Phosphorus loading reductions needed to control blue-green algal blooms in Lake Mendota

Abstract

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]

Eutrophication of aquatic ecosystems: Bistability and soil phosphorus

“…Values of the parameters were estimated for the watershed of Lake Mendota, Wisconsin (10,27). These parameters are consistent with estimates of presettlement phosphorus inputs (28) and modern measurements of phosphorus inputs (29). Two simulations were calculated to illustrate changes of the system over time.…”

Reduction of nutrient loading and biomanipulation as tools in water quality management: Long-term observations on Bautzen Reservoir and Feldberger Haussee (Germany)

“…As the season progresses inedible phytoplankton starts to grow, thereby decoupling the trophic cascade (McQueen et al 1986). Nevertheless, Lake Mendota (Wisc., U.S.) as a result of intensive biomanipulation did show increased summer Secchi readings when Daphnia pulicaria dominated, although average annual P loading was slightly higher (0.85 g P/m 2 yr; Lathrop et al 1998) as suggested by Benndorf (1990;0.6-0.8 g P/m 2 yr). Moreover, Kasprzak et al (1999) have shown that in Wisconsin lakes dominated by D. pulicaria (large), the clear-water phase started earlier, lasted longer, and was usually characterized by greater Secchi readings compared to lakes where D. galeata mendotae (small) prevailed.…”

Invasive species triggers a massive loss of ecosystem services through a trophic cascade

“…To estimate the effect of P loading on water clarity, we made predictions using our MARSS model under varying grazing (pre-and post-Bythotrephes; i.e., high and low grazing from D. pulicaria) and P loading (−99% to +100%) scenarios under post-2009 surface temperature conditions and long-term average P loading conditions. We chose to investigate improving water clarity through P loading reduction as opposed to other methods (e.g., chemical treatment or biological control) because of existing efforts to reduce P loading into Lake Mendota (26,27) and additional benefits to water quality of lakes downstream of Lake Mendota (25).…”

“…1). This ecosystem service provided by D. pulicaria has delivered huge economic benefits, providing recreational value to citizens who have been estimated to be willing to pay US$140 million (present-day value) for 1 m of water clarity (1.6-to 2.6-m change in summer clarity) (26,27).…”