DECOMPOSITION AND MACROINVERTEBRATE COLONIZATION OF AQUATIC AND TERRESTRIAL LEAF MATERIAL IN ALKALINE AND ACID STILL WATER

1. Decomposition of the leaves of Nymphaea alba L. and Betula pubescen s Ehrh. was studied in two ponds of contrasting pH, buffering capacity and trophic level. Rates of carbon loss, concentrations of nitrogen, protein and phenolics and colonization of the leaf material by macroin vertebrates were studied. 2. Decomposition of B. pubescens leaves was slower than that of N. alba in both ponds. Protein and phenolic concen trations were higher in the decomposing N. alba material, whereas no s ignificant difference was found for nitrogen concentrations. 3. Decomp osition of both species was slower in the acid water. The effect of ac id water on decomposition rate was similar for both species. Both nitr ogen and protein concentrations of N. alba remained higher under acid conditions, whereas no effect of study site was observed on the protei n and nitrogen concentrations in B. pubescens litter. The concentratio n of phenolic compounds was higher in N. alba litter from the acid pon d; no such difference was observed for B. pubescens litter. The high c oncentrations of phenolic compounds in N. alba litter from the acid po nd probably induced chemical immobilization (tanning reaction), yieldi ng high concentrations of nitrogen and protein in the litter. 4. Diver sity of the detritivorous macroinvertebrate fauna on the litter of bot h species was extremely low in the acid pond, while a relatively high diversity occurred in the alkaline water. Exclusion of large macrofaun a inhibited decomposition in the alkaline water, while there was no co mparable influence in the acid water. 5. The effects of low pH on the decomposition of both terrestrial and aquatic macrophyte litter appear ed to be caused by interactive effects of low pH on the microbial deco mposition, on the detritivorous macroinvertebrates and, in the case of N. alba, on the resource quality of the detritus.