Dissimilatory iron reduction contributes to anaerobic mineralization of sediment in a shallow transboundary lake.

Dissimilatory iron reduction (DIR) coupled with carbon cycling is increasingly being recognized as an influential process in freshwater wetland soils and sediments. The role of DIR in organic matter (OM) mineralization, however, is still largely unknown in lake sediment environments. In this study, we clarified rates and pathways of OM mineralization in two shallow lakes with seasonal hydrological connectivity and different eutrophic situations. We found that in comparison with the domination of DIR (55%) for OM mineralization in Lake Xiaoxingkai, the contribution of methanogenesis was much higher (68%) in its connected lake (Lake Xingkai). The differences in rates and pathways of sediment OM mineralization between the two lakes were attributed to higher concentrations of carbonate associated iron oxides (Fecarb) in Lake Xiaoxingkai compared to Lake Xingkai (P = 0.002), due to better deposition mixing, more contributions of terrigenous detrital materials, and higher OM content in Lake Xiaoxingkai. Results of structural equation modeling showed that Fecarb and total iron content (TFe) regulated 25% of DIR in Lake Xiaoxingkai and 76% in Lake Xingkai, accompanied by a negative effect of TFe on methanogenesis in Lake Xingkai. The relative abundance and diversity of Fe-reducing bacteria were significantly different between the two lakes, and showed a weak effect on sediment OM mineralization. Our findings emphasize the role of iron minerals and geochemical characterizations in regulating rates and pathways of OM mineralization, and deepen the understanding of carbon cycling in lake sediments.

Restoration recommendations for mitigating habitat fragmentation of a river corridor.

Flow discharge and anthropogenic activities influence the composition and configuration of habitat patches in river ecosystems. Understanding the response of habitat landscapes and the corresponding fish habitat quality is crucial for river management. We investigated the reaction of fish habitat suitability and variant flow discharge performance in examining aquatic habitat patch fragmentation. The hydraulic simulation and fish habitat calculation were used to determine the flow characteristics, habitat conditions, and river landscapes. FRAGSTATS was applied to explore the composition and configuration of habitat patches. Cluster analysis and logistic regression were employed to compute the spatiotemporal variabilities of riverscape indices and establish the relationship between riverscape attributes and fish habitat quality. The results indicate that the changes in specific habitat features are associated with the riverscape indices of total edge (TE), mean nearest-neighbor distance (MNN), interspersion and juxtaposition index (IJI), mean patch size (MPS), and area-weighted mean patch fractal dimension (AWMPFD). The flow discharge is the key to determining habitat fragmentation in rivers, with natural barriers occurring at low flow. In contrast, weirs are anthropogenic obstacles that have significant adverse effects on the downstream corridor. A priority restoration activity to conserve river habitat is to create refuge pools during dry seasons by modifying channel morphology. The positive correlation between habitat suitability and MPS and the negative relationship between habitat suitability and AWMPFD highlight the patch size and shape complexity that are critical indices for pool creation. The prediction of the landscape attributes of the outcomes under different scenarios could support the decision-making in river management. The innovative integrated method presented in this study provides a solid foundation and supports the implementation of nature-based solutions for sustainable river management.

A Framework for Identifying Reference Wetland Conditions in Highly Altered Landscapes.

This paper arose from collaboration and discussions over the past years between the authors about what wetlands should be restored to in landscapes that have been intensively altered due to human activities over many centuries and where reference conditions are lacking. It is not intended as an in-depth review of the thinking about reference conditions, but as an opinion paper, with the goal of stimulating discussions about wetland restoration approaches, particularly in regions around the world with highly altered landscapes where restoration of wetlands has been gaining traction only relatively recently. We first explain why the thinking on reference wetlands is biased towards North America, where large areas exist with wetlands that are relatively unimpacted by anthropogenic activities. We then argue that in regions with few unimpaired wetlands those of fair condition may still be good enough to be used as reference wetlands, and that restored and created wetlands should be considered as well.

Multi-element fingerprinting of soils can reveal conversion of wetlands to croplands.

Little is known about the influence of conversion of wetlands to farmlands on concentrations and distribution of elements other than those most commonly studied, partly because of the lack of stratification in wetland soils. In this study, in the Sanjiang Plain in northeastern China, we determined the concentrations of 63 elements along soil profiles at three depths: 0-20 cm, 20-40 cm, and below the depth to which farmers would plow, at 40-60 cm, under four land uses: natural wetland, drained wetland, wetland converted to soybean field and subsequently to rice paddy field. Based on our previous work, we expected that changes in organic matter content would be an important factor affecting element concentrations, but that changes in land uses also led to decoupling of the influence of organic matter on elements. This would lead to other factors, such as changes in redox conditions, changes in hydrology and mixing of soils due to plowing, becoming more important factors affecting element distributions. Our study confirmed these expectations. Changes in organic matter content directly or indirectly affected many elements, explaining 67% of variation. Arsenic, cobalt, iron and nickel concentrations were dramatically higher and sulfur concentrations lower when wetland was converted to paddy field. Co and Ni were identified as potential chemical indicators of wetland conversion. Our research is the first to use multi-element fingerprinting to study effects of conversion of wetlands to croplands in China and showed that this relatively simple approach highlights the complexity of the many interacting factors in reclamation of wetland soils for agricultural uses.

What are the sympatric mechanisms for three species of terrestrial hermit crab (Coenobita rugosus, C. brevimanus, and C. cavipes) in coastal forests?

Terrestrial hermit crabs play a significant role in coastal ecology. For example, as seed dispersers and debris scavengers in coastal forests, they accelerate the decomposition of organic substances. In the Indo-Pacific Ocean, Coenobita rugosus, C. brevimanus, and C. cavipes are the three most common species of terrestrial hermit crab. Because the mechanisms that contribute to the sympatry of these three species of crab have not been identified, this study investigated the three most likely explanations: niche differences, competition, and predation. The results showed that the three species displayed niche differences in terms of seasonal activity, habitat, utilization of shells, and food preference, suggesting that competition for resources is avoided. The habitat of terrestrial hermit crabs in Taiwan is closely associated with that of humans. Our study helps improve our understanding of the ecology of terrestrial hermit crabs and their conservation.

Multi-Elements in Waters and Sediments of Shallow Lakes: Relationships with Water, Sediment, and Watershed Characteristics.

We measured concentrations of multiple elements, including rare earth elements, in waters and sediments of 38 shallow lakes of varying turbidity and macrophyte cover in the Prairie Parkland (PP) and Laurentian Mixed Forest (LMF) provinces of Minnesota. PP shallow lakes had higher element concentrations in waters and sediments compared to LMF sites. Redundancy analysis indicated that a combination of site- and watershed-scale features explained a large proportion of among-lake variability in element concentrations in lake water and sediments. Percent woodland cover in watersheds, turbidity, open water area, and macrophyte cover collectively explained 65.2 % of variation in element concentrations in lake waters. Sediment fraction smaller than 63 µm, percent woodland in watersheds, open water area, and sediment organic matter collectively explained 64.2 % of variation in element concentrations in lake sediments. In contrast to earlier work on shallow lakes, our results showed the extent to which multiple elements in shallow lake waters and sediments were influenced by a combination of variables including sediment characteristics, lake morphology, and percent land cover in watersheds. These results are informative because they help illustrate the extent of functional connectivity between shallow lakes and adjacent lands within these lake watersheds.

Macrophytes in shallow lakes: relationships with water, sediment and watershed characteristics.

We examined macrophyte-environment relationships in shallow lakes located within the Prairie Parkland and Laurentian Mixed Forest provinces of Minnesota. Environmental variables included land cover within lake watersheds, and within-lake, water and sediment characteristics. CCA indicated that sediment fraction smaller than 63 µm (f<63), open water area, turbidity, and percent woodland and agricultural cover in watersheds were significant environmental variables explaining 36.6% of variation in macrophyte cover. When Province was added to the analysis as a spatial covariate, these environmental variables explained 30.8% of the variation in macrophyte cover. CCA also indicated that pH, f<63, percent woodland cover in watersheds, open water area, emergent vegetation area, and organic matter content were significant environmental variables explaining 43.5% of the variation in macrophyte biomass. When Province was added to the analysis as a spatial covariate, these environmental variables explained 39.1% of the variation in macrophyte biomass. The f<63 was the most important environmental variable explaining variation for both measures of macrophyte abundance (cover and biomass) when Province was added as a spatial covariate to the models. Percent woodland in watersheds, turbidity, open water area, and Ca+Mg explained 34.5% of the variation in macrophyte community composition. Most species showed a negative relationship with turbidity and open water area except for Potamogeton richardsonii, Stuckenia pectinata, and filamentous algae. Our study further demonstrates the extent to which macrophyte abundance and community composition are related to site- and watershed-scale variables including lake morphology, water and sediment characteristics, and percent land cover of adjacent uplands.

Cadmium and associated metals in soils and sediments of wetlands across the Northern Plains, USA.

Cadmium, present locally in naturally high concentrations in the Northern Plains of the United States, is of concern because of its toxicity, carcinogenic properties, and potential for trophic transfer. Reports of natural concentrations in soils are dominated by dryland soils with agricultural land uses, but much less is known about cadmium in wetlands. Four wetland categories - prairie potholes, shallow lakes, riparian wetlands, and river sediments - were sampled comprising more than 300 wetlands across four states, the majority in North Dakota. Cd, Zn, P, and other elements were analyzed by ICP-MS, in addition to pH and organic matter (as loss-on-ignition). The overall cadmium content was similar to the general concentrations in the area's soils, but distinct patterns occurred within categories. Cd in wetland soils is associated with underlying geology and hydrology, but also strongly with concentrations of P and Zn, suggesting a link with agricultural land use surrounding the wetlands.

Uptake and translocation of Ti from nanoparticles in crops and wetland plants.

Bioavailability of engineered metal nanoparticles affects uptake in plants, impacts on ecosystems, and phytoremediation. We studied uptake and translocation of Ti in plants when the main source of this metal was TiO2 nanoparticles. Two crops (Phaseolus vulgaris (bean) and Triticum aestivum (wheat)), a wetland species (Rumex crispus, curly dock), and the floating aquatic plant (Elodea canadensis, Canadian waterweed), were grown in nutrient solutions with TiO2 nanoparticles (0, 6, 18 mmol Ti L(-1) for P. vulgaris, T. aestivum, and R. crispus; and 0 and 12 mmol Ti L(-1) for E. canadensis). Also examined in E. canadensis was the influence of TiO2 nanoparticles upon the uptake of Fe, Mn, and Mg, and the influence of P on Ti uptake. For the rooted plants, exposure to TiO2 nanoparticles did not affect biomass production, but significantly increased root Ti sorption and uptake. R. crispus showed translocation of Ti into the shoots. E. canadensis also showed significant uptake of Ti, P in the nutrient solution significantly decreased Ti uptake, and the uptake patterns of Mn and Mg were altered. Ti from nano-Ti was bioavailable to plants, thus showing the potential for cycling in ecosystems and for phytoremediation, particularly where water is the main carrier.

Phyto (in)stabilization of elements.

The effects of plants (corn, soybean, and sunflower) and fertilizer on mobility of more than 60 elements were assessed in a greenhouse experiment. Unplanted columns with the same soil served as controls. Half the columns received fertilizer and all columns were watered at the same rate. At the end of the experiment, the columns were watered to mimic a rainstorm event such that water drained from the bases of the columns, which was collected and analyzed for element content. Soil from between the roots of the plants was also collected and the water-extractable fraction determined. It was expected that (1) more mobile elements, as measured by water extraction, would be leached from the soils at a higher rate compared to less mobile elements, (2) plants would immobilize most elements, but that some would be immobilized, and (3) that this would depend on plant species. The results led to the following conclusions: plants cause metal mobility to vary over a wide range for a specific soil and do mobilize some elements (e.g., Th) while immobilizing others (e.g., U). The effects depended on plant species for some elements. Water-extractable fractions of elements do not predict mobility.

Multi-element accumulation near Rumex crispus roots under wetland and dryland conditions.

Rumex crispus was grown under wet and dry conditions in two-chamber columns such that the roots were confined to one chamber by a 21 mum nylon mesh, thus creating a soil-root interface ('rhizoplane'). Element concentrations at 3 mm intervals below the 'rhizoplane' were measured. The hypothesis was that metals accumulate near plant roots more under wetland than dryland conditions. Patterns in element distribution were different between the treatments. Under dryland conditions Al, Ba, Cu, Cr, Fe, K, La, Mg, Na, Sr, V, Y and Zn accumulated in soil closest to the roots, above the 'rhizoplane' only. Under wetland conditions Al, Fe, Cr, K, V and Zn accumulated above as well as 3 mm below the 'rhizoplane' whereas La, Sr and Y accumulated 3 mm below the 'rhizoplane' only. Plants on average produced 1.5 times more biomass and element uptake was 2.5 times greater under wetland compared to dryland conditions.

Screening the wetland plant species Alisma plantago-aquatica, Carex rostrata and Phalaris arundinacea for innate tolerance to zinc and comparison with Eriophorum angustifolium and Festuca rubra Merlin.

Several wetland plant species appear to have constitutive metal tolerance. In previous studies, populations from contaminated and non-contaminated sites of the wetland plants Typha latifolia, Phragmites australis, Glyceria fluitans and Eriophorum angustifolium were found to be tolerant to high concentrations of metals. This study screened three other species of wetland plants: Alisma plantago-aquatica, Carex rostrata and Phalaris arundinacea for innate tolerance to zinc. The degree of tolerance was compared to known zinc-tolerant E. angustifolium and Festuca rubra Merlin. It was found that A. plantago-aquatica and P. arundinacea did not posses innate tolerance to zinc, but that C. rostrata was able to tolerate elevated levels of zinc, at levels comparable to those tolerated by E. angustifolium and F. rubra Merlin. The findings support the theory that some wetland angiosperm species tend to be tolerant to exposure to high levels of metals, regardless of their origin.

Influence of Typha latifolia and fertilization on metal mobility in two different Pb-Zn mine tailings types.

Storing metal-rich mine waste (tailings) under submerged and reduced conditions can prevent the release of metals to the water column, but introduction of wetland plants on these sediments may alter the reducing environment through root oxygen diffusion or organic matter accumulation. Fertilization of these wetlands can enhance plant growth, but also may either strengthen reducing conditions via microbial stimulation, or increase the redox potential (Eh) through increased root radial oxygen loss. This long-term study (2.25 years) investigated the porewater As, Fe, and Zn concentrations of waterlogged Pb-Zn tailings from two Irish mines, Silvermines and Tara mines, with addition of Typha latifolia, fertilizer, or both treatments combined. In both tailings types, the fertilized plants showed significantly increased total biomass production, but the plants grew greater biomass in Tara tailings relative to Silvermines tailings even without fertilization. In Tara mines tailings, the addition of plants increased Eh and mobilized Zn; the addition of fertilizer enhanced reducing conditions and increased porewater concentrations of As and soluble sulfides; and the combination of treatments on these tailings resulted in complex interactions. In Silvermines tailings, there were negligible effects of the treatments. For effective sequestration of metals in these tailings, Silvermines would require only water cover, but Tara mines tailings would require either both treatments or neither because each treatment individually would increase solubility of As or Zn. These results show also the necessity of evaluating treatment effects specific to individual tailings, that long-term studies (years) are crucial for tailings equilibration and valid experimental conclusions, and that passive accumulation of organic matter may take decades.

Dimethylsulphoniopropionate (DMSP) and related compounds in higher plants.

Dimethylsulphoniopropionate (DMSP) is produced in high concentrations in many marine algae, but in higher plants only in a few salt marsh grasses of the genus Spartina, in sugar canes (Saccharum spp.), and in the Pacific strand plant Wollastonia biflora (L.) DC. The high concentrations found in higher plants (up to 250 micromol g(-1) dry weight) suggest an important role, but though many functions have been suggested (including methylating agent, detoxification of excess sulphur, salt tolerance, and herbivore deterrent), its actual functions remain unclear. The fact that the ability to produce DMSP in high concentrations is found in species that have no taxonomic or ecological relationship suggests that the compound evolved independently and serves different functions in different plants. This is supported by observations that DMSP in W. biflora behaves differently from that in Spartina species. While DMSP concentrations in W. biflora have been found to increase with increasing salinity, suggesting a role in osmotic control, such a relationship has not been found for DMSP in Spartina species. Recent observations on tissue culture showed that, while undifferentiated tissue of W. biflora produced DMSP, such material of Spartina alterniflora Loisel. did not. Ongoing studies with tissue culture of both species have opened up new avenues of research on DMSP in higher plants, ultimately to elucidate the functions of this enigmatic compound.

Long-term effects of submergence and wetland vegetation on metals in a 90-year old abandoned Pb-Zn mine tailings pond.

A Pb-Zn tailings pond, abandoned for approximately 90 years, has been naturally colonized by Glyceria fluitans and is an excellent example of long-term metal retention in tailings ponds under various water cover and vegetation conditions. Shallow/intermittently flooded areas (dry zone) were unvegetated and low in organic matter (OM) content. Permanently flooded areas were either unvegetated with low OM, contained dead vegetation and high OM, or living plants and high OM. It was expected that either water cover or high OM would result in enhanced reducing conditions and lower metal mobility, but live plants would increase metal mobility due to root radial oxygen loss. The flooded low OM tailings showed higher As and Fe mobility compared with dry low OM tailings. In the permanently flooded areas without live vegetation, the high OM content decreased Zn mobility and caused extremely high concentrations of acid-volatile sulfides (AVS). In areas with high OM, living plants significantly increased Zn mobility and decreased concentrations of AVS, indicating root induced sediment oxidation or decreased sulfate-reduction. This is the first study reporting the ability of wetland plants to affect the metal mobility and AVS in long-term (decades), unmanaged tailings ponds.