Effect of P-Cresol on Leaf Decomposition and Invertebrate Colonization in Experimental Outdoor Streams

Abstract

In 1980, outdoor experimental stream channels at Monticello, Minnesota were continuously dosed for 24 and later 48 h with 8 mg/L p-cresol, a toxic phenolic by-product of coal gasification. The dosage, based on laboratory studies by the Environmental Protection Agency, was expected to stress but not to kill fish and invertebrates in the channels. Two unexpected results occurred: (1) severe depressions in dissolved oxygen (less than 1 mg/L), and (2) mortality to some fish and invertebrate species in the experimental versus reference channels. Because the direct effects on the aquatic ecosystem were inseparable from the indirect effect of reduced dissolved oxygen, two types of dosing regimes were designed in 1981 to partially uncouple the direct from the indirect effects. First, a continuous dose at 8 mg/L for 96 h was administered, then a second intermittent dose at 8 mg/L with temporary cessations when reductions in dissolved oxygen occurred. Leafpacks were placed in experimental and reference channels to monitor community level responses to the toxicant under severe long-term and under moderate short-term dissolved oxygen depletions. Biotic responses to the intermittent dose schedule were less severe than biotic responses to the continuous dose. The intermittently dosed experimental channel and its reference channel were more similar to each other than was the continuously dosed channel as compared with its reference channel with respect to: (1) invertebrate colonization patterns, (2) invertebrate biomass changes over time and (3) leaf biomass changes over time. During the continuous dose, some invertebrate species that were rare colonists on leafpacks became highly abundant (leeches and isopods); other normal colonists became extremely abundant (snails and flat-worms) or diminished (freshwater scuds). These changes in colonization patterns significantly altered biomass of invertebrates on leafpacks over time. Leaf biomass losses were similar over time except in the continuously dosed channel where swamp poplar leaf losses (Populus deltoides Marsh) were significantly smaller during and shortly after the dose.