Arthur J. Gold

Ecological Thresholds: The Key to Successful Environmental Management or an Important Concept with No Practical Application?

An ecological threshold is the point at which there is an abrupt change in an ecosystem quality, property or phenomenon, or where small changes in an environmental driver produce large responses in the ecosystem. Analysis of thresholds is complicated by nonlinear dynamics and by multiple factor controls that operate at diverse spatial and temporal scales. These complexities have challenged the use and utility of threshold concepts in environmental management despite great concern about preventing dramatic state changes in valued ecosystems, the need for determining critical pollutant loads and the ubiquity of other threshold-based environmental problems. In this paper we define the scope of the thresholds concept in ecological science and discuss methods for identifying and investigating thresholds using a variety of examples from terrestrial and aquatic environments, at ecosystem, landscape and regional scales. We end with a discussion of key research needs in this area.

Nitrous oxide production in riparian zones and its importance to national emission inventories

Abstract Riparian zones, which sit at the interface between terrestrial and aquatic components of the landscape, often receive and process large amounts of excess nitrogen (N) that moves out of agricultural fields towards streams. These areas thus have the potential to be “hotspots” of nitrous oxide (N2O) production in the landscape. However, current Intergovernmental Program on Climate Change (IPCC) methodologies for calculating national N2O emission inventories do not explicitly account for riparian N2O production. In this paper, we examine the nature and extent of N2O production in riparian zones, present some new data on N2O production in these areas, and propose a modification to the current IPCC methodology for quantifying N2O emissions from agriculture. We also present an example of how large-scale riparian restoration efforts to achieve agricultural water quality objectives could cause significant changes in regional N2O budgets. Although current data are inadequate to propose a quantitative emission factor for riparian N2O emissions, they suggest that these emissions are likely to be significant in many regions. Specific data on riparian N2O emissions should be collected in association with detailed watershed mass balance studies that allow for evaluation of several aspects of the IPCC methodology at once and provide constraints on the magnitude of fluxes that are difficult to measure, e.g. N2O flux, N2O:N2 ratio. Riparian and wetland restoration projects to reduce NO3− delivery to coastal waters are being considered in many areas of the world. These projects may affect regional and global N2O budgets, but only if they alter the N2O:N2 ratio during denitrification.

Coastal Lagoons and Climate Change: Ecological and Social Ramifications in U.S. Atlantic and Gulf Coast Ecosystems

Lagoons are highly productive coastal features that provide a range of natural services that society values. Their setting within the coastal landscape leaves them especially vulnerable to profound physical, ecological, and associated societal disturbance from global climate change. Expected shifts in physical and ecological characteristics range from changes in flushing regime, freshwater inputs, and water chemistry to complete inundation and loss and the concomitant loss of natural and human communities. Therefore, managing coastal lagoons in the context of global climate change is critical. Although management approaches will vary depending on local conditions and cultural norms, all management scenarios will need to be nimble and to make full use of the spectrum of values through which society views these unique ecosystems. We propose that this spectrum includes pragmatic, scholarly, aesthetic, and tacit categories of value. Pragmatic values such as fishery or tourism revenue are most easily quantified and are therefore more likely to be considered in management strategies. In contrast, tacit values such as a sense of place are more difficult to quantify and therefore more likely to be left out of explicit management justifications. However, tacit values are the most influential to stakeholder involvement because they both derive from and shape individual experiences and beliefs. Tacit values underpin all categories of social values that we describe and can be expected to have a strong influence over human behavior. The articulation and inclusion of the full spectrum of values, especially tacit values, will facilitate and support nimble adaptive management of coastal lagoon ecosystems in the context of global climate change.

Nitrous oxide production in riparian zones and groundwater

This paper addresses the question of whether riparian zones and groundwater are ‘hotspots’ of nitrous oxide (N2O) flux in the landscape. First, we describe how riparian zones and groundwater function as transformers of N, with a particular emphasis on mechanisms of N2O production in these ecosystems. We then present specific data on N2O flux in these ecosystems and attempt to reconcile these data with existing regional scale estimates of N flux for Norway and with estimates of N2O flux for Norway produced using the OECD/IPCC/IEA Phase II methodology for calculation of regional and global N2O budgets. While the OECD/IPCC/IEA approach produces estimates of riparian and groundwater N2O flux that are reasonable, given what we know about regional scale N balances and actual data on N2O flux, it does not allow us to determine if riparian zones and groundwater are ‘hotspots’ of N2O production in the landscape. The approach fails to answer this question because it is unable to account for spatially explicit phenomena such as riparian and groundwater processing of excess agricultural N. Research needs that would allow us to address this question are discussed.

Influence of natural and novel organic carbon sources on denitrification in forest, degraded urban, and restored streams

Organic carbon is important in regulating ecosystem function, and its source and abundance may be altered by urbanization. We investigated shifts in organic carbon quantity and quality associated with urbanization and ecosystem restoration, and its potential effects on denitrification at the riparian–stream interface. Field measurements of streamwater chemistry, organic carbon characterization, and laboratory-based denitrification experiments were completed at two forested, two restored, and two unrestored urban streams at the Baltimore Long-Term Ecological Research site, Maryland, USA. Dissolved organic carbon (DOC) and nitrate loads increased with runoff according to a power-law function that varied across sites. Stable isotopes and molar C:N ratios suggested that stream particulate organic matter (POM) was a mixture of periphyton, leaves, and grass that varied across site types. Stable-isotope signatures and lipid biomarker analyses of sediments showed that terrestrial organic carbon sources in streams...

Denitrification Capacity in a Subterranean Estuary below a Rhode Island Fringing Salt Marsh

Coastal waters are severely threatened by nitrogen (N) loading from direct groundwater discharge. The subterranean estuary, the mixing zone of fresh groundwater and sea water in a coastal aquifer, has a high potential to remove substantial N. A network of piezometers was used to characterize the denitrification capacity and groundwater flow paths in the subterranean estuary below a Rhode Island fringing salt marsh.15N-enriched nitrate was injected into the subterranean estuary (in situ push-pull method) to evaluate the denitrification capacity of the saturated zone at multiple depths (125–300 cm) below different zones (upland-marsh transition zone, high marsh, and low marsh). From the upland to low marsh, the water table became shallower, groundwater dissolved oxygen decreased, and groundwater pH, soil organic carbon, and total root biomass increased. As groundwater approached the high and low marsh, the hydraulic gradient increased and deep groundwater upwelled. In the warm season (groundwater temperature >12 °C), elevated groundwater denitrification capacity within each zone was observed. The warm season low marsh groundwater denitrification capacity was significantly higher than all other zones and depths. In the cool season (groundwater temperature <10.5 °C), elevated groundwater denitrification capacity was only found in the low marsh. Additions of dissolved organic carbon did not alter groundwater denitrification capacity suggesting that an alternative electron donor, possibly transported by tidal inundation from the root zone, may be limiting. Combining flow paths with denitrification capacity and saturated porewater residence time, we estimated that as much as 29–60 mg N could be removed from 11 of water flowing through the subterranean estuary below the low marsh, arguing for the significance of subterranean estuaries in annual watershed scale N budgets.

The effect of a protected area on the tradeoffs between short-run and long-run benefits from mangrove ecosystems

Protected areas are used to sustain biodiversity and ecosystem services. However, protected areas can create tradeoffs spatially and temporally among ecosystem services, which can affect the welfare of dependent local communities. This study examines the effect of a protected area on the tradeoff between two extractive ecosystem services from mangrove forests: cutting mangroves (fuelwood) and harvesting the shrimp and fish that thrive if mangroves are not cut. We demonstrate the effect in the context of Saadani National Park (SANAPA) in Tanzania, where enforcement of prohibition of mangrove harvesting was strengthened to preserve biodiversity. Remote sensing data of mangrove cover over time are integrated with georeferenced household survey data in an econometric framework to identify the causal effect of mangrove protection on income components directly linked to mangrove ecosystem services. Our findings suggest that many households experienced an immediate loss in the consumption of mangrove firewood, with the loss most prevalent in richer households. However, all wealth classes appear to benefit from long-term sustainability gains in shrimping and fishing that result from mangrove protection. On average, we find that a 10% increase in the mangrove cover within SANAPA boundaries in a 5-km2 radius of the subvillage increases shrimping income by approximately twofold. The creation of SANAPA shifted the future trajectory of the area from one in which mangroves were experiencing uncontrolled cutting to one in which mangrove conservation is providing gains in income for the local villages as a result of the preservation of nursery habitat and biodiversity.

The role of interface organizations in science communication and understanding

“Interface” organizations are groups created to foster the use of science in environmental policy, management, and education. Here we compare interface organizations that differ in spatial scale, modes of operation, and intended audience to illustrate their diversity and importance in promoting the application of science to environmental issues. There has been exciting recent growth in the nature and extent of activities by interface organizations and in new methods for science communication and engagement. These developments can help scientists – who face personal and institutional challenges when attempting to convey the results of their research to various audiences – interact with society on specific issues in specific places, and with a wide range of non-traditional audiences. The ongoing mission for these organizations should be to move beyond simply increasing awareness of environmental problems to the creation of solutions that result in genuine environmental improvements.

Nitrous oxide generation, denitrification, and nitrate removal in a seepage wetland intercepting surface and subsurface flows from a grazed dairy catchment

Little is known about seepage wetlands, located within agricultural landscapes, with respect to removing nitrate (NO3 � ) from agricultural catchments, mainly through gaseous emissions of nitrous oxide (N2O) and dinitrogen (N2) via denitrification. These variables were quantified using a push-pull technique where we introduced a subsurface water plume spiked with 15 N-enriched NO3 � and 2 conservative tracers (bromide (Br � ) and sulfur hexafluoride (SF6)) into each of 4 piezometers and extracted the plume from the same piezometers throughout a 48-h period. To minimise advective and dispersive flux, we placed each of these push-pull piezometers within a confined lysimeter (0.5m diameter) installed around undisturbed wetland soil and vegetation. Although minimal dilution of the subsurface water plumes occurred, NO3 � -N concentration dropped sharply in the first 4h following dosing, such that NO3 � -limiting conditions (<2mg/L of NO3-N) for denitrification prevailed over the final 44h of the experiment. Mean subsurface water NO3 � removal rates during non-limiting conditions were 15.7mg/L.day. Denitrification (based on the generation of isotopically enriched N2O plus N2) accounted for only 7% (1.1mg/L.day) of the observed groundwater NO3 � removal, suggesting that other transformation processes, such as plant uptake, were responsible for most of the NO3 � removal. Although considerable increases in 15 N-enriched N2O levelswere initiallyobserved followingNO3 � dosing,no netemissionswere generated over the 48-h study. Our results suggest that this wetland may be a source of N2O emissions when NO3 � concentrations are elevated(non-limited),butcanreadilyremoveN2O(functionasaN2Osink)whenNO3 � levelsarelow.Theseresultsargue for the use of engineered bypass flow designs to regulate NO3 � loading to wetland denitrification buffers during high flow events and thus enhance retention time and the potential for NO3 � -limiting conditions and N2O removal. Although this type of management may reduce the full potential for wetland NO3 � removal, it provides a balance between water quality goals and greenhouse gas emissions. Additional keywords: bromide, denitrification, 15 N, NO3 � removal, N2O, N2, wetland, SF6.

Denitrifying Bioreactors for Nitrate Removal: A Meta-Analysis

Meta-analysis approaches were used in this first quantitative synthesis of denitrifying woodchip bioreactors. Nitrate removal across environmental and design conditions was assessed from 26 published studies, representing 57 separate bioreactor units (i.e., walls, beds, and laboratory columns). Effect size calculations weighted the data based on variance and number of measurements for each bioreactor unit. Nitrate removal rates in bed and column studies were not significantly different, but both were significantly higher than wall studies. In denitrifying beds, wood source did not significantly affect nitrate removal rates. Nitrate removal (mass per volume) was significantly lower in beds with <6-h hydraulic retention times, which argues for ensuring that bed designs incorporate sufficient time for nitrate removal. Rates significantly declined after the first year of bed operation but then stabilized. Nitrogen limitation significantly affected bed nitrate removal. Categorical and linear assessments found significant nitrate removal effects with bed temperature; a of 2.15 was quite similar to other studies. Lessons from this meta-analysis can be incorporated into bed designs, especially extending hydraulic retention times to increase nitrate removal under low temperature and high flow conditions. Additional column studies are warranted for comparative assessments, as are field-based studies for assessing in situ conditions, especially in aging beds, with careful collection and reporting of design and environmental data. Future assessment of these systems might take a holistic view, reviewing nitrate removal in conjunction with other processes, including greenhouse gas and other unfavorable by-product production.