δ13C Of Particulate Organic Matter Research Articles

Amino acid carbon isotope profiles provide insight into lability and origins of particulate organic matter

AbstractIdentifying the phytoplankton origin of particulate organic matter (POM) in the deep ocean is challenging due to the changing phytoplankton composition and varied extent of decomposition by heterotrophic bacteria and zooplankton. Here, we report vertical distributions of amino acid stable carbon isotope values (δ13C) measured in particles in the South China Sea. Carbon‐weighted amino acid δ13C values generally parallel bulk POM δ13C profiles with a positive offset from 2.4‰ in the surface water to 6.0‰ in deep. Accordingly, a lability model is introduced to describe the relative distribution and isotopic linkage of labile and refractory particulate organic carbon. Temporal changes of amino acid δ13C were observed during the microbial decomposition of two phytoplankton strains, the prokaryote cyanobacterium Synechococcus and the eukaryote diatom Thalassiosira weissflogii. Principal component analysis of individual amino acid δ13C values from the decomposing cells separate samples into two components, reflecting the influence of microbial decomposition and taxonomic differences, respectively. These two components were further applied to particles. A major contribution from decomposing phytoplankton to particles below the euphotic zone was observed, with more prokaryote organic matter in the basin and a larger contribution from eukaryotes at stations near the productive northern shelf. Our findings show the applicability of amino acid δ13C values in tracing the lability and phytoplankton origins of POM in samples with varied extents of decomposition in marine environments.

Source apportionment of suspended particulate organic matter in a shallow eutrophic lake of Southwest China using MixSIAR model

Eutrophication has become prominent in many lakes of the world, resulting in a continuous rise of suspended particulate organic matter (POM) that can be of vital consequence for water quality and carbon cycling. While the accumulation and decomposition of POM can enhance nutrient cycling at a rate depending on the property of lake-water organic carbon, it is important to identify the source of suspended POM for evaluating eutrophication and carbon burial. Here we analyzed the distribution of stable isotopic ratios (δ13C and δ15N) and the carbon/nitrogen ratio (C/N) in suspended POM in a shallow eutrophic lake of subtropical China, for apportioning the POM sources using Bayesian mixing model. Specifically, we collected seasonal samples of lake-water POM (>0.7 μm) from 20 sites in Yilong Lake from September 2020 to June 2022, as well as six types of modern samples covering the main carbon sources in the catchment. The spatial surveys showed that in the dry year (low water level) the δ13C of POM was more enriched in the western basin than those in the central and eastern basins, while the seasonal surveys showed the most enriched δ13C of POM in September. However, this spatio-temporal heterogeneity disappeared in the wet year (high water level). It was also found that the δ13C, δ15N and C/N values of POM varied significantly with those of phytoplankton across sites and seasons. Application of the MixSIAR model further showed that phytoplankton was the predominant source of POM in Yilong Lake (79.8 ± 13.4 %), but with large annual difference, 68.9 ± 10.3 % in the dry year and 90.7 ± 3.7 % in the wet year. In comparison, the contribution of the other five endmembers was much more moderate. In particular, the proportion of allochthonous organic carbon in POM was relatively low, with terrestrial plants and soils contributing 6.1 ± 4.8 % and 4.3 ± 2.5 % of POM, respectively. Our quantitative evidence for the dominance of the autochthonous source (i.e., phytoplankton) in lake-water POM over time and space suggested a determining role of eutrophication in the composition of lake organic carbon. Therefore, the coupling of algal blooms with organic carbon cycling in inland waters can be enhanced with continuous catchment development and regional warming.

Spatial and Seasonal Variations of Stable Isotope Ratios of Particulate Organic Carbon and Nitrogen in the Surface Water of the Kuroshio

AbstractSeasonal stable isotope ratios of carbon and nitrogen (δ13C and δ15N) in the particulate organic matter (POM) of the euphotic layer were investigated around the Kuroshio from 2008 to 2019 (n = 474). Significant seasonality in POM δ13C and δ15N were observed in the coastal and offshore areas divided by the northern edge of the Kuroshio. Seasonal mean δ13C was highest during the summer (−23.2 ± 1.1‰ and −23.7 ± 1.1‰ in coastal and offshore areas, respectively) and lowest during the winter (−24.6 ± 0.8‰ and −25.0 ± 0.9‰ in coastal and offshore areas, respectively) in both areas. Seasonal mean δ15N exhibited different spatial variations. The mean δ15N value in the coastal area was the lowest during the winter (1.0 ± 1.9‰) and increased to a similar level (∼3‰) during the other three seasons. In contrast, δ15N in the offshore area near the surface decreased from the spring (2.5 ± 1.5‰) to the summer (1.2 ± 1.7‰). These spatial and seasonal differences in δ15N are significant in generalized linear models and the generalized additive models, and suggest that the nitrogen sources used in biological production differ between coastal and offshore areas and between seasons. Nitrate originating in deeper water and rivers is the main sources used for new production throughout the year in the coastal area. Contributions from atmospheric deposition and nitrogen fixation are significant in the offshore area during the summer, while nitrate originating in deeper water is main during winter, spring, and fall in the offshore area.

Interannual changes in δ15N baseline in a lake–swamp–river food web: effects of summer precipitation in West Siberian forest steppe

Frequency and intensity of precipitation events can alter hydrological conditions of lotic systems and material inflow from the catchment. In this study, interannual changes in food web structure were investigated in a lake–swamp–river system in the semi-arid West Siberian forest-steppe region using carbon and nitrogen stable isotope ratios (δ13C and δ15N). The δ15N signatures of aquatic producers and consumers including zooplankton, benthic invertebrates, and fish sharply increased (up to 7.2‰ in range) in 2009 under high summer precipitation. This would reflect the inflow of 15N-rich nutrients accumulated in the catchment through natural and anthropogenic activities, e.g., sewage, livestock farming, evaporation, and/or microbial denitrification. In 2010, benthic producers including epiphytes and macrophytes were more 13C-enriched, which is likely due to limited supply of dissolved inorganic carbon (DIC) under water stagnation, leading to a large isotopic fractionation during photosynthesis. However, 13C-enrichments in the benthic producers were not reflected to those of aquatic consumers in the habitat. Interannual changes in δ13C of particulate organic matter (POM; a proxy of phytoplankton) were reflected to those of zooplankton, benthic invertebrates, and fish fry, suggesting the trophic importance of autochthonous phytoplankton in the system. These findings suggest that occasional precipitation events in semi-arid forest-steppe regions can modify nutrient flux from the catchment, leading to changes in δ15N baseline in the lotic food web.

Macrophyte-derived detritus in shallow coastal waters contributes to suspended particulate organic matter and increases growth rates of Mytilus edulis

Benthic suspension feeders, such as bivalves, potentially have several different food sources, including plankton and resuspended detritus of benthic origin. We hypothesised that suspension feeders are likely to feed on detritus if it is present. This inference would be further strengthened if there was a correlation between δ13C of suspension feeder tissue and δ13C of particulate organic matter (POM). Since detritus is characterised by high particulate organic matter (POC):chl a ratios, we would also predict a positive correlation between POM δ13C and POC:chl a. We hypothesised that increasing depth and greater distance from shore would produce a greater nutritional reliance by experimentally transplanted blue mussels Mytilus edulis on plankton rather than macrophyte-derived detritus. After deployments of 3 mo duration in 2 different years at depths from 3 to 40 m, M. edulis sizes were positively correlated with POM concentrations. POC:chl a ratios and δ13C of POM and M. edulis gill tissue decreased with increasing depth (and greater distance from shore). δ13C of POM was correlated with δ13C of M. edulis. Our results suggest that detritus comprised a large proportion of POM at shallow depths (<15 m), that M. edulis ingested and assimilated carbon in proportion to its availability in POM, and that growth of M. edulis was higher where detritus was present and POM concentrations were higher.

Carbon Transfer from Cyanobacteria to Pelagic and Benthic Consumers in a Subtropical Lake: Evidence from a 13C Labelling Experiment

Eutrophication of lakes often results in dominance of cyanobacteria, which may potentially lead to serious blooms and toxic water. However, cyanobacterial detritus may act as an important carbon source for aquatic organisms. Using stable isotope carbon (13C) as a tracer, we assessed the carbon transfer from cyanobacteria to pelagic and benthic consumers in a 28-day outdoor mesocosm (~130 L) labelling experiment established in Lake Taihu, China, during a Microcystis aeruginosa bloom. The different organisms were labelled differently after addition of the labelled Microcystis detritus to the water. δ13C of particulate organic matter and of cladoceran zooplankton peaked earlier than for larger invertebrate consumers. Among the pelagic species, Daphnia similis had the highest Δδ13C, while the two snail species Radix swinhoei and Bellamya aeruginosa had lower but similar Δδ13C. The bivalves showed relatively modest changes in δ13C. The δ13C of Anodonta woodiana and Unio douglasiae showed a marginal though not significant increase, while a marked increase occurred for Arconaia lanceolate peaking on day 20, and Corbicula fluminea a slight increase peaking on day 9. Our results suggest that carbon from cyanobacteria can be incorporated by pelagic and some benthic consumers and eventually be transferred to higher trophic levels. Cyanobacterial carbon may, therefore, be considered an important carbon source supporting the entire food web during blooms, even if the cyanobacteria are not consumed directly.

Carbon geochemistry of plankton-dominated samples in the Laptev and East Siberian shelves: contrasts in suspended particle composition

Abstract. Recent Arctic studies suggest that sea ice decline and permafrost thawing will affect phytoplankton dynamics and stimulate heterotrophic communities. However, in what way the plankton composition will change as the warming proceeds remains elusive. Here we investigate the chemical signature of the plankton-dominated fraction of particulate organic matter (POM) collected along the Siberian Shelf. POM (> 10 µm) samples were analysed using molecular biomarkers (CuO oxidation and IP25) and dual-carbon isotopes (δ13C and Δ14C). In addition, surface water chemical properties were integrated with the POM (> 10 µm) dataset to understand the link between plankton composition and environmental conditions. δ13C and Δ14C exhibited a large variability in the POM (> 10 µm) distribution while the content of terrestrial biomarkers in the POM was negligible. In the Laptev Sea (LS), δ13C and Δ14C of POM (> 10 µm) suggested a heterotrophic environment in which dissolved organic carbon (DOC) from the Lena River was the primary source of metabolisable carbon. Within the Lena plume, terrestrial DOC probably became part of the food web via bacteria uptake and subsequently transferred to relatively other heterotrophic communities (e.g. dinoflagellates). Moving eastwards toward the sea-ice-dominated East Siberian Sea (ESS), the system became progressively more autotrophic. Comparison between δ13C of POM (> 10 µm) samples and CO2aq concentrations revealed that the carbon isotope fractionation increased moving towards the easternmost and most productive stations. In a warming scenario characterised by enhanced terrestrial DOC release (thawing permafrost) and progressive sea ice decline, heterotrophic conditions might persist in the LS while the nutrient-rich Pacific inflow will likely stimulate greater primary productivity in the ESS. The contrasting trophic conditions will result in a sharp gradient in δ13C between the LS and ESS, similar to what is documented in our semi-synoptic study.

Effects of watershed land-cover on the biogeochemical properties of estuarine tidal flat sediments: A test in a densely-populated subtropical island

The effects of watershed land cover on the biogeochemical properties of estuarine tidal flat sediment were examined in estuarine tidal flats of 16 watersheds in a densely populated, subtropical island of Japan. Despite the small sizes of the watersheds (<16.5 km2), a redundancy analysis showed that river water quality explained 62% of the cross-estuary variation in the biogeochemical properties of estuarine tidal flat sediment by the first two ordination axes. We also found that the dissolved nutrient concentrations of river water and pheophytin a content of tidal flat sediment were positively related to agricultural and urbanized land cover in the watersheds. These results indicate that human nutrient inputs significantly increase algae-derived deposits in estuaries with relatively more developed watersheds. The δ13C of particulate organic matter (POM) was negatively related to watershed forest cover. This suggests that terrestrially derived-origin POM deposits are substantial in the estuaries connected to watersheds with relatively high forest cover. However, the chemical properties of tidal flat sediment were not related to chemical indicators of POM in the base flow. We hypothesize that substantial terrestrially derived POM is discharged to estuaries of high-forest-cover watersheds during high flow, and this partially controls the chemical properties of estuarine sediments. Our results demonstrate that the chemical properties of estuarine tidal flats are associated with watershed land cover, and that the dominant processes controlling estuarine sediment properties differ among watersheds depending on land cover composition.

The effect of summer monsoon on pelagic and littoral food webs in a large regulated reservoir (Lake Paldang, Korea): A stable isotope approach

To investigate the effect of summer monsoon-derived inputs of particulate organic matter (POM) on patterns of stable isotopes in the food web in a reservoir, we traced dietary sources of carbon and nitrogen in pelagic zooplankton and herbivorous littoral zoobenthos following a monsoonal disturbance in Lake Paldang, a large regulated reservoir in Korea. Carbon and nitrogen stable isotopes were analyzed in POM, zooplankton, sedimentary organic matter, zoobenthos, and piscivorous fishes in pre- and post-summer monsoon periods. Sharp depletion of 13C and enrichment of 15N of POM along with elevated POC/Chl a ratios occurred following the summer monsoon period, reflecting the rapid reduction of phytoplankton biomass and a pulsed supply of terrestrial organic detritus. Herbivorous zooplankton δ13C values, which were initially similar to POM δ13C values in the pre-summer monsoon period, decreased to nearly that of terrestrial organic matter, providing isotopic evidence of allochthonous contribution to zooplankton in the post-summer monsoon period. In contrast, herbivorous zoobenthos in the littoral area and higher fishes appeared to have a similar stable isotope composition between the pre- and post-summer monsoon period. Our findings indicate that although monsoon storms dramatically decrease phytoplankton biomass, the augmented allochthonous material after the summer monsoon offsets this effect temporarily by subsidizing pelagic zooplankton production. Allochthonous subsidies to herbivorous littoral zoobenthos and piscivorous fishes are relatively limited.

Variation and Assimilation of Arctic Riverine Sestonin the Pelagic Food Web of the Mackenzie River Delta and Beaufort Sea Transition Zone

On-going shifts in the arctic climate and landscape have the potential to increase terrestrial particulate organic matter (POM) transported to the Mackenzie delta and coastal Beaufort shelf. If increased terrestrial/freshwater material exports to the coastal zone can be assimilated by the pelagic food web, then this represents a new pool of carbon for the regional food web and could result in a shift in the nature of carbon supporting pelagic productivity. Analysis of the δ13C of POM from the riverine, estuarine, and marine (shelf) zones of the Mackenzie delta shows that the signatures of terrestrial/freshwater carbon does extend onto the shelf, although it diminishes with distance from the Mackenzie River. δ13C values of calanoid copepods, mysid shrimp, and two species of Amphipods varied depending on their environment (river, estuary, or marine) and local POM δ13C signature. From this, we conclude that exported terrestrial POM can be a subsidy for portions of both the estuarine and marine pelagic food web. However because the response of consumer δ13C depended on both species and location, we also conclude that the response to increased riverine POM will be different for estuarine versus marine biota. The implication is that increases in exported terrestrial carbon represent a significant potential shift in coastal pelagic ecosystem structure and function.

Sources and Exchange of Particulate Organic Matter in an Estuarine Mangrove Ecosystem of Xuan Thuy National Park, Vietnam

The spatio-temporal variations in stable isotope signatures (δ13C and δ15N) and C/N ratios of particulate organic matter (POM), and physicochemical parameters in a creek water column were examined in an estuarine mangrove ecosystem of Xuan Thuy National Park, Vietnam. The objective was to examine the factors influencing creek water properties, and the sources and exchange of POM in this important mangrove ecosystem. The diel and seasonal variations in water temperature, flow velocity, pH, dissolved oxygen, and salinity demonstrated that tidal level, season, and biological factors affected the creek water properties. Mangroves had relatively low δ15N and very low δ13C values, with respective average values of 1.5 ± 0.9‰ and −28.1 ± 1.4‰. The low mangrove leaf δ15N indicated minor anthropogenic nitrogen loading to the mangrove forests. A significant positive correlation between POM–δ13C and salinity along the axis of Ba Lat Estuary, Red River, indicated that marine phytoplankton (δ13C value, −21.4 ± 0.5‰) was the predominant source of POM at the estuary mouth. Based on the co-variation of δ13C and C/N ratios, marine phytoplankton and mangrove detritus were predominant in POM of major creeks and small creeks, respectively. During the diurnal tidal cycle, the dynamics of POM were affected by sources of organic matter, tidal energy, and seasonal factors. The contribution of mangrove detritus to POM reached a maximum at the low tide and was enhanced during the rainy season, whereas marine phytoplankton contribution was highest at high tide.

Terrestrial trophic subsidy in sandy beaches: evidence from stable isotope analysis in organic matter sources and isopod Excirolana armata

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 14:129-134 (2012) - DOI: https://doi.org/10.3354/ab00392 NOTE Terrestrial trophic subsidy in sandy beaches: evidence from stable isotope analysis in organic matter sources and isopod Excirolana armata Leandro Bergamino*, Diego Lercari, Omar Defeo Unidad de Ciencias del Mar, Facultad de Ciencias, Iguá 4225, Montevideo 11400, Uruguay *Email: lbergamino@fcien.edu.uy ABSTRACT: We examined the assimilation of terrestrial organic matter (TOM) by the ovoviviparous isopod Excirolana armata inhabiting an exposed sandy beach of Uruguay. The source of this terrestrial input was an artificial freshwater discharge (the Andreoni Canal). We analyzed carbon (δ13C) and nitrogen (δ15N) isotope ratios in the TOM, in seawater particulate organic matter (POM) and in E. armata at 3 beach sites: Andreoni, at the canal mouth, Coronilla, at 1 km from the mouth and Barra, at 13 km from the mouth. Our analysis revealed clear effects of the canal discharge on the beach ecosystem. TOM was observed in the beach-face detrital mixture only at Andreoni; it also showed the lowest isotopic values there. POM δ13C values were also lowest near the canal mouth (increasing with distance from the mouth), indicating the dominance of terrestrial material in the carbon pools of sites closest to the freshwater discharge (viz. Andreoni and Coronilla). The same trend was seen in E. armata. Mixing-model (IsoSource) results for E. armata at Andreoni and Coronilla suggested that TOM was the main contributor to diet at these sites, while POM was the main contributor at Barra. We concluded that E. armata is a useful indicator of TOM assimilation in sandy-beach ecosystems. Our results point up the need, when studying sandy beach food webs, to consider potential modifications by terrestrial allochthonous inputs. KEY WORDS: Trophic subsidy · Excirolana armata · Freshwater discharge · Sandy beaches · Stable isotopes · Uruguay Full text in pdf format PreviousNextCite this article as: Bergamino L, Lercari D, Defeo O (2012) Terrestrial trophic subsidy in sandy beaches: evidence from stable isotope analysis in organic matter sources and isopod Excirolana armata. Aquat Biol 14:129-134. https://doi.org/10.3354/ab00392 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 14, No. 2. Online publication date: January 04, 2012 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2012 Inter-Research.

Marine diatoms sustain growth of bivalves in a Mediterranean lagoon

Carbon stable isotopes and fatty acids were measured in the suspended particulate organic matter (POM) of the Thau lagoon to study its qualitative temporal changes in relation to environmental factors and to identify the food sources of bivalves over a one-yr-cycle in relation to their growth. Reciprocally, the impact of shellfish farming on POM was also studied. Oysters and mussels were sampled and measured for biometry, stable isotopes and fatty acid composition. Water samples were collected at two sites, both inside and outside of the shellfish farming area, to determine concentrations in POM, chlorophyll a (Chl a) and stable isotopes. Carbon isotopes and fatty acids in bivalves reflected seasonal changes in food sources, which varied consistently with the environment. Seasonal changes in δ13C and fatty acids in the bivalves suggested that dietary phytoplankton contribution varied according to season. Terrestrial organic matter and bacteria can contribute to the diet of bivalves during non-bloom periods. Mussels seemed to rely more on diatoms and less on terrestrial organic matter and bacteria than oysters did, particularly when phytoplankton biomass was low during the summer. Although one- and two-yr-old oysters showed similar δ13C, their fatty acid dynamics differed slightly. Periods of high growth rate in bivalves were mainly fuelled by diatoms, thus highlighting the importance of seasonal blooms of microphytoplankton during the critical period of bivalve growth and gamete production. Although there was no significant effect of shellfish farms on Chl a and POM δ13C, consistent differences indicate that stable isotopes could be used successfully to investigate the effects of bivalve aquaculture.

The response of a natural phytoplankton community from the Godavari River Estuary to increasing CO2 concentration during the pre-monsoon period

This paper reports for the first time upon the effects of increasing CO2 concentrations on a natural phytoplankton assemblage in a tropical estuary (the Godavari River Estuary in India). Two short-term (5-day) bottle experiments were conducted (with and without nutrient addition) during the pre-monsoon season when the partial pressure of CO2 in the surface water is quite low. The results reveal that the concentrations of total chlorophyll, the phytoplankton growth rate, the concentrations of particulate organic matter, the photosynthetic oxygen evolution rates, and the total bacterial count were higher under elevated CO2 treatments, as compared to ambient conditions (control). δ13C of particulate organic matter (POM) varied inversely with respect to CO2, indicating a clear signature of higher CO2 influx under the elevated CO2 levels. Whereas, δ13CPOM in the controls indicated the existence of an active bicarbonate transport system under limited CO2 supply. A considerable change in phytoplankton community structure was noticed, with marker pigment analysis by HPLC revealing that cyanobacteria were dominant over diatoms as CO2 concentrations increased. A mass balance calculation indicated that insufficient nutrients (N, P and Si) might have inhibited diatom growth compared to cyanobacteria, regardless of increased CO2 supply. The present study suggests that CO2 concentration and nutrient supply could have significant effects on phytoplankton physiology and community composition for natural phytoplankton communities in this region. However, this work was conducted during a non-discharge period (nutrient-limited conditions) and the responses of phytoplankton to increasing CO2 might not necessarily be the same during other seasons with high physicochemical variability. Further investigation is therefore needed.

Seasonal shifts between autochthonous and allochthonous carbon contributions to zooplankton diets in a subarctic lake

We addressed the extent to which autochthonous and allochthonous carbon sources contribute to zooplankton diet throughout the year in oligotrophic subarctic Lake Saanajärvi. Optical measures of dissolved organic matter (DOM) indicated high water discharge and associated terrestrial DOM during winter and low inputs in summer. Bulk particulate organic matter (POM) showed δ13C values consistent with allochthonous inputs of DOM. The most positive POM δ13C values (−27‰) occurred during winter, when heterotrophic bacteria and nanoflagellates peaked in abundance; the isotopically lighter autotrophic phytoplankton shifted the POM δ13C to −29‰ in summer. The δ13C values of zooplankton were up to 4‰ more negative than those of POM, most likely because detritus and other nonliving material with higher δ13C values comprised 45–90% of POM. The δ13C of the cladoceran Daphnia umbra mirrored the trend of POM δ13C, indicating a dietary shift from heavy dependence on autochthonous carbon in summer to allochthonous or heterotrophic food in winter. The δ13C of the most abundant zooplankton in Lake Saanajärvi, Eudiaptomus graciloides and Cyclops abyssorum, remained low and constant in winter, suggestive of little or no food intake during the 8 months of winter. There are significant differences in the zooplankton utilization of nonphytoplankton carbon in different seasons and among species. Although autochthonous carbon seemed to be mainly responsible for the growth of zooplankton, the ability to utilize allochthonous carbon‐fueled microorganisms explains the survival of Daphnia through winter in northern lakes.

Benthic food-web structure under differing water mass properties in the southern Chukchi Sea

We used carbon and nitrogen stable isotope ratios as markers of food source connections and trophic position to evaluate whether benthic food-web structure varied among water masses with different productivity regimes in the southern Chukchi Sea. Benthic communities and suspended particulate organic matter (POM) were sampled at nine stations located in four water masses during the 2004 Russian-American Long-term Census of the Arctic (RUSALCA) cruise. POM δ13C values were depleted in the relatively unproductive Alaska Coastal Water (ACW, −24.2‰) and at the Russian Coast (RC, −24.5) compared to the enriched signatures of highly productive Anadyr Water (AW, −21.1‰) and the intermediate value (−23.6‰) of Bering Shelf Water (BSW). Corresponding differences in POM C/N reflected higher nutritive content of AW (6.19) compared to ACW (8.45). Carbon isotopic values of sediments were also most depleted in the nearshore waters of the ACW (−24.8‰) and RC (−23.4‰), versus BSW (−22.8‰) and AW (−22.1‰). In addition, the low δ15N values and high C/N ratios associated with sediments under the ACW (2.9‰ and 10.0, respectively) compared to the other three water masses (range 4.5–4.9‰ and 6.8–7.5, respectively) are likely explained by a terrestrial signal associated with the higher freshwater input into the ACW. Consequently, the δ13C value of POM in the ACW is likely driven by a large fraction of refractory material of terrestrial origin, and POM may not always be a reliable baseline for trophic level calculations in the ACW. Excluding POM, δ15N isotope spread among the same 42 taxa of invertebrates and fishes was 8.5‰ in ACW and 7.5‰ in AW, compared to 12.0‰ and 9.6‰ with the inclusion of δ15N POM values. Almost without exception, consumers in the ACW had higher δ15N values than their AW counterparts (average difference 2.5‰). However, food webs in ACW and AW (as well as in the BSW and RC) did not differ substantially in length (four trophic levels) when based on primary consumers as the baseline. The relatively high proportion of consumers within the first trophic level in AW suggests that there is a more direct coupling of benthic consumers to the very high pelagic primary production in these waters, which is also reflected in the high benthic infaunal biomass at low trophic levels (TL2) reported in the literature for this area. We conclude that differences in regional water column productivity in the southern Chukchi Sea may be manifested primarily in the quantitative representation of various trophic levels and less in qualitative characteristics such as food-web length or relative distribution of trophic levels.

Spatial and seasonal variations in δ13C AND δ15N of particulate organic matter in a dam‐controlled subtropical river

AbstractAlteration of stream flow by artificial dams has been observed to be a significant factor for river water environmental changes. Therefore, understanding the biogeochemical processes occurring in the dam‐controlled rivers is important for water resource management. In this paper, δ13C and δ15N signatures of particulate organic matter (POM) in a dam‐controlled subtropical river, Beijiang River, in south China are reported for their spatial and seasonal distributions. POM affected by reservoirs is lighter in δ13C and heavier in δ15N relative to unaffected POM. In April, POM δ13C and δ15N values show less spatial variation in the mainstem, and suggest relatively greater contributions of terrestrial organic matter (OM) to POM. This could be related to the onset of summer monsoon that caused an abrupt increase in terrestrial input to the river by the monsoon‐induced enhancement of rainfall and runoff. In August and December, however, POM isotopic values for the sites affected by the Feilaixia dam reservoir in the middle of the river show marked changes, suggesting aquatic plankton proliferation in the reservoir during the times. Upstream from the reservoirs, POM isotopes are seasonally less varied and suggest mainly terrestrial origin. However, the isotopic signals of aquatic plankton proliferation in the reservoir in August and December is imprinted on the POM isotopic compositions downstream the reservoir, indicating far‐reaching influences of the reservoir on the downstream water environment. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Radiocarbon and stable isotope geochemistry of organic matter in the Amazon headwaters, Peruvian Andes

We used δ13C, δ15N, and Δ14C to determine the sources of riverine organic matter in the Pozuzo River, a tributary of the Pachitea River located in the mountainous headwaters of the Amazon. Particulate organic matter (POM) Δ14C decreased downstream, suggesting that young organic matter (OM) introduced in small headwaters is respired preferentially in rivers or is diluted downstream with older material. The δ13C and δ15N of POM in headwater rivers were heterogeneous and also isotopically distinct from the main stem, suggesting that differences in landscape processes in small catchments impact the concentration and composition of POM in streams. The δ13C of dissolved organic matter (DOM) closely mirrored that of POM in the headwaters, with an apparent decoupling of dissolved and particulate OM in the river main stem. The δ13C of POM sampled in 2004 had a much different relationship with altitude than observed during previous years: In 2004, a very dry year, we observed enrichment from the headwaters to the main stem, and the opposite pattern in 2002, when rain fell heavily throughout the sampling campaign. This suggests that during dry conditions, in‐stream processes (such as resuspension or photosynthesis) may control riverine POM composition as opposed to terrestrial processes, which dominate during high runoff. Thus a decrease in runoff in Andean headwaters could result in less terrestrial POM transfer to rivers in the Andes foothills and lowland Amazon with a corresponding decrease in aquatic CO2 evasion.

Factors controlling seasonal variations in stable isotope composition of particulate organic matter in a softwater eutrophic lake

Weekly water samples were taken to measure stable isotope composition (δ13C and δ15N) of particulate organic matter (POM) in Lake Wauberg, Florida, from June 1994 to May 1995. The average δ13C of POM was ‐19.3%, consistent with an autochthonous origin from phytoplankton production, and exhibited a seasonal pattern that coincided with changes in water temperature, pH, CO2 concentration, and phytoplankton biomass in the surface water. The 13C enrichment in POM was attributed to reduced isotope fractionation due to carbon (C) limitation and the use of an isotopically heavy dissolved inorganic carbon pool supported mainly by atmospheric invasion and anaerobic respiration. Intermittent declines in δ13C of POM were related to the frequent collapses of phytoplankton blooms and increases in CO2 concentration resulting from both increased community respiration and terrestrial loading. Average δ15N of POM was 1.3% and varied within a narrow range (20.1% to 2.5%). No significant correlation between phytoplankton biovolume and the δ15N of POM was found. The low δ15N is indicative of strong N2 fixation, which is in line with the low concentration of dissolved inorganic nitrogen and the presence of high biovolume of N2‐fixing cyanobacteria in the surface water. This study suggests that stable C isotopes are good proxies for surface water CO2 concentration and primary production, while stable N isotopes can be used to indicate N2 fixation.

Temporal variation in body composition (C : N) helps explain seasonal patterns of zooplankton δ13C

Summary1. The stable carbon isotope ratio δ13C is a useful tracer of energy flow in lake food webs, and the zooplankton signature is commonly used to establish a baseline for the pelagic habitat. However, sources of temporal variability in the δ13C of different zooplankton taxa are rarely considered.2. Here, we investigate to what extent temporal variation in the δ13C of particulate organic matter (POM) (&lt;41 μm) and the C : N of zooplankton can explain the temporal variability in δ13C of freshwater zooplankton. We compare temporal patterns of δ13C and C : N for Daphnia, Hesperodiaptomus franciscanus and Leptodiaptomus tyrelli over a 6‐month period at four sites in two oligotrophic lakes.3. In all three taxa, seasonal variation in zooplankton C : N explained more of the variation in zooplankton δ13C than did the δ13C of POM. This suggests that variation in the lipid content of zooplankton can strongly influence temporal variation of δ13C in zooplankton.4. Using these data, we evaluate procedures that estimate the δ13C of only the non‐lipid component of zooplankton. If zooplankton lipids are primarily dietary in origin, than extracting lipids or ‘normalising’δ13C based on C : N will exclude a major dietary source, and therefore may be inappropriate.5. We conclude that temporal variation in body composition (C : N) of zooplankton can significantly influence the temporal variation of zooplankton δ13C signatures.