Regeneration of inorganic phosphorus and nitrogen from decomposition of seston in a freshwater sediment

The mineralization of phosphorus and nitrogen from seston was studied in consolidated sediment from the shallow Lake Arreskov (July and November) and in suspensions without sediment (July). In the suspension experiment, phosphorus and nitrogen were mineralized in the same proportions as they occurred in the seston. During the 30 days suspension experiment, 47 and 43% of the particulate phosphorus and nitrogen, respectively, was mineralized with constant rates.Addition of seston to the sediment had an immediate enhancing effect on oxygen uptake, phosphate and ammonia release, whereas nitrate release decreased due to denitrification. The enhanced rates lasted for 2–5 weeks, while the decrease in nitrate release persisted throughout the experiment. The increase in oxygen uptake (equivalent to 21% of the seston carbon) was, however, only observed in the July experiment. The release of phosphorus and nitrogen from seston decomposing on the sediment surface differed from the suspension experiments. Thus, between 91 and 111% of the phosphorus in the seston was released during the experiments. Due to opposite directed effects on ammonium and nitrate release, the resulting net release of nitrogen was relatively low.A comparison of C/N/P ratios in seston, sediment and flux rates indicated that nitrogen was mineralized faster than phosphorus and carbon. Some of this nitrogen was lost through denitrification and therefore not measurable in the flux of inorganic nitrogen ions. This investigation also suggests that decomposition of newly settled organic matter in sediments have indirect effects on sediment-water exchanges (e.g. by changing of redox potentials and stimulation of denitrification) that modifies the release of mineralized phosphate and nitrogen from the sediment.     

Paleolimnological evaluation of historical trophic state conditions in hypereutrophic Lake Thonotosassa, Florida, USA

We used paleolimnological methods to evaluate historical water quality in Lake Thonotosassa, Hillsborough County, Florida, USA. Sediment mapping shows that organic deposits are unevenly distributed in the lake. Two short (<130 cm) sediment cores from the depositional zone were analyzed for radioisotopes (²¹⁰Pb, ²²⁶Ra, and ¹³⁷Cs), bulk density, organic matter concentration, nutrients (C,N,P), and diatoms. ²¹⁰Pb results indicate that the profiles represent > 100 years of sediment accumulation. There is an abrupt change in sediment composition at about the turn of the century (80 cm depth), above which bulk density decreases and concentrations of organic matter, total C, total N, total P, and ²²⁶Ra activity increase. Diatom-based reconstructions of historical water-column trophic conditions indicate progressive nutrient enrichment in the lake during the past 100 years. Stratigraphic changes in diatom assemblages suggest that anthropogenic nutrient loading converted Lake Thonotosassa from a naturally eutrophic system to a hypereutrophic waterbody after 1900. Given the edaphic setting of Lake Thonotosassa, efforts to mitigate recent anthropogenic impacts will, at best, yield the eutrophic conditions that characterized the lake prior to human disturbance. This study illustrates the importance of paleolimnological data for targeting realistic water quality conditions when lake restoration is contemplated.Journal Series No. R-05019 of the Florida Agricultural Experiment Station     

武汉东湖颗粒悬浮物的结构与元素组成

本研究于1989-1990对武汉东湖营养水平不同的二个湖区的颗粒悬浮物的干物质结构和元素组成进行了分析。综合平均值表明,浮游动物的现存量约为浮游植物的1/4,浮游动物群落以小型的原生动物和轮虫占优势。从年平均值来看,浮游生物的干重占颗粒悬浮物干物质的2.5-7.6%,浮游生物碳量占颗粒悬浮物碳量的4.0-9.8%;颗粒悬浮物的碳/氮比与一般浮游植物的比值相似,但明显大于多数浮游动物;颗粒悬浮物的碳与干物重之比约为一般浮游生物的3/4;颗粒悬浮物的灰分含量约为45%,显着高于除硅藻以外的其它浮游生物。从数量上来看,有机碎屑是东湖生态系统颗粒悬浮物最重要的组成部分,而活体浮游生物只占颗粒悬浮物很小的一部分(<10%);这意味着在东湖来自以浮游植物为核心的食物网的有机碎屑的形成速率显着大于有机碎屑的矿化速率。       

Microbial sulfate reduction in a brackish meromictic steppe lake

Patterns of sulfate reduction were studied in water and sediments of Lake Shira, South Siberia, Russia. The lake was characterized by a high level of sulfate (91-116 mM). The concentration of hydrogen sulfide in the anoxic waters of the lake reached 0.6 mM. In summer the sulfate reduction rate in the water column, measured by radiometric technique, varied from 0.25 to 9.81 mol sulfate l^-1 d^-1. There were two peaks of sulfate reduction activity: just below the chemocline and near the sediment surface. Sulfate reduction rate in the profundal silts ranged from 4.1 to 90.6 mol l^-1 d^-1. The zone of the most active sulfate reduction was restricted to the surface sediment layers. The acceleration of sulfate reduction rate (up to 236 mol l^-1 d^-1) and the increase of density of viable sulfate reducers (up to 2 x 10⁵ cells ml^-1) were recorded in the littoral sediments adjacent to the mouth of the Son River and sewage discharge. It was apparently caused by the input of allochthonous organic substrates and also by a high environmental temperature. On an areal basis, sulfate reduction rate in the water was approximately 8 times higher than that in the profundal sediments. Sulfate reduction was the most important process of anaerobic oxidation of organic carbon in Lake Shira. In summer in the profundal zone of the lake, sulfate reducers were able to mineralize about 67% of the daily integrated primary production of phototrophic and chemotrophic organisms.     

Orthophosphate turnover in East African lakes

Summary Turnover rates of ³²P–PO₄ and concentrations of orthophosphate as soluble reactive phosphorus (SRP) were measured in five East African waters. Rapid incorporation of ³²P–PO₄ by the seston and orthophosphate concentrations below the limit of detectibility were found in Lakes Elmenteita, Naivasha, and Naivasha Crater Lake. Turnover was slow and orthophosphate concentration high in both Lake Nakuru and the Crescent Island Crater basin of Lake Naivasha. Further experiments in Lake Nakuru indicated that colloidal binding of orthophosphate was limited and that particles retained by an 8.0 filter incorporated 66% as much tracer as particles retained by a 0.1 filter. These experiments strengthen our conclusion that a large quantity of orthophosphate is available for algal use in Lake Nakuru.     

Biogeochemical changes in the sediments of Lake Cantara South,a saline lake in South Australia

Biogeochemical studies were undertaken of a 65-cm long sediment core from Lake Cantara South, South Australia. ¹⁴C determinations indicated that the sediments had been deposited over 2000 years. Changes with sediment depth in the concentration or ratio of the following were determined: (i) total organic carbon, total carbonate (inorganic) carbon, total sulfur, total carbon, total inorganic and organic sulfur, atomic C/N, and sulfate/chloride; (ii) n-alkanes; (iii) a highly branched isoprenoid alkane, and (iv) steroids. Interpretation of the changes with sediment depth indicated the nature of changes that took place when the system changed from a protected marine lagoon to an isolated (athalassic) saline lake. This change took place about 1000 years ago.     

Bacterial Diversity among the Sediments of Glacial Lakes in the Western Balkans: Exploring the Impact of Human Population

16S rRNA gene-based metagenomic approach was used to assess the biodiversity of bacterial communities in the sediments of selected glacial lakes in the Western Balkans and to assess the impact of human population on these microbial communities. Sediment samples were collected from three glacial lakes, viz., Plav Lake (in a zone of the highest impact of human population), Black Lake (a zone of medium impact of human population), and Donje Bare Lake (a remote lake with minimal impact of human population).

Canonical correlation analysis analysis indicated correlation between the distance of the lake from urbanized population and bacterial diversity in Donje Bare Lake sediment. Bacterial diversity of Black Lake sediment was correlated with high content of phosphorous and pH value. Chemical compounds exhibiting the most prominent correlation with bacterial diversity of Plav Lake were NH₄-N, K₂O, CaCo₃, and total nitrogen . Additionally, CCA analysis indicated that population density was correlated with biodiversity of bacterial communities in Plav Lake sediment, which is the most exposed to human population. Multivariate regression revealed the highest correlation between the presence of Proteobacteria classes and population density and levels of NH₄-N.

The influence of human population was observed to be important for shaping the sediment communities in addition to biological and chemical factors.     

A comparison of the fatty acid composition of Gammarus lacustris and its food sources from a freshwater reservoir, Bugach, and the saline Lake Shira in Siberia, Russia

We studied fatty acid (FA) composition in samples from bodies and intestinal contents of the littoral amphipod Gammarus lacustris Sars, from the Bugach freshwater reservoir. Simultaneously, samples of seston and bottom sediments were also collected from the reservoir during early August. There were no differences in FA composition of gut contents, seston and sediments of pebbly bottom. Seston was the main food source of Gammarus but some FAs Gammarus got from sediments. The FA composition of G. lacustris and seston from the Bugach freshwater reservoir were compared with those of the animals from the saltwater Lake Shira (Siberia). While FA composition of the two Gammarus populations differed significantly, those of seston were practically similar: the composition of long-chain unsaturated fatty acids, 20:53, 22:63 and 20:46, were significantly higher in animals from saline Shira Lake, whereas 16:1 and 16:0 were higher in the freshwater populations of amphipods from the Bugach freshwater reservoir. Taking into account the relevant literature data, we hypothesise that this difference in C16 acid might be a distinguishing characteristic of FA composition of freshwater and saltwater crustaceans.     

The cycling of nutrients in a closed-basin antarctic lake: Lake Vanda

Lake Vanda is a permanently ice covered, meromictic, closed basin lake, located in the Dry Valley region of Southern Victoria Land, Antarctica. A unique feature of the lake water column structure is that the bottom lake waters exist as a natural diffusion cell. The diffusive nature of these waters allows rates of sulfate reduction, nitrification and denitrification to be calculated from nutrient concentration gradients. Calculation reveals that sulfate reduction is by far the most important anoxic process acting to oxidize organic material. In addition, rate calculations reveal that bottom water nutrient profiles are in steady state. One argument in support of this conclusion is that the calculated rate of nitrification balances the flux of ammonia from the anoxic lake waters. The flux of phosphorus from the reducing waters is several times less than would be predicted from the nitrogen and phosphorus content of decomposing lake seston. Solubility calculations show that phosphorus may be actively removed at depth in Lake Vanda by the formation of hydroxyapatite. It is found that estimated rates of nitrogen and phosphorus removal in the bottom lake waters and sediments roughly balance the riverine input flux. This suggests that throughout the lake a nutrient steady state may exist, and that the anoxic zone may be the most important loci for nutrient removal. Finally, the ratio of nitrogen to phosphorus entering Lake Vanda by riverine input is less than the Redfield ratio of 16/1; in contrast to the lake waters which are strongly phosphorus limited at all depths. This curious aspect of the lake's nutrient chemistry is explained by the presence of preformed nitrogen, which has been concentrated in the deep brine due to several episodes of evaporative concentration.     

A comparison of different methods used for the quantitative evaluation of biomass of freshwater phytoplankton

Although in a strict sense the term phytoplankton biomass only refers to living algal material, in aquatic ecology the term has been associated with a variety of biological and biochemical procedures used to quantify the particulate matter suspended in natural waters. Relative merits of different biomass characteristics have been studied in three Dutch freshwater lakes with great differences in absolute biomass. Parallel determinations have been made of seston dry weight and supplementary elementary and caloric analyses of seston, of chlorophyll-a concentration and supplementary paper chromatographic analyses of pigment extracts, of particle concentration and particle size distribution as studied with an electronic particle counter, and of phytoplankton cell volume as calculated from the results of microscopic enumeration and sizing of algae. In this way an attempt was made to create a detailed picture of the nature of the seston of the three freshwater lakes.Different analytical techniques give strikingly different information, the accuracy of any method is largely dependent on the circumstances present, and different biomass characteristics therefore are only of value in limited spheres. It is suggested to distinguish between total seston characteristics (e.g. seston dry weight, particulate organic carbon, total particle volume) and strictly algological biomass characteristics (e.g. chlorophyll-a concentration, phytoplankton cell volume). The pattern of growth of phytoplankton populations shown by e.g. chlorophyll-a concentration may differ markedly from that indicated by e.g. total particle volume or seston dry weight. Also, to more or less extent the wax and wane of phytoplankton populations may go undetected among the total seston. Apparently, there is no one method of estimating biomass and no conversion factor that may serve for general purposes. In general, unambiguous information on the nature of the seston of natural waters may only be obtained by estimating total seston characteristics and algological biomass characteristics simultaneously. Depending on the objective of the investigation supplementary component analyses should be carried out to guarantee the correct interpretation of the results.     

The influence of particle size distribution and composition on seasonal sedimentation rates in a temperate lake

To evaluate the effect of particle size and composition on seasonal changes in the sedimentation rate, suspended and sedimenting particles were collected from Trout Lake, Wisconsin, USA during the 2002 ice-free season. Particles were characterized with regard to particulate biogenic silicon (PBSi) concentration and distribution between four size classes. The concentration of particulate chlorophyll and chlorophyll degradation products was also measured in water column particles and sediment trap material collected during the first half of the sampling period. The highest rates of mass sedimentation during the stratified period were measured in spring and early summer. Seasonal increases in sedimentation are related to the presence of large colonial diatoms in the water column as indicated by elevated PBSi concentrations. The majority of particulate matter in the water column was found in the smallest (<20 m) size fraction while most of the mass of sediment trap material was in larger size fractions (>20 m). Smaller cells appear to carry out most of the photosynthesis in Trout Lake but larger cells are responsible for seasonal trends in sedimentation. These results may explain how pelagic photosynthesis and sedimentation rates can be quantitatively decoupled across a range of trophic conditions but appear correlated when based on mid-summer measurements.     

Sediment cores from shallow lakes preserve reliable,informative paleoenvironmental archives despite hurricane-force winds

Comparison of sediment cores collected in 1999 and 2013 from shallow Lake Harris, Florida, USA showed that the sediment maintained stratigraphic integrity despite multiple hurricanes passing through the area in 2004. Sediments less than 50 years old displayed small losses of organic matter (OM), total phosphorus (TP) and heat-extractable (HE-P) through time that are unrelated to hurricane impacts. Nevertheless, sediment that accumulated between the two core collection dates contained 4-fold more TP than was lost from the sediment over the 14-year period, indicating that sediments in Lake Harris are a net sink for water-column TP. Persistent, elevated ¹³⁷Cs activity in sediments deposited since the mid-1960s indicates protracted cesium input to the lake from this subtropical watershed. There is also evidence for downward transport of ¹³⁷Cs in the sediment profile between core collection dates. Prolonged export of ¹³⁷Cs from the watershed and downward mobility of the radionuclide in the sediment profile diminish the utility of ¹³⁷Cs as a dating tool in Lake Harris.     

Cesium-137 sediment depth profiles and inventories in Adirondack Lake sediments

Depth distributions and inventories of¹³⁷Cs (mCi km²) were determined in sediment from several fresh water lakes in the New York State Adirondack Preserve. Included were Big Moose and Darts Lakes, part of the North Branch of the Moose River system, as well as North, Sagamore, South, and Woods Lakes and the seepage pond, Tamarack Lake. Comparisons were made between the¹³⁷Cs inventories in these lakes and large inpoundments in the Adirondacks (Hinkley, Great Sacandaga, Stillwater and Cranberry Lake Reservoirs) and other large impoundments and lakes located in various regions of the U.S., especially Cayuga Lake, Ithaca, NY.None of the Adirondack Lakes had¹³⁷Cs distributions with depth in sediment that closely resembled the deposition pattern of weapons testing as a function of time. All of the natural lakes and small impoundments, including the seepage pond, were found to have significantly lower inventories of¹³⁷Cs than expected; while the large reservoirs were generally enhanced in¹³⁷Cs. We suggest that more than one mechanism may be responsible for the low sediment inventories: for the majority of lakes, flushing of¹³⁷Cs out of the lakes during periods of thermal stratification and ice thaw; and for the seepage pond, remobilization of¹³⁷Cs into the water column due to biological recycling.     

Biogeochemical cycling of sulfur and iron in sediments of a south-east Asian mangrove,Phuket Island,Thailand

Benthic sulfate reduction and sediment pools of sulfur and iron were examined during January 1992 at 3 stations in the Ao Nam Bor mangrove, Phuket, Thailand. Patterns of sulfate reduction rates (0–53 cm) reflected differences in physical and biological conditions at the 3 stations, and highest rates were found at the vegetated site within the mangrove (Rhizophora apiculata) forest. Due to extended oxidation of mangrove sediments, a large portion of the added³⁵S-label was recovered in the chromium reducible pools (FeS₂ and S⁰) (41–91% of the reduced sulfur). Pyrite was the most important inorganic sulfur component, attaining pool sizes 50–100 times higher than acid volatile pools (FeS). HCl-extractable (0.5 M HCl) iron pools, including Fe(II)_HCl and Fe(III)_HCl, were generally low and Fe(III)_HCl was only present in the upper surface layers (0–5 cm). Maximum concentrations of dissolved Fe²⁺ (35–285 M) occurred just about the depth where dissolved H₂S accumulated. Furthermore Fe²⁺ and H₂S coexisted only where concentrations of both were low. There was an accumulation of organic sulfur in the deep sediment at 2 stations in the inner part of the mangrove. The reoxidation of reduced sulfides was rapid, and storage of sulfur was minor in the upper sediment layers, where factors like bioturbation, the presence of roots, or tidal mixing enhance oxidation processes.Author of correspondence.     

The sediment column as a record of trophic status: examples from Bosherston Lakes,SW Wales

Bosherston Lakes are a series of interconnected, mesotrophic to hypereutrophic, artificially-created coastal marl lakes in Dyfed, South West Wales. Progressive eutrophication of the lake system has been produced by a high external phosphorus loading which includes phosphorus-rich effluent from a sewage treatment works (STW) in the catchment of the Lakes.Cores were taken from four sites of varying eutrophic status within the Lakes. In the surface sediment layer, organic C, N and P concentrations generally correlate directly with trophic status and reflect distance from the source of P input. At one site, sediment stratigraphy records a clear transition at 20–15 cm depth, marked by a sharp upward increase in porosity, organic C, N, and P, and iron-associated-P; decreases in organic matter C/N, C/P and N/P ratios; a sharp decrease in carbonate, and a change in the subfossil diatom assemblage. Lead-210 dating indicates that this change occurred in the period 1919 to 1938.The diatom stratigraphy and sediment geochemistry suggest that this transition reflects an increase in trophic status at this site, probably as a result of the influx of nutrient-rich water. This took place when the management of the Stackpole estate surrounding the lake system, fell into decline during the period 1919–1938.     

Variation in stable isotope signatures of seston and a zooplanktivorous fish in a eutrophic Chinese lake

Temporal and spatial changes in ¹³C and ¹⁵N of seston (mainly phytoplankton) and isotopic relationship between seston and the lake anchovy (Coilia ectenes) were studied in the large eutrophic freshwater Lake Chaohu in China. Much of the spatial and temporal variation in ¹³C of lake anchovies was explained by variation in seston, indicating a strong link between pelagic primary production and higher order consumers. Because the lake is shallow, there were no significant differences in ¹³C and ¹⁵N of seston between surface and overlying waters. Spatially, the relatively high ¹³C and ¹⁵N of seston in the western part of the lake might be due to high levels of anthropogenically derived N and C introduced from the surrounding cities through sewage drainage systems. The trophic position of the lake anchovy in the food web of Lake Chaohu was estimated to be 2.9–4.1 (3.5 ± 0.4), which agrees well with the previous stomach content analysis suggesting that the lake anchovy fed both on zooplankton and small planktivorous fishes.     

Distribution of iron in Lake Banyoles in relation to the ecology of purple and green sulfur bacteria

The distribution of iron both in suspended sediment and in the water column has been studied during summer stratification in Lake Banyoles. In this lake, near bottom springs, a very fine material suspended sediment remains in suspension. Dissolved Fe²⁺ in interstitial water of this suspended sediment, is related to redox potential and to the bottom water inflow. In the water column, soluble iron is present in the hypolimnion of the six different basins forming Lake Banyoles. Under those conditions Fe²⁺ is partially removed by sulfide produced in the anoxic sediment. In addition, a peak of Fe²⁺ found at the density gradient level in basins C-III, C-IV and C-VI. A three compartment model on the dynamics of the processes involving iron in Lake Banyoles is proposed. The bottom springs supply oxygen to the anoxic hypolimnion affecting chemical processes of the iron cycle. The presence of phototrophic sulfur bacteria in the anoxic monimolimnion of basins C-III and C-IV can be related to the kinetics of Fe²⁺ and sulfide. In C-III sulfide concentration exceeds Fe²⁺ concentration whereas in C-IV sulfide is not detectable and iron reached values up to 60 mM. The presence of phototrophic sulfur bacteria in iron-containing environments with no detectable sulfide is explained by the ability of such microorganisms to use FeS as electron donor instead of H₂S.     

Organic matter processing by a stream-segment ecosystem: Fort River,Massachusetts, U.S.A.

An annual organic matter budget for a 1700 m segment of Fort River (Massachusetts, USA) is presented. Primary production in this fourth order stream exceeds litter input annually, however ecosystem P/R is 0.5. Respiration in excess of gross primary production is supported by allochthonous organic matter imported from upstream reaches. The relative contribution of organic matter size fractions to stream consumers depends upon biologic lability, rate of input, and residence time in the ecosystem. Particles of seston size (1 μm to 1 mm) are most heavily used by consumers, however dissolved organic matter represents the largest input component. Microorganisms are the predominant consumers in this soft-water, nutrient-poor stream ecosystem. A conceptual model for assessing the processing efficiency of stream ecosystems is presented and discussed in terms of several headwater to estuary gradients.     

Sulfur dynamics in mineral horizons of two northern hardwood soils. A column study with 35S

Sulfur dynamics of two Spodosols were ascertained using soil columns constructed from homogenized mineral soil from nothern hardwood ecosystems at the Huntington Forest (HF) in the Adirondack Mountains of New York and Bear Brook Watershed in Maine (BBWM). Columns were leached for 20 weeks with a simulated throughfall solution with³⁵SO₄ ^2-. Sulfur constituents were similar to those of other Spodosols, with the organic S fractions (C-bonded S and ester sulfate) constituting over 90% of total S. HF soil columns had higher total S (14.9 mol S g^-1) than that for the BBWM soil columns (7.4 mol g^-1) primarily due to higher C-bonded S in the former.Initially, adsorbed SO₄ ^- accounted for 5 and 4% of total S for the BBWM and HF soil columns, respectively. After 20 weeks, adsorbed SO₄ ^2- decreased (81%) in BBWM and increased (33%) in HF soil columns. For both HF and BBWM soil columns, C-bonded S increased and ester sulfate decreased, but only for HF columns was there a net mineralization of organic S (5.6% of total S). The greatest decrease in ester sulfate occurred at the top of the columns.Leaching of³⁵S was less than 0.5% of the³⁵S added due to its retention in various S constituents. There was an exponential decrease in³⁵S with column depth and most of the radioisotope was found in C-bonded S (70–88 and 70–91% for BBWM and HF, respectively). The rapid turnover of adsorbed SO^2- ₄ was reflected in its high specific activity (834 and 26 kBq mol^-1 S for BBWM and HF, respectively). The lower specific activity of adsorbed SO₄ ^2- in HF was attributable to greater isotopic dilution by non-radioactive SO^2- ₄ derived from greater organic S mineralization in the HF versus the BBWM columns.Both soil columns initially had high levels of NO^- ₃ which resulted in the generation of H⁺ and net retention of SO₄ ^2- in the early phase of the experiment due to pH dependent sulfate adsorption; later NO₃ ^- decreased and SO₄ ^2- was desorbed. Leaching of NIO₃ ^- and SO₄ ^2- was correlated with losses of Mg²⁺ and Ca²⁺ of which the latter was the dominant cation.Analyses using both S mass balances and radioisotopes corroborate that for BBWM soil columns, SO^2- ₄ adsorption-desorption dominated the S biogeochemistry while in HF soil columns, organic S mineralization-immobilization processes were more important. It is suggested that similar techniques can be applied to soils in the field to ascertain the relative importances of SO₄ ^2- adsorption processes and organic S dynamics.     

Widespread Distribution of <Emphasis Type="Italic">Dehalococcoides mccartyi</Emphasis> in the Houston Ship Channel and Galveston Bay,Texas, Sediments and the Potential for Reductive Dechlorination of PCDD/F in an Estuarine Environment

Sediments in the Houston Ship Channel and upper Galveston Bay, Texas, USA, are polluted with polychlorinated dibenzo-p-dioxins/furans (PCDD/F; ≤46,000 ng/kg dry weight (wt.)) with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic congener, contributing >50 % of the total toxic equivalents (TEQ) at most locations. We measured PCDD/F concentrations in sediments and evaluated the potential for enhanced in situ biodegradation by surveying for Dehalococcoides mccartyi, an obligate organohalide respiring bacterium. Dehalococcoides spp. (98 % similar to D. mccartyi) and 22 other members of the class Dehalococcoidia were predominant 16S ribosomal RNA (rRNA) phylotypes. Dehalococcoides spp. were also present in the active fraction of the bacterial community. Presence/absence PCR screening detected D. mccartyi in sediment cores and sediment grab samples having at least 1 ng/kg dry wt. TEQ at salinities ranging from 0.6 to 19.5 PSU, indicating that they are widespread in the estuarine environment. Organic carbon-only and organic carbon + sulfate-amended sediment microcosm experiments resulted in ～60 % reduction of ambient 2,3,7,8-TCDD in just 24 months leading to reductions in total TEQs by 38.4 and 45.0 %, respectively, indicating that 2,3,7,8-TCDD degradation is occurring at appreciable rates.