When a substance subject to biological
concentration is introduced as a single
dose, as in the microcosm experiment described, the substance is first taken up by
organisms and then moves progressively out of active circulation and into less
active or inactive forms. The concentration of a fertilizer (such as phosphate)
introduced as a single dose will decline toward the same steady-state level,
determined by the functional characteristics of the ecosystem, as existed before its
introduction. When a substance is introduced on a continuing, constant basis,
however, this continuing introduction becomes one of the factors determining the
steady state for the system. A new pattern of pool contents and transfer rates for the
system will develop, based on the new rate of introduction and its relationship to the
system's new rates of turnover and removal.
