The quality and quantity of allochthonous inputs and of benthic organic matter were investigated in a secondorder, perennial mountain stream in the south-west Cape, South Africa, between April 1983 and January 1986. Although the endemic, riparian vegetation is sclerophyllous, low and evergreen, inputs of allochthonous detritus to the stream (434 to 500 g m-2y-1) were similar to those recorded for riparian communities worldwide, as were calorific values of these inputs (9548 to 10 032 KJ mm2y-'). Leaf fall of the riparian vegetation is seasonal, occurring in spring (November) as discharge decreases, resulting in retention of benthic organic matter (BOM) on the stream bed during summer and early autumn (maximum 224 g me2). Early winter rains (May) scoured the stream almost clean of benthic detritus (winter minimum 8 g mm2). Therefore, BOM was predictably plentiful for about half of each year and predictably scarce for the other half. Coarse BOM (CBOM) and fine BOM (FBOM) constituted 46 -64% of BOM standing stock, ultra-fine BOM (UBOM) 16 -33 070 and leaf packs 13 -24%. The mean annual calorific value of total BOM standing stock was 1709 KJ me2. Both standing stocks and total calorific values of BOM were lower than those reported for streams in other biogeographical regions. Values of C:N ratios decreased with decrease in BOM particle size with no seasonal trends. The stream is erosive with a poor ability to retain organic detritus. Its character appears to be dictated by abiotic factors, the most important of which is winter spates.
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,
In a study of a second-order southern African stream, complementary classification and multidimensional scaling (MDS) techniques revealed longitudinal changes in macroinvertebrate community structure, but no temporal changes. Stepwise discriminant analysis and multiple linear regression were used to identify environmental variables correlated with the community changes but produced conflicting results depending on the information used, possibly because of strong correlations between some of the variables. The MDS plot of biotic samples illustrated that potassium levels correlated most strongly with community distribution. Because of the large number of variables now shown worldwide to correlate with faunal distributions, we suggest ways to choose the variables to suit the kind of study to be undertaken. Attempts to assign the invertebrates to functional feeding groups (FFGs) were unsatisfactory as the relevant categories are poorly defined and often inadequate for classifying the fauna. We suggest that until these categories are more clearly defined, and more uniformly applied, concepts relating to FFGs cannot be tested satisfactorily. The stream community in Langrivier is more similar to communities found in two other mediterranean ecosystems than to those in other southern African rivers, probably because of the greater predictability of flow in the former.
The influence of different antecedent conditions on hydrochemical response during storm events was investigated in a small, south-western Cape mountain catchment. Winter and summer storms (four in total) were sampled both before and after the catchment was deliberately burnt. During winter storms, discharge responded rapidly to rainfall, and direct runoff represented the major component of streamflow. Marked lags were observed between rainfall and discharge peaks during the summer storms, and streamflow was dominated by delayed interflow. Chloride, PO: --P and NO, -N exhibited variable response to discharge according to seasonal variations in soil-moisture levels, whereas the response of HCOq , H' and NH2 -N was not influenced by season. The movement of ions appears to be affected more by geochemical processes operating within the soil than by plant-uptake dynamics, as the prescribed burn appeared to have little effect on relationships between ionic concentration and discharge. The findings of the study highlight the complexity of relationships between solute concentration and discharge.
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