Food Quality for Detritivores in Streams of Contrasting pH

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Abstract
Leaves of alder (Alnus glutinosa Gaertner) and beech (Fagus sylvatica L.) were conditioned in an acid and a circumneutral stream. Conditioned leaves in litter bags then underwent a period of further exposure, either having been replaced in the same stream or transferred to the contrasting stream. The respiration rate of microbes on leaf surfaces conditioned in circumneutral water was greater than on those from acid water, for the first 8-16 days of exposure in the same stream. This corresponded to differences in microbial biomass. Carbon-nitrogen ratios were lower in the neutral than the acid-conditioned leaves of both species. For alder, differences due to leaf conditioning persisted for about 14 days of exposure in the same stream. Aluminium, scarce at first, accumulated markedly on all leaves during conditioning. During further exposure aluminium content increased in alder only, the rate being greatest in the acid stream. Invertebrate animals colonized the leaf bags in both streams, although somewhat erratically in the circumneutral stream. Leaf species were alike in this, irrespective of conditioning. However, in the acid stream, there was faster colonization of neutral than there was of acid-conditioned alder over the first 8 days of exposure, when differences in food quality were most marked. Feeding on acid or neutral-conditioned leaves of both species produced no significant difference in the growth of the caddis Lepidostoma hirtum (Fabricius), but the stonefly Nemurella pictetii (Klapalek) grew faster on neutral-conditioned litter than on acid-conditioned litter. Reduction in food quality could account for the reduced productivity of acid streams, and also for some of the differences in their invertebrate species composition, but the distribution of N. pictetii, particularly abundant in acid streams, is not due to food quality.