Assessment of the spatial and temporal variations of water quality for agricultural lands with crop rotation in China by using a HYPE model

Many water quality models have been successfully used worldwide to predict nutrient losses from anthropogenically impacted catchments, but hydrological and nutrient simulations with little data are difficult considering the transfer of model parameters and complication of model calibration and validation. This study aims (i) to assess the performance capabilities of a new and relatively more advantageous model-hydrological predictions for the environment (HYPE) to simulate stream flow and nutrient load in ungauged agricultural areas by using a multi-site and multi-objective parameter calibration method and (ii) to investigate the temporal and spatial variations of total nitrogen (TN) and total phosphorous (TP) concentrations and loads with crop rotation using the model for the first time. A parameter estimation tool (PEST) was used to calibrate parameters, which shows that the parameters related to the effective soil porosity were most sensitive to hydrological modeling. N balance was largely controlled by soil denitrification processes, whereas P balance was influenced by the sedimentation rate and production/decay of P in rivers and lakes. The model reproduced the temporal and spatial variations of discharge and TN/TP relatively well in both calibration (2006–2008) and validation (2009–2010) periods. The lowest NSEs (Nash-Suttclife Efficiency) of discharge, daily TN load, and daily TP load were 0.74, 0.51, and 0.54, respectively. The seasonal variations of daily TN concentrations in the entire simulation period were insufficient, indicated that crop rotation changed the timing and amount of N output. Monthly TN and TP simulation yields revealed that nutrient outputs were abundant in summer in terms of the corresponding discharge. The area-weighted TN and TP load annual yields in five years showed that nutrient loads were extremely high along Hong and Ru rivers, especially in agricultural lands

Hydrology, limnology and environmental feasibility of the Pareja Limno-reservoir

Sastre Merlín, Antonio, codir.Los grandes embalses en áreas de clima mediterráneo generan una serie de impactos ambientales y económicos negativos. Una actuación innovadora para mitigarlos es la construcción de pequeños diques en sectores de cola de dichos embalses, que dan lugar a la aparición de una masa de agua diseñada para mantener un nivel constante y para la que hemos propuesto el término "limnoembalse". El Limnoembalse de Pareja, primero diseñado con una doble finalidad ambiental y recreativa, fue construido en 2006 junto al pueblo homónimo y es alimentado por el río Ompólveda. Se ubica en el área del embalse de Entrepeñas (provincia de Guadalajara), afectado por los citados impactos. A pesar del interés de esta infraestructura, parece oportuno adquirir conocimiento sobre su comportamiento. Esta tesis doctoral tiene como objetivo caracterizar la cuenca del río Ompólveda y el Limnoembalse de Pareja, evaluando su viabilidad ambiental. La aportación del río Ompólveda supone un 10% de la precipitación media, con un 60-70% de flujo de base que mantiene un caudal permanente en ausencia de lluvias. Sin embargo, el mantenimiento de un nivel de agua constante en el limnoembalse no queda garantizado durante el estiaje en años secos. Además, la simulación de escenarios de cambio climático con el modelo SWAT apunta a una disminución de la aportación de hasta un 50%, lo que supondría un descenso notable del nivel del agua y cuestiona la viabilidad hidrológica de la infraestructura. Por otra parte, el estudio hidrogeoquímico de la cuenca reveló que la disolución de calcita y yeso son los procesos dominantes, evolucionaldo la composición del agua de bicarbonatada cálcica a sulfatada cálcica y aumentando su conductividad. El Limnoembalse de Pareja sigue una dinámica de lago monomíctico templado, con un agua ligeramente alcalina y de alta conductividad. Las comunidades de fitoplancton, zooplancton y microorganismos están dominadas por diatomeas, rotíferos y oligotrofos, respectivamente. La calidad del agua resulta adecuada para los usos previstos del limnoembalse. Diversos indicadores sugirieron un estatus oligo-mestorófico y un estado ecológico Muy Bueno. Las concentraciones de microorganismos fecales fueron acordes con la legislación. No obstante, la calidad puede empeorar en invierno (mayor transporte de nutrientes y microorganismos) y en verano (mayor conductividad y proliferación de algas). Además, las simulaciones de cambio climático predicen un deterioro futuro del estado trófico. También se ha estudiado el régimen de erosión de la cuenca del río Ompólveda y el aterramiento del limnoembalse mediante una metodología in-situ, sencilla y económica. La erosión bruta media en la cuenca se estima en 6 T ha-1 año -1. No obstante, la tasa anual de aterramiento en el Limnoembalse de Pareja fue aproximadamente un 0,29%. Ello supone que supone que sólo un 3,9% de los sedimentos movilizados en la cuenca alcanzan el limnoembalse -posiblemente debido a la baja conectividad-, lo que garantizaría la viabilidad ambiental del Limnoembalse de Pareja desde el punto de vista del riesgo de aterramiento. Los resultados obtenidos en esta tesis doctoral podrían tener relevancia en el ámbito de la gestión del agua, puesto que aportan conocimiento y metodologías para la evaluación multidisciplinar de estas nuevas masas de agua

Hydrology, limnology and environmental feasibility of the Pareja Limno-reservoir

Sastre Merlín, Antonio, codir.Los grandes embalses en áreas de clima mediterráneo generan una serie de impactos ambientales y económicos negativos. Una actuación innovadora para mitigarlos es la construcción de pequeños diques en sectores de cola de dichos embalses, que dan lugar a la aparición de una masa de agua diseñada para mantener un nivel constante y para la que hemos propuesto el término "limnoembalse". El Limnoembalse de Pareja, primero diseñado con una doble finalidad ambiental y recreativa, fue construido en 2006 junto al pueblo homónimo y es alimentado por el río Ompólveda. Se ubica en el área del embalse de Entrepeñas (provincia de Guadalajara), afectado por los citados impactos. A pesar del interés de esta infraestructura, parece oportuno adquirir conocimiento sobre su comportamiento. Esta tesis doctoral tiene como objetivo caracterizar la cuenca del río Ompólveda y el Limnoembalse de Pareja, evaluando su viabilidad ambiental. La aportación del río Ompólveda supone un 10% de la precipitación media, con un 60-70% de flujo de base que mantiene un caudal permanente en ausencia de lluvias. Sin embargo, el mantenimiento de un nivel de agua constante en el limnoembalse no queda garantizado durante el estiaje en años secos. Además, la simulación de escenarios de cambio climático con el modelo SWAT apunta a una disminución de la aportación de hasta un 50%, lo que supondría un descenso notable del nivel del agua y cuestiona la viabilidad hidrológica de la infraestructura. Por otra parte, el estudio hidrogeoquímico de la cuenca reveló que la disolución de calcita y yeso son los procesos dominantes, evolucionaldo la composición del agua de bicarbonatada cálcica a sulfatada cálcica y aumentando su conductividad. El Limnoembalse de Pareja sigue una dinámica de lago monomíctico templado, con un agua ligeramente alcalina y de alta conductividad. Las comunidades de fitoplancton, zooplancton y microorganismos están dominadas por diatomeas, rotíferos y oligotrofos, respectivamente. La calidad del agua resulta adecuada para los usos previstos del limnoembalse. Diversos indicadores sugirieron un estatus oligo-mestorófico y un estado ecológico Muy Bueno. Las concentraciones de microorganismos fecales fueron acordes con la legislación. No obstante, la calidad puede empeorar en invierno (mayor transporte de nutrientes y microorganismos) y en verano (mayor conductividad y proliferación de algas). Además, las simulaciones de cambio climático predicen un deterioro futuro del estado trófico. También se ha estudiado el régimen de erosión de la cuenca del río Ompólveda y el aterramiento del limnoembalse mediante una metodología in-situ, sencilla y económica. La erosión bruta media en la cuenca se estima en 6 T ha-1 año -1. No obstante, la tasa anual de aterramiento en el Limnoembalse de Pareja fue aproximadamente un 0,29%. Ello supone que supone que sólo un 3,9% de los sedimentos movilizados en la cuenca alcanzan el limnoembalse -posiblemente debido a la baja conectividad-, lo que garantizaría la viabilidad ambiental del Limnoembalse de Pareja desde el punto de vista del riesgo de aterramiento. Los resultados obtenidos en esta tesis doctoral podrían tener relevancia en el ámbito de la gestión del agua, puesto que aportan conocimiento y metodologías para la evaluación multidisciplinar de estas nuevas masas de agua

Water Resource Variability and Climate Change

Climate change affects global and regional water cycling, as well as surficial and subsurface water availability. These changes have increased the vulnerabilities of ecosystems and of human society. Understanding how climate change has affected water resource variability in the past and how climate change is leading to rapid changes in contemporary systems is of critical importance for sustainable development in different parts of the world. This Special Issue focuses on “Water Resource Variability and Climate Change” and aims to present a collection of articles addressing various aspects of water resource variability as well as how such variabilities are affected by changing climates. Potential topics include the reconstruction of historic moisture fluctuations, based on various proxies (such as tree rings, sediment cores, and landform features), the empirical monitoring of water variability based on field survey and remote sensing techniques, and the projection of future water cycling using numerical model simulations

Modelling temporal dynamics of discharge and nutrient loading from a mixed land use catchment, and interactions with a eutrophic, temperate lake under climate change

Understanding anthropogenic–induced changes in catchment water discharge and nutrient loads is critical for eutrophication assessment and sustainable management of receiving environments. Anthropogenic activities have increased nutrient export from terrestrial systems to lakes, where they may lead to eutrophication. Impacts of excess nutrients may be exacerbated by a warming climate. A variety of catchment models has been developed to gain insight into the temporal and spatial variations in discharge, and suspended sediment and nutrient transport in response to climate forcing and rainfall–runoff. These models can be used to predict the effects of different land management strategies and climate change on discharge and losses of particulate and dissolved constituents of the discharge. The integration of individual components of the modelling framework, including climate, catchment and aquatic ecosystem models, enables simulation and prediction of present and future states of freshwater ecosystems, including their spatial and temporal dynamics. The study area for this thesis is the Lake Rotorua catchment (~410 km²; Bay of Plenty, North Island, New Zealand). Commencement in 1991 of spray irrigation of treated wastewater (10 mm d⁻¹) from Rotorua city in the Whakarewarewa Forest was envisaged as a solution to eutrophication of Lake Rotorua (surface area ~80 km2) where treated wastewater (to secondary treatment level) had previously been discharged. The Waipa Stream draining the irrigated area (~2 km²) discharges to the Puarenga Stream, ultimately entering Lake Rotorua. The Puarenga Stream is the second–largest surface inflow to Lake Rotorua and drains a catchment of 77 km². Land use in the Puarenga Stream catchment is mostly plantation forest within which there are 16 blocks for spray irrigation of wastewater. The catchment has an area of pastoral farmland (8 km²) that is typically fertilised with nitrogen (N) and phosphorus (P), as well as being irrigated with cowshed washdown which also contributed N and P. The overarching aim of this study was to utilize advanced modelling technologies to simulate the discharge and sediment and nutrient loads from a mixed land use catchment of Puarenga Stream, part of which is spray–irrigated with wastewater in Waipa Stream catchment, and to model and understand the impacts and effects of different management regimes on the receiving waterbody; a temperate eutrophic lake (Rotorua). To achieve this, the study encompassed three main areas of research: 1) a process–based catchment model (Soil and Water Assessment Tool) application in the Puarenga catchment of Lake Rotorua under different hydrologic conditions, testing the influence of parameter sensitivity; 2) improvements to the catchment model (SWAT) to represent high–frequency (daily and hourly) variability of nutrient discharges and to simulate different land and wastewater irrigation management strategies; and 3) an application of the improved catchment model (from (2) above) combined with the lake model (DYRESM–CAEDYM) to predict the response of Lake Rotorua to future climate in 2090 and catchment nutrient discharge. The objective of the first research component (Chapter 2) was to examine the applicability of SWAT2009 model (version rev488) to the Puarenga catchment. The research included quantifying model performance and parameter sensitivity during different hydrologic conditions. A Sequential Uncertainty Fitting (SUFI–2) procedure was used to auto–calibrate unknown parameter values in the SWAT2009 model for years 2004–2008. Model validation was performed using: 1) monthly instantaneous measurements of suspended sediment (SS), total phosphorus (TP) and total nitrogen (TN) concentrations (1994–1997); and 2) daily discharge–weighted mean concentrations calculated from high–frequency event–based samples for concentrations of SS (nine events), TP and TN (both 14 events) at 1 h or 2 h frequency (2010–2012). Model error associated with quick–flow was underestimated (44% bias for SS, 70% bias for TP) compared with monthly measurements derived predominantly from base flow measurements (< 1% bias for SS, 24% bias for TP). The use of low–frequency base flow measurements for model calibration provided poor simulation results for “flashy” lower–order streams. The model results highlight the importance of using high–frequency, event–based monitoring data for calibration, to alleviate the potential for underestimation of storm–driven fluxes. A manual procedure (one–at–a–time sensitivity analysis) was used to quantify parameter sensitivity for the two hydrologically–separated regimes. Parameters relating to tuning of main channel processes (e.g., lateral flow slope length and travel time) were more sensitive for base flow estimates (particularly discharge and SS), while those relating to overland processes (e.g., Manning's n value for overland flow) were more sensitive for the quick flow estimates. Separating discharge and loads of sediments and nutrients into a base flow and a quick flow component provided important insights into uncertainties in parameter values. This research has important implications for performance of hydrological models applied to catchments with large fluctuations in stream flow, and in cases where models are used to examine scenarios that involve substantial changes to the existing flow regime. The SWAT2009 model described in Chapter 2 did not have algorithms to simulate a complex irrigation operation. The objective of the second research chapter (Chapter 3) was therefore to develop a capability to simulate the irrigated sub–catchment and examine alternatives for managing the wastewater. A modified version of the SWAT2012 code (rev629) using hourly routing algorithms was adapted to the Waipa Stream sub–catchment within the Puarenga catchment. A similar configuration to Chapter 2 was applied for the modelling except that a finer temporal resolution of rainfall records was used in Chapter 3. Hourly records at Kaituna rain gauge, which is outside of the irrigated sub–catchment, were used to allocate weekly records at Red Stag gauge, which is within the irrigated sub–catchment, to hourly rainfall values. The modified SWAT2012 model was run at an hourly time step for a 10–year (2003–2012) period using the daily irrigation routine, then calibrated and validated by comparing weekly average predictions with measurements. The optimised values of parameters were different from those in Chapter 2. A range of statistical metrics indicated that the SWAT2012 model performed well using hourly routing with respect to 10–year (2003–2012) daily simulations that were averaged to the weekly measurements for comparison of discharge (r ≥ 0.81; p TP > TN variability). Simulations were run using daily outputs for an unirrigated scenario and for a range of other management options including changes in the area, frequency and amount of irrigation. Increasing the irrigation area decreased TP and TN loads in the simulation. The impact of changing irrigation frequency from daily to one day each week was small for annual TP load simulations. Annual TN load increased considerably under weekly irrigation. Compared with low–frequency, high–volume wastewater applications (once every seven days), the current strategy of daily wastewater irrigation minimises TN leaching and reduces saturation of the subsurface layer. Improvements to the SWAT2012 model and the use of hourly routing to capture high–frequency (daily and hourly) variability of nutrient discharges and simulations of different wastewater irrigation management regimes may assist with future strategies to mitigate P and N losses from the irrigated area by refining the area, timing, frequency and amount of irrigation. In Chapter 4 the primary objective was to combine the modified SWAT2012 model from Chapter 3 with the lake model (DYRESM–CAEDYM version 4.0) to simulate the trophic state of Lake Rotorua (mean depth 10.8 m), in response to nutrient load reductions from wastewater–irrigated forest and farmland in the Puarenga Stream catchment under present and future climates. Initial parameter values required for the setup of both models were based on the monitoring data that were measured close to the start date of the simulation period. A range of statistical metrics indicated that the SWAT2012 model performed well (r ≥ 0.88, p 0.05) for the 4–year (2006–2010) simulation period. SWAT2012 model simulations were used for the Puarenga Stream input to the DYRESM–CAEDYM model of Lake Rotorua while other inflows used either measured data or values derived from other studies. Considering the 1.5–year lake residence time for Lake Rotorua, the DYRESM–CAEDYM model was validated using monthly data collected at two sites during 2008–2010. The DYRESM–CAEDYM model performed well (r ≥ 0.63; p < 0.01) for surface water TP and TN concentrations in both the calibration and validation periods, but not for bottom–water nutrient concentrations. Effects of land management practice were then examined by simulating four nutrient application scenarios relating to wastewater irrigation and farmland fertilisation within the Puarenga catchment. Under the scenario of removing nutrient applications from both wastewater irrigation and farmland fertilisation, nutrient load reductions were 39.5% for TP and 75.2% for TN in the Puarenga catchment but these had much lesser effect on nutrient concentrations in the lake, with reduction of 3.5% for TP, 5.7% for TN, and 4.1% for chlorophyll a (Chl a; as a proxy for phytoplankton biomass) in surface waters. Based on the Intergovernmental Panel on Climate Change Fifth Assessment report, for the projected future climate of 2090 under the RCP8.5 scenario (equivalent to a short–wave radiation increase of 8.5 W m-2), annual mean precipitation and solar radiation increase by 2.8% and 1.4%, respectively, humidity decreases by 0.6%, and air temperature increases by 2.7 °C. Downscaled climate projections for 2090 were derived from 22 general circulation models and used as input to SWAT and DYRESM–CAEDYM models of the catchment and lake, respectively. Simulations using a projected climate for 2090 had moderate impact on catchment nutrient loads (6% increase for TP, 7.6% decrease for TN), but concentrations in surface waters were predicted to increase by 45.9% for TP, 44.5% for TN, and 44.9% for Chl a from 2010 to 2090, suggesting that future climate change would increase eutrophication. Increased water temperatures would cause more frequent and longer periods of thermal stratification in polymictic lakes such as Rotorua, which would likely result in greater depletion of dissolved oxygen and possible anoxia of hypolimnetic waters. This overarching effect of climate change is likely to be through a physical response of the lake in the form of increased stratification and greater levels of internal nutrient loading. This thesis has demonstrated the effects of different hydrologic conditions on SWAT2009 model performance and parameter sensitivity using an application to a small, mixed land use catchment, Lake Rotorua, New Zealand. By using the hourly routing algorithms and modifying relevant model code to simulate complex catchment irrigation operations, the SWAT2012 model performance was improved, particularly for high–frequency simulation of SS, TP and TN loads to the receiving lake. Finally, the modified SWAT2012 model combined with the lake model (DYRESM–CAEDYM) predicted that future climate change should be factored into assessments of the future trophic state of Lake Rotorua

Book of abstracts TropiLakes2015: Tropical lakes in a changing environment: water, land, biology, climate and humans

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Addressing Uncertainty in TMDLS: Short Course at Arkansas Water Resources Center 2001 Annual Conference

Management of a critical natural resource like water requires information on the status of that resource. The US Environmental Protection Agency (EPA) reported in the 1998 National Water Quality Inventory that more than 291,000 miles of assessed rivers and streams and 5 million acres of lakes do not meet State water quality standards. This inventory represents a compilation of State assessments of 840,000 miles of rivers and 17.4 million acres of lakes; a 22 percent increase in river miles and 4 percent increase in lake acres over their 1996 reports. Siltation, bacteria, nutrients and metals were the leading pollutants of impaired waters, according to EPA. The sources of these pollutants were presumed to be runoff from agricultural lands and urban areas. EPA suggests that the majority of Americans-over 218 million-live within ten miles of a polluted waterbody. This seems to contradict the recent proclamations of the success of the Clean Water Act, the Nation\u27s water pollution control law. EPA also claims that, while water quality is still threatened in the US, the amount of water safe for fishing and swimming has doubled since 1972, and that the number of people served by sewage treatment plants has more than doubled

Assessing the Impacts of Land-Use and Climate Change for Water Resource Management

Sustainable management of water resources is a challenging interdisciplinary problem requiring the integration of fields such as hydrology, ecology, sociology, and public policy. In the past decade, there has been a great effort to understand how issues such as climate change and land-use change for biofuel feedstock production will affect water resources. This dissertation assesses the impacts of climate change and land-use change for water resource management in Kansas using an interdisciplinary approach and tools such as the Soil and Water Assessment Tool (SWAT), social surveys, and geospatial analysis. The SWAT model is used to simulate corn and grain sorghum biofuel-based land-use scenarios to assess water quality impacts and sustainability indicators in the Perry Lake and the Kanopolis Lake watersheds in Kansas. Modeling results suggest that corn scenarios produced significantly greater water quality impacts than grain sorghum scenarios, but that corn had a much higher crop yield, particularly in the Perry Lake watershed, and thus can provide more ethanol production potential per land, water, and nutrient input, which are efficiency metrics often used in agricultural studies. Overall, grain sorghum may be a more sustainable feedstock crop in drier climates and corn may be more sustainable in wetter climates. The sustainability measures utilized in this study allow for comparison between crops and between watersheds, yet they are typically not included in the current biofuel-based land-use analyses. This study shows the potential of integrating water quality analysis with sustainability indicators to develop a richer assessment of the trade-offs and benefits of landscape change for biofuel feedstock development. The impact of climate change was assessed in three ways: first, with a review of the potential climate change impacts for reservoirs and a discussion of the potential in-lake and watershed management strategies for mitigation; second, with a social survey that explores perceptions of Kansas water managers towards climate change and planning for climate impacts; and third, with a study of the influence of reservoir management on greenhouse gas emissions from a tributary of the Three Gorges Reservoir in China. The review of climate change impacts for reservoirs found that the sustainability of reservoir services will be threatened by climate change, but that there are a variety of management tools that may be able to mitigate impacts. The social survey demonstrated that anthropogenic climate change is a contentious issue within the state of Kansas, but that water managers believe it is important to consider future climate change in their planning efforts. Survey results, along with a review of key Kansas water management plans, suggest that Kansas water managers are indeed responsive to climate variability and are starting to integrate climate variability into planning efforts. The study of reservoir greenhouse gas emissions suggest that both CO2 and CH4 fluxes were influenced by reservoir water level and exhibited distinct patterns that correspond to the reservoir operation cycle. Over 90% of CO2 effluxes occurred during the high water period, whereas the 58% of CH4 effluxes occurred during the low water period. Results suggest that reservoir operations altered the hydraulic retention time, which along with water temperature, controlled the synthesis and decomposition of carbon in the backwater system

Texas A&M University – Lake Granbury and Bosque River Assessment Final Scientific/Technical Report

The “Texas A&M University – Lakes Granbury and Waco, and Bosque River Assessment” project was developed to address two separate water quality issues in Central Texas. Prior work in each waterbody has identified major water quality concerns and laid the ground work for the activities conducted in this project. Lake Granbury and the Bosque River both serve vital roles in the Brazos River watershed. Lake Granbury is a reservoir constructed on the main stem of the Brazos River in Hood County, Texas and provides a potable water supply for over 250,000 area residents, cooling water for a natural gas fired and a nuclear power plant, vital flood control for the city of Waco and a critical economic stimulus for the city of Granbury and surrounding areas. The Bosque River, also in the Brazos River watershed plays a vital role in Central Texas as well; it feeds Lake Waco and supplies water for 200,000 Central Texans. Lake Granbury has experienced recent toxic blooms of Prymnesium parvum (Golden algae) that have resulted in massive fish kills and concerns about general water quality. Lake Waco, a reservoir constructed on the main stem of the Bosque River, has viable P. parvum populations, but does not experience harmful blooms. Thus, comparisons between Lakes Granbury and Waco allow for comparisons of environmental conditions leading to bloom formation. Both Lake Granbury and Lake Waco are critical to this region as being primary water supplies, sources of revenue and recreational hotspots. This project addressed these water quality issues by providing critical information about the relationships between the contaminants (Golden algae and nutrients) and environmental factors in the respective watersheds. In Lakes Granbury and Waco, various plankton, nutrient and water quality samples were collected at fixed-location stations, and high-resolution spatial maps were generated using an on-board dataflow technology of various plankton and water quality parameters. Linkages between the toxic Golden algae blooms and environmental conditions were examined. In addition, a numerical model of Lake Granbury was developed and continues to be refined. Bloom forming processes were investigated with this model. In the Bosque River, elevated nutrient levels have lead to increased aquatic vegetation growth and subsequent taste and odor problems in Lake Waco. This project enlists physically based computer modeling to determine the nutrient and sediment removal capabilities of implementing recommended Best Management Practices (BMPs) throughout the watershed. Results of this project indicate areas where specific management strategies will provide the most pollutant control for the cost to implement the practice. Each project is focused on addressing specific water quality issues in a specific water body, but each will provide valuable information that can be used to correct water quality concerns in other watersheds with similar problems. Ultimately, the project will result in improved water quality for consumptive, recreational, and industrial uses and will help to sustain the economic stimulus resulting from these water bodies

An integrated modelling approach to investigate the dynamics of Planktothrix rubescens blooming in a medium-sized pre-alpine lake (North Italy)

In a medium-sized pre-alpine lake (Lake Pusiano, North Italy) the cyanobacterium Planktothrix rubescens has strongly dominated the phytoplankton assemblage since 2000 despite improvements in water quality, similarly to what happened in many pre-alpine lakes. The ecological success of the ubiquitous harmful species has been ascribed to largely depend on its eco-physiological traits, lakes re-oligotrophication (and increasing N:P ratios) as well as climate oscillations. Whatever the viewpoint, it has been the dominating algal species over the last two decades but the scientific community is debating about the crucial factors determining the dynamics. A great difficulty is certainly the comprehension of the effects due to human pressures at different scales. Also the natural changes and the interactions within the ecosystem may cause a high uncertainty. The present research focused on the necessity to solve some of the most paradoxical features about P.rubescens large success. An intensive field campaign was conducted to evaluate distributions of phytoplankton taxa, as well as P. rubescens, using spectrally-resolved fluorescence measurements and cell enumeration. These provided a high spatially and temporally resolved database, suitable to calibrate and validate a coupled three-dimensional hydrodynamic and ecological model for lakes ecosystem. The simulations revealed the fundamental role of physiological features. They led to characteristic vertical patterns of distribution, notably a deep chlorophyll maximum, and a visible influence of lake hydrodynamic processes, particularly during high-discharge inflows in summer stratification. The simulations were used to examine growth-limiting factors that help to explain its increased prevalence during a re-oligotrophication phase. A long-term series (1960-2010), assessed over measured data, was reconstructed for some ecological indicators. A natural external phosphorus load was simulated by a hydrological and nutrients transport model (SWAT), after it was calibrated on a natural sub-basin. Data by a paleolimnological survey were used to initialize the lake ecological model to reproduce the past conditions. A specific statistical technique (Spectral Singular Analysis) was used to isolate the trend of air temperature daily series, avoiding the periodic climatic fluctuations. Four different scenarios were simulated to characterize different levels of local and global pressure on lake ecology, combining each alternative driver into the lake model. The integrated lake-basin tool was also proposed as a dynamic tool to simulate the biogeochemical cycle in an alternative pristine ecological state. The output for phosphorus reference conditions was compared to the results by the most traditional methods (previously assessed for subalpine lakes). After decades of lake eutrophication, the simulated temperatures warming did not enhance P. rubescens blooming. Conversely, a positive relation was found when the pressure from the catchment (e.g. phosphorus pollution) was switched off by the simulation, as emerged by the Mann-Kendall statistics on daily model output. In other words, the global warming may have different effects on P. rubescens dynamics, depending on the trophic evolution of a lake. The simulation of a pristine condition projected the lake into an oligo-mesotrophy, as the results of equilibrium between the external phosphorus loading and the trasformations across the internal exchanging pools. The simulation of lake transparency and productivity depicted a good ecological state, but the hypolimnetic waters remained anoxic during the thermal stratification, as confirmed by the paleolimnological survey (to pre-industrial age). P. rubescens persisted in that conditions, but its growth resulted strongly limited by low phosphorus concentration, resulting in a low productivity