A Model Study of the Effect of Weather Forcing on the Ecology of a Meromictic Siberian Lake

Abstract

We used a Lake Shira numerical model to estimate the response of the ecosystem of a saline meromictic lake to variations in weather parameters during the growing season. The sensitivity analysis of the model suggests that compared to other external (nutrient infl ows) and internal (spring biomasses of food-web components) factors, weather parameters are among the most infl uential for both mixolimnetic (phyto- and zooplankton) and monimolimnetic (purple sulfur bacteria, sulfur reducing bacteria and hydrogen sulfi de) food-web components. Calculations with diff erent weather scenarios shows how changes in the water temperature and mixing depth aff ect mixolimnetic and monimolimnetic food-web omponents and the depth of the oxic-anoxic interface in a meromictic lake. When weather forcing stimulates an increase in the biomass of food-web components in the mixolimnion, it produces cascading eff ects that lead to three results: 1) a higher content of detritus in the water column; 2) a higher content of hydrogen sulfi de in the monimolimnion; 3) raising of the oxic-anoxic interface closer to the water-air surface. This cascading eff ect is complicated by the negative correlation between two light dependent primary producers located at diff erent depths—phytoplankton in the mixolimnion and purple sulfur bacteria at the oxic-anoxic interface. Thus, weather conditions that stimulate higher phytoplankton biomass are associated with a higher detritus content and lower biomass of purple sulfur bacteria, a higher content of hydrogen sulfi de and a shallower oxic-anoxic interface. The same weather conditions (higher wind, lower cloud cover, and lower air temperature) promote a scenario of less stable thermal stratifi cation. Thus, our calculations suggest that weather parameters during the summer season strongly control the mixing depth, water temperature and the mixolimnetic food web. An eff ect of biogeochemical and physical interactions on the depth of the oxic anoxic interface is also detectable. However, intra- and interannual climate and weather eff ects will be more important for the control of meromixis stability.