Using marsh organs to test seed recruitment in tidal freshwater marshes.

Premise: Seed recruitment niches along estuarine elevation gradients are seldom experimentally field-tested under tidal regimes of the Pacific Northwest of North America. Addressing this knowledge gap is important to better understand estuary restoration and plant community response to sea level rise. Methods: Germination was tested in marsh organ mesocosms across an elevation gradient (0.5-1.7 m above mean sea level). Seeds were sown on sterile peat moss, and the tops of pipes were secured with horticultural "frost cloth" to ensure no experimental seeds were washed out and no new seeds were introduced. The trials tested artificial and overwinter chilling regimes, as well as the presence and/or absence of a near-neighbor transplant. Results: Carex lyngbyei had significant elevation-driven germination after overwinter and artificial chilling. Schoenoplectus tabernaemontani had near-significant germination across elevation after overwinter chilling, and germination in the absence of competition was significantly greater than with a near-neighbor transplant. Discussion: Carex lyngbyei had the highest germination rate at higher elevations, which suggests restricted seed recruitment potential and required clonal expansion to extend into lower marsh elevations. Identifying species-specific recruitment niches provides insight for restoration opportunities or invasive species monitoring, as well as for estuary migration under sea level rise.

Relationships between ecosystem properties and sea-level rise vulnerability of tidal wetlands of the U.S. Mid-Atlantic.

Tidal wetlands in the Mid-Atlantic, USA, are experiencing high rates of relative sea level rise, and it is unclear whether they will be resilient in the face of future flooding increases. In a previous study, we found 80% of our study areas in tidal freshwater and salt marshes in the Delaware Estuary and Barnegat Bay had elevation change rates lower than the 19-year increase in mean sea level. Here, we examine relationships between marsh elevation dynamics and abiotic and biotic parameters in order to assess their utility as indicators of vulnerability to relative sea level rise. We further apply a range of marsh vulnerability indicators including elevation change rates to evaluate their ability to corroborate marsh habitat change over the last 30 years. Of the field measurements, soil bulk density and belowground plant biomass were among the strongest predictors of elevation change and accretion dynamics across all marsh types and settings. Both tidal freshwater and salt marshes tended to have higher rates of elevation increase and surface accretion in areas where soil bulk density and live belowground biomass were higher. Nine of the ten marshes experienced a net loss of area from the 1970s to 2015 ranging from 0.05 to 14%. Although tidal freshwater marshes were low in elevation and experienced variable elevation change rates, marsh area loss was low. Conversely, salt marshes closest to the coast and perched high in the tidal frame with a higher degree of human modification tended to experience the greatest marsh loss, which incorporated anthropogenic impacts and edge erosion. Thus, our regional assessment points to the need for a comprehensive understanding of factors that influence marsh resilience including human modifications and geomorphic settings.

Divergent responses of wetland methane emissions to elevated atmospheric CO2 dependent on water table.

Elevated atmospheric CO2 may have consequences for methane (CH4) emissions from wetlands, yet the magnitude and direction remain unpredictable, because the associated mechanisms have not been fully investigated. Here, we established an in situ macrocosm experiment to compare the effects of elevated CO2 (700 ppm) on the CH4 emissions from two wetlands: an intermittently inundated Calamagrostis angustifolia marsh and a permanently inundated Carex lasiocarpa marsh. The elevated CO2 increased CH4 emissions by 27.6-57.6% in the C. angustifolia marsh, compared to a reduction of 18.7-23.5% in the C. lasiocarpa marsh. The CO2-induced increase in CH4 emissions from the C. angustifolia marsh was paralleled with (1) increased dissolved organic carbon (DOC) released from plant photosynthesis and (2) reduced (rate of) CH4 oxidation due to a putative shift in methanotrophic community composition. In contrast, the CO2-induced decrease in CH4 emissions from the C. lasiocarpa marsh was associated with the increases in soil redox potential and pmoA gene abundance. We synthesized data from worldwide wetland ecosystems, and found that the responses of CH4 emissions to elevated CO2 was determined by the wetland water table levels and associated plant oxygen secretion capacity. In conditions with elevated CO2, plants with a high oxygen secretion capacity suppress CH4 emissions while plants with low oxygen secretion capacity stimulate CH4 emissions; both effects are mediated via a feedback loop involving shifts in activities of methanogens and methanotrophs.

Coupling between sediment biogeochemistry and phytoplankton development in a temperate freshwater marsh (Charente-Maritime, France): Evidence of temporal pattern.

In freshwater systems, sediment can be an important source for the internal loading of PO4. The limiting character of this element in such system leads to consider this phenomenon in terms of eutrophication risks and water quality stakes. A four-months follow-up (January, March, April and May 2019) was carried out in a strong phosphate (PO4) limited secondary channel from an artificial irrigation system of Charente Maritime (France) to link the mobilization of remineralization products in the upper 6 cm layer of sediment (conventional core slicing/centrifugation and DET probes) and the phytoplankton biomass dynamics in the water column. Results showed congruent patterns between the temporal succession of the organic matter mineralization processes in the sediment and the primary biomass dynamics in the water column. In January and March (considered in winter), PO4 proved to be retained by adsorption onto iron oxides in anoxic sediment since pore water nitrate inhibited for about a month the respiration of metal oxides in the first cm of sediment, thus limiting PO4 availability and the phytoplankton growth. In April and May (early spring), after exhaustion of pore water nitrate, the dissolutive reduction of iron oxides released PO4 into pore water generated a significant diffusive outgoing flux from the sediment to the water column with a maximum in April (-1.10E-04±2.81E-05 nmol cm-2 s-1). This release coincided with the nanophytoplankton bloom (5.50 µg Chla L-1) and a potential increase of PO4 concentration in the water column. This work provides some insight on the importance of benthic-pelagic coupling in anthropogenic systems. This conceptual model has to be deployed on other sites of interest where internal loading of P takes precedence over external inputs and nitrate mitigation drives its benthic recycling and ultimately its bioavailability. This is to be essential to characterize the aquatic environment quality in order to limit eutrophication risks.

Mercury adsorption in the Mississippi River deltaic plain freshwater marsh soil of Louisiana Gulf coastal wetlands.

Mercury adsorption characteristics of Mississippi River deltaic plain (MRDP) freshwater marsh soil in the Louisiana Gulf coast were evaluated under various conditions. Mercury adsorption was well described by pseudo-second order and Langmuir isotherm models with maximum adsorption capacity of 39.8 mg g-1. Additional fitting of intraparticle model showed that mercury in the MRDP freshwater marsh soil was controlled by both external surface adsorption and intraparticle diffusion. The partition of adsorbed mercury (mg g-1) revealed that mercury was primarily adsorbed into organic-bond fraction (12.09) and soluble/exchangeable fraction (10.85), which accounted for 63.5% of the total adsorption, followed by manganese oxide-bound (7.50), easily mobilizable carbonate-bound (4.53), amorphous iron oxide-bound (0.55), crystalline Fe oxide-bound (0.41), and residual fraction (0.16). Mercury adsorption capacity was generally elevated along with increasing solution pH even though dominant species of mercury were non-ionic HgCl2, HgClOH and Hg(OH)2 at between pH 3 and 9. In addition, increasing background NaCl concentration and the presence of humic acid decreased mercury adsorption, whereas the presence of phosphate, sulfate and nitrate enhanced mercury adsorption. Mercury adsorption in the MRDP freshwater marsh soil was reduced by the presence of Pb, Cu, Cd and Zn with Pb showing the greatest competitive adsorption. Overall the adsorption capacity of mercury in the MRDP freshwater marsh soil was found to be significantly influenced by potential environmental changes, and such factors should be considered in order to manage the risks associated with mercury in this MRDP wetland for responding to future climate change scenarios.

Vegetation recovery in an oil-impacted and burned Phragmites australis tidal freshwater marsh.

In-situ burning of oiled marshes is a cleanup method that can be more effective and less damaging than intrusive manual and mechanical methods. In-situ burning of oil spills has been examined for several coastal marsh types; however, few published data are available for Phragmites australis marshes. Following an estimated 4200gallon crude oil spill and in-situ burn in a Phragmites tidal freshwater marsh at Delta National Wildlife Refuge (Mississippi River Delta, Louisiana), we examined vegetation impacts and recovery across 3years. Oil concentrations in marsh soils were initially elevated in the oiled-and-burned sites, but were below background levels within three months. Oiling and burning drastically affected the marsh vegetation; the formerly dominant Phragmites, a non-native variety in our study sites, had not fully recovered by the end of our study. However, overall vegetation recovery was rapid and local habitat quality in terms of native plants, particularly Sagittaria species, and wildlife value was enhanced by burning. In-situ burning appears to be a viable response option to consider for future spills in marshes with similar plant species composition, hydrogeomorphic settings, and oiling conditions. In addition, likely Phragmites stress from high water levels and/or non-native scale insect damage was also observed during our study and has recently been reported as causing widespread declines or loss of Phragmites stands in the Delta region. It remains an open question if these stressors could lead to a shift to more native vegetation, similar to what we observed following the oil spill and burn. Increased dominance by native plants may be desirable as local patches, but widespread loss of Phragmites, even if replaced by native species, could further acerbate coastal erosion and wetland loss, a major concern in the region.

Methanogenic Community Was Stable in Two Contrasting Freshwater Marshes Exposed to Elevated Atmospheric CO2.

The effects of elevated atmospheric CO2 concentration on soil microbial communities have been previously recorded. However, limited information is available regarding the response of methanogenic communities to elevated CO2 in freshwater marshes. Using high-throughput sequencing and real-time quantitative PCR, we compared the abundance and community structure of methanogens in different compartments (bulk soil, rhizosphere soil, and roots) of Calamagrostis angustifolia and Carex lasiocarpa growing marshes under ambient (380 ppm) and elevated CO2 (700 ppm) atmospheres. C. lasiocarpa rhizosphere was a hotspot for potential methane production, based on the 10-fold higher abundance of the mcrA genes per dry weight. The two marshes and their compartments were occupied by different methanogenic communities. In the C. lasiocarpa marsh, archaeal family Methanobacteriaceae, Rice Cluster II, and Methanosaetaceae co-dominated in the bulk soil, while Methanobacteriaceae was the exclusively dominant methanogen in the rhizosphere soil and roots. Families Methanosarcinaceae and Methanocellaceae dominated in the bulk soil of C. angustifolia marsh. Conversely, Methanosarcinaceae and Methanocellaceae together with Methanobacteriaceae dominated in the rhizosphere soil and roots, respectively, in the C. angustifolia marsh. Elevated atmospheric CO2 increased plant photosynthesis and belowground biomass of C. lasiocarpa and C. angustifolia marshes. However, it did not significantly change the abundance (based on mcrA qPCR), diversity, or community structure (based on high-throughput sequencing) of methanogens in any of the compartments, irrespective of plant type. Our findings suggest that the population and species of the dominant methanogens had weak responses to elevated atmospheric CO2. However, minor changes in specific methanogenic taxa occurred under elevated atmospheric CO2. Despite minor changes, methanogenic communities in different compartments of two contrasting freshwater marshes were rather stable under elevated atmospheric CO2.

[Short-term Effects of Nitrogen Deposition on CO2 and CH4 Fluxes from Wetlands in the Minjiang River Estuary].

In order to evaluate the effects of nitrogen deposition on CO2 and CH4 fluxes from a Cyperus malaccensis marsh in the Min River estuary at Daoqingzhou, Fujian, static chamber-gas chromatography (GC) techniques were used. CO2 and CH4 fluxes at a control (CK)[0 g·(m2·a)-1], N1[24 g·(m2·a)-1], N2[48 g·(m2·a)-1], and N3[96 g·(m2·a)-1] were measured and analyzed and the related environmental factors were recorded. The results show that,① compared to the control treatment, the effects of nitrogen deposition on the CO2 fluxes for N1 and N3 increased by 20.30% and 4.06%, respectively, whereas the CO2 fluxes for N2 reduced by 10.05%, furthermore, the CO2 fluxes under different treatments were not significantly different (P>0.05), except for the N2 and N3 treatments in December (P<0.05). ② Compared to the control treatment, the effects of nitrogen deposition on CH4 fluxes at N1, N2, and N3 were increased by 64.51%, 30.23%, and 80.57%, respectively, whereas the CH4 fluxes under different treatments were not significantly different (P>0.05). ③ There was significant positive correlation between CO2 and CH4 fluxes and soil temperature (P<0.05), however, the CO2 and CH4 fluxes were not significant with regards to soil pH and soil EC (P>0.05).

Effects of P addition on plant C:N:P stoichiometry in an N-limited temperate wetland of Northeast China.

Phosphorus (P) enrichment induced by anthropogenic activities results in modified plant nutrient status, which potentially alters the stoichiometry of carbon (C), nitrogen (N), and P in plants. However, how increased P availability changes plant C:N:P stoichiometry at different hierarchical scales is unclear in N-limited ecosystems. In this study, we conducted a four-level P addition experiment (0, 1.2, 4.8, and 9.6gPm(-)(2)year(-1)) to elucidate the effect of P enrichment on plant C:N:P stoichiometric ratios at both the species and community levels in a freshwater wetland in the Sanjiang Plain, Northeast China. We found that species- and community-level plant C:N:P stoichiometry responded consistently to six years of P addition, although there was a shift in species dominance. Phosphorus addition increased plant N and P concentrations and thus decreased C:N, C:P, and N:P ratios irrespective of the P addition levels. These similar change trends at different scales resulted from the identical responses of plant N and P concentrations in different species to P addition. Moreover, plant N concentration exhibited an increasing trend with increasing P addition levels, whereas plant C:N ratio showed a declining trend. At the community level, P addition at the rates of 1.2, 4.8, and 9.6gPm(-2)year(-1) decreased the C:N ratio by 24%, 27%, and 34%; decreased the C:P ratio by 33%, 35%, and 38%; and decreased the N:P ratio by 12%, 10%, and 6%, respectively. Our results indicate that the stoichiometric responses to P addition are scale-independent, and suggest that altered plant C:N:P stoichiometry induced by P enrichment would stimulate organic matter decomposition and accelerate nutrient cycles in N-limited temperate freshwater wetlands.

Connecting carbon and nitrogen storage in rural wetland soil to groundwater abstraction for urban water supply.

We investigated whether groundwater abstraction for urban water supply diminishes the storage of carbon (C), nitrogen (N), and organic matter in the soil of rural wetlands. Wetland soil organic matter (SOM) benefits air and water quality by sequestering large masses of C and N. Yet, the accumulation of wetland SOM depends on soil inundation, so we hypothesized that groundwater abstraction would diminish stocks of SOM, C, and N in wetland soils. Predictions of this hypothesis were tested in two types of subtropical, depressional-basin wetland: forested swamps and herbaceous-vegetation marshes. In west-central Florida, >650 ML groundwater day(-1) are abstracted for use primarily in the Tampa Bay metropolis. At higher abstraction volumes, water tables were lower and wetlands had shorter hydroperiods (less time inundated). In turn, wetlands with shorter hydroperiods had 50-60% less SOM, C, and N per kg soil. In swamps, SOM loss caused soil bulk density to double, so areal soil C and N storage per m(2) through 30.5 cm depth was diminished by 25-30% in short-hydroperiod swamps. In herbaceous-vegetation marshes, short hydroperiods caused a sharper decline in N than in C. Soil organic matter, C, and N pools were not correlated with soil texture or with wetland draining-reflooding frequency. Many years of shortened hydroperiod were probably required to diminish soil organic matter, C, and N pools by the magnitudes we observed. This diminution might have occurred decades ago, but could be maintained contemporarily by the failure each year of chronically drained soils to retain new organic matter inputs. In sum, our study attributes the contraction of hydroperiod and loss of soil organic matter, C, and N from rural wetlands to groundwater abstraction performed largely for urban water supply, revealing teleconnections between rural ecosystem change and urban resource demand.

Effects of simulated drought on the carbon balance of Everglades short-hydroperiod marsh.

Hydrology drives the carbon balance of wetlands by controlling the uptake and release of CO2 and CH4 . Longer dry periods in between heavier precipitation events predicted for the Everglades region, may alter the stability of large carbon pools in this wetland's ecosystems. To determine the effects of drought on CO2 fluxes and CH4 emissions, we simulated changes in hydroperiod with three scenarios that differed in the onset rate of drought (gradual, intermediate, and rapid transition into drought) on 18 freshwater wetland monoliths collected from an Everglades short-hydroperiod marsh. Simulated drought, regardless of the onset rate, resulted in higher net CO2 losses net ecosystem exchange (NEE) over the 22-week manipulation. Drought caused extensive vegetation dieback, increased ecosystem respiration (Reco ), and reduced carbon uptake gross ecosystem exchange (GEE). Photosynthetic potential measured by reflective indices (photochemical reflectance index, water index, normalized phaeophytinization index, and the normalized difference vegetation index) indicated that water stress limited GEE and inhibited Reco . As a result of drought-induced dieback, NEE did not offset methane production during periods of inundation. The average ratio of net CH4 to NEE over the study period was 0.06, surpassing the 100-year greenhouse warming compensation point for CH4 (0.04). Drought-induced diebacks of sawgrass (C3 ) led to the establishment of the invasive species torpedograss (C4 ) when water was resupplied. These changes in the structure and function indicate that freshwater marsh ecosystems can become a net source of CO2 and CH4 to the atmosphere, even following an extended drought. Future changes in precipitation patterns and drought occurrence/duration can change the carbon storage capacity of freshwater marshes from sinks to sources of carbon to the atmosphere. Therefore, climate change will impact the carbon storage capacity of freshwater marshes by influencing water availability and the potential for positive feedbacks on radiative forcing.

Recovery of freshwater marsh vegetation after a saltwater intrusion event.

Greenhouse mesocosms of freshwater marsh vegetation were exposed to a simulated saltwater intrusion event followed by a recovery period during which water levels and interstitial water salinity were adjusted over a range of conditions. Virtually all above-ground vegetation, including the three dominant species, Sagittaria lancifolia L., Leersia oryzoides (L.) Swartz, and Panicum hemitomon Schultes, was killed by the initial saltwater intrusion event. P. hemitomon did not recover, but S. lancifolia and L. oryzoides, as well as many of the other species initially present, exhibited some ability to recover depending on post-saltwater intrusion conditions. Increasingly harsh recovery conditions (for freshwater marsh vegetation), including more reduced soil conditions, higher interstitial salinities, and higher interstitial sulfide concentrations were associated with decreased live above-ground biomass and species richness. The effect of elevated salinity on vegetative recovery became more pronounced under flooded conditions. This experiment illustrates that the response of a freshwater marsh community to the long-term disturbance effect of a transient saltwater intrusion event will be strongly influenced by post-intrusion salinity and water levels.

Seasonal flooding, soil salinity and primary production in northern prairie marshes.

Hydrologic regime is an important control of primary production in wetland ecosystems. I investigated the coupling of flooding, soil salinity and plant production in northern prairie marshes that experience shallow spring flooding. Field experiments compared whitetop (Scolochloa festucacea) marsh that was: (1) nonflooded, (2) flooded during spring with 25 cm water and (3) nonflooded but irrigated with 1 cm water · day-1. Pot culture experiments examined whitetop growth response to salinity treatments. The electrical conductivity of soil interstitial water (ECe) at 15 cm depth was 4 to 8 dS· m-1 lower in flooded marsh compared with nonflooded marsh during 2 years. Whitetop aboveground biomass in flooded marsh (937 g · m-2, year 1; 969 g · m-2, year 2) exceeded that of nonflooded marsh (117 g · m-2 year 1; 475 g · m-2, year 2). Irrigated plots had lower ECe and higher aboveground biomass than nonflooded marsh. In pot culture, ECe of 4.3 dS · m-1 (3 g · L-1 NaCl) reduced total whitetop biomass by 29 to 44% and ECe of 21.6 dS · m-1 (15 g · L-1 NaCl) reduced biomass by more than 75%. Large reductions of ECe and increases of whitetop growth with irrigation indicated that plants responded to changes in soil salinity and not other potential environmental changes caused by inundation. The results suggest that spring flooding controls whitetop production by decreasing soil salinity during spring and by buffering surface soils against large increases of soil salinity after mid-summer water level declines. This mechanism can explain higher marsh plant production under more reducing flooded soil conditions and may be an important link between intermittent flooding and primary production in other wetland ecosystems.