Three different sizes of marine microcosms were used to study the influence of two features of spatial scale on the chemical fate and ecological effects of the pesticide Kepone. Increasing the size of microcosms reduced the ratio of wall surface area to volume of contained sea water, but increased the number of benthic species due to increasing sample size. Other features of spatial scale, such as water turbulence, water turnover, etc., were held constant. Intact water-column and benthic communities from a north-temperate marine system were coupled together in 9.1-, 35.0-, and 140.0-L containers. Kepone at 20.4 nmol/L was added to these microcosm systems over a 30-d period. A 3 x 2 factorial design was used to discern the effects of size and Kepone. In the absence of Kepone the phytoplankton community exhibited excessive growth relative to the field system for all system sizes. Growth was directly related to the size of microcosms. In addition, the time required to achieve maximum algal biomass was also directly related to size. Release of a growth-stimulating compound(s) from fouling organisms settling on the microcosm walls and size-dependent increases in benthic species provided the best explanation for the observed phytoplankton dynamics. (Copyright (c) 1991 by the Ecological Society of America.)