Whole-Lake Model for the Distribution of Sediment-Derived Chemical Species

Abstract

The distribution of CH₄, ΣCO₂, and NH₃-N below the thermocline of Lake 227 can be reproduced using a simple numerical model. The model uses a constant vertical eddy diffusion coefficient and assumes the sediments to be the sole source of the chemical species. Analogs dependent on oxygen concentration can effectively represent conditions at the interface of the modeled region and the shallower depths in the lake. The best tit value for the vertical eddy diffusion coefficient is 3.1 × 10⁻³ cm²∙s⁻¹. This is in good agreement with other independent measurements of this phenomena. The best fit fluxes of CH₄, dissolved inorganic carbon and NH₃-N are respectively: 13 × 10⁻³ mol∙m⁻²∙d⁻¹, 7.5 × 10⁻³ mol∙m⁻²∙d⁻¹, and 1.65 × 10⁻³ mol∙m⁻²∙d⁻¹. The flux of methane is similar to that found in other small productive northern lakes and the ratio of CH₄:ΣCO₂ production of 1.73 falls in the range of values established for fermentation of mixed organic materials and sediments. The flux of ΣCO₂ is 50–75% of the bicarbonate flux (10–15 × 10⁻³ mol∙m⁻²∙d⁻¹) found previously in Linsley Pond, Connecticut.Key words: sediment–water interactions, methane, CO₂, and ammonia fluxes, diffusion from sediments, Experimental Lakes Area