Hurricane effects on subtropical pine rocklands of the Florida Keys

We investigate the effects of Hurricane Wilma??s storm surge (23?C24 October 2005) on the dominant tree Pinus elliottii var densa (South Florida slash pine) and rare plant species in subtropical pine rocklands of the Lower Florida Keys. We examine the role of elevation on species abundance in 1995 (Hurricane Betsy in 1965), 2005 (Hurricane Georges in 1998), and 2008 (Hurricane Wilma in 2005) to investigate if hurricanes influence abundance by eliminating plants at lower elevation on Big Pine Key, the largest island in the Lower Florida Keys. We compare densities before and after Hurricane Wilma over the 2005?C2008 sampling period and examine the role of elevation on changes in pine and rare species densities three years after Hurricane Wilma. We use elevation to assess the impact of hurricanes because elevation determined whether a location was influenced by storm surge (maximum surge of 2 m) in the Lower Florida Keys, where pine rocklands occur at a maximum elevation of 3 m. In 1995 (30 years after a major storm), elevation did not explain the abundance of South Florida slash pine or Chamaecrista lineata, but explained significant variation in abundance of Chamaesyce deltoidea. The latter two species are rare herbaceous plants restricted to pine rocklands. In 2008, 3 years after Hurricane Wilma, the positive relationship between elevation and abundance was strongest for South Florida slash pine, C. deltoidea, and C. lineata. Effects of Hurricane Wilma were not significant for rare species with wider distribution, occurring in plant communities adjacent to pine rocklands and in disturbed rocklands. Our results suggest that hurricanes drive population dynamics of South Florida slash pine and rare species that occur exclusively in pine rocklands at higher elevations. Rare species restricted to pine rocklands showed dramatic declines after Hurricane Wilma and were eliminated at elevations <0.5 m. Widely distributed rare species did not show significant changes in density after Hurricane Wilma. Abundance increased with elevation for South Florida slash pine and C. lineata after the hurricane. In an environment influenced by sea level rise, concrete plans to conserve pine ecosystems are warranted. Results from this study will help define conservation strategies by strengthening predictive understanding of plant responses to disturbance in the backdrop of sea level rise.     

A framework for validation of seismic response analyses using seismometer array recordings

A framework for the validation of computational models used to predict seismic response based on observations from seismometer arrays is presented. The framework explicitly accounts for the epistemic uncertainty related to the unknown characteristics of the ‘site’ (i.e. the problem under consideration) and constitutive model parameters. A mathematical framework which makes use of multiple prediction–observation pairs is used to improve the statistical significance of inferences regarding the accuracy and precision of the computational methodology and constitutive model. The benefits of such a formal validation framework include: (i) development of consistent methods for determination of constitutive model parameters; (ii) rigorous, objective, and unbiased assessment of the validity of various constitutive models and computational methodologies for various problem types and ground motion intensities; and (iii) an improved understanding of the uncertainties in computational model assumptions, constitutive models and their parameters, relative to other seismic response uncertainties such as ground motion variability. Details regarding the implementation of such a framework to achieve the aforementioned benefits are also addressed.     

Reinterpreting the Importance of Oxygen‐Based Biodegradation in Chloroethene‐Contaminated Groundwater

Chlororespiration is common in shallow aquifer systems under conditions nominally identified as anoxic. Consequently, chlororespiration is a key component of remediation at many chloroethene‐contaminated sites. In some instances, limited accumulation of reductive dechlorination daughter products is interpreted as evidence that natural attenuation is not adequate for site remediation. This conclusion is justified when evidence for parent compound (tetrachloroethene, PCE, or trichloroethene, TCE) degradation is lacking. For many chloroethene‐contaminated shallow aquifer systems, however, nonconservative losses of the parent compounds are clear but the mass balance between parent compound attenuation and accumulation of reductive dechlorination daughter products is incomplete. Incomplete mass balance indicates a failure to account for important contaminant attenuation mechanisms and is consistent with contaminant degradation to nondiagnostic mineralization products like CO₂. While anoxic mineralization of chloroethene compounds has been proposed previously, recent results suggest that oxygen‐based mineralization of chloroethenes also can be significant at dissolved oxygen concentrations below the currently accepted field standard for nominally anoxic conditions. Thus, reassessment of the role and potential importance of low concentrations of oxygen in chloroethene biodegradation are needed, because mischaracterization of operant biodegradation processes can lead to expensive and ineffective remedial actions. A modified interpretive framework is provided for assessing the potential for chloroethene biodegradation under different redox conditions and the probable role of oxygen in chloroethene biodegradation.     

Uranium speciation and stability after reductive immobilization in aquifer sediments

It has generally been assumed that the bioreduction of hexavalent uranium in groundwater systems will result in the precipitation of immobile uraninite (UO₂). In order to explore the form and stability of uranium immobilized under these conditions, we introduced lactate (15 mM for 3 months) into flow-through columns containing sediments derived from a former uranium-processing site at Old Rifle, CO. This resulted in metal-reducing conditions as evidenced by concurrent uranium uptake and iron release. Despite initial augmentation with Shewanella oneidensis, bacteria belonging to the phylum Firmicutes dominated the biostimulated columns. The immobilization of uranium (∼1 mmol U per kg sediment) enabled analysis by X-ray absorption spectroscopy (XAS). Tetravalent uranium associated with these sediments did not have spectroscopic signatures representative of U-U shells or crystalline UO₂. Analysis by microfocused XAS revealed concentrated micrometer regions of solid U(IV) that had spectroscopic signatures consistent with bulk analyses and a poor proximal correlation (μm scale resolution) between U and Fe. A plausible explanation, supported by biogeochemical conditions and spectral interpretations, is uranium association with phosphoryl moieties found in biomass; hence implicating direct enzymatic uranium reduction. After the immobilization phase, two months of in situ exposure to oxic influent did not result in substantial uranium remobilization. Ex situ flow-through experiments demonstrated more rapid uranium mobilization than observed in column oxidation studies and indicated that sediment-associated U(IV) is more mobile than biogenic UO₂. This work suggests that in situ uranium bioimmobilization studies and subsurface modeling parameters should be expanded to account for non-uraninite U(IV) species associated with biomass.     

The Growth of Estuarine Resources (Zostera marina, Mercenaria mercenaria, Crassostrea virginica, Argopecten irradians, Cyprinodon variegatus) in Response to Nutrient Loading and Enhanced Suspension Feeding by Adult Shellfish

While many coastal ecosystems previously supported high densities of seagrass and abundant bivalves, the impacts of overfishing, eutrophication, harmful algal blooms, and habitat loss have collectively contributed to the decline of these important resources. Despite improvements in wastewater treatment in some watersheds and subsequent reduced nutrient loading to neighboring estuaries, seagrass and bivalve populations in these locations have generally not recovered. We performed three mesocosm experiments to simultaneously examine the contrasting effects of nutrient loading and historic suspension-feeding bivalve densities on the growth of eelgrass (Zostera marina), juvenile bivalves (northern quahogs, Mercenaria mercenaria; eastern oysters, Crassostrea virginica; and bay scallops, Argopecten irradians), and juvenile planktivorous fish (sheepshead minnow, Cyprinodon variegatus). High nutrient loading rates led to significantly higher phytoplankton (chlorophyll a) levels in all experiments, significantly increased growth of juvenile bivalves relative to controls with lower nutrient loading rates in two experiments, and significantly reduced the growth of eelgrass in one experiment. The filtration provided by adult suspension feeders (M. mercenaria and C. virginica) significantly decreased phytoplankton levels in all experiments, significantly increased light penetration and the growth of eelgrass in one experiment, and significantly decreased the growth of juvenile bivalves and fish in two experiments, all relative to controls with no filtration from adult suspension feeders. These results demonstrate that an appropriate level of nutrient loading can have a positive effect on some estuarine resources and that bivalve filtration can mediate the effects of nutrient loading to the benefit or detriment of different estuarine resources. Future ecosystem-based approaches will need to simultaneously account for anthropogenic nutrient loading and bivalve restoration to successfully manage estuarine resources.     

Relocation effects of dredged marine sediments on mercury geochemistry: Venice lagoon,Italy

Understanding the biogeochemical process of Hg is critical in the overall evaluation of the ecological impacts resulting from the reuse of Hg-contaminated dredged sediment. Sediment banks (V1 and V2) were constructed with freshly dredged sediments from a navigational channel in Venice Lagoon, Italy, with the goal of clarifying potential differences in the biogeochemistry of Hg between the reused dredged sediments and those from surrounding sites (SS1 and S2). Toward this purpose, Hg and monomethylmercury (MMHg) concentrations, and Hg methylation rates (MMRs) in the surface 2.5 cm sediments were monitored, along with ammonium, iron, sulfate and sulfide concentrations in the pore waters of banks and surrounding sites from November 2005 to February 2007. Pore water analyses indicate that the bank sediments are characterized by lower levels of sulfate and iron, and by higher levels of ammonium and sulfide compared to the surrounding sediments. With respect to Hg speciation, the fractions of MMHg in total Hg (%MMHg/Hg) and the MMRs were significantly lower in the bank V1 compared to those in the reference site SS1, whereas the %MMHg/Hg and the MMRs were similar between V2 and S2. A negative correlation is found between the logarithm of the particle-water partition coefficient of Hg and the MMR, indicating that the reduced MMRs in V1 are caused by the limited concentrations of dissolved Hg. Organic matter appears to play a key role in the control of MMR via the control of Hg solubility.     

The Contribution of Groundwater Discharge to Nutrient Exports from a Coastal Catchment: Post-Flood Seepage Increases Estuarine N/P Ratios

Four months of daily nutrient and radon (a natural groundwater tracer) observations at the outlet of a heavily drained coastal wetland illustrated how episodic floods and diffuse groundwater seepage influence the biogeochemistry of a sub-tropical estuary (Richmond River, New South Wales, Australia). Our observations downstream of the Tuckean Swamp (an acid sulphate soil floodplain) covered a dry stage, a flood triggered by a 213-mm rain event and a post-flood stage when surface water chemistry was dominated by groundwater discharge. Significant correlations were found between radon and ammonium and N/P ratios and between radon and dissolved organic nitrogen (DON) during the post-flood stage. While the flood lasted for 14?% of the time of the surface water time series, it accounted for 18?% of NH₄, 32?% of NO _x , 66?% of DON, 58?% of PO₄ and 55?% of dissolved organic phosphorus (DOP) catchment exports. Over the 4-month study period, groundwater fluxes of 35.0, 3.6, 36.3, 0.5 and 0.7?mmol?m^?2?day^?1 for NH₄, NO _x , DON, PO₄ and DOP, respectively, were estimated. The groundwater contribution to the total surface water catchment exports was nearly 100?% for ammonium, and <20?% for the other nutrients. Post-flood groundwater seepage shifted the system from a DON to a dissolved inorganic N-dominated system and doubled N/P ratios in surface waters. We hypothesise that the Richmond River Estuary N/P ratios may reflect a widespread trend of tidal rivers and estuaries becoming more groundwater-dominated and phosphorus-limited as coastal wetlands are drained for agriculture, grazing and development.     

Comparison of isotope pairing and N2:Ar methods for measuring sediment denitrification—Assumption, modifications, and implications

Denitrification has been measured during the last few years using two different methods in particular: isotope pairing measured on a triple-collector isotopic ratio mass spectrometer and N₂:Ar ratios measured on a membrane inlet mass spectrometer (MIMS). This study compares these two techniques in short-term batch experiments. Rates obtained using the original N₂∶Ar method were up to 3 to 4 times higher than rates obtained using the isotope pairing technique due to O₂ reacting with the N₂ during MIMS analysis. Oxygen combines with N₂ within the mass spectrometer ion source forming NO⁺ which reduces the N₂ concentration. The decrease in N₂ is least at lower O₂ concentrations and since oxygen is typically consumed during incubations of sediment cores, the result is often a pseudo-increase in N₂ concentration being interpreted as denitrification activity. The magnitude of this ocygen effect may be instrument specific. The reaction of O₂ with N₂ and the subsequent decrease in N₂ was only partly correctly using an O₂ correction curve for the relationship between N₂ and O₂ concentrations. The O₂ corrected N₂∶Ar denitrification rates were lower, but still did not match the isotope pairing rates and the variability between replicates was much higher. Using a copper reduction column heated to 600°C to remove all of the O₂ from the sample before MIMS analysis resulted in comparable rates (slightly lower), and comparable variability between replicates, to the isotope pairing technique. The N₂:Ar technique determines the net N₂ production as the difference between N₂ production by denitrification and N₂ consumption by N-fixation, while N-fixation has little effect on the isotope pairing technique which determines a rate very close to the gross N₂ production. When the two different techniques were applied on the same sediment, the small difference in rates obtained by the two methods seemed to reflect N-fixation as also supported from measurements of ethylene production in acetylene enriched sediment cores. The N₂:Ar and isotope pairing techniques may be combined to provide simultaneous measurements of denitrification and N-fixation. Both techniques have several assumptions that must be met to achieve accurate rates; a number of tests are outlined that can be applied to demonstrate that these assumptions are being meet.     

Iron and Sulfur Chemistry in a Stratified Lake: Evidence for Iron-Rich Sulfide Complexes

A four month study of a man-made lake used for hydroelectric power generation in northeastern Pennsylvania USA was conducted to investigate seasonal anoxia and the effects of sulfide species being transported downstream of the power generation equipment. Water column analyses show that the system is iron-rich compared to sulfide. Total Fe(II) concentrations in the hypolimnion are typically at least twice the total sulfide levels. In situ voltammetric analyses show that free Fe(II) as [Fe(H₂O)₆]²⁺ or free H₂S as H₂S/HS^- are either not present or at trace levels and that iron-rich sulfide complexes are present. From the in situ data and total Fe(II) and H₂S measurements, we infer that these iron-rich sulfide complexes may have stoichiometries such as Fe₂SH³⁺ (or polymeric forms of this and other stoichiometries). These iron-rich sulfide complexes appear related to dissolution of the iron-rich FeS mineral, mackinawite, because IAP calculations on data from discrete bottle samples obtained from bottom waters are similar to the pK_sp of mackinawite. Soluble iron-sulfide species are stable in the absence of O₂ (both in lake waters and the pipeline) and transported several miles during power generation. However, iron-sulfide complexes can react with O₂ to oxidize sulfide and can also dissociate releasing volatile H₂S when the waters containing them are exposed to the atmosphere downstream of the powerplant. Sediment analyses show that the lake is rich in oxidized iron solids (both crystalline and amorphous). Fe concentrations in FeS solids are low (<5 μmole/gr_{dry wt}) and the pyrite concentration ranges from about equal to the solid FeS to 30 times the solid FeS concentration. The degree of pyritization is below 0.12 indicating that pyrite formation is limited by free sulfide, which can react with the iron-rich sulfide complexes.