Functional properties of a microcosm of the freshwater benthic zone and the effects of 2,4-dichlorophenol

Abstract

A sediment-water microcosm system was evaluated that was designed to address some ecosystem functions selecteda priori for their significance to man's use of lakes: (a) the decomposition of organic carbon, (b) secondary production, and (c) the binding or degradation of toxic chemicals. 2,4-Dichlorophenol (DCP) was chosen as a ‘typical’ toxic chemical. The microcosms were undisturbed cores recovered from a high-deposition area of Lake Michigan and dosed with a trace amount of14C-alanine. The measurements found to be interpretable in terms of ecosystem functions were (a) turnover of non-acid-volatile radiocarbon, (b) turnover of particulate radiocarbon, (c) rate of attachment of radiocarbon-labelled organisms to glass surfaces, (d) sediment penetration of radiocarbon, (e) mortality of benthic fauna (especially pontoporeia), and (f) toxicant concentrations. Lethal toxicity of DCP toPontoporeia sp. in the microcosms agreed with that determined in controlled bioassays (96-hr LC50: 2.5 mg/L). Functions of the microbial community were initially inhibited by 81 mg/L DCP, but recovered in six days. The rates of radiocarbon turnover were reduced throughout the 21-day experiments even at the lowest concentration of DCP, 1.0 mg/L. These experiments have shown that a microcosm approach can yield quantitative estimates of the effects of toxicants on the functional properties of lake benthic ecosystems.