Nicholas D. Ward

Adult attention-deficit hyperactivity disorder: Assessment guidelines based on clinical presentation to a specialty clinic

Of 143 adults presenting for attention-deficit hyperactivity disorder (ADHD) evaluation, 46 (32%) clearly met diagnostic criteria, 46 (32%) clearly did not meet diagnostic criteria, and another 51 (36%) with current ADHD-like features did not meet criteria due to either a lack of childhood history and/or complicating severe psychiatric or substance abuse comorbidity. The three groups were similar in demographics, psychiatric diagnosis, psychiatric symptom severity, and functional impairment. Compared with the group not meeting ADHD criteria, patients with ADHD had more frequent histories of learning disability in childhood, poorer reading scores on the Wide-Range Achievement Test (WRAT), poorer scores on the Continuous Performance Test (CPT), and higher scores on the Wender-Utah Rating Scale (WURS) for ADHD. Patients in the ambiguous ADHD category had higher rates of current substance abuse than the other two groups. While this group resembled the non-ADHD group in having a low incidence of learning disability and normal reading scores, their poor performance on the CPT and high scores on the WURS more closely resembled those of ADHD patients. These findings suggest that there are a few rating scales, testing instruments, and lifetime history characteristics that help to clarify the difficult diagnostic distinction between adult patients who do and do not have ADHD.

Fate of the Amazon River dissolved organic matter in the tropical Atlantic Ocean

Constraining the fate of dissolved organic matter (DOM) delivered by rivers is a key to understand the global carbon cycle, since DOM mineralization directly influences air-sea CO2 exchange and multiple biogeochemical processes. The Amazon River exports large amounts of DOM, and yet the fate of this material in the ocean remains unclear. Here we investigate the molecular composition and transformations of DOM in the Amazon River-ocean continuum using ultrahigh resolution mass spectrometry and geochemical and biological tracers. We show that there is a strong gradient in source and composition of DOM along the continuum, and that dilution of riverine DOM in the ocean is the dominant pattern of variability in the system. Alterations in DOM composition are observed in the plume associated with the addition of new organic compounds by phytoplankton and with bacterial and photochemical transformations. The relative importance of each of these drivers varies spatially and is modulated by seasonal variations in river discharge and ocean circulation. We further show that a large fraction (50–76%) of the Amazon River DOM is surprisingly stable in the coastal ocean. This results in a globally significant river plume with a strong terrigenous signature and in substantial export of terrestrially derived organic carbon from the continental margin, where it can be entrained in the large-scale circulation and potentially contribute to the long-term storage of terrigenous production and to the recalcitrant carbon pool found in the deep ocean.

Positive priming of terrestrially derived dissolved organic matter in a freshwater microcosm system

The role of priming processes in the remineralization of terrestrially derived dissolved organic carbon (TDOC) in aquatic systems has been overlooked. We provide evidence for TDOC priming using a lab-based microcosm experiment in which TDOC was primed by the addition of 13C-labeled algal dissolved organic carbon (ADOC) or a 13C-labeled disaccharide (trehalose). The rate of TDOC remineralization to carbon dioxide (CO2) occurred 4.1 ± 0.9 and 1.5 ± 0.3 times more rapidly with the addition of trehalose and ADOC, respectively, relative to experiments with TDOC as the sole carbon source over the course of a 301 h incubation period. Results from these controlled experiments provide fundamental evidence for the occurrence of priming of TDOC by ADOC and a simple disaccharide. We suggest that priming effects on TDOC should be considered in carbon budgets for large-river deltas, estuaries, lakes, hydroelectric reservoirs, and continental shelves.

The reactivity of plant‐derived organic matter and the potential importance of priming effects along the lower Amazon River

Here we present direct measurements of the biological breakdown of 13C-labeled substrates to CO2 at seven locations along the lower Amazon River, from Obidos to the mouth. Dark incubation experiments were performed at high and low water periods using vanillin, a lignin phenol derived from vascular plants, and at the high water period using four different 13C-labeled plant litter leachates. Leachates derived from oak wood were degraded most slowly with vanillin monomers, macrophyte leaves, macrophyte stems, and whole grass leachates being converted to CO2 1.2, 1.3, 1.7, and 2.3 times faster, respectively, at the upstream boundary, Obidos. Relative to Obidos, the sum degradation rate of all four leachates was 3.3 and 2.6 times faster in the algae-rich Tapajos and Xingu Rivers, respectively. Likewise, the leachates were broken down 3.2 times more quickly at Obidos when algal biomass from the Tapajos River was simultaneously added. Leachate reactivity similarly increased from Obidos to the mouth with leachates breaking down 1.7 times more quickly at Almeirim (midway to the mouth) and 2.8 times more quickly across the river mouth. There was no discernible correlation between in situ nutrient levels and remineralization rates, suggesting that priming effects were an important factor controlling reactivity along the continuum. Further, continuous measurements of CO2, O2, and conductivity along the confluence of the Tapajos and Amazon Rivers and the Xingu and Jaraucu Rivers revealed in situ evidence for enhanced O2 drawdown and CO2 production along the mixing zone of these confluences.

Temporal variation in river nutrient and dissolved lignin phenol concentrations and the impact of storm events on nutrient loading to Hood Canal, Washington, USA

Rapid rainfall events can be responsible for a large proportion of annual nutrient and carbon loading from a watershed. The bioavailability of organic matter during these rapid loading events increases, suggesting that storms play a relevant role in the mobilization of potentially labile terrestrial carbon. A high correlation between river discharge rates and dissolved and particulate nutrient and carbon concentrations during autumn and winter storms was observed in several temperate Pacific Northwest rivers. Dissolved and particulate lignin concentrations also increased with river discharge; for example, in October 2009 dissolved lignin concentrations increased roughly 240% with a 200% increase in river discharge. During these storms a unique phenolic composition was observed for dissolved lignin that was rapidly mobilized from surface soils relative to the base flow of dissolved lignin. The observed increase in Ad/Al ratios with discharge indicates that rapidly mobilized dissolved lignin is more degraded than the base flow of dissolved lignin. Similarly, a marked increase in C/V ratios and decrease in the S/V ratio of dissolved lignin phenols with increasing river discharge was observed. These results may indicate a difference in source between mobilized and base flow pools, or, more likely, preferential degradation and mobilization/retention of specific lignin phenols. The cumulative results from this year-long data set indicate that a shallow nutrient-rich pool of particulate and dissolved organic matter accumulates in watersheds during periods of soil-saturation deficiency (summer). Autumn and winter storms mobilize this pool of accumulated nutrients from surface soils, which is exhausted with successive winter storms.

Oxidative mitigation of aquatic methane emissions in large Amazonian rivers

The flux of methane (CH4 ) from inland waters to the atmosphere has a profound impact on global atmospheric greenhouse gas (GHG) levels, and yet, strikingly little is known about the dynamics controlling sources and sinks of CH4 in the aquatic setting. Here, we examine the cycling and flux of CH4 in six large rivers in the Amazon basin, including the Amazon River. Based on stable isotopic mass balances of CH4 , inputs and outputs to the water column were estimated. We determined that ecosystem methane oxidation (MOX) reduced the diffusive flux of CH4 by approximately 28-96% and varied depending on hydrologic regime and general geochemical characteristics of tributaries of the Amazon River. For example, the relative amount of MOX was maximal during high water in black and white water rivers and minimal in clear water rivers during low water. The abundance of genetic markers for methane-oxidizing bacteria (pmoA) was positively correlated with enhanced signals of oxidation, providing independent support for the detected MOX patterns. The results indicate that MOX in large Amazonian rivers can consume from 0.45 to 2.07 Tg CH4 yr(-1) , representing up to 7% of the estimated global soil sink. Nevertheless, climate change and changes in hydrology, for example, due to construction of dams, can alter this balance, influencing CH4 emissions to atmosphere.

Impact of Wetland Decline on Decreasing Dissolved Organic Carbon Concentrations along the Mississippi River Continuum

Prior to discharging to the ocean, large rivers constantly receive inputs of dissolved organic carbon (DOC) from tributaries or fringing floodplains and lose DOC via continuous in situ processing along distances that span thousands of kilometers. Current concepts predicting longitudinal changes in DOC mainly focus on in situ processing or exchange with fringing floodplain wetlands, while effects of heterogeneous watershed characteristics are generally ignored. We analyzed results from a 17-year time-series of DOC measurements made at 7 sites and three expeditions along the entire Mississippi River main channel with DOC measurements made every 17 km. The results show a clear downstream decrease in DOC concentrations that was consistent throughout the entire study period. Downstream DOC decreases were primarily (~63-71%) a result of constant dilutions by low-DOC tributary water controlled by watershed wetland distribution, while in situ processing played a secondary role. We estimate that from 1780 to 1980 wetland loss due to land-use alterations caused a ca. 58% decrease in in DOC concentrations in the tributaries of the Mississippi River. DOC reductions caused by watershed wetland loss likely impacted the capacity for the river to effectively remove nitrogen via denitrification, which can further exacerbate coastal hypoxia. These findings highlight the importance of watershed wetlands in regulating DOC longitudinally along the headland to ocean continuum of major rivers.

Dissolved Organic and Inorganic Carbon Flow Paths in an Amazonian Transitional Forest

As a raindrop falls from the atmosphere, over vegetation and forest canopies, and enters soils and streams it experiences a dynamic exchange of carbon constituents with the surrounding environment. Understanding the magnitude and dynamics of carbon export in above and below ground flow paths is critical for constraining the influence of terrestrial and aquatic ecosystems on global carbon cycling. Here we examine the concentration and flux of dissolved organic and inorganic carbon (DOC and DIC) in rainfall, throughfall, stemflow, overland flow, soil solution, groundwater, and stream water in an Amazonian transitional forest located near the arc of deforestation. Rainfall was enriched in DOC by interactions with atmospheric particles derived from both biogenic and anthropogenic emissions, resulting in a flux of 82.3 kg C ha-1 yr-1, which was the largest flux from each respective flow path. Forest throughfall, stemflow, and direct overland flow mobilized DOC from products of terrestrial primary production and decomposition. Net throughfall represented the second largest DOC flux (68.4 kg C ha-1 yr-1), whereas stemflow and overland flow only had a flux of 1.5 and 3.9 kg C ha-1 yr-1, respectively. Much of the DOC in above ground flow paths was removed from solution as the rain percolated through soil layers due to both biological decomposition and sorption/desorption to mineral surfaces, resulting in low concentrations in stream and groundwater (2.6 ± 2.4 mg L-1 and 1.45 ± 0.3 mg L-1, respectively) relative to throughfall (43.9 ± 5.2 mg L-1) and stemflow (30.6 ± 2.7 mg L-1). The flux of DIC in each respective flow path was generally lower than for DOC, and likely driven by atmospheric gas exchange and inputs from respiration and decomposition. DOC concentrations in above ground flow paths were highest during the first rainfall after a dry period, whereas DIC concentrations generally increased throughout the rainy season as soil moisture increased.

Evaluation of Primary Production in the Lower Amazon River Based on a Dissolved Oxygen Stable Isotopic Mass Balance

The Amazon River outgasses nearly an equivalent amount of CO2 as the rainforest sequesters on an annual basis due to microbial decomposition of terrigenous and aquatic organic matter. Most research performed in the Amazon has been focused on unraveling the mechanisms driving CO2 production since the recognition of a persistent state of CO2 supersaturation. However, although the river system is clearly net heterotrophic, the interplay between primary production and respiration is an essential aspect to understanding the overall metabolism of the ecosystem and potential transfer of energy up trophic levels. For example, an efficient ecosystem is capable of both decomposing high amounts of organic matter at lower trophic levels, driving CO2 emissions, and accumulating energy/biomass in higher trophic levels, stimulating fisheries production. Early studies found minimal evidence for primary production in the Amazon River mainstem and it has since been assumed that photosynthesis is strongly limited by low light penetration attributed to the high sediment load. Here, we test this assumption by measuring the stable isotopic composition of O2 (δ18O-O2) and O2 saturation levels in the lower Amazon River from Obidos to the river mouth and its major tributaries, the Xingu and Tapajos rivers, during high and low water periods. An oxygen mass balance model was developed to estimate the input of photosynthetic oxygen in the discrete reach from Obidos to Almeirim, midway to the river mouth. Based on the oxygen mass balance we estimate that primary production occurred at a rate of 0.39 ± 0.24 g O m3 d-1 at high water and 1.02 ± 0.55 g O m3 d-1 at low water. This translates to 41 ± 24% of the rate of O2 drawdown via respiration during high water and 67 ± 33% during low water. These primary production rates are 2-7 times higher than past estimates for the Amazon River mainstem. It is possible that at high water much of this productivity signal is the result of legacy advection from floodplains, whereas limited floodplain connectivity during low water implies that most of this signal is the result of in situ primary production in the Amazon River mainstem.

Patterns of Bacterial and Archaeal Gene Expression through the Lower Amazon River

Analysis of metatranscriptomic and metagenomic datasets from the lower reaches of the Amazon River between Obidos and the river mouth revealed microbial transcript and gene pools dominated by Actinobacteria, Thaumarchaeota, Bacteroidetes, Acidobacteria, Betaproteobacteria, and Planctomycetes. Three mainstem stations spanning a 625 km reach had similar gene expression patterns (transcripts gene copy-1) across a diverse suite of element cycling genes, but two tributary-influenced stations at the mouth of the Tapajos River and near the Tocantins River at Belem had distinct transcriptome composition and expression ratios, particularly for genes encoding light-related energy capture (higher) and iron acquisition and ammonia oxidation (lower). Environmental parameters that were useful predictors of gene expression ratios included concentrations of lignin phenols, suspended sediments, nitrate, phosphate, and particulate organic carbon and nitrogen. Similar to the gene expression data, these chemical properties reflected highly homogeneous mainstem stations punctuated by distinct tributary-influenced stations at Tapajos and Belem. Although heterotrophic processes were expected to dominate in the lower Amazon, transcripts from photosynthetic bacteria were abundant in tributary-influenced regions, and transcripts from Thaumarcheota taxa genetically capable of chemosynthetic ammonia oxidation accounted for up to 21% of the transcriptome at others. Based on regressions of transcript numbers against gene numbers, expression ratios of Thaumarchaeota populations were largely unchanged within the mainstem, suggesting a relatively minor role for gene regulation. These quantitative gene and transcript inventories detail a diverse array of energy acquisition strategies and metabolic capabilities for bacteria and archaea populations of the world’s largest river system.