotopic signatures, suggesting that it strongly relies on foods within its own habitat. 3. The most promising potential foods for detritivores were the dead leaf stalks of a dominant plant in the VC and BPOM in the SL and carpet. An isotopic mixing model (IsoSource version 1.3.1) estimated that aquatic predators rely on unknown trophic sources with higher δ¹³C than detritus, whereas terrestrial predators forage on allochthonous as well as autochthonous prey, suggesting that the latter predators might play key roles in coupling between habitats. 4. Our stable isotope approach revealed that immobile detritivores are confined to their small patchy habitats but that heterogeneous trophic pathways can be coupled by mobile predators, stressing the importance of habitat heterogeneity and predator coupling in characterising food webs in bog ecosystems.     

Inferring the ecology of willow warblers during their winter moult by sequential stable isotope analyses of remiges

We present a comparison of feather stable isotope (δ¹³C, δ¹⁵N) patterns representing the habitat and diet conditions for two subspecies of willow warblers Phylloscopus trochilus that breed in parapatry, but winter in different regions of sub‐Saharan Africa. Previous analyses have shown that on average winter moulted innermost primaries (P1) show subspecific differences in δ¹⁵N values, although individuals show substantial variation for both δ¹³C and δ¹⁵N within the subspecies. We examined whether corresponding variation in the timing of the winter moult, as reflected by consistent intra‐wing correlations for individual's δ¹³C and δ¹⁵N values, could explain some of the previously observed isotopic variation. Further, differential subspecific adaptations to winter precipitation patterns across Africa might result in a variable degree of site fidelity or itinerancy during moult. We found no consistent trend in isotopic values from innermost to outermost primaries, thus inter‐individual variation in the timing of moult does not explain the subspecific isotopic variation for P1. Patterns in wing feather δ¹³C and δ¹⁵N values indicated that 41% of the individuals from both subspecies shifted their diet or habitats during winter moult. Importantly, despite well‐documented itinerancy in willow warblers during the winter, 59% of the individuals had feather isotope values consistent with stable use of habitats or diets during winter moult. Repeatability analyses suggest that individuals of both subspecies initiate moult in similar habitats from year‐to‐year while feeding on isotopically similar diets.     

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.     

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.