We evaluated the reductions in P loading needed to control blue-green algal blooms in Lake Mendota, Wisconsin. After developing a 21-year loading data set, we used a P mass balance model expressed as a difference equation with an annual time step indexed from mid-April. We defined and estimated a loss parameter λ as the proportion of the lake's April P concentration lost through sedimentation and outflow during the following year. Using the distribution of annual λ's and input loadings, we predicted the steady-state distribution of April P concentrations that would result from scenarios of altered inputs due to changes in management practices. These results were then linked to the probability of summer blue-green algal blooms. For no load reduction, the probability of a bloom (>2 mg algaeиL -1 ) on any summer day is about 60%. This probability decreases to 20% with a load reduction of 50%. Our approach illustrates how managers can consider reducing the frequency of extreme events like algal blooms, which may correspond more to the public's perception of lake water quality than average conditions. Résumé : Nous avons évalué les réductions de la charge en P nécessaires pour éliminer les efflorescences d'algues bleues dans le lac Mendota, au Wisconsin. Après avoir établi une série de données sur la charge couvrant une période de 21 ans, nous avons utilisé un modèle du bilan massique de P exprimé sous la forme d'une équation à différences finies avec la mi-avril comme pas de temps annuel. Nous avons défini et estimé un paramètre de perte λ correspondant à la proportion de la teneur du lac en P au mois d'avril qui est perdue par sédimentation et par évacuation au cours de l'année suivante. En utilisant la distribution des λ annuels et des charges d'entrée, nous avons prédit la distribution en état d'équilibre des teneurs en P d'avril qui résulteraient de scénarios de modification de la charge suite à des changements dans les méthodes de gestion. Nous avons alors établi un lien entre les résultats obtenus et la probabilité d'efflorescences estivales d'algues bleues. Sans réduction de charge, la probabilité d'une efflorescence (>2 mg d'alguesиL -1 ) pendant n'importe quel jour d'été se situait à environ 60%. Cette probabilité diminue à 20% lorsque la charge est réduite de 50%. Notre approche illustre une façon pour les gestionnaires d'envisager la réduction de la fréquence d'événements extrêmes comme les efflorescences phytoplanctoniques, qui, aux yeux du grand public, peuvent sembler plus révélateurs de la qualité de l'eau d'un lac que les conditions moyennes. [Traduit par la Rédaction]

The Wisconsin Phosphorus Index (WPI) is one of several P indices in the United States that use equations to describe actual P loss processes. Although for nutrient management planning the WPI is reported as a dimensionless whole number, it is calculated as average annual dissolved P (DP) and particulate P (PP) mass delivered per unit area. The WPI calculations use soil P concentration, applied manure and fertilizer P, and estimates of average annual erosion and average annual runoff. We compared WPI estimated P losses to annual P loads measured in surface runoff from 86 field-years on crop fields and pastures. As the erosion and runoff generated by the weather in the monitoring years varied substantially from the average annual estimates used in the WPI, the WPI and measured loads were not well correlated. However, when measured runoff and erosion were used in the WPI field loss calculations, the WPI accurately estimated annual total P loads with a Nash-Sutcliffe Model Efficiency (NSE) of 0.87. The DP loss estimates were not as close to measured values (NSE = 0.40) as the PP loss estimates (NSE = 0.89). Some errors in estimating DP losses may be unavoidable due to uncertainties in estimating on-farm manure P application rates. The WPI is sensitive to field management that affects its erosion and runoff estimates. Provided that the WPI methods for estimating average annual erosion and runoff are accurately reflecting the effects of management, the WPI is an accurate field-level assessment tool for managing runoff P losses.

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This work describes a simple, passive sampling system for measuring runoff, sediment, and chemical losses from typical agricultural fields. The sampler consists of a 5 to 7 m wide runoff collector connected to a series of multislot divisors. These divisors split the flow into aliquots, providing a continuous sampling during the runoff event. Divisors were located in a wooden box below ground level. With an adequate pump, this system can operate in fields with a slope gradient as low as 2%, and can stay in the field during winter to record first snowmelt-generated runoff. A radio transmitter reports by telemetry the occurrence and magnitude of any runoff event, and indicates when the system should be sampled and emptied. This article includes a description of the equipment, advantages, and disadvantages based on 2 yr of operation, and examples of data collected.

The Conservation Reserve Program (CRP) is a federal program that encourages the planting of cool‐ or warm‐season grass cover on highly erodible croplands and along stream corridors. We sought to determine whether fish community structure in coldwater streams was associated with CRP and other agricultural land use changes in southwestern Wisconsin. We compared coldwater fish index of biotic integrity (IBI) scores and species richness in streams located in areas of relatively high (21.3% of land area; high‐CRP area) versus relatively low (12.1% of land area; low‐CRP area) CRP participation. All of the streams were sampled in the 1970s before implementation of the CRP and again at the same locations after implementation, from 2000 to 2005. Pre‐CRP fish communities were characterized by a relatively high diversity of eurythermal species and low coldwater IBI scores. We found significant increases in coldwater IBI scores over time in streams within the high‐CRP area relative to streams within the low‐CRP area. Fish populations in streams within the high‐CRP area shifted from eurythermal and tolerant species before CRP implementation to stenothermal, cool‐ and coldwater species after implementation. Ecological responses within the high‐CRP streams also included a reduction in species richness. Without intensive monitoring of watershed nutrients, the fish community changes cannot be mechanistically linked to specific land use practices. However, we demonstrate that IBI scores and species richness were correlated with phosphorus loading estimates and that predicted phosphorus reductions were greater within the high‐CRP grassland area. The estimated phosphorus loading declines reflected reduced cropland areas and reduced density of dairy farms. The estimates did not capture all environmental factors, such as trends in production of hogs and cattle or streamflow regime changes associated with conservation practices. We conclude that the combination of extensive grassland management, livestock reductions, and other long‐term agricultural land use changes benefited the coldwater fish communities within the high‐CRP area.

Identification of areas contributing disproportionatelyhigh amount of pollutants (i.e., critical source areas (CSAs)) to streamsis important to efficiently and effectivelytarget best management practices (BMPs). Process-based models are commonly used to identify CSAs and evaluate the impact of alternative management practices on pollutant load reductions. The objective of this study was to use the Soil and Watershed Assessment Tool (SWAT) to identify CSAs at the subwatershed level and evaluate the impact of alternative BMPs on sediment and total phosphorus (TP) load reductions in the Pleasant Valley watershed (50 km 2 ) in South Central Wisconsin (USA). The Nash-Sutcliffe efficiency, percent bias, and coefficient of determination ranged from 0.58 to 0.71, -12.87 to 38.33, and 0.67 to 0.79, respectively, indicating that SWAT was able to predict stream flow, sediment and TP loadings at a monthly time-step with sufficient accuracy. Based on the SWAT simulation results, annual Average (2006Average ( -2012 subwatershed yield for sediment and TP ranged from 0.06 to 3.14 tonsha -1 yr -1 and 0.04 to 1.9 kg ha -1 yr -1 ,