Biogeochemical model. The biogeochemical model (almlab.mit.edu/mystic.html),
inspired by Hunter et al.22, was run with Matlab (version 8) and supporting Python
scripts (version 2.7). Details on the mechanics, implementation and parameter
values are provided in the Supplementary Information. Briefly, the water under the
thermocline is modelled as 17 linked compartments, one per metre depth. Within
each compartment, a minimal set of abstracted chemical species interconvert
through a minimal set of modelled primary and secondary redox reactions
(Supplementary Fig. 5 and Supplementary Tables 1–3). Primary oxidation rates
follow a formulation informed by the relative favourability of electron acceptors.
Secondary oxidation rates follow simple mass action rate forms. Chemical species are
transported between adjacent compartments via bulk diffusion (for all species) and
settling (for biomass and oxidized iron). The outside world is modelled by constant
source terms: oxygen and biomass are added in the uppermost compartment (at the
thermocline), and methane is added in the lowermost compartment (at the
sediment). The resulting set of ordinary differential equations is solved numerically.
We intended to model the general distribution of chemical and biological species
in the lake. Because the model is conceptual, it includes many simplifications
compared to the aquifer model. First, transport is modelled compartment-bycompartment,
using ordinary differential equations rather than partial differential
equations.We greatly reduced the number of simulated chemical species (from25 to 9).
Many simulated chemical species consist of multiple chemical species found in
nature (for example, the modelled oxidized sulfur species includes hydrogen sulfide
(H2S), bisulfide (HS−) and sulfide (S2−); there is only one modelled carbon species).
Other chemical species found in nature are not treated in the model (for example,
elemental sulfur (S0) and all manganese compounds) because less is known about
their importance to the lake’s biogeochemistry. The primary redox reactions are
borrowed almost exactly from the aquifer model (excepting some parameter changes
and the removal of manganese as an electron acceptor). The secondary redox
reactions are similar to those in the aquifer model (except some parameter changes,
the removal of some reactions and the addition of iron oxidation on nitrate).
Precipitation–dissolution, acid dissolution and adsorption reactions relevant in the
groundwater system were part of the original aquifer model but were not simulated
here. Further details about these alterations of the original model are included in
Supplementary Sections I.2–I.5.