### 2.25.7.1. Competition for CH4 and O2[¶](#competition-for-ch4-and-o2 "Permalink to this headline")

For each time step, the unlimited CH4 and O2 demands in each model depth interval are computed. If the total demand over a time step for one of the species exceeds the amount available in a particular control volume, the demand from each process associated with the sink is scaled by the fraction required to ensure non-negative concentrations. Since the methanotrophs are limited by both CH4 and O2, the stricter limitation is applied to methanotroph oxidation, and then the limitations are scaled back for the other processes. The competition is designed so that the sinks must not exceed the available concentration over the time step, and if any limitation exists, the sinks must sum to this value. Because the sinks are calculated explicitly while the transport is semi-implicit, negative concentrations can occur after the tridiagonal solution. When this condition occurs for O2, the concentrations are reset to zero; if it occurs for CH4, the surface flux is adjusted and the concentration is set to zero if the adjustment is not too large.

