Impacts of Land-use Changes on the Hydrology of the Grande River Basin Headwaters,Southeastern Brazil

Land-use changes affect soil water balance. The Upper Grande River Basin (UGRB) headwaters have undergone intense modifications in land use. This study was conducted to simulate, using the LASH model, the impacts on the hydrological regime in the UGRB with five land-use trends: S₁ and S₂ – reforestation with eucalyptus covering 20 % and 50 %, respectively, from the current grassland area; S₃ – reforestation with eucalyptus covering 100 % of the current grassland area only in the sub-basins where this trend is predominant; S₄ and S₅ deforestation of 30 % and 70 % of the forest remnants in the Mantiqueira Range region for the cultivation of grasslands, respectively. Results demonstrate that runoff would be reduced due to the land-use changes by 51.65 mm yr^?1, 110.29 mm yr^?1 and 59.48 mm yr^?1 for scenarios S₁, S₂ and S₃, respectively. However, scenarios S₄ and S₅ could increase streamflow by 57.63 mm year^?1 and 156.78 mm year^?1, respectively. This indicates that land-use changes might make the basin more prone to flooding and other hazards associated with increased runoff.     

Temporal Variability of Water Footprint for Maize Production: The Case of Beijing from 1978 to 2008

The water footprint (WF) of crop production is a comprehensive indicator that can reflect water consumption types, quantities and environmental impacts during the crop growth period. This study assesses interannual variability of green, blue and grey WFs of maize production in Beijing from 1978 to 2008. Results indicate that: (1) The multi-year average WF of maize was 1,031 m³ ton^?1 which was 56 % green, 25 % blue, and 19 % grey; (2) the climate experienced a warm-dry period in Beijing during the period from 1978 to 2008, and this lead to the increase of crop water requirement and irrigation water requirement for maize with trends of 0.52 mm a^?1 and 2.86 mm a^?1, respectively; (3) under the combined effects of climate change and agricultural inputs, the total WF and green WF presented decreasing trends. The blue and grey WFs had clear increasing trends; (4) statistical analysis revealed that interannual variability of green and blue WFs were caused by both climatic factors (effective precipitation) and non-climatic (agricultural inputs) factors. The grey WF was mainly associated with non-climatic factors, such as chemical fertilizers consumption.     

Approach for Estimating Available Consumable Water for Human Activities in a River Basin

Water is vital for economic development and environmental sustainability in arid and semi-arid basins. Management of water resource requires good understanding of available water for human consumption. Evapotranspiration (ET) is an important component of the hydrological cycle and represents the amount of water lost to the atmosphere in a basin. This study proposes a new approach to estimate available consumable water for human activities (ACW) in a basin based on precipitation, natural ET, and uncontrollable outflow, thus capping water use for human consumption in a basin. The ACW is illustrated for the Hai Basin in North China, where the average ACW from 2001 to 2012 for the entire basin is estimated at 31.97?×?10⁹m³ yr.^?1, varying between 18.61?×?10⁹m³yr^?1 in 2002 and 42.60?×?10⁹m³yr^?1 in 2003. A water balance analysis for the basin indicates that the aquifer water depletion in Hai Basin for 2001–2012 is 5.23?×?10⁹m³yr^?1. Compared to existing water resources assessment, ACW provides an easier approach to water management planning as no hydrological data are required, only data on precipitation and ET, supported by landcover data.     

An Estimate of Basin‐Wide Denitrification Based on Floodplain Inundation in the Atchafalaya River Basin,Louisiana

Maximizing the reduction of nitrate to dinitrogen gas (denitrification) has been advocated as a means to decrease nitrate pollution that causes eutrophication and hypoxia in estuaries worldwide. Managing this flux in bottomland forest wetlands of the Mississippi River could potentially reduce the world's second largest hypoxic zone. We used published denitrification rates, geospatial data on habitat area and inundation frequency, water level records (1963–2011), and average monthly temperatures to estimate annual denitrification in the Atchafalaya River Basin, the principal distributary of the Mississippi River. Denitrification rates ranged from 5394 kg N year^?1 (3.07 kg N km^?2 year^?1) in 1988 to 17 420 kg N year^?1 (9.92 kg N km^?2 year^?1) in 1981, and rates were consistently higher in fall compared with those in spring. Total NO₃^? denitrified in the basin was negligible compared with total NO₃^? entering the Gulf of Mexico. If all N denitrified in the basin instead entered the Gulf, the hypoxic zone was predicted to increase only 5.07 km² (0.06%). This negligible effect of the basin on N dynamics in the Gulf agrees with other mass balance and isotopic studies in the region. Copyright © 2014 John Wiley & Sons, Ltd.     

Effectiveness of Rainwater Harvesting in Runoff Volume Reduction in a Planned Industrial Park,China

It is urgent to effectively mitigate flood disasters in humid mountainous areas in southeastern China for the increasing flood risk under urbanization and industrialization. In this study, a rural district with an area of 13.39 km² that planning to build an industrial park covering an area of 7.98 km² in Changting was selected to estimate the potential of collectable rainwater and the extent to which runoff volume can potentially be mitigated by rainwater harvesting. In addition, the optimum cistern capacity of a rainwater harvesting system in the planned industrial park was evaluated using daily water balance simulation and cost-efficiency analysis. The results showed that rainwater harvesting in the planned industrial park has great potential. The annually collectable rainwater is approximately 9.8?×?10⁶ m³ and the optimum cistern capacity is determined to be 0.9?×?10⁶ m³. With the optimum cistern capacity, the annual rainwater usage rate is 0.99, showing neither financial savings nor deficits. Rainwater harvesting can reduce 100 % of runoff volume in the cases of critical rainfall storm (50 mm) and annual average maximum daily rainfall (111.2 mm), and 58 % of runoff volume in the case of maximum daily rainfall (233.6 mm), respectively. All surface runoff can be collected and stored in the cisterns when rainfall amount is less than 135.5 mm in a rainstorm event.     

The Compensation Mechanism and Water Quality Impacts of Agriculture-Urban Water Transfers: A Case Study in China’s Chaobai Watershed

Water transfer from agriculture to urban uses will likely become increasingly common worldwide. The objectives of this study were to evaluate the effects of converting paddy rice to dry land crops (PPRDC) on local farmers’ income in China’s Chaobai watershed and to analyze the responses of surface water quality to the change in cropping system. An on-site investigation of 485 households and water quality data from 1999 to 2008 are presented in this study. The cost of cultivation as a percentage of the total revenue was 22.3 % for rice and 30.1 % for corn, and the calculated compensation level (CCOM) should be 6172.3 Yuan?ha^?1. PPRDC provided farms with stable income without regard to drought and flood: thus, farmers responded positively to PPRDC, and 76.7 % of farmers expected to continue PPRDC. Inflow water quality was improved in terms of total nitrogen (TN) and nitrate (NO₃–N) because of the reduction in fertilizer and pesticides after PPRDC, but there was no significant change in total phosphorus (TP). PPRDC not only reallocates water resource to different users but also improves water quality through crop adjustment. A dynamic compensation mechanism based on changing crop price is required for the future water transfer project.     

Water Saving and Energy Reduction through Pressure Management in Urban Water Distribution Networks

Water shortages and climate change are worldwide issues. Reduction in water leakage in distribution networks as well as the associated energy saving and environmental impacts have recently received increased attention by scientists and water industries. Pressure management has been proposed as a cost-effective approach for reduction in water leakage. This study conducted a real-world water pressure regulation experiment to establish the pressure-leakage relationship in a district metering area (DMA) of the water distribution network in Beijing, China. Results showed that flow into the DMA was sensitive to inlet water pressure. A 5.6 m reduction in inlet pressure (from 38.8 m to 33.2 m) led to an 83 % reduction (12.1 l/s) in minimal night flow, which is a good approximator of leakage. These reductions resulted in 62,633 m³ of water saved every year for every km pipe, as well as associated savings of 1.1?×?10⁶ MJ of energy and 68 t of CO₂ equivalent greenhouse gas emissions. The results of this study provide decision makers with advice for reducing leakage in water distribution networks with associated energy and environmental benefits.     

Resolving Trans-jurisdictional Water Conflicts by the Nash Bargaining Method: A Case Study in Zhangweinan Canal Basin in North China

Trans-jurisdictional conflict in both water quantity and quality is a general concern in large river basins. In this paper, the relative utility function combined with the asymmetric Nash bargaining method was established to analyze the trans-jurisdictional conflict between water quantity and water quality in the Zhangweinan Canal Basin in China. The basin was divided into four conflict stakeholders, namely, Shanxi, Hebei, Henan, and Shandong Provinces, based on administrative unit. The water usage and pollutant discharge scheme for multiple stakeholders was optimized using the established model to satisfy the environmental flow and water quality objectives at the identified conflict sections. The results indicated that the total water consumption was reduced from 4.38?×?10⁹?m³ in 2007 to 1.97?×?10⁹?m³, and that the allowable COD and NH₃-N discharged into the river was less than 3.8?×?10⁴?t and 4.3?×?10³?t, respectively. About 81.1 % of COD and 76.1 % of NH₃-N should be further reduced compared with the values in 2007.     

Modeling Water Resources and River-Aquifer Interaction in the Júcar River Basin,Spain

The paper presents how to solve some practical problems of water planning in a medium/large river basin, such as: the water resources assessment and its spatial-temporal variability over the long-short term, the impact of human activities on the water cycle, due to groundwater pumping and water returns into aquifers, the river-aquifer interactions and the aquifer depletion. It is based on the use of a new monthly conceptual distributed water balance model -PATRICAL- that includes the surface water (SW), groundwater (GW) behavior and the river-aquifer interaction. The model is applied to the Júcar River Basin District (RBD) in Spain (43,000 km²), with more than 250 aquifers, including catchments with humid climates (Júcar RBD northern), semiarid and arid catchments (southern). The model has a small number of parameters and obtains a satisfactory performance in SW and GW behavior. It has been calibrated/validated using monthly streamflows and two additional elements not generally used in models for large river basins, GW levels and river-aquifer interactions. In the hydrological time series of the Júcar RBD headers a statistical change point in the year 1979/80 is detected. It is due to changes in precipitation patterns and represents a 40 % of reduction in streamflows in relation with the previous period. The impact of GW pumping in all aquifers is determined, the ‘Mancha Oriental’ aquifer produces a significant reduction in streamflows of the Júcar river –around 200–250 hm³/year. The GW level in the ‘Villena-Benejama’ aquifer -Vinalopo Valley- has declined more than 200 m in last 30 years.     

A Spatial Multi-Criteria Analysis Approach for Locating Suitable Sites for Construction of Subsurface Dams in Northern Pakistan

Pakistan is an agricultural country with an increasing interest for hydropower. Water management problems such as sedimentation and evaporation have been of high concern for surface water reservoirs for many years. Therefore, groundwater storage through subsurface dams could be promising, especially considering the monsoon rainfall and seasonal river flows in Pakistan. The paper aims to develop and test a methodology to locate suitable sites for construction of subsurface dams using spatial multi-criteria analysis (SMCA) in the northern parts of Pakistan. For the study, spatial data on geology, slope, land cover, soil depth and topographic wetness index (TWI) was used. Two weighting techniques, i.e. the analytic hierarchy process (AHP) and the factor interaction method (FIM), were employed and compared. The sensitivity of the two methods as well as of the model parameters was analysed. The suitability map derived from AHP yielded about 3 % (16 km²) of the total area as most suitable, about 4 % (22 km²) as moderately suitable and about 0.8 % (5 km²) as least suitable. The suitability map derived from FIM identified about 2.7 % (14 km²) of the total area as most suitable, about 4 % (22 km²) as moderately suitable and about 1 % (7 km²) as least suitable. The sensitivity analyses suggested that AHP was a more robust weighting technique than FIM and that land cover was the most sensitive factor. The methodology presented here shows promising results and could be used in early planning to locate suitable sites for construction of subsurface dams.     

Agricultural Multi-Water Source Allocation Model Based on Interval Two-Stage Stochastic Robust Programming under Uncertainty

Numerous uncertainties and complexities exist in the agricultural irrigation water allocation system, that must be considered in the optimization of water resources allocation. In this paper, an agricultural multi-water source allocation model, consisting of stochastic robust programming and two-stage random programming and introducing interval numbers and random variables to represent the uncertainties, was proposed for the optimization of irrigation water allocation in Jiamusi City of Heilongjiang Province, China. The model could optimize the water allocaton to different crops of groundwater and surface water. Then, the optimal target value and the optimal water allocation of different water sources distributed to different crops could be obtained. The model optimized the economic benefits and stability of the agricultural irrigation water allocation system via the introduction of a the penalty cost variable measurement to the objective function. The results revealed that the total water shortage changed from [18.6, 32.3]?×?10⁸ m³ to [15.7, 26.2]?×?10⁸ m³ at a risk level ω from zero to five, indicating that the water shortage decreased and the reliability improved in the agricultural irrigation water allocation system. Additionally, the net economic benefits of irrigation changed from [287.21, 357.86]?×?10⁸ yuan to [253.23, 301.32]?×?10⁸ yuan, indicating that the economic benefit difference was reduced. Therefore, the model can be used by decision makers to develop appropriate water distribution schemes based on the rational consideration of the economic benefit, stability and risk of the agricultural irrigation water allocation system.     

Model Based Assessment of Nitrate Pollution of Water Resources on a Federal State Level for the Dimensioning of Agro-environmental Reduction Strategies

The main goal of the project was to assess nitrogen pollution of surface waters and groundwater in the Federal State of North Rhine Westphalia (NRW), Germany. For this purpose the hydro(geo-)logical models GROWA-DENUZ/WEKU were coupled to the agro-economic model RAUMIS in order to assess the diffuse nitrogen loads and to approaches to determine the nitrogen loads from point sources. In this way the complex socio-economic interrelations and hydrological/hydrogeological interdependencies were simultaneously. The model network was applied consistently across the whole territory of NRW. At first the actual N inputs into groundwater and surface waters resulting from diffuse sources and point sources were assessed. For the relevant diffuse input pathways (groundwater runoff, drainage runoff and natural interflow) this was done in a spatial resolution of 100 m???100 m. In the case of point source inputs information from municipal waste water treatment plants, industrial effluents, rainwater sewers and combined sewer overflows has been considered. For NRW an actual total N input into surface waters of ca. 117.000 t???a^?1?N has been quantified. As the inputs via natural interflow (ca. 30 %), groundwater runoff (ca. 26 %) and drainage systems (ca. 18 %) hold the largest portion, it is evident that measures to control nitrate pollution have to focus on the inputs from diffuse sources. For this purpose, initially the development of the agrarian sector according to the Common Agricultural Policy, CAP until 2015 including supplementary measures and other impact factors has been analysed. The impact of this so-called baseline scenario 2015 was predicted for both, the diffuse N surpluses and the N pollution of groundwater and surface waters. It could be shown that the baseline projections for the agricultural sector through 2015 may lead to decrease of the diffuse N inputs into groundwater by ca. 13.500 t???a^?1?N and an overall decrease of the diffuse N inputs into surface waters by ca. 25.000 kg???ha^?1???a^?1?N. Based on the baseline scenario 2015 the additional N reduction to guarantee nitrate concentrations in groundwater below the EU-threshold value of 50 mg???l^?1 NO₃ was determined by means of a backward model calculation. This was done using the predicted nitrate concentrations in the leachate 2015 for the individual 100 m???100 m grids as starting points. In this way for the whole territory of NRW an additional N reduction beyond the baseline scenario 2015 of ca. 12.000 t???a^?1?N has been assessed. Model results indicate that additional N reduction measures don’t have to be implemented area-covering in order to be efficient, but in certain subareas only. It is suggested that in these subareas the available financial resources for the implementation of N reduction measures shall be used for individual, i.e. regionally adapted nitrate reduction measures.     

Delineation of Groundwater Potential Zones of Coastal Groundwater Basin Using Multi-Criteria Decision Making Technique

Delineation of groundwater potential zones (GWPZ) has been performed for a coastal groundwater basin of eastern India. The groundwater potential zone index (GWPZI) map is generated by using Analytic Hierarchy Process (AHP) from different influencing features, e.g., Land Use/Land Cover (LU/LC), soil (S), geomorphology (GM), hydrogeology (HG), surface geology (SG), recharge rate (RR), drainage density (DD), rainfall (RF), slope (Sl), surface water bodies (SW), lineament density (LD), and Normalized Difference Vegetative Index (NDVI). Recharge rate values are estimated from hydrological water balance model. Overlay weighted sum method is used to integrate all thematic feature maps to generate GWPZ map of the study area. Four zones have been identified for the coastal groundwater basin [very good: 36.39 % (273.53 km², good: 43.57 % (327.47 km²), moderate: 18.27 % (137.30 km²), and poor: 1.77 % (13.27 km²)]. Areas in north to south-west and south-east direction show very good GWPZ due to the presence of low drainage density. GWPZ map and well yield values show good agreement. Sensitivity analysis reveals that exclusion/absence of rainfall and lineament density increases the poor groundwater potential zones. Omission of hydrogeology, soils, surface geology, and NDVI show maximum increase in good GWPZ. Obtained GWPZ map can be utilized effectively for planning of sustainable agriculture. This analysis demonstrates the potential applicability of the methodology for a general coastal groundwater basin.     

Sensitivity of Ancient Lake Ohrid to Local Anthropogenic Impacts and Global Warming

Human impacts on the few ancient lakes of the world must be assessed, as any change can lead to an irreversible loss of endemic communities. In such an assessment, the sensitivity of Lake Ohrid (Macedonia/Albania; surface area A = 358 km², volume V = 55 km³, > 200 endemic species) to three major human impacts—water abstraction, eutrophication, and global warming—is evaluated.It is shown that ongoing eutrophication presents the major threat to this unique lake system, even under the conservative assumption of an increase in phosphorus (P) concentration from the current 4.5 to a potential future 9 mg P m⁻³. Eutrophication would lead to a significant reduction in light penetration, which is a prerequisite for endemic, deep living plankton communities. Moreover, a P increase to 9 mg P m⁻³ would create deep water anoxia through elevated oxygen consumption and increase in the water column stability due to more mineralization of organic material. Such anoxic conditions would severely threaten the endemic bottom fauna. The trend toward anoxia is further amplified by the predicted global warming of 0.04°C yr^–1, which significantly reduces the frequency of complete seasonal deep convective mixing compared to the current warming of 0.006°C yr⁻¹. This reduction in deep water exchange is triggered by the warming process rather than by overall higher temperatures in the lake. In contrast, deep convective mixing would be even more frequent than today under a higher temperature equilibrium, as a result of the temperature dependence of the thermal expansivity of water. Although water abstraction may change local habitats, e.g., karst spring areas, its effects on overall lake properties was shown to be of minor importance.     

Silica Regeneration Processes in Nearshore Southern Lake Michigan

The seasonal depletion of dissolved silica to levels that limit diatom production is particularly critical in Lake Michigan's nearshore zone where diatom biomass is greatest, and where silica regeneration from sediments is not well-understood. In our study, intact, medium-fine sand cores, collected from an 11 m deep site in nearshore Lake Michigan during July-August 1980, released soluble reactive silica (SRS) at a mean rate of 2,707 ± 122 (SE) μg Si cm^?2 yr^?1 when incubated in darkness and at 12° C. This measured SRS release was greater than a diffusive flux (270 ± 49 (90% C.I.) μg Si cm^?2 yr^?1) estimated from SRS pore water profiles and physical sediment properties. SRS release from individual cores was not correlated with abundance of most macroinvertebrates (chironomids, pisidiid clams, or oligochaetes). However, a significant (P < 0.05) and inverse relationship between SRS release and Pontoporeia hoyi densities implied that amphipods suppressed SRS release through mixing and burial of a surficial floe layer, where most dissolution of biogenic silica occurs. Moreover, SRS release rates measured from our coarse-grained nearshore sediments were comparable to rates reported for fine-grained offshore material and further implicate dissolution of surficial biogenic silica as the source of remineralized SRS. Because nearshore areas of Lake Michigan undergo strong seasonal variations in temperature and diatom production, and because significant riverine silica inputs exist, we cannot extrapolate our results on a lakewide, or season-long basis. The data, however, strongly imply that nearshore sediments are an important participant in the Lake Michigan silica cycle.     

Bank stabilization,riparian land use and the distribution of large woody debris in a regulated reach of the upper Missouri River,North Dakota,USA

Large woody debris (LWD) is an important component of ecosystem structure and function in large floodplain rivers. We examined associations between LWD distribution and riparian land use, bank stabilization (e.g. riprap revetment), local channel geomorphology, and distance downriver from the dam in the Garrison Reach, a regulated reach of the upper Missouri River in North Dakota, USA. We conducted a survey of shoreline‐associated LWD in the reach during typical summer flow conditions. Reach‐wide LWD density was 21.3 pieces km^?1 of shoreline, of which most pieces (39% ) were ‘beached’ between the waterline and the bankfull level, 31% of pieces had evidence of originating at their current location (anchored), 18% of pieces were in deep water (>1 m), and 13% were in shallow water. LWD density along unstabilized alluvial (sand/silt) shorelines (27.3 pieces km^?1) was much higher than along stabilized shorelines (7.2 pieces km^?1). LWD density along forested shorelines (40.1 pieces km^?1) was higher than along open (e.g. rangeland, crop land; 9.2 pieces km^?1) or developed (e.g. residential, industrial; 7.8 pieces km^?1) shorelines. LWD density was highest overall along unstabilized, forested shorelines (45 pieces km^?1) and lowest along open or developed shorelines stabilized with a blanket‐rock revetment (5.5 pieces km^?1). Bank stabilization nearly eliminated the positive effect of riparian forest on LWD density. A predicted longitudinal increase in LWD density with distance from the dam was detected only for deep LWD (including snags) along unstabilized alluvial shorelines. Partial resurvey in the summer following the initial survey revealed a reduction in total LWD density in the reach that we attribute to an increase in summer flow between years. Changes in riparian management and land use could slow the loss of LWD‐related ecosystem services. However, restoration of a natural LWD regime in the Missouri River would require naturalization of the hydrograph and modification of existing bank stabilization and channel engineering structures. Copyright © 2004 John Wiley & Sons, Ltd.     

Kinetics of Red Clay Bluff Dissolution in Western Lake Superior

The dissolution of red clay bluff samples from the southwestern Lake Superior shoreline area in Lake Superior water or deionized water is studied by following the aqueous concentrations of reactive silica over a three month period. The dissolution process is initially rapid, followed by a first-order dissolution process (k = 9.4 × 10^?7 sec^?1) up to about thirty days. After about 30 days, the rate of dissolution of the bluff material follows linear kinetics (k = 5.4 × 10^?8 mg SiO₂/gram of bluff per second).     

Changes in the Coastline and Water Level of the Akşehir and Eber Lakes Between 1975 and 2009

The Ak?ehir and Eber Lakes, relatively shallow, small freshwater lakes with an area of 361 km² and 150 km² and average depth of 7 m and 2 m (1998), respectively in southwestern Turkey, have experienced a severe decline in water levels in recent decades. This study aimed to investigate coastline and water level changes of lakes and identify the causes for the decline in lake levels. Nine Landsat images from different times, monthly temperature, precipitation, discharge, lake level records and population data were used to analyze these changes. From 1975 to 2009, the water surface areas of the Ak?ehir and Eber Lakes decreased from 356,929 to 126,482 km² and from 119,882 to 85,663 km², a loss of 64.5% and 28.4% over the 34-year period, respectively. From 1975 to 2004, the Ak?ehir Lake level declined by 2.67 m from 956.02 m to 953.35 and the Eber Lake level declined by 2.03 m from 966.75 m to 964.72 m from 1975 to 2004 based on ground lake level data (in situ). The results of the temperature and precipitation analysis showed that although the annual mean climatic factors vary substantially, they show small increasing trend over the record periods. Annual discharge records on the Akarçay River and its tributaries decreased over the basin during the same period. Irrigation systems, three dams and seven pounds built in recent decades for agricultural irrigation and domestic use, made the major impact on lowering the lake levels because they derive water from the river for human use upstream of the lakes’ catchments. Population growth, rising water consumption for agricultural and domestic purposes and building dams has led to lake levels declining. The change of lake levels might depend more on anthropogenic factors than on climatic factors.     

Water conservation through trade: the case of Kenya

This study quantifies and maps the water footprint of Kenya from both production and consumption perspectives and estimates the country’s virtual water export and import. Kenya’s virtual water export related to trade in agricultural products was 4.1 km³/y; its virtual water import was 4.0 km³/y. The average export earning per unit of water consumed or polluted in producing agricultural export products was USD 0.25/m³, while the average expenditure on imported commodities per unit of virtual water imported was USD 0.10/m³. In addition to increasing water productivity in crop production, Kenya can mitigate its water scarcity by increasing imports of water-intensive products such as cereals and exports of high-value products such as cut flowers, vegetables, spices and tea.     

Sediment geochemistry and contributions to carbon and nutrient cycling in a deep meromictic tropical lake: Lake Malawi (East Africa)

The biogeochemical functioning of large tropical lakes differs substantially from temperate lakes, yet remains poorly understood. We characterized the carbon, nitrogen, and phosphorus cycling in the water column and sediments of a deep meromictic tropical Lake Malawi (East Africa) by measuring geochemical distributions and compiling whole-lake geochemical budgets. Four locations (100 to 650 m water depth) were characterized. The results reveal that sediments contribute significantly to lake-wide biogeochemical budgets. Sedimentation rates have significantly increased in recent decades. While the export efficiency of organic matter from photic zone to deep sediments is low (14%), organic carbon is buried in the anoxic sediments with high efficiency (27–46%). Area-specific rates of carbon mineralization (4.1 mmol m^? 2 d^? 1) are similar to those in temperate well-oxygenated large lakes and marine sediments in similar water depths. Ammonium effluxes from sediments (0.44 mmol m^? 2 d^? 1) contribute 29% to the total nitrogen inputs into the water column, while sediment denitrification (0.035 mmol m^? 2 d^? 1) and burial of organic nitrogen (0.27 mmol m^? 2 d^? 1) remove 28% of total inputs in the lake. The recycling efficiency of phosphorus in anoxic sediments is high (73%). P effluxes average 0.037 mmol m^? 2 d^? 1, suggesting a large and previously unquantified contribution (42%) to water column P inputs. The results underscore the importance of sediments in the geochemical budgets of even large lakes and suggest trends in lacustrine carbon cycling that hold across a wide range of environments.