WRS Masters Defense

Title: Stratification Stability of Tropical Lakes and Their Responses to Climate Change: Lake Towuti (Indonesia)

Abstract: Tropical lakes have different physical dynamics than temperate lakes. They are less likely to experience thermal convection and can be permanently stratified by temperature gradients alone (thermogenic meromixis). Climate research suggests that tropical lakes are most susceptible to increasing surface temperature and stratification with future prediction of increasing air temperature. Tropical meromictic lakes are also important sites for paleoclimate research, as the anoxic and stagnant bottom layer preserves laminated sediments. Ancient Lake Towuti, located in Sulawesi island in Indonesia, is believed to be meromictic sustained without profound salinity gradient. This project aims to investigate the strength and dynamics of thermogenic meromixis in tropical lakes. To address this problem, it simulates the physical dynamics of the lake via the use of a 1-D k-epsilon hydrodynamic model Simstrat. The simulations reveal that Lake Towuti may be oligomictic rather than meromictic, as was previously assumed. More generally, tropical lakes much deeper than 80 m are likely to be oligo- or meromictic, whereas shallower lakes are likely to be warm monomictic. Air temperature stands out to be the most influential meteorological variable affecting stratification. Oligomixis may be regulated by a negative feedback that plays out over multiple years: A full water column mixing event will decrease the bottom water temperature of a lake and eventually increase surface-bottom temperature difference and average stability. This leads to increasing difficulties for whole-water column mixing. The temporal variation in the Lake Towuti stratification is only weakly correlated with the ENSO index. During the Last Glacial Maximum (LGM), Lake Towuti likely experienced colder and drier climate, which lowers lake level, and induces more frequent whole-water column mixing. A 1-D biogeochemical reaction-transport model, which was provided with the output of hydrodynamic simulations for the intensity of turbulent mixing, further confirmed a possibility of the whole water column mixing in Lake Towuti as recently as 2008. A weak mixing event lasting a few weeks during the dry season would be consistent with the observed chemical profiles of O2, H2S, and Fe2+. Coexisting dissolved iron and hydrogen sulfide at low concentrations (micro Molar level) likely indicate observations reflecting a transient state. Findings suggest that, under the future climate characterized by warmer air temperatures, Lake Towuti is likely to be more stable. Stronger stability and isolation of monimolimnion water would lead to further accumulation of dissolved iron. Similar tropical lakes whose stability are controlled primarily by water temperature are expected to have increasing stability in the future as well.