Experiments with naturally derived, self-developing microcosms were performed to determine the coefficient of variation for dissolved nitrogen and phosphorous, conductivity, dissolved organic carbon, pH, photosynthesis-to-respiration ratio, and final algal biomass. Ten replicate tanks in each of three treatments were set up with 8 liters of standard reference water. Twenty of the tanks were flow-through and ten were static. Coefficients of variation (CV) for any one sampling time for ten replicates ranged from 0 to 219 percent, but for most parameters, the CV was consistently less than 20 percent. Methoxychlor was introduced into the microcosms after 73 days to obtain preliminary information for further experiments and to test modeling techniques. The methoxychlor disappeared rapidly and produced no apparent effects on the study parameters.