McClain Publications

Michael E. McClain

Abstracts

Melesse A, V Nangia, X Wang & M McClain. 2007. Wetland Restoration Response Analysis using MODIS and Groundwater Data. Sensors 7, 1916-1933.

Vegetation cover and groundwater level changes over the period of restoration are the two most important indicators of the level of success in wetland ecohydrological restoration. As a result of the regular presence of water and dense vegetation, the highest evapotranspiration (latent heat) rates usually occur within wetlands. Vegetation cover and evapotranspiration of large areas of restoration like that of Kissimmee River basin, South Florida will be best estimated using remote sensing technique than point measurements. Kissimmee River basin has been the area of ecological restoration for some years. The current ecohydrological restoration activities were evaluated through fractional vegetation cover (FVC) changes and latent heat flux using Moderate Resolution Imaging Spectroradiometer (MODIS) data. Groundwater level data were also analyzed for selected eight groundwater monitoring wells in the basin. Results have shown that the average fractional vegetation cover and latent heat along 10 km buffer of Kissimmee River between Lake Kissimmee and Lake Okeechobee was higher in 2004 than in 2000. It is evident that over the 5-year period of time, vegetated and areas covered with wetlands have increased significantly especially along the restoration corridor. Analysis of groundwater level data (2000-2004) from eight monitoring wells showed that, the average monthly level of
groundwater was increased by 20 cm and 34 cm between 2000 and 2004, and 2000 and 2003, respectively. This change was more evident for wells along the river.

Jacobs SM, JS Bechtold, HC Biggs, NB Grimm, S Lorentz, ME McClain, RJ Naiman, SS Perakis, G Pinay, MC Scholes. In press . Nutrient vectors and riparian processing in African semiarid savanna ecosystems . Submitted to Ecosystems.

This article describes vectors for nitrogen and phosphorus delivery to riparian zones in semiarid African savannas, the processing of nutrients in the riparian zone and the effect of disturbance on these processes. Semiarid savannas exhibit sharp seasonality, high spatial heterogeneity and complex hillslope hydrology, all of which ultimately impact nutrient fluxes between riparian, upland and aquatic environments. Dissolved and particulate nutrients accumulate in soils during dry winters, and are mobilized and transported in overland flow to riparian zones by summer rains. Strong environmental drivers in African savannas, such as fire and herbivory, enhance nitrogen, phosphorus and sediment transport to lower slope positions by shaping vegetative spatial patterns. Nutrients and sediments are also deposited in the riparian zone during seasonal and intermittent floods while, during the dry season, subsurface movement of water from the stream into riparian soils and vegetation further enrich riparian zones with nutrients. Nutrients are immobilized in the riparian corridor through microbial and plant uptake, while dissimilatory processes such as denitrification may be important where labile nitrogen and carbon are in adequate supply and physical conditions are suitable, such as in seeps, wallows created by animals, ephemeral wetlands and stream edges. Interaction between temporal hydrologic connectivity and spatial heterogeneity are disrupted by disturbances, which may convert certain riparian patches from sinks to sources for nitrogen and phosphorus. As the landscapes of semiarid southern Africa are modified for socioeconomic purposes there is an increasingly important need to understand the role of riparian zones in regulating the movements of nutrients and sediments from uplands to streams. The scientific challenges are to provide a basic understanding of riparian biogeochemistry to adequately address the temporal and spatial impact of disturbances and to apply this knowledge to better land and water management in this region. An integrated, multidisciplinary approach applied in protected as well as human-disturbed ecosystems in southern Africa is essential for underpinning a strong environmental basis for sustainable human-related expansion.

Mena CA, R Bilsborrow & ME McClain. Socioeconomic drivers of deforestation in the Napo River Basin of Ecuador. 2006. Environmental Management.

Investigations of land use land cover (LULC) change and forest management are limited by a lack of understanding of how socioeconomic factors affect land-use. This also makes difficult to predict future deforestation, which is especially important in the Amazon basin where large tracts of natural forest are being converted to managed uses. The objectives of this research are: (a) to quantify deforestation in the Northern Ecuadorian Amazon (NEA) between 1986-1996 and 1996-2002 and (b) to determine the significance and magnitude of the effects of socioeconomic factors on deforestation rates at both the parroquia (parish) and finca (farm) levels. Annual deforestation rates are quantified via satellite image processing and geographic information systems. Linear spatial lag regression analyses are then used to explore relationships between socioeconomic factors and deforestation. Socioeconomic factors were obtained, at the finca level, from a detailed household survey carried out in 1990 and 1999, and at the parroquia level based on data from 1990 and 2001 Ecuadorian National Census of Population and Housing. We find that the average annual deforestation rate was 2.49 and 1.78%/yr for 1986-1996 and 1996-2002 respectively. Among others, at the parroquia level, variables representing demographic factors (i.e., population density) and accessibility factors (i.e., road density) were found to be significantly related to deforestation. At the farm level, household size, distance by main road to cities, education, and hired labor are factors related to deforestation. The findings of this research demonstrate both the severity of deforestation in the Northern Ecuadorian Amazon and the array of factors affecting deforestation in the tropics.

Saunders TJ, ME McClain & CA Llerena. The N and P biogeochemistry of terrestrial-aquatic flowpaths in a small montane catchment of the Peruvian Amazon. In press. Hydrological Processes.

Dissolved N and P dynamics along two upland-stream transects in a first-order montane (2414m) rainforest catchment (Wara) of the Peruvian Amazon were studied in order to characterize spatio-temporal controls on terrestrial-aquatic nutrient fluxes. Surface and subsurface waters along the Wara transects were sampled during both baseflow and stormflow conditions from March 2002-March 2003. Under baseflow conditions we found strong terrestrial controls on stream N, P, and dissolved organic carbon (DOC) concentrations. Median NO3- concentrations consistently decreased (~95%) between upland soilwater (dry-15.5µM and wet-32.5µM), and the stream (dry-0.8µM and wet-1.7µM) despite significant seasonal fluctuations of NO3- in the upland. During the dry
season, concentrations of dissolved organic N (DON) also decreased markedly from the upland to the stream, however, despite this decrease, DON remained the dominant component of total dissolved N (TDN) from the upland to the stream (97% to 75%, respectively). Dissolved organic P (DOP) and soluble reactive phosphorus (SRP) concentrations generally followed a spatial trend inverse to N. Low P concentrations were found in the upland and while the highest P concentrations were found within the stream. Similar to DON, DOP also dominated total dissolved P concentrations from the upland to the stream (88% and 92% respectively). Stoichiometric ratios change drastically from the upland (DOC:DON=0.7, DON:DOP=734, DOC:DOP=512, TIN:SRP=166; dry season) to the stream (DOC:DON=31, DON:DOP=3, DOC:DOP=96, TIN:SRP=12; dry season) indicating a de-coupling of nutrient composition between terrestrial and aquatic systems and seem to suggest that decomposition, and possibly primary productivity, is P-limited within upland sites while the stream is limited by N. Regardless, we conclude that under baseflow conditions, strong terrestrially-based mechanisms control in-stream nutrient concentrations and buffer seasonal changes occurring in the upland. In contrast, storm data indicate that precipitation events short-circuit terrestrial controls thereby exporting a pulse of nutrients to the stream.

Townsend-Small A, ME McClain & JA Brandes. Contributions of carbon and nitrogen from the Andes mountains to the Amazon River: Evidence from an elevational gradient of soils, plants, and river material. Limnology and Oceanography 50(2): 672–685.

We determined the carbon (C) and nitrogen (N) elemental and stable isotopic composition of riverine and terrestrial organic matter (OM), as well as the concentration of dissolved organic C (DOC), δ15NO3- and δ18O of river water along an altitudinal (4043 to 720 meters above sea level [masl]) transect in the Andes of Peru. Plant δ13C increased with increasing elevation, but unlike previous studies, foliar δ13C and %N were negatively correlated. Soil δ13C values did not exhibit similar trends and were enriched by 1-3‰ over plants. Isotopically, riverine fine particulate OM (FPOM, < 60 µm) resembled soils, and coarse particulate OM (CPOM, > 60 µm) resembled leaves. Both FPOM and CPOM exhibited OM levels beyond those attributable to sorption. Percent OC and N of soils and FPOM were positively correlated with altitude, and highlight a trend of sequential downstream dilution of OM with inorganic material. FPOM began to resemble plant OM isotopically at lower altitudes, perhaps due to increased plant and surface soil inputs to lower rivers. The compositional similarity of POM to terrestrial plants and soils indicates that the dominant processes affecting riverine OM are occurring on the landscape, not within the river. Dissolved OC (< 0.2 µm) concentration, δ15NO3-, and δ18O of H2O are variable in high altitude tributaries but approach constant values downstream. Elemental and isotopic analyses of riverine OM suggest compositional differences between size fractions, similar to the lower Amazon; however, unlike previous studies, we have found significant within-stream changes with altitude in OM composition.

Grimm NB, SE Gergel, WH McDowell, EW Boyer, CL Dent, P Groffman, S Hart, J Harvey, C Johnston, E Mayorga, ME McClain, G Pinay. Merging aquatic and terrestrial perspectives of nutrient biogeochemistry. Accepted by Oecologia.

Although biogeochemistry is an integrative discipline, terrestrial and aquatic sub-disciplines have developed somewhat independently of one another. Physical and biological differences between aquatic and terrestrial ecosystems explain this history. In both aquatic and terrestrial biogeochemistry, key questions and concepts arise from a focus on nutrient limitation, ecosystem nutrient retention, and controls of nutrient transformations. Current understanding is captured in conceptual models for different ecosystem types, which share some features and diverge in other ways. Distinctiveness of sub-disciplines has been appropriate in some respects and has fostered important advances in theory. On the other hand, lack of integration between aquatic and terrestrial biogeochemistry limits our ability to deal with biogeochemical phenomena across large landscapes in which connections between terrestrial and aquatic elements are important. Separation of the two approaches also has not served attempts to scale up or to estimate fluxes from large areas based on plot measurements. Understanding connectivity between the two system types and scaling up biogeochemical information will rely on coupled hydrologic and ecological models, and may be critical for addressing environmental problems associated with locally, regionally, and globally altered biogeochemical cycles.

Décamps H, G Pinay, RJ Naiman, GE Petts, ME McClain, A Hillbricht-Ilkowska, TA Hanley, RM Holmes, J Quinn, J Gibert, AM Planty Tabacchi, F Schiemer, E Tabacchi & M Zalewski. Riparian zones: Where biogeochemistry meets biodiversity in management practice. Submitted to the Polish Journal of Ecology.

Riparian zones are well known for their inherent ecological properties related to biogeochemical cycles, biodiversity, and catchment management. This article seeks to highlight the role of riparian processes on biogeochemical cycles and biodiversity under different climatic conditions. Their role is investigated by focusing on: i) the lateral ecotone between land and water systems, ii) their longitudinal corridor structure and, iii) the dry-wet cycles. This information is then used to suggest the value of riparian zones in landscape management. We emphasize the key roles of the ecotonal structure, longitudinal connectivity and timing of the occurrence of wet-dry cycles for riparian zones to process nitrate fluxes and to maintain high levels of biodiversity at the landscape scale. In the context of the worldwide transformations of flow regimes, the deterioration of water quality and loss of biodiversity, restoring riparian zones is both a key objective and a formidable challenge that implies envisioning the consequences of management actions on the long term, considering entire river basins, and paying attention to other goods and services that riparian systems can bring to human societies.

McClain ME and Cossio RE. The Use and Conservation of Riparian Zones in the Rural Peruvian Amazon. Environmental Conservation 30: 242-248.

River margins are valued for agriculture in the western Amazon due to their fertile soils and level surfaces. Riparian forests along river margins also provide valuable ecosystem services by protecting water quality and providing resources to aquatic organisms. Because inhabitants of the region rely on these aquatic resources, riparian deforestation may have unintended negative feedbacks on the health and wellbeing of rural communities. We surveyed 79 households of mixed cultural background to investigate how riparian zones were used, what mechanisms were in place for their conservation, and how local people valued them. We found that maize, beans, and peanuts were cultivated preferentially in riparian zones, complementing the manioc and plantains grown on upland soils. We also found that people value riparian zones for their ecosystem services and generally leave a protective buffer of forest along rivers. Our results are encouraging and suggest that through proper management both the agricultural and ecological values of riparian zones can be preserved.

McClain ME, EW Boyer, CL Dent, SE Gergel, NB Grimm, PM Groffman, SC Hart, JW Harvey, CA Johnston, E Mayorga, WH McDowell, G Pinay. Biogeochemical hot spots and hot moments at the interface of terrestrial and aquatic ecosystems. Submitted to Ecosystems

Rates and reactions of biogeochemical processes vary in space and time to produce both hot spots and hot moments of elemental cycling. We define biogeochemical hot spots as patches that exhibit disproportionately high reaction rates relative to the surrounding matrix, while hot moments are short periods of time that exhibit disproportionately high reaction rates relative to longer intervening time periods. As has been appreciated by ecologists for decades, hot spot and hot moment activity is often enhanced at terrestrial-aquatic interfaces. Using examples from the C and N cycles, we show that hot spots occur where hydrological flow paths converge with substrates or other flow paths containing complementary or missing reactants. Hot moments occur when episodic hydrological flow paths reactivate and/or mobilize accumulated reactants. By focusing on the delivery of specific missing reactants via hydrologic flow paths, we forge a more mechanistic understanding of the factors that create hot spots and hot moments. Such a mechanistic understanding is necessary to identify biogeochemical hot spots at broader spatio-temporal scales and to factor them into quantitative models. We suggest that resource managers incorporate both natural and constructed biogeochemical hot spots into water quality management. Lastly, we point out that key research needs are to assess the potential importance of hot spot and hot moment phenomena in the cycling of different bioactive elements and to assess the importance of hot spot activities in large-scale systems.

Sobieraj, JA, Elsenbeer H and McClain ME. in press, The cation and silica chemistry of a Subandean river basin in western Amazonia. Hydrological Processes.

We sampled river water at 13 locations in the Pichis basin, a 10,500 km2 large rainforest-covered drainage basin in Peru, to assess the influence of lithologic variability and seasonality on water chemistry. The concentrations of major cations and silica show a strong seasonal dependence and a remarkable variability over short distances that is only weakly reduced in the wet season; cation concentrations in streams differ by up to 100% within a few kilometers. The lowest cation concentrations were associated with relatively cation-depleted upper Tertiary and lower Quaternary formations, whereas relatively cation-rich lower Tertiary and Jurassic formations left a clear calcium and sodium signal in the respective rivers. Cluster analysis, in conjunction with boxplots, suggests that the sampling locations can be segregated into 3 groups based on similarities of their geochemical signals. According to the criteria defined by Stallard and Edmond (1983), one river is classified as a Group 2 river with 200<TZ+<450 eq/l, whereas all other rivers fall into Group 3 with 450<TZ+<3000 eq/l. Based on a comparison with other studies at different sections of the Amazon mainstream, the river chemistry of our study area is relatively enriched in K+, Mg2+, and Ca2+ , and, consequently, has a higher TZ+ value, while being relatively depleted in silica. The influence of lithologic variability on water chemistry must be considered in land-use change studies even at watershed areas of 26 km2 - 3,382 km2

McClain ME, Aparicio LM & Llerena CA. 2001, Water use and protection in rural communities of the Peruvian Amazon. Water International 26: 400-410. (PDF version)

Inhabitants of the Peruvian Amazon enjoy plentiful water and other aquatic resources that enhance their wellbeing in many ways. However, the intimate and generally unbuffered connection between the region's inhabitants and their water resources leads to complex negative feedbacks when these resources are mismanaged. Due to severe water problems in other more populated parts of the country, the Peruvian government currently devotes little attention to water management in the Amazon. Thus, organized management is mainly left to individual communities and households. This study reports on the results of 351 interviews of households in the Pachitea Basin of the central Peruvian Amazon. Our aim is to quantify the use of water and other aquatic resources among different social groups and within different geographical settings of the region. With these data we evaluate and identify priorities for community-driven water management in the region. We found that 50-90% of households take their water directly from primary sources, 35-94% transport it manually to their homes, and 50-75% practice only the simplest form of treatment (boiling). Indigenous households tended to rely less on water infrastructure and water treatment. Fish and an assortment of other aquatic and riparian resources were important inputs to all social groups and in all geographic settings. Disposal of wastes in nearby water bodies was also found to be widespread. We conclude that water management efforts in the Pachitea basin should focus on the protection of water quality in rivers and streams through careful disposal of wastes away from water bodies and preservation of natural water purification features such as riparian forests and wetlands. We also recommend that a basinwide master plan be developed which empowers end-users and integrates more detailed plans developed at community and association levels.

Hedges JI, Mayorga E, Tsamakis E, McClain ME, Aufdenkampe A, Benner R, Opsahl S, Black B, Pimentel T, Quintinilla J & Maurice L. 2000, Organic matter in Bolivian tributaries of the Amazon river: A comparison to the lower mainstem. Limnology and Oceanography 45: 1449-1466.

We determined the concentrations and compositions of coarse particulate (>63 m), fine particulate (0.1-63 m), and dissolved (0.001-0.1 m) organic matter collected along a river reach extending from a first-order stream in the Bolivian Andes, through the Beni River system, to the lower Madeira and Amazon Rivers. Dissolved organic carbon (DOC) concentrations increased down the total reach from ~80 to 350 uM. The percentage of total DOC with a molecular weight greater than ~1,000 atomic mass units that could be isolated by ultrafiltration also increased downstream from 40 to 80%. Weight percentages of organic carbon in the ultrafiltered isolates also grew downstream from 5% at the uppermost station to 37% in the Amazon mainstem. Organic carbon composed only 0.4-1.2 weight percentage of the total mass of the fine particulate fraction, which accounted for 70-80% of the total organic carbon (TOC) in transport through the highly turbid (~600-2000 mg L-1) Beni sequence. Observed compositional differences were related primarily to the size fractions in which the organic matter occurred. On average, coarse particulate organic material exhibited an atomic C:N of 24, whereas ultrafiltered DOM was nitrogen poor, (C:N)a = 34, and fine particulate material was nitrogen rich, (C:N)a = 15. The lignin and stable-carbon isotopic compositions of these fractions indicate tree leaves and other nonwoody tissues from C3 land plants as predominant sources. Three molecular parameters demonstrate that the coarse, fine, and dissolved fractions of individual water samples are increasingly degraded downstream. Elemental nitrogen, amino acids, and basic amino acids are all preferentially associated with fine minerals. Observed geographical patterns included more positive 13C values in particulate organic matter from high altitude sites and an increase in the abundance and degradation of ultrafiltered dissolved organic matter down the drainage system. Many of these compositional patterns are imprinted within materials carried by low-order, high altitude tributaries and appear to reflect processes occuring on the landscape.

Downing JA, McClain ME, Twilley R, Melack JM, Elser J, Rabalais NN, Lewis WM, Turner RE, Corredor J, Soto D, Yanez-Aranciba A, Kopaska JA & Howarth RW. 1999, The impact of accelerating land-use change on the N-cycle of tropical aquatic ecosystems: Current conditions and projected changes. Biogeochemistry 46: 109-148.

Published data and analyses from temperate and tropical aquatic systems are used to summarize knowledge about the potential impact of land-use alteration on the nitrogen biogeochemistry of tropical aquatic ecosystems, identify important patterns and recommend key needs for research. The tropical N-cycle is traced from pre-disturbance conditions through the phases of disturbance, highlighting major differences between tropical and temperate systems that might influence development strategies in the tropics. Analyses suggest that tropical freshwaters are more frequently N-limited than temperate zones, while tropical marine systems may show more frequent P limitation. These analyses indicate that disturbances to pristine tropical lands will lead to greatly increased primary production in freshwaters and large changes in tropical freshwater communities. Increased freshwater nutrient flux will also lead to an expansion of the high production, N- and light-limited zones around river deltas, a switch from P- to N-limitation in calcereous marine systems, with large changes in the community composition of fragile mangrove and reef systems. Key information gaps are highlighted, including data on mechanisms of nutrient transport and atmospheric deposition in the tropics, nutrient and material retention capacities of tropical impoundments, and N/P coupling and stoichiometric impacts of nutrient supplies on tropical aquatic communities. The current base of biogeochemical data suggests that alterations in the N-cycle will have greater impacts on tropical aquatic ecosystems that those already observed in the temperate zone.

Lewis WM, Melack JM, McDowell WH, McClain ME & Richey JE. 1999. Nitrogen yields from undisturbed watershed in the Americas. Biogeochemistry 46: 149-162.

Yields of total fixed nitrogen and nitrogen fractions are summarized for thirty-one watersheds in which anthropogenic disturbance of the nitrogen cycle, either through land use or atmospheric deposition, is negligible or slight. These yields are taken as representative of background conditions over a broad range of watershed areas, elevations, and vegetation types. The data set focuses on watersheds of the American tropics, but also includes information on the Gambia River (Africa) and some small watersheds in the Sierra Nevada of California. For the tropical watersheds, total nitrogen yield averages 5.1 kg ha -1 y-1. On average, 30% of the total is particulate and 70% is dissolved. Of the dissolved fraction, an average of 50% is organic and 50% is inorganic, of which 20% is ammonium and 80% is nitrate. Yields are substantially lower than previously estimated for background conditions. Yields of all nitrogen fractions are strongly related to runoff, which also explains a large percentage of variance in yield of total nitrogen (r2 = 0.85). For total nitrogen and nitrogen fractions, yield increases at about two-thirds the rate of runoff; concentration decreases as runoff increases. There is a secondary but significant positive relationship between elevation and yield of DIN. Ratios DON/TDN and PN/TN both are related to watershed area rather tan runoff; DON/TDN decreases and PN/TN increases toward higher stream orders. The analysis suggests for tropical watersheds the existence of mechanisms promoting strong homeostasis in the yield of N and its fractions for a given moisture regime, as well as predictable downstream change in proportionate representation N fractions. Yields and concentrations for small tropical watersheds are much larger than for the few temperate ones with which comparisons are possible.

McClain ME, Richey JE, Brandes JA and Pimentel TP. 1997, Dissolved organic matter and terrestrial-lotic linkages in the central Amazon basin. Global Biogeochemical Cycles 11: 295-311.

We evaluate the hypothesis that decomposition and adsorption reactions operating in upland soils of headwater catchments control the concentration and composition of dissolved and fine particulate organic matter in rivers of the Amazon basin. In two contrasting first-order catchments characteristic of the central Amazon basin, we analyzed plant, litter, soil, groundwater, and streamwater chemistry. Our results indicate that clear and persistent differences exist in the concentration and elemental composition of dissolved organic matter (DOM) in stream and groundwaters from the two catchments, due mainly to corresponding differences in soil texture and chemistry. Within the more oxide and clay rich Oxisols underlying terra firme forest, groundwater DOM concentrations were uniformly low ( 120 µMC) and C/N ratios averaged 10. Conversely, within the oxide and clay deficient Spodosols underlying campinarana forest, groundwater DOM concentrations were greatly elevated ( 3000 µMC) and C/N ratios averaged near 60. We found that, in the terra firme/Oxisol terrain, the majority of DOM contributions to the stream derived from the riparian zone, while in the campinarana/Spodosol terrain, upland groundwater contributions could account for the concentration and composition of DOM in the stream. The implications of our findings are that, in the terra firme terrains which dominate the region, upland soil profiles are not the site of definitive processes which impart compositional signatures to organic matter carried by the largest rivers of the Amazon basin, as was hypothesized. Instead, we suggest that definitive reactions are focused primarily in the river corridor.

Richey JE, Wilhelm SR, McClain ME, Victoria RL, Melack JM & Araujo-Lima C. 1997, Organic matter and nutrient dynamics in river corridors of the Amazon basin and their response to anthropogenic change. In Press, Ciencia e Cultura 49: 98-110.

River corridors of the Amazon basin are a rich and vulnerable part of the basin. They include not only river channels, but also riparian zones, flooded savannas, and extensive floodplains of the large rivers. From a biogeochemical perspective, these extra-channel areas filter material derived from uplands and regulate inputs to the river channels. They are also loci for anaerobic processes and trace gas production. Based on research in the Amazon and elsewhere, deforestation and pasture formation are expected to increase fluxes of sediment, organic matter and nutrients to river corridors. Where the margins of river corridors are intact, their filtering abilities may buffer river channels, and thus downstream reaches, from significant upland inputs. However, river corridors of the Amazon are themselves sites of deforestation and agricultural use, and although it is difficult to predict the biogeochemical impacts of development in these areas, increased downstream fluxes of most constituents are possible. In this paper we examine the potential impacts of land-use change on river corridors of the basin. We then propose a research approach to investigate the course and consequences of these changes. The approach examines river corridors at three scales: Small watershed (<10 km2), mesoscale (~10,000 km2) and whole basin (7 by 10 km2). Research sites should be nested, such that the understanding gained at one scale may be translated to the next. Development of quantitative models of river-related processes is advocated, as these models may be linked to similar upland and atmospheric models, thereby facilitating basin-wide analysis of land-use impacts.

Brandes JA, McClain ME and Pimentel TP. 1996, N-15 evidence for the origin and cycling of inorganic nitrogen in a small Amazonian catchment. Biogeochemistry 34:45-56

The d15N composition of the dominant form of dissolved inorganic nitrogen (DIN) was determined in upland groundwater, riparian groundwater, and stream water of the Barro Branco catchment, Amazônas, Brazil. The d15N composition of organic nitrogen in riparian and upland leaf litter was also determined. The data for these waters could be divided into three groups: upland groundwater DIN predominately composed of NO3- with d15N values averaging 6.25±0.9‰ ; riparian groundwater DIN primarily composed of NH4+ with d15N values averaging 9.17±1.0‰, and stream water DIN predominately composed of NO3- with d15N values averaging 4.52±0.8‰. Nitrate samples taken from the stream source and from the stream adjacent to the groundwater transects showed a downstream increase in d15N from 1.0‰ to 4.5‰. Leaf litter samples averaged 3.5±1.2‰.
The observed patterns in isotopic composition, together with previously observed inorganic nitrogen species and concentration shifts between upland, riparian and stream waters, suggest that groundwater DIN is not the primary source of DIN to the stream. Instead, the isotopic data suggest that remineralization of organic nitrogen within the stream itself may be a major source of stream DIN, and that the majority of DIN entering the stream via groundwater flowpaths is removed at the riparian-stream interface.

McClain ME and Richey JE. 1996, Regional-scale linkages of terrestrial and lotic ecosystems in the Amazon basin: A conceptual model for organic matter. Arch. Hydrobiol. Suppl. 113 Large Rivers 10: 111-125

Terrigenous organic matter originating at the margins of streams in headwater regions of the Amazon basin accounts for a large part of the organic matter carried by the major tributaries of the basin. The purpose of this paper it to articulate our present understanding of terrestrial to lotic transfers in these headwater regions in the form of a conceptual model. On a regional scale, the headwaters of the Amazon basin may be subdivided into four dominant terrain types: (1) terra firme forests developed on oxisols and ultisols, (2) campina forest developed on spodosols, (3) savanna developed on oxisols, ultisols, and alfisols, and (4) montane forest developed on ultisols and inceptisols. Within these terrains terrigenous organic matter is transferred to streams via direct litterfall and blow-in, groundwater baseflow, stormflow, and seepage from fringing wetlands.
Based on the limited available data and data from other systems, our current conceptual model is as follows. Direct litterfall contributions from overhanging canopies are similar across the basin and on the order of 0.7 kg/m2/yr. Blow-in contributions are probably on the order of 20% of direct litterfall fluxes. Groundwater baseflow contributions of organic matter (OM) are strongly correlated with soil type and fall into two distinct classes, one draining the campina terrain which is characterized by high dissolved organic carbon (DOC) concentrations (> 20 mg/l) and another draining terra firme, savanna, and montane terrains which is characterized by lower DOC concentrations (< 5 mg/l). Groundwater OM contributions are also compositionally distinct, with elevated proportions of hydrophobic organic molecules in groundwater draining campina. Stormflow contributions across the basin are dominated by saturation overland flow originating in riparian areas, and transferred OM consists primarily of litter washed in from the surrounding forest floor and material flushed from fringing wetlands. In montane terrains, however, there may also be a significant erosive input of soil OM. Contributions of OM from fringing wetlands are most prevalent in lowland terrains where broad, flat riparian zones provide ample sites for wetland development. Both stormflow and seepage from fringing wetlands transfer a wide spectrum of OM composition, ranging from freshly fallen leaves to heavily decomposed and refractory dissolved molecular forms.

McClain ME, Richey JE and Victoria RL. 1995, Andean contributions to the biogeochemistry of the Amazon river system. Bulletin de l'Institut Francais d'Etudes Andines 24: 425-437.

The biogeochemistry of Andean rivers may play a significant role in determining the basin-wide biogeochemistry integrated into the mainstem Amazon River of Brazil. Available data for organic C, NO3-, and PO43- in Andean rivers are highly variable and reveal no clear spatial or altitudinal patterns in concentrations. In general, concentrations measured in Andean rivers are similar to those reported in the mainstem Amazon river and its major tributaries. Explanations of processes which alter Andean-derived particulates and solutes as they exit the Cordillera are only speculative at this time, but their net effect is to diminish Andean signals through decomposition and dilution by lowland-derived material. Analyses of 13C in particulate and dissolved organic matter of the mainstem Amazon provide evidence that some fraction of Andean derived material persists within the river system, ultimately to be discharged to the Atlantic Ocean. In 1994 a new collaborative research program was launched to further characterize the biogeochemistry of Andean rivers.

McClain ME, Richey JE and Pimentel TP. 1994, Groundwater nitrogen dynamics at the terrestrial-lotic interface of a small catchment in the Central Amazon Basin. Biogeochemistry 27: 113-127

Processes operating at the terrestrial-lotic interface may significantly alter dissolved nitrogen concentrations in groundwater as a result of shifting redox conditions and microbial communities. We monitored concentrations of total dissolved nitrogen, NO3-, NH4+, O2 and Fe2+ for 10 months along two transects tracing groundwater flow from an upland (terra firme) forest, beneath the riparian forest, and into the stream channel of a small Central Amazonian catchment. Our aim was to examine the role of near-stream processes in regulating groundwater transfers of dissolved nitrogen from terrestrial to lotic ecosystems in the Central Amazon. We found pronounced compositional differences in inorganic nitrogen chemistry between upland, riparian, and stream hydrologic compartments. Nitrate dominated (average 89% of total inorganic nitrogen; TIN) the inorganic nitrogen chemistry of oxygenated upland groundwater but decreased markedly upon crossing the upland-riparian margin. Conversely, NH4+ dominated (average 93% of TIN) the inorganic chemistry of apparently anoxic riparian groundwater; NH4+ and TIN concentrations decreased markedly across the riparian-stream channel margin. In the oxygenated streamwater, NO3- again dominated (average 82% of TIN) inorganic nitrogen chemistry. Denitrification followed by continued ammonification is hypothesized to effect the shift in speciation observed at the upland-riparian margin, while a combination of several processes may control the shift in speciation and loss of TIN observed at the riparian-stream margin. Dissolved organic nitrogen concentrations did not vary significantly between upland and riparian groundwater, but decreased across the riparian-stream margin.
Our data suggest that extensive transformation reactions focused at the upland and stream margins of the riparian zone strongly regulate and diminish transfers of inorganic nitrogen from groundwater to streamwater in the catchment. This suggestion questions the veracity of attempts in the literature to link stream nitrogen chemistry with nutrient status in adjacent forests of similar catchments in the Central Amazon. It also complicates efforts to model nitrogen transfers across terrestrial-lotic interfaces in response to deforestation and changing climate.

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