Modeling the climate-induced changes of lake ecosystem structure under the cascade impacts of hurricanes and droughts

Lake Okeechobee, located in south Florida and originated about 6000 years ago during oceanic recession, is the second largest freshwater lake contained entirely within the United States. During recent years, four major hurricanes, Irene (1999), Frances (2004), Jeanne (2004), and Wilma (2005), made landfall near Lake Okeechobee. As a result, the lake's hydrodynamic patterns, water level, and water quality have been extensively influenced by hurricanes in the past decade. The direct landfall of Hurricane Wilma on the Lake Okeechobee area led to a drastic structural change to the ecosystem, resulting in an abrupt population decrease of submerged aquatic vegetation (SAV). During the same time period, south Florida also suffered from intermittent droughts in 2000–2001 and 2006–2008 due to climate variability. Drought stress and hurricane impacts induce complex ecosystem responses, including pervasive changes in species composition and productivity that may persist as changes in habitat, lake trophic state, light penetration depth, water levels, and nutrient cycling. After the 2006–2008 drought, decreased turbidity due to reduced sediment hydrodynamics, along with the increased runoff and nutrient inflows to the aquatic ecosystem, led to a range of ecological responses associated with gradual increases in primary productivity and the recovery of SAV in lake shore environments. The ecosystem then recovered after the population increase of SAV, triggering active ecodynamics among phytoplankton, zooplankton, and fish that had abruptly vanished after successive hurricanes in 2004 and 2005. To address such a unique structural change in the lake ecosystem, this study developed a structurally dynamic model to quantify the ecodynamics of the four main aquatic system organisms (SAV, phytoplankton, zooplankton, and fish) based on a thermodynamics index (exergy) and elucidated the ecosystem recovery pathways under the coupled impact of hurricanes and droughts.