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Introduction
Additional affiliations
May 2010 - present
Publications
Publications (43)
Background
About 30 % of vegetables in China are produced in intensively managed greenhouses comprising flood irrigation and extreme rates of nitrogen fertilizers. Little is known about denitrification N losses.
Methods
Soil denitrification rates were measured by the acetylene inhibition technique applied to anaerobically incubated soil samples. F...
Recent studies show that tropical hydroelectric reservoirs may be responsible for substantial greenhouse gas emissions to the atmosphere, yet emissions from the surface of released water downstream of the dam are poorly characterized if not neglected entirely from most assessments. We found that carbon dioxide (CO 2 ) emission downstream of Kariba...
Drip fertigation increases water and nitrogen use efficiency, and reduces nitrogen leaching in comparison to conventional flood irrigation and over fertilization in plastic greenhouse vegetable production. However, it is unknown whether this also improves yield stability, possibly through improved root growth. A two-factor field trial was conducted...
Sub-Saharan Africa (SSA) is home to approximately ¼ of the global livestock population, which in the last 60 years has increased by factors of 2.5-4 times for cattle, goats and sheep. An important resource for pastoralists, most livestock live in semi-arid and arid environments , where they roam during the day and are kept in enclosures (or bomas)...
The Namatala Wetland in Uganda faces severe degradation from agricultural development and urbanization. Besides the Namatala River and tributary rural streams, the wetland receives surface water from Mbale town and wastewater from two sets of wastewater stabilization ponds. The objective of this study was to examine water quality, and sediment and...
Greenhouse gas (GHG) emission estimates from tropical African rivers are underrepresented in global datasets, resulting in uncertainties in their contributions to global emissions. To better constrain the contribution of rivers and streams to GHG emissions from tropical landscapes and to determine possible underlying controlling processes, we imple...
Anthropogenic activities have led to increases in nitrous oxide (N2O) emissions from river systems, but there are large uncertainties in estimates due to lack of data in tropical rivers and rapid increase in human activity. We assessed the effects of land use and river size on N2O flux and concentration in 46 stream sites in the Mara River, Kenya d...
Agricultural intensification may accelerate the loss of wetlands, increasing the concentrations of nutrients and sediments in downstream water bodies. The objective of this study was to assess the effects of land use and land cover and river discharge on water quality in the Migina catchment, southern Rwanda. Rainfall, discharge and water quality (...
Headwater streams influence the biogeochemical characteristics of large rivers and play important roles in regional and global carbon budgets. The combined effects of seasonality and land use change on the biogeochemistry of headwater streams, however, are not well understood. In this study we assessed the influence of catchment land use and season...
Effective management of water quality in large rivers requires information on the influence of activities within the catchment (urban and rural) throughout the whole river basin. However, traditional water quality monitoring programmes undertaken by individual agencies normally relate to specific objectives, such as meeting quality criteria for was...
The tropical savannas of Africa have witnessed a dramatic reduction in native large mammalian herbivore populations. The consequences of these changes for terrestrial-aquatic food-web linkages are poorly documented. We used natural abundances of stable carbon and nitrogen isotopes (δ13C, δ15N) to determine spatial and temporal patterns in the impor...
This chapter studies the response of lakes near the Arctic Long Term Ecological Research (LTER) to changing environmental conditions. It presents data from an extensive series of manipulative experiments that provides insight into how lakes respond to changes in the environment. It describes experiments that include multiyear fertilization treatmen...
Data on the functional composition of invertebrates in tropical streams are needed to develop models
of ecosystem functioning and to assess anthropogenic effects on ecological condition. We collected macroinvertebrates during dry and wet seasons from pools and riffles in 10 open- and 10 closed-canopy Kenyan highland streams. We classified macroinve...
We examined controls of benthic dinitrogen (N2) fixation and primary production in oligotrophic lakes in Arctic Alaska, Toolik Field Station (Arctic Long-Term Ecological Research Site). Primary production in many oligotrophic lakes is limited by nitrogen (N), and benthic processes are important for whole-lake function. Oligotrophic lakes are increa...
Relationships between hydrology, ecosystem function and livelihood outcomes in Nyando papyrus wetland in western Kenya were integrated in a Bayesian Network model with 34 nodes which was populated with results of ecohydrological and socio-economic research and stakeholder consultations. Scenarios for dry and wet seasons were evaluated. For a curren...
It is increasingly recognized that in order to accurately determine
terrestrial carbon budgets, we must understand how dissolved organic
matter (DOM) in rivers is lost via microbial respiration. The amount of
respiration that can be supported - or lability - is related to DOM
quality, but little is known about this relationship. Fluorescence
spectr...
Legacy effects of past land use and disturbance are increasingly recognized, yet consistent definitions of and criteria for
defining them do not exist. To address this gap in biological- and ecosystem-assessment frameworks, we propose a general metric
for evaluating potential legacy effects, which are computed by normalizing altered system function...
In order to construct accurate terrestrial carbon budgets, it is
necessary to understand how land use and river processing affect the
export and quality of organic matter. Fluorescence spectroscopy is
commonly used to characterize dissolved organic matter (DOM) quality,
but how fluorescence characteristics relate to the functional properties
of DOM...
Human demands for ecosystem services (e.g. provision of food and water; regulation of waste) change over space and time as society, economy, and environment evolve. The distribution of population relative to watershed boundaries determines supply and demand of ecosystem services, which in turn affects watershed water and nutrient budgets. A watersh...
Humans can affect the amount and quality of dissolved organic carbon (DOC) inputs to aquatic ecosystems by altering land use (e.g. by agricultural uses and impervious surfaces) and by the production of sewage and animal waste. These changes may affect stream ecosystem metabolism and contribute to the eutrophication of downstream ecosystems. The pre...
Nitrogen provides a fundamental building block for life. However, some nitrogen species (e.g., NO~3~) cause water quality degradation and contribute to climatic warming as a greenhouse gas (e.g., N~2~O). Denitrification is a central process in the nitrogen cycle, transforming water-soluble nitrogen to gaseous nitrogen, thereby removing nitrogen fro...
This study examines the role of fluvial wetlands (those connected to the stream via floodplains) in controlling DOC quality and export in urban systems. Human activity in urban basins may impact both DOC quality and export, but river-wetland complexes may dampen this signature. These dynamics are important to understand because the processing of DO...
River networks are comprised of a wide array of linked aquatic ecosystem types (e.g. channels of different sizes with associated transient storage zones, ponds, lakes, reservoirs, and floodplains). Recent field and modeling results demonstrate that river channels become less effective at retaining nutrients under high flow conditions when most nutr...
This study assessed whether grazing by the snail, Lymnaea elodes, limits benthic dinitrogen (N2) fixation and primary production in nitrogen (N)-limited oligotrophic lakes near Toolik Field Station on the North Slope of Alaska. We also tested whether snail excretion increased N and the ratio of N and phosphorus (P) supply ratio to benthic algae, wh...
Dissolved organic matter (DOM) is exported from watersheds to downstream ecosystems where it can contribute to eutrophication problems by enhancing microbial respiration and lowering oxygen levels. DOM quality affects microbial respiration; however, little is known about how watershed processes affect the quality of DOM export. In order to document...
Atmospheric nitrogen (N) fixation is a key N input to arctic ecosystems, but relatively few estimates of annual N-fixation rates are available. We measured N-fixation of plant-soil cores by the acetylene reduction technique at different topographic positions in an upland tundra watershed, Imnavait Creek, through two growing seasons in order to eval...
Understanding the biogeochemistry of suburban basins is becoming increasingly important due to the rapidly accelerating suburban sprawl in many parts of the US. In southeastern New Hampshire, population density is expected to increase by 50% over 20 years, and most of the development will occur as low-density suburban home lots with wells and on-si...
We investigated the influence of nitrogen (N) and phosphorus (P) on N2-fixation and abundance of two of the most common N2-fixing arctic lichens, Peltigera aphthosa and P. polydactyla, in two common moist upland tundra types, acidic and non-acidic tundra, at Toolik Lake, Alaska. Acidic tundra has higher N and lower P availability than non-acidic tu...
In nitrogen-(N) limited ecosystems, N-fixing cyanobacteria are an important source of primary production that is independent of N limitation. In an N-limited Venezuelan piedmont stream, we measured N-fixation in the presence and absence of the migratory benthic feeding fish, Prochilodus mariae. Prochilodus clear the sediment from the substratum whe...
We examined how physical and chemical factors and fish populations affected the presence and density of zooplankton species in 104 Alaskan arctic lakes. Five fish species and nine zooplankters were sampled from the lakes. Lake depth was related to the distribution of zooplankton in that smaller zooplankton species were found in deep lakes and large...
Primary production in most oligotrophic lakes in the Arctic is N-limited. Because water clarity is high and littoral zones are large, benthic production and N-fixation are important to whole-lake processes. Our objectives were to determine nutrient and landscape controls of benthic primary production and N-fixation in high-latitude arctic lakes at...
Inlbflnadon on age and gfo$th is es\enlial for the conserlation and mrnagement offish species. Age is often estimaied using the banding structure in orolrths. but the technique used c.rn influence the estimate. especially in slow-gfolving, long-lived species. Counts ofiranslucent bands ft{)Ill both whole rnd thin-sectioned Afctic grayling('l hrndl1...
Thesis (M.S.)--University of Minnesota, 1998. Includes bibliographical references (leaves 24-29).
We examined how physical and chemical factors and fish populations affected the presence and density of zooplankton species in 104 Alaskan arctic lakes. Five fish species and nine zooplankters were sampled from the lakes. Lake depth was related to the distribution of zooplankton in that smaller zooplankton species were found in deep lakes and large...
Projects
Projects (5)
The overall objective of this study is to determine the effect of livestock movement within agro-pastoral systems on spatial and temporal dynamics of water quality and GHG emissions from livestock watering points in Taita Taveta County, Kenya. The specific objectives are:
1. determine water quality and physical characteristics of watering points (sizes of stream, age and size of pond/ pans, etc) that are precursors to GHGs emissions in different livestock production systems
2. determine the spatial and temporal dynamics of GHGs emissions and soil carbon stocks in riparian areas and sediments in watering points in different livestock production systems
3. determine soil carbon stocks in riparian zones and sediments of watering points for livestock in different livestock production systems
4. Empower the participation of livestock farmers through citizen science to identify and adopt livestock management practices for improved water quality and reduced GHG emissions
Storage to stabilize water resource availability is necessary to increase water, food and energy security, reduce hazards, and adapt to climate change. Regulation of water resources using dams and reservoirs played a major role in socioeconomic development during the 20th Century, but practices of the time often led to undesired environmental and social impacts. Thousands more dams and reservoirs are planned for construction in the next decades, mainly in Asia, Africa, and Latin America, and there is a threat that many of the unwanted impacts experienced in the last century will be repeated in this new wave of dam construction, perhaps with even more severe consequences.
S-MultiStor is a three years initiative to investigate and demonstrate improved approaches to sustainable multi-purpose storage, aiming to make a measurable impact on sustainable development. The initiative is global in scope but includes concentrated activates in the Zambezi Basin of southern Africa, the Magdalena Basin of Colombia, and the Irrawaddy Basin of Myanmar. It creates a common research and innovation platform where researchers, planners, resource managers, and other decision-makers collaborate in the co-design and co-development of research and innovation activities.
website: http://smultistor.nl/
