Bulk Particulate Organic Matter Research Articles

Drifting along: using diatoms to track the contribution of microbial mats to particulate organic matter transport in a glacial meltwater stream in the McMurdo Dry Valleys, Antarctica.

Flow pulses mobilize particulate organic matter (POM) in streams from the surrounding landscape and streambed. This POM serves as a source of energy and nutrients, as well as a means for organismal dispersal, to downstream communities. In the barren terrestrial landscape of the McMurdo Dry Valleys (MDV) of Antarctica, benthic microbial mats occupying different in-stream habitat types are the dominant POM source in the many glacier-fed streams. Many of these streams experience daily flow peaks that mobilize POM, and diatoms recovered from underlying stream sediments suggest that mat-derived diatoms in the POM are retained there through hyporheic exchange. Yet, 'how much' and 'when' different in-stream habitat types contribute to POM diatom assemblages is unknown. To quantify the contribution of different in-stream habitat types to POM diatom assemblages, we collected time-integrated POM samples over four diel experiments, which spanned a gradient of flow conditions over three summers. Diatoms from POM samples were identified, quantified, and compared with dominant habitat types (i.e., benthic 'orange' mats, marginal 'black' mats, and bare sediments). Like bulk POM, diatom cell concentrations followed a clockwise hysteresis pattern with stream discharge over the daily flow cycles, indicating supply limitation. Diatom community analyses showed that different habitat types harbor distinct diatom communities, and mixing models revealed that a substantial proportion of POM diatoms originated from bare sediments during baseflow conditions. Meanwhile, orange and black mats contribute diatoms to POM primarily during daily flow peaks when both cell concentrations and discharge are highest, making mats the most important contributors to POM diatom assemblages at high flows. These observations may help explain the presence of mat-derived diatoms in hyporheic sediments. Our results thus indicate a varying importance of different in-stream habitats to POM generation and export on daily to seasonal timescales, with implications for biogeochemical cycling and the local diatom metacommunity.

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.

Paired Synoptic and Long-Term Monitoring Datasets Reveal Decadal Shifts in Suspended Sediment Supply and Particulate Organic Matter Sources in a River-Estuarine System

The San Francisco Estuary, in central California, has several long-running monitoring programs that have been used to reveal human-induced changes throughout the estuary in the last century. Here, we pair synoptic records of particulate organic matter (POM) composition from 1990–1996 and 2007–2016 with more robust long-term monitoring program records of total suspended sediment (TSS) concentrations generally starting in the mid-1970s to better understand how POM and TSS sources and transport have shifted. Specifically, POM C:N ratios and stable isotope values were used as indicators of POM source and to separate the bulk POC pool into detrital and phytoplankton components. We found that TSS and POC sources have shifted significantly across the estuary in time and space from declines in terrestrial inputs. Landward freshwater and brackish water sites, in the Delta and near Suisun Bay, witnessed long-term declines in TSS (32 to 52%), while seaward sites, near San Pablo Bay, recorded recent increases in TSS (16 to 121%) that began to trend downwards at the end of the record considered. Bulk POM C:N ratios shifted coeval with the TSS concentration changes at nearly all sites, with mean declines of 12 to 27% between 1990–1996 and 2007–2016. The widespread declines in bulk POM C:N ratios and inferred changes in POC concentrations from TSS trends, along with the substantial declines in upstream TSS supply through time (56%), suggest measurable reductions in terrestrial inputs to the system. Changes in terrestrial TSS and POM inputs have implications for biotic (e.g., food web dynamics) and abiotic organic matter cycling (e.g., burial, export) along the estuarine continuum. This work demonstrates how human-generated environmental changes can propagate spatially and temporally through a large river-estuary system. More broadly, we show how underutilized monitoring program datasets can be paired with existing (and often imperfect) synoptic records to generate new system insight in lieu of new data collection.

Seasonal variations in biochemical (biomolecular and amino acid) compositions and protein quality of particulate organic matter in the Southwestern East/Japan Sea

The biochemical compositions of marine particulate organic matter (POM) can provide significant information to understanding the physiological conditions of phytoplankton and food quality for their potential consumers. We investigated the seasonal variations in biomolecular and amino acid (AA) compositions of the bulk POM in the southwestern East/Japan Sea from four different sampling months (February, April, August, and October) in 2018. In terms of the biomolecular composition of the POM, overall carbohydrates (CHO) were predominant among three biomolecules accounting for 48.6% followed by lipids (LIP; 35.5%) and proteins (PRT; 15.9%) in the East/Japan Sea. However, markedly seasonal differences in the biomolecular composition of POM were found from February to October, which could be due to seasonally different conditions favorable to phytoplankton growth. Dominant AA constituents to trace POM lability were glycine (GLY), alanine (ALA), and glutamic acid (GLU), suggesting that our POM was the mixtures of decomposing and fresher materials. Furthermore, the significantly negative correlation between the proportion of total essential amino acids (EAAs) and PRT composition (r = -0.627, p< 0.01) was probably reflected by nutrient availability to phytoplankton partitioning EAAs or non-essential AAs (NEAAs). The different biomolecular compounds under un- or favorable growth conditions for phytoplankton could determine the nutritional quality of POM as potential prey as well as degradation status of POM. Therefore, the biochemical compositions of phytoplankton-originated POM hold important ecological implications in various marine ecosystems under ongoing climate changes.

Effects of river inputs on particulate organic matter composition and distributions in surface waters and sediments of the Mersin Bay, Northeastern Mediterranean Sea

Terrestrial inputs-induced eutrophication in the P-depleted Northeastern (NE) Mediterranean shelf waters has led to changes in particulate organic matter (POM) composition and distributions in the water column and surface sediments. The present study aimed to understand the impacts of terrestrial nutrient and POM inputs on coastal eutrophication, bulk POM composition and concentrations in surface waters and sediments of the Mersin Bay, located at the Cilician Basin of NE Mediterranean Sea. The present results showed that the regional rivers and wastewater discharges markedly increased nutrient concentrations, leading to enhanced eutrophication and POM concentrations surface waters of the Mersin inner bay. The C/N and C/P molar ratios derived from the regressions of POM data in surface waters were 6.92 and 129, respectively. The higher C/N and C/P molar ratios in the surface sediments of productive Mersin Bay indicated selective decay of N, P-containing organic matter buried in muddy sediments. Close positive correlations were found in sediment organic matter and mud contents, indicating the crucial role of fine particles in transport and enrichment of organic matter in the muddy shelf sediments of NE Mediterranean Sea.

Varying influence of phytoplankton biodiversity and stoichiometric plasticity on bulk particulate stoichiometry across ocean basins

Concentrations and elemental ratios of suspended particulate organic matter influence many biogeochemical processes in the ocean, including patterns of phytoplankton nutrient limitation and links between carbon, nitrogen and phosphorus cycles. Here we present direct measurements of cellular nutrient content and stoichiometric ratios for discrete phytoplankton populations spanning broad environmental conditions across several ocean basins. Median cellular carbon-to-phosphorus and nitrogen-to-phosphorus ratios were positively correlated with vertical nitrate-to-phosphate flux for all phytoplankton groups and were consistently higher for cyanobacteria than eukaryotes. Light and temperature were inconsistent predictors of stoichiometric ratios. Across nutrient-rich and phosphorus-stressed biomes in the North Atlantic, but not in the nitrogen-stressed tropical North Pacific, we find that a combination of taxonomic composition and environmental acclimation best predict bulk particulate organic matter composition. Our findings demonstrate the central role of plankton biodiversity and plasticity in controlling linkages between ocean nutrient and carbon cycles in some regions.

Toward Determining the Spatio-Temporal Variability of Upper-Ocean Ecosystem Stoichiometry From Satellite Remote Sensing

The elemental stoichiometry of particulate organic carbon (C), nitrogen (N), and phosphorus (P) connects the C fluxes of biological production to the availability of the limiting nutrients in the ocean. It also influences the marine food-web by modulating zooplankton’s feeding behavior and organic matter decomposition by bacteria. Despite its importance, there is a general paucity of information on how the global C:N:P ratio evolves seasonally and interannually, and large parts of the global ocean remain devoid of observational data. Here, a new method combining satellite ocean-color data with a cellular-trait-based model to characterize the spatio-temporal variability of the phytoplankton stoichiometry in the surface mixed layer of the ocean is presented. This new method is demonstrated specifically for the C:P ratio. The approach was applied to phytoplankton growth rates and chlorophyll-to-carbon ratios derived from MODIS-Aqua and maps of temperature-dependent nutrient limitation to generate global and seasonal maps of upper-ocean phytoplankton C:P. Taking it a step further, we determined the C:P of the bulk particulate organic matter, using MODIS-Aqua estimates of particulate organic carbon and phytoplankton biomass. Our results are within 95% confidence interval of available data for both horizontal distributions and time series, indicating our new method’s viability in accurately quantifying seasonally resolved global ocean bulk C:P. We anticipate the new hyperspectral capabilities of the NASA PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) mission will facilitate the determination of phytoplankton stoichiometry for different size classes and further enhance the predictability of marine-ecosystem stoichiometry from space.

Trophic transfer of persistent toxic substances through a coastal food web in Ulsan Bay, South Korea: Application of compound-specific isotope analysis of nitrogen in amino acids

Trophic magnification factor (TMF) of persistent toxic substances (PTSs: Hg, PCBs, PAHs, and styrene oligomers (SOs)) in a coastal food web (12 fish and four invertebrates) was determined in Ulsan Bay, South Korea. The nitrogen stable isotope ratios (δ15N) of amino acids [δ15NGlu−Phe based on glutamic acid (δ15NGlu) and phenylalanine (δ15NPhe)] were used to estimate the trophic position (TPGlu−Phe) of organisms. The TPGlu−Phe of organisms ranged from 1.64 to 3.69, which was lower than TP estimated by δ15N of bulk particulate organic matter (TPBulk: 2.46–4.21). Mercury and CB 138, 153, 187, and 180 were biomagnified through the whole food web (TMF > 1), while other PTSs, such as PAHs and SOs were not (biodilution of SOs firstly reported). In particular, the trophic transfer of PTSs was pronounced in the resident fish (e.g., rock bream, sea perch, Korean rockfish). Of note, CB 99, 101, 118, and 183 were additionally found to be biomagnifying PTSs in these species. Thus, fish residency appears to represent an important factor in determining the TMF of PTSs in the coastal environment. Overall, δ15NGlu−Phe provided accurate TPs of organisms and could be applied to determine the trophic transfer of PTSs in coastal food webs.

Molecular Nature of Marine Particulate Organic Iron-Carrying Moieties Revealed by Electrospray Ionization Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FTICRMS)

Marine sinking particulate organic matter (POM), acting as a link between surface primary production and burial of organic matter in marine sediments, undergoes a variety of physical and biochemical alterations on its way to the deep ocean, resulting in an increase in its un-characterizable proportion with diagenesis. Further, the binding ligands in POM for iron, an essential nutrient to marine life and tightly coupled with organic matter, has rarely been studied. In the current study, we employed an approach combining sequential extraction with ultrahigh resolution mass spectrometry (ESI-FTICRMS), in order to explore and unravel the chemical characteristics of organic matter compounds relevant to marine particle flux within the mesopelagic and deep ocean, with a focus on the potential iron-carrying molecules. With increasing depth, POM increases in aliphaticity, and decreases in intensity-normalized O/C ratios, aromatics, and carboxylic-rich alicyclic molecules (CRAM)-like compounds. The potential iron-carrying molecules account for ~14% of total identified molecules, and appear to have been incorporated into the marine particles via ion complexation, hydrophobic interaction, and/or interlayered ‘occlusion’. The relative abundance of iron-binding organic molecules in these three operationally-defined categories changes with depth: “surficially-complexed” fraction decreases with depth, the “interlayered-occluded” fraction increases to a comparable extent and “hydrophobic interaction” fraction occurs at all depths. Collectively, the potential iron-carrying organic molecules exhibit a set of unique molecular characteristics: a relatively lower average H/C ratio and a higher O/C ratio compared to bulk POM, a dominance of aromatics, black carbon-like compounds and CRAM-like compounds, and minor amounts of aliphatics. These molecules exhibit partial similar molecular features as precursors formed from photochemical reactions in the surface ocean, but they have been greatly modified by flux processes. Noticeably, a minor fraction of these iron-carrying molecules (<1%) was identified to contain hydroxamate-like moieties (N(OH)-COOH), the key functionality of one of the strongest iron-binding ligands in the dissolved phase. This agrees with improved spectrophotometric results and corroborates their presence in the POM. These hydroxamate-like moieties play an important role in controlling the distributions and fluxes of Fe and particle-reactive radionuclides with similar chemical complexing properties as Fe (e.g., thorium) in the ocean.

The use of amino sugars for assessing seasonal dynamics of particulate organic matter in the Yangtze River estuary

The particulate amino sugars (PAS), organic carbon (POC) and nitrogen (PN), the chlorophyll a (Chl-a), and the heterotrophic bacterial cell count (HBC) in the Yangtze River Estuary (YRE) were determined to study the biological source, bioavailability and bacterial contribution of particulate organic matter (POM) under the influence of algal bloom during the spring and autumn of 2017. The contents of PAS were 2.6–69.6 nmol/(mg dw) and made up 5.29 ± 2.55% of POC and 5.39 ± 2.77% of PN. The strong positive correlations between Chl-a and POC, PN and PAS and their similar distributions indicated that phytoplankton was a major source of POM and PAS in the YRE. When algal blooms occurred, the high ratios of glucosamine to galactosamine (GlcN/GalN) (~10.9) and the high carbon and nitrogen normalized yields of PAS (%PAS-POC and %PAS-PN, respectively) clearly indicated the planktonic (especially zooplankton) source of POM, although the high muramic acid (Mur) contents implying high contents of fresh bacterial organic matter (OM) were also found. Additionally, the high proportion of fresh plankton- and bacterial-derived OM indicated that the POM was more labile when algal bloom occurred. In contrast, when the primary production was low, the lower %PAS-POC and GlcN/GalN ratios (< 3) in the POM demonstrated the decreased contribution of planktonic OM and the relatively elevated contribution of bacterial OM. In addition, the higher %PAS-PN and the lower DO concentrations in seawater indicated that the POM experienced some degree of degradation but was still at an early stage in the nearshore site in autumn. In spring, the Changjiang diluted water (CDW) carried considerable plankton-derived POM into the estuary and influenced the distribution of POM. In autumn, the terrestrial OM might become more bacterial-derived and had less influence on the bulk POM of the estuary. The bacterial OM estimated by the Mur accounted for an average of 10.8 ± 3.7% and 40.5 ± 19.1% of the POC in spring and autumn, respectively, which was approximately two times higher than the results based on the heterotrophic bacterial cell count (HBC), indicating that most of the bacterial OM in the POM was detritus. In summary, the content, source and bioavailability of POM were significantly affected by algal bloom and the CDW, and the amino sugars represent a reliable tool to indicate the source and bioavailability of POM.

Fluvial influence on the biochemical composition of particulate organic matter in the Laptev and Western East Siberian seas during 2015

Here, we investigated the elemental (C/N ratio) and isotopic signatures (δ13C) and major biomolecules (carbohydrates, proteins, and lipids) and their relative abundance (i.e., the biochemical composition) in particulate organic matter (POM) to assess their origin and fate in the Laptev and western East Siberian seas during late summer/fall of 2015. In addition, we compared our results with the summer data of 2013 collected from Laptev and northwestern East Siberian seas. In accordance with the observed hydrological structure (i.e., a northward, warmer, diluted freshwater plume than previously observed in 2013), the more depleted δ13C (−28.2 ± 0.9‰) and higher C/N ratio (10.8 ± 2.0) than those of 2013 signalled that fluvially released terrestrial organic carbon (TerrOC) was the main source of the POM, unlike in 2013, when phytoplankton was the dominant source (δ13C = −24.9 ± 1.0‰, C/N ratio = 7.6 ± 2.4; Ahn et al., 2019). During the offshore transport of heterogeneous TerrOC, carbohydrates seem to be the primary contributor to the bulk POM as a result of selective degradation and hydrodynamic sorting. Despite the TerrOC-dominated system in 2015, some marine influence was also found. The estimated phytoplankton biomass was low and comparable among the study sites. In addition, the presence of resting spores and high ammonium concentrations within the water column may suggest senescent and, to some extent, degrading conditions of the resident phytoplankton. In this regard, carbohydrate concentrations and freshwater content were significantly correlated (r = 0.79, p < 0.01), suggesting that carbohydrates are useful inferences of freshwater within overall study sites, at least when the marine influence is similar or low.

Terrestrial support of zooplankton biomass in northern rivers

AbstractThe contribution of terrestrially derived carbon to micro‐crustacean zooplankton biomass (i.e., allochthony) has been previously studied in lakes, reservoirs, and estuaries, but little is known about zooplankton allochthony in rivers. In lacustrine environments, allochthony is regulated by distinct selective feeding behavior of different taxa. However, we hypothesized that restricted possibility for selective grazing in turbulent environments such as rivers would decouple zooplankton from specific microbial and algal food resources, such that their allochthony would mirror the terrestrial contribution to the surrounding bulk particle pool. We tested this idea by analyzing allochthony in 13 widely distributed Swedish rivers, using a dual‐isotope mixing model. Zooplankton biomasses were generally low, and allochthony in different micro‐crustacean groups (Cladocera, Cyclopoida, Calanoida) varied from 2% to 77%. As predicted, there were no correlations between allochthony and variables indicating the supply of algal and microbial food resources, such as chlorophyll a and bacterial production. Instead, the allochthony was generally similar to the share allochthonous contribution in bulk particulate organic matter, with relationships close to the 1 : 1 line. The zooplankton community allochthony was strongly regulated by the ecosystem metabolic balance between production and respiration, which in turn was dependent upon the ratio between total autochthonous organic carbon concentrations and water color. Our study for the first time shows that micro‐crustacean allochthony is regulated differently in rivers compared to in lacustrine systems, and points to inefficient support of zooplankton biomass by algal resources in turbulent waters.

Use of flow cytometry and stable isotope analysis to determine phytoplankton uptake of wastewater derived ammonium in a nutrient-rich river

Abstract. Anthropogenic alteration of the form and concentration of nitrogen (N) in aquatic ecosystems is widespread. Understanding availability and uptake of different N sources at the base of aquatic food webs is critical to establishment of effective nutrient management programs. Stable isotopes of N (14N, 15N) are often used to trace the sources of N fueling aquatic primary production, but effective use of this approach requires obtaining a reliable isotopic ratio for phytoplankton. In this study, we tested the use of flow cytometry to isolate phytoplankton from bulk particulate organic matter (POM) in a portion of the Sacramento River, California, during river-scale nutrient manipulation experiments that involved halting wastewater discharges high in ammonium (NH4+). Field samples were collected using a Lagrangian approach, allowing us to measure changes in phytoplankton N source in the presence and absence of wastewater-derived NH4+. Comparison of δ15N-POM and δ15N-phytoplankton (δ15N-PHY) revealed that their δ15N values followed broadly similar trends. However, after 3 days of downstream travel in the presence of wastewater treatment plant (WWTP) effluent, δ15N-POM and δ15N-PHY in the Sacramento River differed by as much as 7 ‰. Using a stable isotope mixing model approach, we estimated that in the presence of effluent between 40 and 90 % of phytoplankton N was derived from NH4+ after 3 days of downstream transport. An apparent gradual increase over time in the proportion of NH4+ in the phytoplankton N pool suggests that either very low phytoplankton growth rates resulted in an N turnover time that exceeded the travel time sampled during this study, or a portion of the phytoplankton community continued to access nitrate even in the presence of elevated NH4+ concentrations.

Estimation of different source contributions to sediment organic matter in an agricultural-forested watershed using end member mixing analyses based on stable isotope ratios and fluorescence spectroscopy

The two popular source tracing tools of stable isotope ratios (δ13C and δ15N) and fluorescence spectroscopy were used to estimate the relative source contributions to sediment organic matter (SeOM) at five different river sites in an agricultural-forested watershed (Soyang Lake watershed), and their capabilities for the source assignment were compared. Bulk sediments were used for the stable isotopes, while alkaline extractable organic matter (AEOM) from sediments was used to obtain fluorescent indices for SeOM. Several source discrimination indices were fully compiled for a range of the SeOM sources distributed in the catchments of the watershed, which included soils, forest leaves, crop (C3 and C4) and riparian plants, periphyton, and organic fertilizers. The relative source contributions to the river sediment samples were estimated via end member mixing analysis (EMMA) based on several selected discrimination indices. The EMMA based on the isotopes demonstrated that all sediments were characterized by a medium to a high contribution of periphyton ranging from ~30% to 70% except for one site heavily affected by forest and agricultural fields with relatively high contributions of terrestrial materials. The EMMA based on fluorescence parameters, however, did not show similar results with low contributions from forest leaf and periphyton. The characteristics of the studied watershed were more consistent with the source contributions determined by the isotope ratios. The discrepancy in the EMMA capability for source assignments between the two analytical tools can be explained by the limited analytical window of fluorescence spectroscopy for non-fluorescent dissolved organic matter (FDOM) and the inability of AEOM to represent original bulk particulate organic matter (POM).

Influence of Major Storm Events on the Quantity and Composition of Particulate Organic Matter and the Phytoplankton Community in a Subtropical Estuary, Texas

Variations in the freshwater inflow regimes of estuaries due to perturbations, such as storm events, alter the source and composition of particulate organic matter (POM) and the phytoplankton community, which are key links in estuarine carbon and nitrogen cycling. To evaluate the impact of varying freshwater discharge on POM quantity and composition and the phytoplankton community, monthly samples of surface water were collected at four long-term monitoring stations from 2012-2015 in a subtropical estuarine system, the Mission-Aransas Estuary, Texas (USA). This system is characterized by a semiarid climate, drought, and sporadic precipitation events. We analyzed organic carbon and nitrogen content, carbon and nitrogen stable isotopes, and chlorophyll a and accessory pigments of the POM samples. Following a prolonged dry spell (2012-2015), consecutive major storm events in spring 2015 led to a dramatic freshening of the entire estuary. Large increases in particulate organic carbon (POC) concentrations and decreases in δ13C values of POC over several weeks following the storms suggest an increase in in-situ production at lower salinities as a result of increased freshwater inflow. These changes in bulk POM coincided with an increase in chlorophyll a concentration, further indicating a significant contribution by phytoplankton to the elevated POM. Concurrently, pigment biomarkers revealed a significant (10-fold) increase in the cyanobacterial pigment, zeaxanthin, and further 16S rRNA analysis showed that Cyanobium spp. was responsible for the observed bloom. The combination of environment conditions, including freshening, high temperature, and high nutrients, likely contributed to the cyanobacteria bloom. These results show that episodic rain events can substantially affect estuarine phytoplankton community composition, which impacts the available energy resources for secondary production and thus may have ecosystem-wide implications on productivity.

Carbon and nitrogen stable isotope ratios of pelagic zooplankton elucidate ecohydrographic features in the oligotrophic Red Sea

Although zooplankton occupy key roles in aquatic biogeochemical cycles, little is known about the pelagic food web and trophodynamics of zooplankton in the Red Sea. Natural abundance stable isotope analysis (SIA) of carbon (δ13C) and N (δ15N) is one approach to elucidating pelagic food web structures and diet assimilation. Integrating the combined effects of ecological processes and hydrography, ecohydrographic features often translate into geographic patterns in δ13C and δ15N values at the base of food webs. This is due, for example, to divergent 15N abundances in source end-members (deep water sources: high δ15N, diazotrophs: low δ15N). Such patterns in the spatial distributions of stable isotope values were coined isoscapes. Empirical data of atmospheric, oceanographic, and biological processes, which drive the ecohydrographic gradients of the oligotrophic Red Sea, are under-explored and some rather anticipated than proven. Specifically, five processes underpin Red Sea gradients: (a) monsoon-related intrusions of nutrient-rich Indian Ocean water; (b) basin scale thermohaline circulation; (c) mesoscale eddy activity that causes up-welling of deep water nutrients into the upper layer; (d) the biological fixation of atmospheric nitrogen (N2) by diazotrophs; and (e) the deposition of dust and aerosol-derived N. This study assessed relationships between environmental samples (nutrients, chlorophyll a), oceanographic data (temperature, salinity, current velocity [ADCP]), particulate organic matter (POM), and net-phytoplankton, with the δ13C and δ15N values of zooplankton collected in spring 2012 from 16°28′ to 26°57′N along the central axis of the Red Sea. The δ15N of bulk POM and most zooplankton taxa increased from North (Duba) to South (Farasan). The potential contribution of deep water nutrient-fueled phytoplankton, POM, and diazotrophs varied among sites. Estimates suggested higher diazotroph contributions in the North, a greater contribution of POM in the South, and of small phytoplankton in the central Red Sea. Consistent variation across taxonomic and trophic groups at latitudinal scale, corresponding with patterns of nutrient stoichiometry and phytoplankton composition, indicates that the zooplankton ecology in the Red Sea is largely influenced by hydrographic features. It suggests that the primary ecohydrography of the Red Sea is driven not only by the thermohaline circulation, but also by mesoscale activities that transports nutrients to the upper water layers and interact with the general circulation pattern. Ecohydrographic features of the Red Sea, therefore, aid in explaining the observed configuration of its isoscape at the macroecological scale.

Characterization of particulate organic matter in the Lena River delta and adjacent nearshore zone, NE Siberia – Part I: Radiocarbon inventories

Abstract. Particulate organic matter (POM) derived from permafrost soils and transported by the Lena River represents a quantitatively important terrestrial carbon pool exported to Laptev Sea sediments (next to POM derived from coastal erosion). Its fate in a future warming Arctic, i.e., its remobilization and remineralization after permafrost thawing as well as its transport pathways to and sequestration in marine sediments, is currently under debate. We present one of the first radiocarbon (14C) data sets for surface water POM within the Lena Delta sampled in the summers of 2009–2010 and spring 2011 (n = 30 samples). The bulk Δ14C values varied from −55 to −391 ‰ translating into 14C ages of 395 to 3920 years BP. We further estimated the fraction of soil-derived POM to our samples based on (1) particulate organic carbon to particulate nitrogen ratios (POC : PN) and (2) on the stable carbon isotope (δ13C) composition of our samples. Assuming that this phytoplankton POM has a modern 14C concentration, we inferred the 14C concentrations of the soil-derived POM fractions. The results ranged from −322 to −884 ‰ (i.e., 3060 to 17 250 14C years BP) for the POC : PN-based scenario and from −261 to −944 ‰ (i.e., 2370 to 23 100 14C years BP) for the δ13C-based scenario. Despite the limitations of our approach, the estimated Δ14C values of the soil-derived POM fractions seem to reflect the heterogeneous 14C concentrations of the Lena River catchment soils covering a range from Holocene to Pleistocene ages better than the bulk POM Δ14C values. We further used a dual-carbon-isotope three-end-member mixing model to distinguish between POM contributions from Holocene soils and Pleistocene Ice Complex (IC) deposits to our soil-derived POM fraction. IC contributions are comparatively low (mean of 0.14) compared to Holocene soils (mean of 0.32) and riverine phytoplankton (mean of 0.55), which could be explained with the restricted spatial distribution of IC deposits within the Lena catchment. Based on our newly calculated soil-derived POM Δ14C values, we propose an isotopic range for the riverine soil-derived POM end member with Δ14C of −495 ± 153 ‰ deduced from our δ13C-based binary mixing model and δ13C of −26.6 ± 1 ‰ deduced from our data of Lena Delta soils and literature values. These estimates can help to improve the dual-carbon-isotope simulations used to quantify contributions from riverine soil POM, Pleistocene IC POM from coastal erosion, and marine POM in Siberian shelf sediments.

Biogeochemical characteristics of settling particulate organic matter in Lake Superior: A seasonal comparison

To assess settling particulate organic matter (POM) seasonality and its availability to the benthic community, settling particulate matter was studied in terms of mass fluxes and main biogeochemical characteristics (including organic carbon (OC), nitrogen, and stable carbon and nitrogen isotopic values) at two Lake Superior offshore sites over the course of a year. Fourier transform infrared spectroscopy (FTIR) and hydrolysis, extraction, and derivatization were used to provide further compositional information. Carbon and nitrogen content, isotopic and wet chemical data, and FTIR spectra show that summer particulate material is mainly autochthonous, with higher proportions of amide and carbohydrate. FTIR shows that spring particulate material contains relatively high proportions of clay minerals, indicating major sources from sediment resuspension and/or spring runoff. Distinct amino acid distributions at the two sites, revealed by principal component analysis (PCA) based on amino acid mol% composition, possibly result from differences in OM sources and the degree of degradation occurring at the two sites. Carbohydrate (PCHO), total hydrolyzable amino acid (THAA) and FTIR data suggest that the nutritional value of bulk POM to benthic heterotrophs should be lower in spring than summer-fall, although both periods exhibited high sinking fluxes of total mass and OC. Due to sediment resuspension events and an oxic water column, organic matter eventually buried in Lake Superior’s sediments has probably experienced extensive alteration due to several cycles through the water column and the bacterially-active sediment-water interface.

Assigning hydrogen, carbon, and nitrogen isotope values for phytoplankton and terrestrial detritus in aquatic food web studies

Studies designed to assess the resources supporting aquatic consumers using stable isotope analysis require measurements of the potential end members (basal resources). While some basal resources are easily measured, it is often difficult to physically separate phytoplankton (one potential end member) from other components in seston. Further, terrestrial materials entering aquatic ecosystems undergo diagenetic change, potentially altering isotope composition and making it difficult to assign end member values. We tested techniques for determining the isotopic hydrogen (δ2H), carbon (δ13C), and nitrogen (δ15N) values of terrestrial and phytoplankton end members in seston. Long term in situ leaf decomposition experiments were performed. No appreciable change was found in the isotope values of degraded material (mean change 3.6‰ for δ2H, 0.0‰ for δ13C, and −0.1‰ for δ15N). We conclude that the isotope values of terrestrial plant material can be used to assign end members for terrestrial detritus. Using samples collected from 10 lakes with phytoplankton-dominated seston, we compared 3 published methods for estimating the δ13C and δ15N of phytoplankton. One method, which corrected bulk particulate organic matter (POM) isotope values based on a δ2H mixing model, accurately predicted measured phytoplankton δ13C. Another method, which used a C:N mixing model to correct bulk POM, also performed well. A new method, proposed here, modified seston isotope values using the difference in C:N of phytoplankton and terrestrial material in a δ2H mixing model and correctly predicted measured phytoplankton δ15N. We recommend estimating phytoplankton δ13C and δ15N by correcting bulk POM using a δ2H mixing model, with the C:N modification proposed here for δ15N.

Stable carbon isotope signatures of zooplankton in some reservoirs in Korea

Dissolved organic carbon (DOC) concentrations and zooplankton and particulate organic matter (POM) values were measured in five reservoirs in Korea. Zooplankton and POM showed large range from -33‰ to -22‰ and a significant difference among the reservoirs. One eutrophic reservoir, Lake Masan, showed unique characteristics with the highest zooplankton density, the highest , and the highest DOC. Zooplankton was similar to POM , implying that zooplankton occupies substantial portion of POM or that zooplankton isotopic composition is related to selective grazing and assimilation of food sources from bulk POM. Except Lake Masan zooplankton values were negatively correlated to DOC concentration in four reservoirs with mostly forest land use. This pattern can be probably attributed to intensive agricultural land use in the watershed of Lake Masan compared to the mostly forest land use in the other watersheds. Understanding the relationship between zooplankton values and the origin of organic matter associated with watershed characteristics will be valuable to better understand trophic relationships in reservoirs in the summer monsoon region.