Sediment Mixing Rates Research Articles

Invasive mussels reduce community bioturbation but do not affect oxygen penetration or nutrient fluxes in organic‐poor Great Lakes sediments

Abstract Invasive zebra and quagga (dreissenid) mussels have disrupted nutrient cycling and benthic macrofauna communities in the Laurentian Great Lakes and other invaded ecosystems. Dreissenids are now the dominant benthic macroinvertebrate in the Great Lakes, replacing the formerly dominant native bioturbating amphipod Diporeia spp. Dreissenids and Diporeia interact with their environment in fundamentally different ways, and the consequences of this functional shift in benthic community structure on benthic–pelagic coupling are not well understood, particularly in unproductive offshore lake regions. To determine how functional biology and benthic community structure impact sediment mixing and biogeochemistry in low particulate organic matter (POM) lake regions, we conducted a 6‐week sediment microcosm experiment with dreissenids, Diporeia and oligochaete worms—the second most common Great Lakes benthic macroinvertebrate. We found that sediment mixing rate and depth varied significantly among the taxa. Diporeia mixed sediment the deepest and strongest, followed by oligochaetes, while dreissenids did not appreciably mix sediment. Despite these differences, we found no significant variations among treatments in sediment oxygen penetration depth, sediment respiration (oxygen uptake) or nutrient dynamics. Our results suggest that dreissenids mix sediment less than native Great Lakes taxa, but that differential mixing rates may not measurably affect nutrient and oxygen dynamics in low‐POM sediments. Therefore, mussel effects in these areas may be manifested more through direct mechanisms rather than via altered sediment geochemistry.

Luminescence dating of cyclone-induced washover fans at Point Lefroy (NW Australia)

Reliable age dating of coastal sedimentary landforms is crucial for inferring storm frequencies and magnitudes from geological archives. However, in highly energetic coastal settings, radiocarbon dating is often biased by reworking and/or poorly constrained marine reservoir effects. Due to this, most cyclone-driven sediment archives from the semiarid coast of NW Australia – a region frequently affected by tropical cyclones but with a historical record limited to ∼150 a, and therefore strongly in need of long-term data inferred from geological evidence – are affected by chronological inaccuracies. Optically stimulated luminescence dating (OSL) may overcome these shortcomings by dating the transport of sediment directly. In turn it may be related to other challenges when applied to cyclone deposits from semiarid environments. The cyclone-induced washover fans at Point Lefroy, NW Australia, are composed of a heterogeneous mixture of coral fragments, shell hash and siliciclastic sand. This makes them particularly prone to high dose scatter resulting from a combination of partial bleaching, sediment mixing and dose-rate heterogeneity. The washover fans are further characterised by a discontinuous nature of cyclone deposition, as indicated by erosional features and macroscopic brunification horizons. By using a combination of quartz single grain dating, autoradiography, alpha counting and gamma spectrometry, sediment mixing and dose rate heterogeneity are identified as the main sources of dose scatter. The resulting chronology allows us to discriminate at least four well constrained phases of washover fan activity at ∼180, ∼360, ∼870, and ∼1300 a ago. Older but less well constrained activity phases occurred ∼1950, ∼2300, and ∼2830 a ago. While these phases of increased cyclone activity correlate with depositional units separated by potential palaeosols, OSL ages, quasi-continuous portable OSL reader measurements and gamma spectrometry measured with increased sampling resolution point to deposition of distinct washover units within a very short period of time. However, unambiguous discrimination between deposition of individual units by single events and deposition by several cyclones within periods of only a few decades is currently not possible.

Solute transport into the Jiulong River estuary via pore water exchange and submarine groundwater discharge: New insights from 224Ra/228Th disequilibrium

Pore water exchange (PEX) and submarine groundwater discharge (SGD) represent two mechanisms for solute transport from the seabed into the coastal ocean. However, their relative importance remains to be assessed. In this study, we pursued the recently developed 224Ra/228Th disequilibrium approach to quantify PEX fluxes of 224Ra into the Jiulong River estuary, China. By constructing a full mass balance of water column 224Ra, we were allowed to put various source terms, i.e., SGD, diffusive and advective pore water flow (PEX), and river input in a single context. This led to the first quantitative assessment of the relative importance of PEX vs. SGD in the delivery of solutes into an estuary. We carried out two surveys in the Jiulong River estuary: one in January 2014 (winter survey), the other in August 2014 (summer survey). By virtue of a 1-D mass balance model of 224Ra in the sediment column, we demonstrated that PEX fluxes of 224Ra were highly variable, both temporally and spatially, and can change by 1–2 orders of magnitude in our study area. Moreover, we identified a strong correlation between 224Ra-based irrigation rate and 234Th-based sediment mixing rate. Our results highlighted irrigation as the predominant PEX process for solute transfer across the sediment–water interface.Total PEX flux of 224Ra (in 1010dpmd−1) into the Jiulong River estuary was estimated to be 22.3±3.0 and 33.7±5.5 during the winter and summer surveys, respectively. In comparison, total SGD flux of 224Ra (in 1010dpmd−1) was 11.3±8.6 and 49.5±16.3 in the respective seasons. By multiplying the PEX fluxes of 224Ra by the ratio of the concentration gradients of component/224Ra at the sediment–water interface, we quantified the total PEX fluxes of dissolved inorganic carbon (DIC) and nutrients (NH4+, NO3−, and H4SiO4) into the Jiulong River estuary. In the meantime, net export of DIC and nutrients via SGD were estimated by multiplying the SGD fluxes of 224Ra by the DIC (nutrients)/224Ra ratios in the SGD end-members around this area. Our results revealed that PEX-driven fluxes of solutes rival net SGD input and river input in an estuary. An additional new finding is that water column NO3− in the surface estuary was effectively sequestered due to SGD, probably as a result of intense denitrification occurring in the anoxic subterranean estuary.

Tracking trends in eutrophication based on pigments in recent coastal sediments

Summary Eutrophication in two different coastal areas – the Gulf of Gdansk (southern Baltic) and the Oslofjord/Drammensfjord (Norway) – both subject to human pressure and with restricted water exchange with adjacent seas, was investigated and compared. Sediment cores (up to 20 cm long) were collected at 12 stations using a core sampler, 6 in each of the two areas, and divided into sub-samples. The physicochemical parameters characterizing the adjacent water column and near-bottom water, i.e. salinity, oxygen concentration and temperature, were measured during sample collection. Chlorophylls-a, -b and -c, their derivatives and selected carotenoids were determined for all the samples, as were additional parameters characterizing the sediments, i.e. Corg, Ntot, δ13C and δ15N, grain size. 210Pb activity was also determined and on that basis sediment mixing and accumulation rates were estimated. The distribution of pigments in sediments was related to environmental conditions, the sampling site location and sediment characteristics. The results are in agreement with other observations that eutrophication in the Gulf of Gdansk has increased, especially since the 1970s, whereas in the Oslofjord it decreased during the same period. The pigments are better preserved in inner Oslofjord sediments than in those from the Gulf of Gdansk. The results demonstrate that the sum of chloropigments-a in sediments calculated per dry weight of sediments is a valuable measure of eutrophication, providing that the monitoring site is selected properly, i.e. sediments are hypoxic/anoxic and non-mixed. Besides, the results confirm previous observations that the percentages of particular chlorophyll-a derivatives in the sum of chloropigments-a are universal markers of environmental conditions in a basin. The ratios of chloropigments-b and chlorophylls-c to the sum of chloropigments-a (ΣChlns-b/ΣChlns-a; Chls-c/ΣChlns-a) may by applied as complementary markers of freshwater and marine organic matter input, respectively.

Mass-balance control on the interaction of axial and transverse channel systems

AbstractWe present a geometric, sediment mass-balance model for the interaction of axial and transverse alluvial systems in a subsiding basin. By comparing the model result with a flume experiment that employed a simplified half-graben tectonic geometry with axial and transverse sediment sources, we quantify rates of axial-transverse erosional sediment mixing. In the experiment, the lateral migration rate of the axial-transverse boundaries due to the sediment mixing scales with sediment supplies delivered by transverse drainages, but not with water (or sediment) discharge from the axial channel or with tectonic tilting rate. Using an empirical lateral erosion rate, the model shows how sediment supply partitioning among the axial, hanging-wall, and footwall drainages controls the width and the location of the axial-channel belt. Comparing the modeling results with field cases demonstrates that transverse sediment fluxes could slow the axial-channel migration or even reverse the movement against the tectonic forcing.

Sediment bioavailable organic matter, deposition rates and mixing intensity in the Setúbal–Lisbon canyon and adjacent slope (Western Iberian Margin)

The role of the Setúbal–Lisbon canyon in accumulation and transport of labile organic matter from the coastal sea and ocean surface water towards the deep sea was assessed by investigating the distribution of organic matter of different quality in sedimentary aggregates and surface sediments of the canyon and adjacent slopes. Total hydrolysable amino acids (THAA) and organic carbon (Corg) were measured from aggregates, and contents of Corg, chlorophyll a (chl a), phaeopigments (phaeo), chloroplastic pigment equivalents (CPE) from sediments. As indices of organic matter (OM) quality THAA:Corg, degradation index (DI), chl a:phaeo, chl a:Corg and C:N ratio were determined. Sediment profiles of chl a and the isotope 210 of lead ( 210Pb) were used as tracers in a transport model to estimate deposition rates and background levels of the tracers, and sediment mixing rates (Db). Whereas bulk Corg contents of canyon and slope sediments were practically similar at all depths, higher contents of THAA, chl a and CPE, as well as higher THAA:Corg, DI and chl a:Corg, in aggregates and sediments from the upper reaches of the canyon indicate that labile organic matter accumulates in the upper canyon. This is confirmed by higher chl a and 210Pb deposition and Db calculated from the model. Hence, the Setúbal–Lisbon canyon, specially the upper region, acts as a natural trap of organic matter that is transported to the region via lateral transport and vertical settling from primary productivity. Organic matter might be further transported in downward canyon direction via rebound processes. The chl a and 210Pb profiles reveal active sediment mixing by physical processes and/or animal reworking.

Intertidal mangrove mudflat 240+239Pu signatures, confirming a 210Pb geochronology on the southeastern coast of Brazil

A sediment core was taken to determine if sediment accumulation rates could be conducted using 240+239Pu signatures in the coastal mangrove mudflats of southeastern Brazil. The results from this study show that 240+239Pu fallout activities are sufficient and well preserved in the coastal sediments of this region. Sediment accumulation rates determined from the 240+239Pu signatures were 4.4 mm/year and 4.1 from 210Pb (CIC) method. A sediment mixing coefficient rate was calculated using chlorophyll-a profile (9.5 cm2).

Joined by geochemistry, divided by history: PCBs and PBDEs in Strait of Georgia sediments

Polychlorinated biphenyls (PCBs) are relict contaminants, while polybrominated diphenyl ethers (PBDEs) are in increasing use. Using sediment cores collected in the Strait of Georgia, we demonstrate that the surface sediment concentration of PCBs is largely determined by environmental processes, such as sediment accumulation and mixing rates, while that of PBDEs is strongly influenced by proximity to source. The Iona Island wastewater outfall appears to be a primary pathway for PBDEs. As well, Vancouver Harbour is highly contaminated with both classes of chemical. BDE-209, the main component of deca-BDE, is the dominant PBDE congener. Environmental debromination is not evident. Currently, the ranges of the surface concentration of PCBs and PBDEs are similar to one another, but that will change in the future, as the concentration of PBDEs continues to rise. The experience with PCBs suggests that if PBDEs were banned today, it would take decades for inorganic sediment to bury them.

Monitoring compared with paleolimnology: implications for the definition of reference condition in limed lakes in Sweden

Surface water acidification was identified as a major environmental problem in the 1960s. Consequently, a liming program was launched in Sweden in the 1970s. The primary purpose of liming is to restore conditions that existed prior to acidification. To reach this goal, as well as achieve 'good status' (i.e. low levels of distortion resulting from human activity) in European freshwaters until 2016 under the European Union Water Framework Directive, lake data are required to define reference conditions. Here, we compare data from chemical/biological monitoring of 12 limed lakes with results of paleolimnological investigations, to address questions of reference conditions, acidification, and restoration by liming. Using diatom-based lake-water pH inferences, we found clear evidence of acidification in only five of the 12 lakes, which had all originally been classified as acidified according to monitoring data. After liming, measured and diatom-inferred pH agree well in seven lakes. The sediment record of three of the five remaining lakes gave ambiguous results, presumably due to sediment mixing or low sediment accumulation rates. It is difficult to determine whether liming restored the lakes to a good status, especially as some of the lakes were not acidified during the twentieth century. In addition to acid deposition, other factors, such as natural lake and catchment ontogeny or human impact through agricultural activity, influence lake acidity. This study shows that monitoring series are usually too short to define reference conditions for lakes, and that paleolimnological studies are useful to set appropriate goals for restoration and for evaluation of counter measures.

The cycling and oxidation pathways of organic carbon in a shallow estuary along the Texas Gulf Coast

The cycling and oxidation pathways of organic carbon were investigated at a single shallow water estuarine site in Trinity Bay, Texas, the uppermost lobe of Galveston Bay, during November 2000. Radio-isotopes were used to estimate sediment mixing and accumulation rates, and benthic chamber and pore water measurements were used to determine sediment-water exchange fluxes of oxygen, nutrients and metals, and infer carbon oxidation rates. Using 7Be and 234Th XS, the sediment-mixing coefficient ( D b) was 4.3 ± 1.8 cm 2 y −1, a value that lies at the lower limit for marine environments, indicating that mixing was not important in these sediments at this time. Sediment accumulation rates ( S a), estimated using 137Cs and 210Pb XS, were 0.16 ± 0.02 g cm −2 y −1. The supply rate of organic carbon to the sediment-water interface was 30 ± 3.9 mmol C m −2 d −1, of which ∼10% or 2.9 ± 0.44 mmol C m −2 d −1was lost from the system through burial below the 1-cm thick surface mixed layer. Measured fluxes of O 2 were 26 ± 3.8 mmol m −2 d −1 and equated to a carbon oxidation rate of 20 ± 3.3 mmol C m −2 d −1, which is an upper limit due to the potential for oxidation of additional reduced species. Using organic carbon gradients in the surface mixed layer, carbon oxidation was estimated at 2.6 ± 1.1 mmol C m −2 d −1. Independent estimates made using pore water concentration gradients of ammonium and C:N stoichiometry, equaled 2.8 ± 0.46 mmol C m −2 d −1. The flux of DOC out of the sediments (DOC efflux) was 5.6 ± 1.3 mmol C m −2 d −1. In general, while mass balance was achieved indicating the sediments were at steady state during this time, changes in environmental conditions within the bay and the surrounding area, mean this conclusion might not always hold. These results show that the majority of carbon oxidation occurred at the sediment-water interface, via O 2 reduction. This likely results from the high frequency of sediment resuspension events combined with the shallow sediment mixing zone, leaving anaerobic oxidants responsible for only ∼10–15% of the carbon oxidized in these sediments.

Controls on the δ13C of dissolved inorganic carbon in marine pore waters: An integrated case study of isotope exchange during syndepositional recrystallization of biogenic carbonate sediments (South Florida Platform, USA)

Abstract The carbon isotope systematics of marine carbonates, organic matter and dissolved inorganic carbon (DIC) play a critical role in quantifying carbonate dissolution fluxes from modern deep-ocean sediments to paleoocean–atmospheric modeling. However, there is a growing body of evidence that C mass and isotope balances in marine pore waters appear incompatible, suggesting that some processes other than mass transport, carbonate dissolution, and organic matter decomposition have significantly increased the value of δ13C(DIC). We present a comprehensive data set of pore water and sediment geochemistries in biogenic carbonates from well-characterized depositional environments of the South Florida platform. Pore water elemental and δ13C(DIC) values are integrated with δ13C values of carbon sources (seawater, organic and inorganic carbon), sediment mixing rates (210Pb profiles), microbial sulfate reduction rates (SRR) (radiotracer 35SO42−), and incubation experiments spiked with low δ13C(DIC) to estimate the rate and extent of C isotope exchange. Together, these data indicate that biogenic carbonates undergo extensive syndepositional recrystallization at rates comparable to net dissolution rates, permitting significant exchange between isotopically depleted organic C and isotopically enriched inorganic C pools. Significant amounts of net carbonate dissolution are common in the pore waters of these low-Fe sediments, as manifested by Ca2+/Cl− ratios increased by up to 25% relative to overlying seawater. Despite rapid microbial SRR, degrees of pore water SO42− reduction usually are maintained below 5% by H2S oxidation, the main acid source for dissolution. These processes increase pore water DIC concentrations by more than 6 mM, over a 5-fold increase relative to overlying seawater values. Pore water δ13C(DIC) values are usually greater than −5‰, and sometimes as high as +2‰, despite decomposition of organic matter with low δ13C values (−9‰ to −15‰ VPDB). Pore water δ13C(DIC) values thus require inputs from inorganic C that exceed those from organic C, even though less than 25% of the DIC can be accounted for by the amount of net carbonate dissolution indicated by Ca2+/Cl− ratios. Incubation experiments attain DIC concentrations in excess of 20 mM during closed system SO42− reduction, but δ13C(DIC) values remain buffered between –4‰ and –6‰. Exchange of 12C for 13C during carbonate recrystallization in pore waters is required to reconcile the significant imbalance between mass and isotope budgets. Past work and data presented here for Sr2+/Ca2+ in pore waters show that recrystallization of biogenic aragonites and high-Mg calcites from South Florida platform sediments can occur without significant mineralogic change. Although these phases are metastable relative to lower Mg-calcite, the drive for recrystallization does not appear to be mineralogic stabilization. Instead, the high surface areas of many biogenic carbonates from algal and foraminiferal sources (about 2 m2 g−1) contribute excess surface free energy that is released by recrystallization to larger crystallites of the same mineralogy. Studies of carbonate dissolution and precipitation kinetics have firmly established that high ratios of solid surface area: solution volume causes reaction of CO2 with grain surfaces to become the rate-limiting step. Thus, in sediment-pore water systems containing carbonates with high specific surface areas, pore water δ13C(DIC) may be buffered either by the value of the bulk CaCO3, producing δ13C(DIC) values approaching +2‰, or by the relatively large fractionation factor between CO2 and solid CaCO3 (Δ∼−8‰), producing δ13C(DIC) values that approach −6‰ VPBD. The observed magnitude of isotopic exchange between organic and inorganic carbon pools during the earliest stages of sedimentation has ramifications for the interpretation of the C isotopic record of marine carbonate rocks and contributes significant uncertainty to reconstructions of paleoocean and atmospheric chemistries.

The Santonian-Campanian phosphatic chalks of England and France

Phosphatic chalks were deposited during the mid-Santonian to early Campanian between 85 Ma and 83 Ma in NE France and southern England. The beds contain abundant brown granular phosphate, largely consisting of phosphate-filled and coated foraminiferal tests, phosphatized macrofossil fragments, faecal pellets, phosphatized intraclasts and vertebrate remains. Phosphate contents typically range from 5–20% P 2 O 5 . Beds 1–20 m thick occur within elongate 1 km long, 250 m wide and 30 m deep erosional basins (‘cuvettes’), cut into the underlying Coniacian-Santonian white chalks. Several superimposed cuvettes with consistent orientations may occur. Cuvettes are floored by phosphatized, bored and encrusted hardgrounds, and contain unique faunal assemblages that include ahermatypic colonial corals, and encrusting oyster and worm patches. Belemnites, fish and reptile remains are common. Extensive reworking by currents and biota occurred during the deposition of the phosphate. Gravity flow led to the dislocation and redeposition of basal hardground blocks and the formation of synsedimentary slump folds. Phosphatic chalks accumulated in water depths of between 70 m and a few hundred metres in oxic, current-swept areas of an open epicontinental sea of normal salinity. High surface productivity, moderate organic-carbon fluxes, reduced sedimentation rates and high rates of sediment mixing led to the precipitation of phosphate within hardground surfaces and carbonate grains. Bacterial decomposition of organic matter raised phosphate contents in pore waters and promoted the geochemical conditions necessary for phosphate precipitation. Phosphate precipitates, now transformed to carbonate fluorapatite, include um-sized ovoid bodies and massive coatings interpreted to be of microbial origin. Phosphatization was a rapid process occurring under mixed aerobic/anaerobic redox conditions in the upper decimetre of the sediment column, close to the sediment-water interface. Phosphatization took place predominantly within the confines of the cuvettes, where phosphate grains became concentrated by current winnowing and transport. Phosphatic chalk sedimentation occurred on the NE and northern margins of the Anglo-Paris Basin following regressive events that enabled bottom currents to erode the cuvettes and maintain productivity and the other sedimentary conditions necessary for phosphate accumulation.

Effects of commercial otter trawling on the physical environment of the southeastern Bering Sea

The effects of commercial bottom trawling on the physical environment were investigated in a sandy habitat at ∼26-m water depth in the southeastern Bering Sea. We compared an area closed to trawling for 10 years with an adjacent commercial fishery area, examined the immediate effects of experimental trawling on sediment properties, and compared bottom trawling to natural disturbance by waves. The sediments in the top 3 cm of the fished area were slightly better sorted, less variable, and contained fewer fine grains than those of the closed area. Relative to controls, no shift in means was detected immediately after experimental trawling, but significant increases in variability occurred for several grain size and organic matter parameters. Wave disturbance to the study area was persistent and of greater magnitude than disturbance associated with commercial trawling. However, the trawl season occurred at a time of year when sediment resuspension by waves was diminished. Previous work suggested that such trawling-related factors have important biological consequences and account for the different benthic communities observed between trawled and closed areas. Given the physical setting, it is difficult to explain our subtle but significant results by purely physical processes. We hypothesize that the differences in sediment characteristics could result from a biological feedback process, where trawling results in different communities which in turn result in different sediment mixing and bioturbation rates, thereby influencing grain size parameters and labile carbon distributions. Functional differences in the benthic communities are consistent with the observed differences in sedimentary characteristics. This study is unique in identifying potential trawling effects in sandy high-energy environments, and suggests that interactions between physical and biological processes should be addressed in future studies.

Geochronological reconstruction of 137Cs transport from the Coruh river to the SE Black Sea: comparative assessment of radionuclide retention in the mountainous catchment area

The deposition record of 137Cs was traced in the SE Black Sea sediments adjacent to the Coruh river mouth in comparison with the earlier studied chronology of 137Cs deposition in front of the Danube delta (NW Black Sea). In both cases, the 137Cs profiles showed two subsurface peaks attributable to maximum fallout of ‘bomb’ and Chernobyl radionuclides. The Coruh profile revealed a larger contribution of ‘bomb’ 137Cs in comparison with the Chernobyl input, suggesting different coverage of NW and SE Black Sea regions with the Chernobyl fallout. The 137Cs-derived dating showed that maximum deposition of particulate bound 137Cs in sediments adjacent to the Coruh river mouth was delayed for ∼14 yr relative to date of Chernobyl accident, reflecting a buffer effect of the watershed soils. This transit time is 3 times longer than in the Danube catchment area, indicating a difference in retention processes in these mountainous (Coruh) and lowland (Danube) river basins. The 137Cs profile in Coruh sediments showed penetration of 137Cs to much greater depth than would be expected from 137Cs fallout chronology, suggesting the sediment mixing rate of 1.3 cm 2 yr −1. This value was used to evaluate deposition chronology of 137Cs, applying the model developed for pulse fallout case. Comparing the measured and modelled data has allowed differentiation of the flood-induced discharge of the 137Cs-containing suspended matter and the slower transit of eroded soil particles from the contaminated catchment areas. The obtained results may be used for the prediction of period when the pollutants, deposited over the river basins, can reach the Black Sea.

Accumulation of anthropogenic and natural radionuclides in bottom sediments of the Northwest Pacific Ocean

Abstract Activities of anthropogenic radionuclides in bottom sediments of the Northwest-Central and West Caroline basins of the Northwest Pacific Ocean are reported. Down-core distributions of natural radionuclides were also investigated in order to elucidate the sedimentation processes of anthropogenic radionuclides in the seafloor. In the Northwest-Central Basin, the inventories of 239+240Pu and 241Am decrease from south to north while the opposite trend is observed for 137Cs and 90Sr. This latitudinal gradient in the inventories of artificial radionuclides reflects higher fluxes of 239+240Pu and 241Am near the weapons test sites and transport of 137Cs and 90Sr over longer distances, as well as the regional differences in sediment-mixing and accumulation rates. In the West Caroline Basin, relatively low inventories of 239+240Pu and 241Am were observed due to the topographic isolation of deep water. Sediment-mixing rates were calculated using down-core distributions of 239+240Pu, 241Am, 137Cs and 90Sr based on two distinct models (continuous and constant) of tracer input and with a third model with pulsed input during the peak fallout period. The continuous input model yielded mixing coefficients comparable to those derived from excess 210Pb profiles within an order of magnitude.

Tracing organic matter sources of estuarine tidal flat nematodes with stable carbon isotopes

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 234:127-137 (2002) - doi:10.3354/meps234127 Tracing organic matter sources of estuarine tidal flat nematodes with stable carbon isotopes Tom Moens1,*, Caroline Luyten1, Jack J. Middelburg2, Peter M. J. Herman2, Magda Vincx1 1University of Gent, Biology Department, Marine Biology Section, K.L. Ledeganckstraat 35, 9000 Gent, Belgium 2Netherlands Institute for Ecology, Centre for Estuarine and Coastal Ecology, Korringaweg 7, PO Box 140, 4400 AC Yerseke, The Netherlands *E-mail: tom.moens@rug.ac.be ABSTRACT: The present study explores the use of stable carbon isotopes to trace organic matter sources of intertidal nematodes in the Schelde estuary (SW Netherlands). Stable carbon isotope signatures of nematodes from a saltmarsh and 4 tidal flat stations were determined in spring and winter situations, and compared to isotope ratios of organic matter sources within the estuary. Nematodes collected from fine sandy tidal flat sediments in late spring and during mild and sunny late winter weather had δ13C values consistent with microphytobenthos as their prime carbon source. Nematodes from a silty station and individuals sampled under cold and dark winter conditions had δ13C values intermediate between those of microalgae and particulate organic matter. The isotopic signatures of nematodes from the saltmarsh station were intermediate between those of microalgae, Spartina anglica-litter and particulate organic matter. An in situ H13CO3- spike experiment at 2 tidal flat stations corroborated the importance of microphytobenthos as a carbon source for nematodes, yet at the same time contradicted the hypothesis that direct grazing would be the main pathway of microalgal carbon to nematode consumers. A laboratory experiment adding 13C-labeled algal detritus to sediment microcosms demonstrated the nematodes¹ ability to rapidly utilize settling organic matter too. Incorporation of carbon from phytodetritus by subsurface nematodes in both enrichment experiments was high, but, compared to carbon utilization by surface individuals, showed time-lags largely consistent with sediment mixing rates. A combination of observed natural isotope signatures and experimental results suggests that tidal flat nematodes preferentially utilize labile organic carbon derived from microphytobenthos or settling phytoplankton; organic matter from terrestrial or riverine origin does not contribute significantly to the diet of nematodes at our study sites. KEY WORDS: Food web · Nematodes · Microphytobenthos · Phytoplankton · Intertidal estuarine 13C Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 234. Online publication date: June 03, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Particle Mixing by Freshwater Infaunal Bioirrigators: Midges (Chironomidae: Diptera) and Mayflies (Ephemeridae: Ephemeroptera)

Mechanisms and rates of sediment mixing by the infaunal chironomids Coelotanypus sp. and Chironomus sp., and the mayfly nymph, Hexagenia spp., were quantitatively investigated in the laboratory using a multiple 137Cs tracer layer microcosm technique. These insect larvae construct and irrigate burrows up to 10 cm beneath the sediment-water interface. Obliteration of tracer layers by these larvae was attributed to mixing by both diffusive (random smearing)—and feeding (directed particle motion)—style processes. Biodiffusion coefficients were obtained from the rate of widening of the tracer layers and feeding rates were obtained from the rate of decrease in activity of the tracer layers. The biodiffusion coefficients and feeding rates normalized by experimental population densities were used to compare the results of these experiments with those obtained in similar experiments using other organisms. The results suggest that of the organisms studied here, only Chironomus sp. mix sediments by biodiffusion at rates comparable to conveyor-belt deposit feeding oligochaete worms on a per individual basis. Individual rates of non-local mixing by feeding for the chironomid and mayfly larvae in these experiments are similar to or less than those reported for oligochaetes in similar experiments. These results indicate that although chironomid larvae and mayfly nymphs are burrow irrigators, they may also deposit-feed or at least rework sediments in a non-local fashion. Some of the differences in reported individual biodiffusion coefficients and feeding rates may be related to organism size, because larger organisms exhibited greater biodiffusion coefficients and feeding rates per individual. The values for the biodiffusion coefficients on a per individual basis were multiplied by approximate in situ population densities to evaluate the relative importance of each of these species at natural densities. The results indicate that the sparse populations of bivalves, some chironomids ( Coelotanypus sp.), and mayflies cause a relatively small amount of mixing compared to the more abundant amphipods, tubificid oligochaetes, and Chironomus sp. Surprisingly, the bioirrigator Chironomus sp. appears to be as important in mixing sediment as depositfeeding conveyor-belt tubificid oligochaetes; however, whereas sediment mixing by tubificid oligochaetes is primarily the result of feeding, sediment mixing by Chironomous sp. appears to be largely the result of burrow construction.

Influence of particle type and faunal activity on mixing of di(2-ethylhexyl)phthalate (DEHP) in natural sediments

The influence of faunal activity and particle type on sediment-mixing processes of the particle-bound organic contaminant di(2-ethylhexyl) phthalate (DEHP) were examined over a 48 d experimental period with natural box-cosm sediments. A dual-labelling radiotracer technique using 14 C-DEHP and 51 Cr determined the fate and utilization of DEHP associated with either sediment or phytoplankton particles. Particle-mixing was estimated using a 1-dimensional transient-state biodiffusion model and expressed as Db coefficients. Sediment 51 Cr depth profiles fitted this simple biodiffusion model well, indicating randomized, vertical particle mixing. Particle reworking rates were approximately 9-fold higher in faunated treatments compared to defaunated control sediments, thus quantifying the importance of benthic fauna as agents of physical transport. Biodiffusion rates in faunated box-cosms were significantly higher in phytoplankton compared to sediment 14 C-DEHP particle treatments (p < 0.05). This was attributed to an experimental urchin effect resulting from natural variations in the density of the large macrofaunal urchin Echinocardium cordatum between particle treatments. Sediment-mixing rates were independent of particle type when this urchin effect was removed. Examination of 14 C and 51 Cr relative depth profiles showed the 2 tracers to be coupled in defaunated sediments and decoupled with animals present. This effect was also independent of particle-type, indicating no selective transport associated with the particle-type treatments. The decoupling of tracers in faunated treatments, with 14C-DEHP having consistently higher depth-weighted mean values than the 51 Cr tracer, suggested a decrease in surface 14 C-DEHP concentrations through degradation processes at the sediment-water interface. Sediment-mixing rates increased significantly with increasing total community biomass, indicating that size (biomass) may be the single most important community parameter determining sediment-mixing intensity. Downward particle transport was strongly correlated with E cordatum abundance (p < 0.01), the dominant species in the benthic community in terms of size. E. cordatum densities also correlated strongly with the number of benthic infaunal species present in the box-cosms, with maximum infaunal species numbers occurring at intermediate E. cordata densities. This is in accordance with theories on intermediate disturbance. The 2 14 C-DEHP particle-type treatments had no significant effect on E. cordata body sizes or DEHP body burden. The large urchin E. cordatum clearly dominated sediment-mixing, and did so in a 'non-selective' manner.

Sedimentary fluxes of 90Sr, 137Cs, 239,240Pu and 210Pb in the East Sea (Sea of Japan)

Sediment cores collected from the deep basins of the East Sea (Sea of Japan) provide an ongoing and historical record of artificial radionuclides contamination into one of the most highly publicized radioactive waste dumping areas in the world ocean. The depth distributions of 90Sr, 137Cs and 239,240Pu in sediment cores were investigated with the aid of 210Pb-derived sediment accumulation and mixing rates in the deep basins of the East Sea (Sea of Japan). Five box core samples were collected from the northern Yamato Ridge, Korea Plateau, Ulleung and Japan Basins below 1000-m depth. Sediment inventories of 137Cs and 239,240Pu are inversely correlated with water depth and linearly correlated with sediment accumulation rates. The inventories of these nuclides are linearly correlated with the accumulation rates of organic carbon in sediments. The 238Pu/ 239,240Pu activity ratios in sediments are 0.036±0.009 suggesting that most of the Pu to the study area is derived from the global fallout. The activity ratios of 239,240Pu/ 137Cs, and 90Sr/ 137Cs in bottom sediments are much lower than those of global fallout due to the differences of particle affinity and biological uptake of these nuclides. Sediment inventories of 90Sr and 137Cs constitute <4% of the anticipated inventories from the global fallout, while those of 239,240Pu constitute 30–150% of the anticipated inventories from the global fallout. The residence time of the dissolved 239,240Pu in the study area is estimated to be 200–400 years based on the sediment inventory and/or sediment accumulation rate, and water column inventory.

Sediment mixing and accumulation in a mangrove ecosystem: Evidence from 210Pb, 234Th and 7Be.

210Pb, 234Th and 7Be activities were measured to establish sediment accumulation rates, estimate sediment mixing rates, and determine the depth of the sediment mixed layer in the Sepetiba Bay mangrove ecosystem near Rio de Janeiro City, Brazil. Three sediment cores were collected from Enseada das Garcas, a typical exposed tidal flat region with a sequence of sedimentary features. The seaward edge of this sequence is a mud flat with the landward portion covered with Spartina alterniflora followed by mangrove vegetation. An additional core was collected on an overwash island near Barra de Guaratiba, which is cover with mangroves without a mud flat or Spartina alterniflora sequence. Sediment accumulation rates were determined to range up to 1.8 cm/yr with the Spartina alterniflora having the maximum rate. Mixing rates were estimated for the Spartina alternifloracore at 40 cm2/yr based on 210Pb and 7Be from the upper mixed region of the core. The 234Th activity in this core suggested that either mixing or the input of 234Th were not in steady state. The sediment mixed region depth ranged from 4 cm to greater than 30 cm. At the Enseada das Garcas site the mixing depth decreased in the landward direction (i.e. mud flats > 30 cm, Spartina alterniflora 11 cm, mangroves 4 cm). Along with this decrease in sediment mixing depth was a shift from physical to biological mixing. The Barra de Guaratiba core had a sediment mixed layer of 13 cm as a result of physical and intense biological activity.