Modeling the response of primary production and sedimentation to variable nitrate loading in the Mississippi River plume

Increases in nitrate loading to the Mississippi River watershed during the last 50 years are considered responsible for the increase in hypoxic zone size in Louisiana-Texas shelf bottom waters. There is currently a national mandate to decrease the size of the hypoxic zone to 5000 km² by 2015, mostly by a 30% reduction in annual nitrogen discharge into the Gulf of Mexico. We developed an ecosystem model for the Mississippi River plume to investigate the response of organic matter production and sedimentation to variable nitrate loading. The nitrogen-based model consisted of nine compartments (nitrate, ammonium, labile dissolved organic nitrogen, bacteria, small phytoplankton, diatoms, micro- and mesozooplankton, and detritus), and was developed for the spring season, when sedimentation of organic matter from plume surface waters is considered important in the development of shelf hypoxia. The model was forced by physical parameters specified along the river-ocean salinity gradient, including residence time, light attenuation by dissolved and particulate matter, mixed layer depth, and dilution. The model was developed using measurements of biological biomasses and nutrient concentrations across the salinity gradient, and model validation was performed with an independent dataset of primary production measurements for different riverine NO₃ loads. Based on simulations over the range of observed springtime NO₃ loads, small phytoplankton contributed on average 80% to primary production for intermediate to high salinities (>15), and the main contributors to modeled sedimentation at these salinities were diatom sinking, microzooplankton egestion, and small phytoplankton mortality. We investigated the impact of limiting factors on the relationship between NO₃ loading and ecosystem rates. Model results showed that primary production was primarily limited by physical dilution of NO₃, followed by abiotic light attenuation, light attenuation due to mixing, and diatom sinking. Sedimentation was mainly limited by the first three of these factors. Neither zooplankton grazing or plume residence times acted as limiting factors of ecosystem rates. Regarding nutrient reductions to the watershed, simulations showed that about half of the percent decrease in NO₃ load was reflected in decreased plume sedimentation. For example, a 30% decrease in NO₃ load resulted in a 19% decrease in average plume primary production and a 14% decrease in sedimentation. Finally, our model results indicated that the fraction of primary production exported from surface waters is highly variable with salinity (7–87%), a finding which has important implications for predictive models of hypoxic zone size that assume a constant value for this ratio.     

Estimating restorable wetland water storage at landscape scales

Globally, hydrologic modifications such as ditching and subsurface drainage have significantly reduced wetland water storage capacity (i.e., volume of surface water a wetland can retain) and consequent wetland functions. While wetland area has been well documented across many landscapes and used to guide restoration efforts, few studies have directly quantified the associated wetland storage capacity. Here, we present a novel raster‐based approach to quantify both contemporary and potential (i.e., restorable) storage capacities of individual depressional basins across landscapes. We demonstrate the utility of this method by applying it to the Delmarva Peninsula, a region punctuated by both depressional wetlands and drainage ditches. Across the entire peninsula, we estimated that restoration (i.e., plugging ditches) could increase storage capacity by 80%. Focusing on an individual watershed, we found that over 59% of restorable storage capacity occurs within 20 m of the drainage network, and that 93% occurs within 1 m elevation of the drainage network. Our demonstration highlights widespread ditching in this landscape, spatial patterns of both contemporary and potential storage capacities, and clear opportunities for hydrologic restoration. In Delmarva and more broadly, our novel approach can inform targeted landscape‐scale conservation and restoration efforts to optimize hydrologically mediated wetland functions.     

Implementing marine reserve networks: A comparison of approaches in New South Wales (Australia) and New Zealand

Marine reserve networks are an essential and effective tool for conserving marine biodiversity. They also have an important role in the governance of oceans and the sustainable management of marine resources. The translation of marine reserve network theory into practice is a challenge for conservation practitioners. Barriers to implementing marine reserves include varying levels of political will and agency support and leadership, poorly coordinated marine conservation policy, inconsistencies with the use of legislation, polarised views and opposition from some stakeholders, and difficulties with defining and mapping conservation features. The future success of marine reserve network implementation will become increasingly dependent on: increasing political commitment and agency leadership; greater involvement and collaboration with stakeholders; and the provision of resources to define and map conservation features. Key elements of translating marine reserve theory into implementation of a network of marine reserves are discussed based on approaches used successfully in New Zealand and New South Wales (Australia).     

Global Sensitivity Analysis Techniques for Probabilistic Ground Water Modeling

Global sensitivity analysis techniques are better suited for analyzing input-output relationships over the full range of parameter variations and model outcomes, as opposed to local sensitivity analysis carried out around a reference point. This article describes three such techniques: (1) stepwise rank regression analysis for building input-output models to identify key contributors to output variance, (2) mutual information (entropy) analysis for determining the strength of nonmonotonic patterns of input-output association, and (3) classification tree analysis for determining what variables or combinations are responsible for driving model output into extreme categories. These techniques are best applied in conjunction with Monte Carlo simulation-based probabilistic analyses. Two examples are presented to demonstrate the applicability of these methods. The usefulness of global sensitivity techniques is examined vis-a-vis local sensitivity analysis methods, and recommendations are provided for their applications in ground water modeling practice.     

Polar Compounds from the Dissolution of Weathered Diesel

Hydrocarbon and nonhydrocarbon components dissolving in water from fresh diesel and field samples of highly weathered diesel (spilled up to 50 years ago) from two sites were investigated. The fresh and weathered diesels were equilibrated with water using a slow-stirring method, and the product and equilibrated aqueous water samples analyzed by a range of analytical procedures. The water phase equilibrated with weathered diesels had higher total dissolved organics concentrations (96 and 8.6 mg/L at the two sites) compared to the water phase equilibrated with fresh diesel (average of 3.4 mg/L). Compound class characterization of dissolved organics in water from the weathered diesel showed that polar components were a significant compound class (98% and 42% at the two sites) and appeared largely as an unresolved complex mixture (UCM) in the total ion chromatograms (TICs). Identification of 1-adamantanol in the polar fraction of both weathered diesel samples (3.6 and 0.3 μg/L at the two sites) suggested that at least some of the associated polar components are from a petroleum source. The analysis of total petroleum hydrocarbons (TPH) is aimed at measuring only dissolved carbon and hydrogen-containing compounds, and dissolved polar compounds present as a UCM are often assumed to be from natural organic matter (NOM) and removed. This may result in a gross underestimation of the total soluble organic material in water associated with weathered diesels. In addition, the risk posed by these fuel-derived polar compounds is unknown.     

Regional beryllium-10 production rate calibration for late-glacial northeastern North America

The major uncertainty in relating cosmogenic-nuclide exposure ages to ages measured by other dating methods comes from extrapolating nuclide production rates measured at globally scattered calibration sites to the sites of unknown age that are to be dated. This uncertainty can be reduced by locating production rate calibration sites that are similar in location and age to the sites to be dated. We use this strategy to reconcile exposure age and radiocarbon deglaciation chronologies for northeastern North America by compiling ¹⁰Be production rate calibration measurements from independently dated late-glacial and early Holocene ice-marginal landforms in this region. ¹⁰Be production rates measured at these sites are 6–12% lower than predicted by the commonly accepted global ¹⁰Be calibration data set used with any published production rate scaling scheme. In addition, the regional calibration data set shows significantly less internal scatter than the global calibration data set. Thus, this calibration data set can be used to improve both the precision and accuracy of exposure dating of regional late-glacial events. For example, if the global calibration data set is used to calculate exposure ages, the exposure-age deglaciation chronology for central New England is inconsistent with the deglaciation chronology inferred from radiocarbon dating and varve stratigraphy. We show that using the regional data set instead makes the exposure age and radiocarbon chronologies consistent. This increases confidence in correlating exposure ages of ice-marginal landforms in northeastern North America with glacial and climate events dated by other means.     

Incorporation of conceptual and parametric uncertainty into radionuclide flux estimates from a fractured granite rock mass

Detailed numerical flow and radionuclide simulations are used to predict the flux of radionuclides from three underground nuclear tests located in the Climax granite stock on the Nevada Test Site. The numerical modeling approach consists of both a regional-scale and local-scale flow model. The regional-scale model incorporates conceptual model uncertainty through the inclusion of five models of hydrostratigraphy and five models describing recharge processes for a total of 25 hydrostratigraphic–recharge combinations. Uncertainty from each of the 25 models is propagated to the local-scale model through constant head boundary conditions that transfer hydraulic gradients and flow patterns from each of the model alternatives in the vicinity of the Climax stock, a fluid flux calibration target, and model weights that describe the plausibility of each conceptual model. The local-scale model utilizes an upscaled discrete fracture network methodology where fluid flow and radionuclides are restricted to an interconnected network of fracture zones mapped onto a continuum grid. Standard Monte Carlo techniques are used to generate 200 random fracture zone networks for each of the 25 conceptual models for a total of 5,000 local-scale flow and transport realizations. Parameters of the fracture zone networks are based on statistical analysis of site-specific fracture data, with the exclusion of fracture density, which was calibrated to match the amount of fluid flux simulated through the Climax stock by the regional-scale models. Radionuclide transport is simulated according to a random walk particle method that tracks particle trajectories through the fracture continuum flow fields according to advection, dispersion and diffusional mass exchange between fractures and matrix. The breakthrough of a conservative radionuclide with a long half-life is used to evaluate the influence of conceptual and parametric uncertainty on radionuclide mass flux estimates. The fluid flux calibration target was found to correlate with fracture density, and particle breakthroughs were generally found to increase with increases in fracture density. Boundary conditions extrapolated from the regional-scale model exerted a secondary influence on radionuclide breakthrough for models with equal fracture density. The incorporation of weights into radionuclide flux estimates resulted in both noise about the original (unweighted) mass flux curves and decreases in the variance and expected value of radionuclide mass flux.     

A reevaluation of in situ cosmogenic 3He production rates

³He is among the most commonly measured terrestrial cosmogenic nuclides, but an incomplete understanding of the ³He production rate has limited robust interpretation of cosmogenic ³He concentrations. We use new measurements of cosmogenic ³He in olivine from a well-dated lava flow at Tabernacle Hill, Utah, USA, to calibrate the local ³He production rate. The new ³He measurements (n = 8) show excellent internal consistency and yield a sea level high latitude (SLHL) production rate of 123 ± 4 at g^?1 yr^?1 following the Lal (1991)/Stone (2000) scaling model [Lal, D., 1991. Cosmic ray labeling of erosion surfaces: in situ nuclide production rates and erosion models. Earth and Planetary Science Letters, 104, 424–439.; Stone, J.O., 2000. Air pressure and cosmogenic isotope production. Journal of Geophysical Research, 105, 23753–23759.]. We incorporate the new measurements from Tabernacle Hill in a compilation of all published production rate determinations, characterizing the mean global SLHL production rates (e.g. 120 ± 9.4 at g^?1 yr^?1 with Lal (1991)/Stone (2000)). The internal consistency of the global ³He production rate dataset is as good as the other commonly used cosmogenic nuclides. Additionally, ³He production rates in olivine and pyroxene agree within experimental error. The ³He production rates are implemented in an age and erosion rate calculator, forming a new module of the CRONUS-Earth web-based calculator, a simple platform for cosmogenic nuclide data interpretation [Balco, G., Stone, J., Lifton, N.A., and Dunai, T.J., 2008. A complete and easily accessible means of calculating surface exposure ages or erosion rates from ¹⁰Be and ²⁶Al measurements. Quaternary Geochronology, 3, 174–195.]. The ³He calculator is available online at http://www.cronuscalculators.nmt.edu/.     

(Re)making space for kiwi: beyond ‘fortress conservation’ in Northland

Mainstream conservation has been long dominated globally by the protected area paradigm. This approach has been widely challenged in recent years, and new conservation initiatives have emerged. The situation is mirrored in New Zealand, where ongoing biodiversity loss has prompted reappraisal. Within this context, we highlight the plight of kiwi, focusing on Northland and efforts there to (re)make space for this iconic bird which is at risk of extinction. While the state has primarily responded by fortifying ‘islands’ on public conservation land, Far North communities are working in a variety of ways and localities, both within and beyond ‘the fortress’, to secure a future for the ‘people's bird’.     

Steady convective exchange flows down slopes

: Horizontal exchange flows driven by destabilising buoyancy fluxes through the surface waters of lakes and coastal regions of oceans are important in understanding the transport of nutrients, micro-organisms and pollutants from littoral to pelagic zones. Our interest here is in the discharge flow driven by cooling or destabilising forcing at the water surface in a water body with variable depth due to sloping bottom topography. Flow visualisation studies and measurements in a laboratory model enabled us to develop scaling arguments to predict the dependency of discharge upon surface forcing and the angle of bottom slope. The results were used to interpret both the laboratory measurements and field data from a small shallow lake with sloping sides and an essentially flat bottomed interior, as well as published results from the literature. The steady state horizontal exchange can be described by Q = 0.24 B^1/3 (l tan //(1 + tan /))^4/3, where Q is the discharge rate per unit length of shoreline, / is the angle of the bottom slope, B is the surface buoyancy flux and l is the horizontal length of the forcing region over the slope. The flushing timescale of the wedge shaped littoral region was given by Ff ~ l ^2/3 (1 + tan /) ^4/3/ (B tan / ^1/3. While the buoyancy flux in the field is almost never constant in space or time and the slope from the shore is seldom uniform, we found that the exchange rate was relatively insensitive to buoyancy flux changes and only moderately sensitive to slope.