Mobility of iron in the estuaries of the Rhine and the Ems relevant to plant-availability problems

Studies were undertaken to obtain information on the iron mobilization processes in the sediments of the rivers Rhine and Ems, both located in western Europe. On their way from the fresh-water tidal area to the marine environment these sediments loose a considerable amount of their iron. The iron is released from the sediment by means of biodegradation products of the organic matter which dissolve the iron as organic iron complexes. The major functional groups of these organic compounds responsible for the iron mobilization are carboxyls and phenolic hydroxyls. From the sediments of the river Ems greater amounts of organic iron compounds are dissolved than from sediments of the river Rhine. Also fewer organic compounds are released from marine sediments than from fresh-water sediments, which indicates a diminished iron mobility in the marine area of the delta. Besides this the organic compounds from the marine sediments show an impoverishment in their functional groups. The sediments of the rivers Rhine and Ems are also distinguished by a different occupation of functional groups in the organic compounds. On account of a number of experiments the mobilization capacity of these river systems have been discussed.From a viewpoint of plant nutrition the mobility of iron in deposits of different ripening stages was also investigated.     

The distribution of macroinvertebrate assemblages in a reach of the River Allier (France), in relation to riverbed characteristics

Investigations on large canalised rivers, for example the Danube, have shown that transported particulate matter, which is typically inorganic, is predominantly deposited in waters near the river’s main channel. This investigation deals with the lower section of the River Havel (NE Germany), a canalised lowland river with a very flat floodplain. This river is highly polluted by nutrients from urban areas (Berlin) and a long chain of river lakes produces high concentrations of phytoplankton. Due to the high proportion of planktogenic detritus, it was hypothesised that greater quantities of nutrient-rich fine particulate organic matter (FPOM) would be deposited in floodplain waters located further from the main channel than has been reported for large rivers. The total nutrient, P-binding metal (Fe, Al, and Mn), organic and inorganic carbon (TOC, TIC) contents of the upper organic sediment layer (0–4 cm) were analysed in samples collected from 48 floodplain water and river sites. The sediment bulk density, calculated on the basis of dry mass content and loss on ignition, was used to characterise the waters according to the impact of the river current. The results showed that the variability of total phosphorus (TP) was best explained by the variability of total iron (TFe, R² = 0.52). The floodplain water sediments could clearly be separated into two groups on the basis of the sediment particle size composition, and of the element ratios TOC:TP, TN:TP, primarily TFe:TP. The sediments from impounded river sections and from mouth sections of backwaters (approx. 100–200 m) were characterised by a high proportion particles from the 0.1 to 0.5 mm size fraction and by homogeneous, low TFe:TP, TOC:TP and TN:TP ratios. Sediments from distal sections of backwaters and of oxbow lakes tended to exhibit high element ratios with much higher variability. These results were interpreted as a spatially limited impact of the river on the floodplain water sediments. Contrary to expectation, the phosphorus bound in river seston was predominantly and very homogeneously deposited in the impounded river and mouth sections of backwaters. This implies that the inundation of the floodplain waters during spring floods seems to have no important material impact on the sediments in waters of low hydrological connectivity with the River Havel.     

Influence of quantity and lability of sediment organic matter on the biomass of two isoetids,Littorella uniflora and Echinodorus repens

1. Despite real improvement in the water quality of many previously eutrophic lakes, the recovery of submerged vegetation has been poor. This lack of recovery is possibly caused by the accumulation of organic matter on the top layer of the sediment, which is produced under eutrophic conditions. Hence, our objective was to study the combined effects of quantity and lability of sediment organic matter on the biomass of Echinodorus repens and Littorella uniflora and on the force required to uproot plants of L. uniflora. 2. Lake sediments, rich in organic matter, were collected from four lakes, two with healthy populations of isoetids and two from which isoetids had disappeared. The four lake sediments were mixed with sand to prepare a range of experimental sediments that differed in quantity and lability of sediment organic matter. Two isoetid species, E. repens and L. uniflora, were grown in these sediments for 8 weeks. Sediment quality parameters, including elemental composition, nutrient availability and mineralisation rates, were determined on the raw sources of sediment from the lakes. Porewater and surface water were analysed for the chemical composition in all mixtures. At the end of the experiment, plants were harvested and their biomass, tissue nutrient concentration and (for L. uniflora) uprooting force were measured. 3. For both species, all plants survived and showed no signs of stress on all types of sediment. The biomass of E. repens increased as the fraction of organic matter was increased (from 6 to 39% of organic content, depending upon sediment type). However, in some of the sediment types, a higher fraction of organic matter led to a decline in biomass. The biomass of L. uniflora was less responsive to organic content and was decreased significantly only when the least labile sediment source was used to create the gradient of organic matter. The increase in shoot biomass for both species was closely related to higher CO₂ concentrations in the porewater of the sediment. The force required to uproot L. uniflora plants over a range of sediment organic matter fitted a Gaussian model; it reached a maximum at around 15% organic matter and declined significantly above that. 4. Increasing organic matter content of the sediment increased the biomass of isoetid plants, as the positive effects of higher CO₂ production outweighed the negative effects of low oxygen concentration in more (labile) organic sediments. However, sediment organic matter can adversely affect isoetid survival by promoting the uprooting of plants.     

Key role of streambed moisture and flash storms for microbial resistance and resilience to long‐term drought

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Factors controlling bacterial production in marine and freshwater sediments

We collected benthic bacterial production data measured by ³H thymidine incorporation (TTI) (25 studies), frequency of dividing cells (FDC) (3 studies), dark-C0₂ assimilation (1 study) and ³H-adenine uptake (2 studies) from the literature, which included 18 marine, 6 river, and 2 lake studies. In all of the studies that used the TTI method, ³H-DNA was isolated and incubations were carried out at in situ temperatures. Most of the researchers also determined ³H-DNA extraction efficiencies and isotope dilution, thus interpretable estimates of bacterial production were used in the analysis. In marine sediments, bacterial production rates were linked to bacterial biomass, bacterial abundance, sediment organic matter, temperature, and sediment chlorophyll a, with these variables explaining between 40% and 68% of the variation in production rates. Simple relationships between production and bacterial biomass or bacterial abundance, or between production and sediment organic matter, were improved by also including temperature in the analysis of marine sediments. Sediment organic matter explained an appreciable fraction (58%) of the observed production in freshwater sediments. Temperature was the most powerful predictor of the observed variability in specific growth rates (r ² = 0.48 and r ² = 0.58) in marine and freshwater sediments, respectively. Thus, bacterial production and specific growth rates are most closely linked to substrate supply and temperature in marine and freshwater sediments. Offprint requests to: B. C. Sander.     

典型冷暖季沉水植物凋落物分解特性及沉积物微生物群落变化

为研究湿地沉水植物腐败分解对水体的污染状况,选择典型沉水植物金鱼藻（暖季植物）和菹草（冷季植物）进行了为期60 d的凋落物分解实验。结果表明金鱼藻和菹草凋落物分解规律相似,0—15 d快速分解,15—60 d缓慢分解,60 d凋落物失重率分别达到60.43%和66.72%。菹草的有机物释放量明显高于金鱼藻,N和P释放量相反,分解释放的N主要是NH4+-N和有机氮。三维荧光光谱（Excitation-Emission Matrix Spectroscopy, EEMs）结合平行因子分析法解析出一种类色氨酸物质C2和3种类腐殖质物质C1、C3、C4,易降解的类色氨酸有机物先增加后减少,难降解的类富里酸和类腐殖酸有机物逐渐增加。EEMs和四种组分的最大荧光强度百分比表明,溶解性有机物（Dissolved organic matter, DOM）在0—15 d以易降解有机物为主,15—60 d以难降解有机物为主。两种植物凋落物分解释放的DOM含量及特性不同,整体上呈低腐殖化特征,可能是水中难降解DOM的一个重要来源。植物凋落物的分解促进了沉积物中微生物的丰富度,降低了微生物的多样性;参与分解的主要微生物包括4 d时的Pseudomonas属（26%—35%）、15 d和30 d时的Malikia属（>8%）和Bacillus属（2.6%—9%）,分解难降解有机物的微生物逐渐增加,如Flavobacterium属;沉积物中微生物群落结构的变化受营养物质可利用性变化的影响。分析发现植物凋落物分解对水质的影响具有阶段性,0—15 d,N和P释放量增加暂时导致了水质恶化;15—60 d,N和P释放量降低,难降解有机物含量逐渐增加,可能会加剧水体甚至是沉积物的腐殖化程度。因此,在植物衰亡期应及时打捞或者做好植物平衡收割管理,避免因植物大量腐败导致水质恶化。     

Geochemical partitioning and bioavailability of copper to aquatic plants in an artificial oxide-organic sediment

This study investigated the effects of competition between binding substrates (organic matter and iron oxide) and between metals (cadmium and copper), on the partitioning of sedimentary copper and its subsequent bioavailability to an aquatic plant. Organic matter and a synthesized iron oxide, ferrihydrite, were added singly and in combination to a series of sand sediments, which were then dosed with environmentally realistic concentrations of cadmium and copper and planted with rice,Oryza sativa. Organic matter controlled copper partitioning and bioavailability, whereas the synthetic ferrihydrite bound negligible amounts of either metal, even in the absence of organic matter. As organic matter concentrations increased, operationally-defined leachable copper decreased, organic-associated copper increased and the survival of rice plants improved in an approximately linear fashion. At a nominal starting copper concentration of 5.8 μg g dry wt⁻¹, plant survival after four weeks averaged 0–8% in sediments without organic matter, 25% in a sediment containing 0.18% organic matter and 58% in a sediment containing 0.36% organic matter. These results suggest that organic-associated forms of copper are unavailable to plants, and that the operational definition of ‘leachable’ copper (extracted with dilute ammonium acetate) adequately represents the species of copper that is (are) available to plants. Our study using a well-characterized artificial sediment supports the copper fractionation patterns and correlations between copper partitioning and bioavailability reported from the heterogeneous, poorly characterized sediments of natural lake and river sediments.     

Relative influence of bioturbation and predation on organic matter processing in river sediments: a microcosm experiment

1. Our objective was to measure the effects of bioturbation and predation on the physical characteristics and biogeochemical processes in river sediments. 2. We investigated the impacts of tubificid worms tested separately and together with an omnivore (Gammarus pulex), which does feed on tubificids, on sediment distribution, water flux, sediment organic carbon, biofilm biomass and microbial activities, and the concentrations of dissolved oxygen, dissolved organic carbon, PO, NO, NO and NH in slow filtration sand–gravel columns. We hypothesised that gammarids, which exploit the top 2–3 cm of the sediment, would modify the impact of worms at the sediment surface. 3. In experiments both with and without gammarids, bioturbation by the tubificids modified both the distribution of surface particles in the sediment column and water flux. In addition, microbial aerobic (oxygen consumption) and anaerobic (denitrification and fermentative decomposition of organic matter) processes in the sediment were stimulated in the presence of tubificid worms. However, G. pulex did not affect either the density or bioturbation activity of the tubificid worms. 4. Bioturbation by the benthos can be a major process in river habitats, contributing to the retention of organic matter in sediment dynamics. The presence of at least one predator had no effect on bioturbation in sediments. In such systems, physical heterogeneity may be sufficient for tubificids to escape from generalist predators, though more specialised ones might have more effect.     

Composition of organic matter in sediments facing a river estuary (Tyrrhenian Sea): relationships with bacteria and microphytobenthic biomass

The relationships between the biochemical composition of sediment organic matter and bacteria and microphytobenthic biomass distribution, were investigated along the coast of Northern Tuscany (Tyrrhenian Sea). Organic matter appeared to be of highly refractory composition. Among the three main biochemical classes, proteins were the major component (0.96 mg g^-1 sediment d.w.) followed by total carbohydrates (0.81 mg g^-1 sediment d.w.) and lipids (8.1 µg g^-1 sediment d.w.). Bacterial number in surface sediments (0–2 cm) ranged from 1.7 to 24.5 × 10⁸ cells g^-1 of sediment dry weight showing a strong decrease with sediment depth. In surface sediments, significant correlations were found between bacterial biomass and protein concentration. Bacterial activity (measured by the frequency of dividing cells) was significantly related to lipid concentration. Bacterial and microphytobenthic biomass accounted for 3.1 and 18.1% respectively of the sediment organic carbon. In surface sediments bacterial lipids accounted, on average, for 27 % of total lipids, whereas bacterial proteins and carbohydrates accounted for 2.5 and 0.5% of total proteins and carbohydrates, respectively.The benthic degradation process indicated that lipids were a highly degradable compound (about 35% in the top 10 cm). Carbohydrate decreased for 25.6% in the top 10 cm, whereas proteins increased with depth, thus indicating that this compound may resist to diagenetic decomposition.These data suggest that specific organic compounds need to be measured rather than bulk carbon and nitrogen measurements in order to relate microbial biomass to the quality of organic matter.     

The impact of the kelp (Laminaria hyperborea) forest on the organic matter content in sediment of the west coast of Norway

Sediment cores were collected from an area off the west coast of Norway, where a well-established kelp community made up mainly of Laminaria hyperborea is found. Chemical analyses of the sectioned sediment cores were made, which included organic carbon, carbohydrates and phenols. These were used to collate the sediments in order to establish whether the different sites had a common origin and if this was kelp-related. The organic matter content in the surface sediments appears to be related to the water depth, which determines the degree of perturbation at the sediment-water boundary and hence the sedimentation of the organic matter. The relation between carbohydrates and phenols in the sediment appears to indicate a common origin. However, the carbohydrate and phenol content in the sediment organic fraction appears to closely resemble (to be analogous) to that of the kelp Laminaria hyperborea. Although there are some diagenetic and decomposition changes in the subsurface (historical) organic matter (that may alter its findings), this study provides good evidence that the deposits were supplied by the kelp forest in the region. The sedimentation and export of kelp-derived material is of particular significance in terms of benthic ecology and production and may also play a significant role in the global carbon budget.     

Contrasted ecological niches shape fungal and prokaryotic community structure in mangroves sediments

Mangroves are forest ecosystems located at the interface between land and sea where sediments presented a variety of contrasted environmental conditions (i.e. oxic/anoxic, non-sulfidic/sulfidic, organic matter content) providing an ideal ecosystem to study microbial communities with niche differentiation and distinct community structures. In this work, prokaryotic and fungal compositions were investigated during both wet and dry seasons in New Caledonian mangrove sediments, from the surface to deeper horizons under the two most common tree species in this region (Avicennia marina and Rhizophora stylosa), using high-throughput sequencing. Our results showed that Bacteria and Archaea communities were mainly shaped by sediment depth while the fungal community was almost evenly distributed according to sediment depth, vegetation cover and season. A detailed analysis of prokaryotic and fungal phyla showed a dominance of Ascomycota over Basidiomycota whatever the compartment, while there was a clear shift in prokaryotic composition. Some prokaryotic phyla were enriched in surface layers such as Proteobacteria, Euryarchaeota while others were mostly associated with deeper layers as Chloroflexi, Bathyarchaeota, Aminicenantes. Our results highlight the importance of considering fungal and prokaryotic counterparts for a better understanding of the microbial succession involved in plant organic matter decomposition in tropical coastal sediments.     

The ecological significance of sulfur in the energy dynamics of salt marsh and coastal marine sediments

Sulfur is an important element in the metabolism of salt marshes and subtidal, coastal marine sediments because of its role as an electron acceptor, carrier, and donor. Sulfate is the major electron acceptor for respiration in anoxic marine sediments. Anoxic respiration becomes increasingly important in sediments as total respiration increases, and so sulfate reduction accounts for a higher percentage of total sediment respiration in sediments where total respiration is greater. Thus, sulfate accounts for 25% of total sediment respiration in nearshore sediments (200 m water depth or less) where total respiration rates are 0.1 to 0.3gCm^–1 day^–1 , for 50% to 70% in nearshore sediments with higher rates of total respiration (0.3 to 3gCm^–2 day^–1), and for 70% to 90% in salt marsh sediments where total sediment respiration rates are 2.5 to 5.5gcm^–2 day^–1 .During sulfate reduction, large amounts of energy from the respired organic matter are conserved in inorganic reduced sulfur compounds such as soluble sulfides, thiosulfate, elemental sulfur, iron monosulfides, and pyrite. Only a small percentage of the reduced sulfur formed during sulfate reduction is accreted in marine sediments and salt marshes. When these reduced sulfur compounds are oxidized, energy is released. Chemolithoautotrophic bacteria which catalyze these oxidations can use the energy of oxidation with efficiencies (the ratio of energy fixed in organic biomass to energy released in sulfur oxidation) of up to 21–37% to fix CO₂ and produce new organic biomass.Chemolithoautotrophic bacterial production may represent a significant new formation of organic matter in some marine sediments. In some sediments, chemolithoautotrophic bacterial production may even equal or exceed organoheterotrophic bacterial production. The combined cycle of anaerobic decomposition through sulfate reduction, energy conservation as reduced sulfur compounds; and chemolithoautotrophic production of new organic carbon serves to take relatively low-quality organic matter from throughout the sediments and concentrate the energy as living biomass in a discrete zone near the sediment surface where it can be readily grazed by animals.Contribution from a symposium on the role of sulfur in ecosystem processes held August 10, 1983, at the annual meeting of the A.I.B.S., Grand Forks, ND; Myron Mitchell, convenor.     

Massive uprooting of Littorella uniflora (L.) Asch. during a storm event and its relation to sediment and plant characteristics

During spring storms massive uprooting of Littorella uniflora occurred in a shallow Dutch softwater lake. The aim of this study was to test whether changes in plant morphology and sediment characteristics could explain the observed phenomenon. Uprooting was expected to occur in plants having a high shoot biomass and low root to shoot ratio (R:S), growing on sediments with a high organic matter content. Normally, uprooting of the relative buoyant L. uniflora is prevented by an extensive root system, expressed as a high R:S. This was studied by sampling floating and still rooted L. uniflora plants, as well as sediment and sediment pore water, along a gradient of increasing sediment organic matter content. Increasing organic matter content was related to increasing L. uniflora shoot biomass and consequently decreasing R:S. Furthermore, the results indicated that uprooting indeed occurred in plants growing on very organic sediments and was related to a low R:S. The increased shoot biomass on more organic sediments could be related to increased sediment pore water total inorganic carbon (TIC; mainly CO₂) availability. Additionally, increased phosphorus availability could also have played a role. The disappearance of L. uniflora might lead to higher nutrient availability in the sediments. It is suggested that this could eventually promote the expansion of faster‐growing macrophytes.     

Bacterial abundance and production in river sediments as related to the biochemical composition of particulate organic matter (POM)

The major proportion of heterotrophic activity in running waters islocalized on the solid surfaces of sediments in the benthic and hyporheic zoneand is dominated by microorganisms. However, this assertion is based on thestudies of small streams, and little is known about the microbial metabolism oforganic matter in river ecosystems. We therefore explored the relationshipsbetween bacterial abundance and production and the gradients of organic matterquality and quantity in sediments of a sixth-order lowland river (Spree,Germany). We found vertical gradients of detrital variables (particulateorganicmatter (POM), particulate organic carbon (POC), nitrogen (PN), and protein) andof bacterial variables (abundance, production, turnover time, and proportion ofbacterial carbon in total POC) in two different sediment types. These gradientswere steeper in stratified sediments than in the shifting sediments. Detritalvariables correlated strongly with bacterial abundance and production. The bestcorrelation was found for detrital variables indicating substrate quantity andquality (r_S = 0.90 for PN with abundance). Although bacterialbiomasscomprised only 0.7% of the POC (1.9% of PN, 3.4% of the protein) in sediments,the turnover of sedimentary organic carbon was fast (median = 62d), especially in the shifting sediments. Our findings demonstratethat sediment dynamics significantly foster organic carbon metabolism in riversystems. Thus, these sediments, which are typical for lowland rivers, stronglyinfluence the metabolism of the whole ecosystem.     

Removal of agricultural non-point source pollutants by ditch wetlands: implications for lake eutrophication control

Ditches grown with nature reed (Phragmites communis Trin) and wild rice (Zizania latifolia Turcz) were selected to study the removal capacity of agricultural non-point source pollutants so as to find a way to alleviate eutrophication in Lake Taihu. Ditches sediment from depths below 40 cm can accumulate organic matter and total nitrogen (TN). TN is correlated positively to organic matter in reed populated sediment and wild rice populated sediment. This suggests that the main composition of TN is organic nitrogen derived from plant decomposition. A significant negative relationship between TN and pH was found in reed and wild rice sediments. Seasonal harvest of helophyte vegetation is an effective method to remove N and P from wetlands. Organic matter and TN concentrations in water and sediments (0–20 cm) in areas where reeds were removed are lower than non-harvested areas (control). Reeds and wild rice have high uptake ability of nitrogen (N) and phosphorus (P). However, the low economic value of these plants will not stimulate voluntary harvest of farmers. Zizania caduciflora Turez Hand-mazt is a kind of vegetable widely cultivated in ditches around the lake. It can also absorb N and P effectively. Thus, large scale cultivation of Z. caduciflora to replace nature plants may improve water quality.     

Polychaete distribution on the southwestern Atlantic continental shelf

Diversity and distribution of polychaetes on the southeastern Brazilian Shelf (SBS) adjacent to the city of Santos were studied. The study area is situated in the vicinity of Santos Bay with its estuarine complex and other small rivers along the coast. A total of 16,274 specimens belonging to 214 polychaete species were found at 21 sampling stations during two different periods. Two communities occur along the shelf and were related firstly to sediment characteristics. Prionospio dayi (Spionidae) was dominant in the inner shelf community that was characterized by well-sorted, very fine sand. The outer shelf community was found in muddy sediments with high organic matter content and was dominated by subsurface deposit feeders, like Petersenaspis capillata (Sternaspidae) and Leitoscoloplos kerguelensis (Orbiniidae), and by large carnivores such as Aglaophamus sp. (Nephtyidae) and Sigambra sp. (Pilargidae). A transition zone of mixed sediments presenting high diversity and richness was found between these communities. The characteristics of bottom water and food quality in the sediment were important to divide the inner shelf into two sub-areas. Nearshore sampling stations (A group) were more influenced by freshwater input, mainly during summer. This influence is not conspicuous at stations close to 30?m depth (B group). However the effect of South Atlantic Central Water mass (SACW) intrusion on the benthic system was observed on the inner shelf – B group during the summer. The summer intrusion of SACW onto the inner shelf may explain the higher quality of organic matter that resulted in an increase in polychaete abundance and richness.     

Flow regulation by dams affects ecosystem metabolism in Mediterranean rivers

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Temperature response of litter and soil organic matter decomposition is determined by chemical composition of organic material

The global soil carbon pool is approximately three times larger than the contemporary atmospheric pool, therefore even minor changes to its integrity may have major implications for atmospheric CO₂ concentrations. While theory predicts that the chemical composition of organic matter should constitute a master control on the temperature response of its decomposition, this relationship has not yet been fully demonstrated. We used laboratory incubations of forest soil organic matter (SOM) and fresh litter material together with NMR spectroscopy to make this connection between organic chemical composition and temperature sensitivity of decomposition. Temperature response of decomposition in both fresh litter and SOM was directly related to the chemical composition of the constituent organic matter, explaining 90% and 70% of the variance in Q₁₀ in litter and SOM, respectively. The Q₁₀ of litter decreased with increasing proportions of aromatic and O‐aromatic compounds, and increased with increased contents of alkyl‐ and O‐alkyl carbons. In contrast, in SOM, decomposition was affected only by carbonyl compounds. To reveal why a certain group of organic chemical compounds affected the temperature sensitivity of organic matter decomposition in litter and SOM, a more detailed characterization of the ¹³C aromatic region using Heteronuclear Single Quantum Coherence (HSQC) was conducted. The results revealed considerable differences in the aromatic region between litter and SOM. This suggests that the correlation between chemical composition of organic matter and the temperature response of decomposition differed between litter and SOM. The temperature response of soil decomposition processes can thus be described by the chemical composition of its constituent organic matter, this paves the way for improved ecosystem modeling of biosphere feedbacks under a changing climate.     

Differences in Sediment Organic Matter Composition and PAH Weathering between Non-Vegetated and Recently Vegetated Fuel Oiled Sediments

We examined polycyclic aromatic hydrocarbon (PAH) attenuation in contaminated field sediments after only 2 years of plant growth. We collected sediments from vegetated and non-vegetated areas at the Indiana Harbor Canal (IHC), an industrialized area with historic petroleum contamination of soils and sediments. PAH concentrations, PAH weathering indices, and organic matter composition in sediments colonized by Phragmites, cattails, or willow trees were compared to the same indices for non-vegetated sediments. We hypothesized that bulk sediment and humin fractions with measurable increases in plant organic matter content would show measurable changes to PAH attenuation as indicated by more weathered PAH diagnostic ratios or reduced PAH concentrations. Carbon-normalized PAH concentrations were lower in vegetated bulk sediments but higher in vegetated humin fractions relative to non-vegetated sediment fractions. Total organic carbon content was not indicative of more weathered N₃/P₂ ratios or reduced PAH concentrations in vegetated sediment fractions. More weathered N₃/P₂ ratios were observed with increased modern carbon (plant carbon) content of vegetated sediment fractions. Phragmites sediments contained more modern carbon (plant carbon) and more weathered PAH ratios [C₃-naphthalenes and C₂-phenanthrenes (N₃/P₂)] than willow, cattail, and non-vegetated sediments.