The non-equilibrium, stable-isotope simulator (NESIS) calculates the dynamics in the isotopic signature of the stocks of an element using the output from any parent model that predicts the flux rates and stocks of that element based on a mass balance approach. The software alleviates the need to recode the parent model to incorporate isotopes. The parent model can be a simple mass balance spreadsheet of the system or a standalone executable. The isotopic simulations use a linear, donor-controlled approximation of the fluxes in the parent model, which are updated for each time step. These approximations are based on the output of the parent model, so no modifications to the parent model are required. However, all fluxes provided to the NESIS must be gross fluxes (net fluxes allow only one-way isotope movement when the movement is actually in two directions), and the user must provide the initial isotopic signature for all stocks, the fractionation associated with each flux, and the isotopic signature of any flux originating from outside the system. For models based on net fluxes, the model output can often be converted to gross fluxes with a few simple assumptions (e.g., an assumed ratio of net to gross fluxes).
The software can also be used for radioisotopes if the half-life is long enough and simulation duration short enough to ignore losses caused by radioactive decay. The NESIS can be used for isotopes with shorter half-lives if the isotope is added in the simulations all at once rather than spread out through time; the results simply need to be corrected by removing the same proportion of isotope from all pools to account for the radioactive decay.


The NESIS model is written in Borland Delphi (version 5, service pack 1) for IBM PC compatibles. The file below is a compressed file containing the full source code, a Windows executable, sample files for running NESIS, and directions for using NESIS.

Compressed Nesis v1.3.2


Rastetter, E.B., B.L. Kwiatkowski, and R.B. McKane. 2005. A Stable Isotope Simulator that Can Be Coupled to Existing Mass-Balance Models. Ecological Applications 15:1772-1782. doi:

This material is based upon work supported by the National Science Foundation under grants #OPP-9318529, OPP-9732281, DEB-9509613, and DEB-0108960 and the Environmental Protection Agency under grants RFQ-RT-00-00107 and QT-RT-00-001667. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or the Environmental Protection Agency.