The Formation of Particles in Freshwater Leachates of Dead Leaves

Leaves of similar morphology and condition release a similar quantity of soluble organic material. However, the quantity of material that precipitates, the rate of precipitation and the size of particles that result depend on leaf species and water chemistry. A microbial componcnt and an abiotic component are apparent in the precipitates; the latter bears a similarity to particulate material from scawatcr and soils. It is now well established that allochthonous organic matter is important in the productivity of streams ( Hynes 1970; Fisher and Likens 1972; Cummins 1973). Previous work in this laboratory has been concerned with the fate of the autumnal leaves that fall into water. Our earlier experiments were with leaves that had been leached to constant weight before being used, and this represented a considerable loss of material ( Kaushik and Hynes 1971). In 1968, therefore, we began a study of the leachate, with particular reference to the finding that organic particles are formed when bubbles are passed through seawater (Riley 1963, 1970). These seawater particles are variable in size and shape, but they are described as being usually either semitransparent flakes or globular aggregations with mean diameters of 25-50 p. It has been suggested that they are formed by adsorption of organic matter onto the surfaces of the bubbles, which then burst and leave them in the water, We felt that similar particles might also be formed by turbulence such as occurs in streams. Saunders ( 1969) had already tried to form particles by passing bubbles through lake water, but with little success, and had concluded that the mechanism does not appear to be important in freshwater. There are, however, several reports of micropar’ This investigation was supported by an opcrating grant from the National Research Council of Canada. ticulate organic detritus in both lotic and lentic freshwater habitats (Egglishaw 1964; Pfistcr et al. 1969). Nykvist ( 1963 and earlier papers) has studied the release of soluble materials from dead leaves and has mentioned the formation of “clots” in the solutions, although he did not investigate them further. Also, when McConnell (1968) studied the effects of water extracts from oak leaves on the growth of Xenopus larvae, he concluded that the larvae fed on particles that had probably been formed from the solutions by microbial action, Soil scientists have long been aware of the complexing of dissolved organic matter with metals, which may at times result in the formation of particles. Schnitzer and Poapst (1967) have stated that heavy summer rains may bring concentrations of organic matter into the soil solution as high as several thousand parts per million. Some of this material enters streams, the amount depending on the concavity or convexity of the drainage slope and the permeability of the soil ( Freeze 1972)) and these factors probably also influence the quantities and the sizes of any accompanying particles. Fisher and Likens (1972) have recently shown that a large proportion of the organic matter that comes in from an undisturbed watershed is actually in solution. The fate then of the dissolved material, both from the soil and leached from leaves, and the probability that at least some of it becomes particulate and may form food for fine particle feeders such as Simuliidae LIMNOLOGY AND OCEANOGRAPHY 968 NOVEMBER 1973, V. 18(6) FARTICJLE FORMATION IN FRESHWATER 969 Table 1. Composition of standard water in mg liter-l