Leaf‐pack dynamics in a southern African mountain stream

Abstract

1. The occurrence, composition and invertebrate fauna of naturaUy-occurring leaf packs were studied over 24 months in Langrivier, a second-order mountain stream in the south-western Cape, South Africa. Langrivier is shallow and fast-tlowing and stores very low levels of allochthonous detritus, although natural leaf packs form an obvious part of the energy base in the stream throughout the year.2. The occurrence and size of the packs were influenced mainly by stream discharge and by the timing and character of leaf fall froin riparian trees. Packs were smallest (minimum dry mass 17 g, minimum volume 1.7xU)-^ m-) in winter when discharge was high, and largest (maximum dry mass lyi g, maximum volume 4.2x 10"' m') in spring w^hen discharge decreased and leaf fall from the evergreen riparian trees began. Through the year the packs covered a mean 0.41 % ofthe stream bed and had a mean abundance of 0.46 packs m-of stream bed. They were ephemeral, lasting on average < 1.7 months and yet accounted for 29% of the stored detritus in the system. Wood was the dominant component of packs, and leaves at ali stages of decomposition were present throughout the year.3. The ratio of numbers of invertebrates in packs: numbers of individuals in the benthos was very low (0.002-0.030). presumably because of the rarity and small size of the packs. Nevertheless, the density of invertebrates per unit area covered by leaf packs was consistently much higher than the density in an equivalent area of the benthos, except during peak leaf fall (October to December).4. Experiments were undertaken with artificial leaf packs in order to determine the extent to which these simulated natural packs. Although both natural and artificial leaf packs contained a high proportion of Plecoptera (46% and 29% respectively), the natural packs contained high numbers of simuliid larvae (33% of total), whereas artificial packs had a high percentage of chironomid larvae (62%), Several other taxa regularly occurred in both types of pack but in very low numbers. In addition,

An assessment of mesh bags and plastic leaf traps as tools for studying macroinvertebrate assemblages in natural leaf packs

“…Recently, however, these assumptions have been questioned, but the results of two experiments devised to test their validity with respect to invertebrate colonization are conflicting . King et al (1987b) found significant differences between invertebrate assemblages colonizing natural packs and mesh bags, whereas Mutch et al . (1983) found that they were similar .…”

“…Plastic traps broke the surface of the water, physically obstructing flow and forming large obstacles for a drifting animal to hit, whereas mesh bags were totally submerged for the duration of the experiment, and offered less resistance to the flow of the river . King et al (1987b) andMutch et al (1983) observed that numbers of invertebrates in mesh bags were higher than those in benthic accumulations . They attributed this to the way that their mesh bags were positioned, completely submerged and in direct contact with the river bed, facilitating their colonization from the benthos .…”

“…The mesh bags used by King et al (1987b) were colonized by twice as many animals per unit mass as natural leaf packs after placement in their study stream at the end of summer . In the south western Cape of South Africa, where the experiment took place, maximum leaf fall occurs in spring/early summer, as the temperature increases and flow rate decreases (King et al, 1987a) ; therefore, the leaves in natural leaf packs had the whole summer to decompose before the placement of the mesh bags .…”

Yeast and Macroinvertebrate Communities Associated with Leaf Litter Decomposition in a Second Order Stream

“…Since P. meyeri is the only shredder present, we believe that the type of leaves has only an indirect effect on the presence of most invertebrates. The evidence for leaf preferences of some invertebrates, in spite of the opportunistic character of most colonisers (KING et al, 1987), has already been pointed out for oligotrophic headwaters streams .…”

A preliminary assessment of the effects of a small south african inter-basin water transfer on discharge and invertebrate community structure

“…The fauna of the Berg will almost certainly have become adapted to the warm waters and low flows naturally encountered during summer months in the Western Cape (King et al, 1987(King et al, , 1988. Due to the IBT, 56% of the 36 taxa recorded above and below the siphon in the Berg River during March were common to both sites at this time, while the corresponding figure for May was 52% of 31 taxa: these values are greater than that reported by O'Keeffe and de Moor (1988) for the Great Fish River (33%).…”