Particulate organic matter in a mountain stream in the south-western Cape,South Africa

The quality and quantity of allochthonous inputs and of benthic organic matter were investigated in a second-order, 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 m^–2y^–1). 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 m^–2). Early winter rains (May) scoured the stream almost clean of benthic detritus (winter minimum 8 g m^–2). 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% and leaf packs 13–24%. The mean annual calorific value of total BOM standing stock was 1709 KJ m^–2. 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 (CBOM 27–100; FBOM 25–27; UBOM 13–19) 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.     

Nitrogen transformations in a small mountain stream

Ammonium, urea, and nitrate were added to Bear Brook, a second and third order stream in the Hubbard Brook Experimental Forest, New Hampshire. Removal of ammonium and urea during downstream transport coincided with the release of nitrate. Nitrate removal did not occur when it was added alone or with dissolved organic carbon. Laboratory experiments showed that coarse particulate organic material (detritus) and bryophytes taken from the streambed were active in the removal of ammonium from enriched stream water, and in the release of nitrate upon the addition of ammonium.The patterns of removal and release observed in these experiments suggest a biologically mediated, oxidation process. Budgetary calculations show that the in-stream transformation of nitrogen inputs during summer and autumn could represent 12 to 25 percent of the nitrogen exported as nitrate during winter and spring from heterotrophic streams like Bear Brook. This type of internal cycling affects the timing and form of nitrogen export from small streams draining forested watersheds in the northeastern United States.     

Fire and the chemistry of a South African mountain stream

The effects of a late-summer prescribed burn on the chemistry of a second-order mountain stream in the south-western Cape, South Africa, were investigated. Nitrate concentrations in stream water were significantly higher during the winter of the post-burn year. Increased concentrations of chloride, bicarbonate, polyphenols and potassium and decreased sodium concentrations were also recorded. Concentrations of ammonium, phosphate, calcium, magnesium, total dissolved solids and hydrogen ions were not significantly affected by the burn. Ionic export from the catchment was generally greater in the post-burn year. Apart from nitrate, however, values probably lie within the natural range of year-to-year variation. It is predicted that enhanced losses of nitrate will decrease progressively with the recovery of the vegetation and the re-establishment of soil/plant nutrient cycles. Atmospheric losses of nutrients in smoke were unquantified, but may be of more significance to site productivity than losses through surface runoff, which, in the case of nitrogen, appear to be compensated by precipitation inputs.     

Fire and the dynamics of allochthonous detritus in a South African mountain stream

SUMMARY.

• 1 Input of allochthonous material, standing stocks of benthic organic matter (BOM) and suspended paniculate organic matter (POM) were measured in a south-western Cape mountain stream from March 1986 to February 1988. The surrounding fynbos-dominated catch-ment was subjected to a prescribed burn in March 1987.
• 2 Litter-fall in the pre-burn year exhibited a distinct seasonal pattern, with peak falls during the early summer. Although the riparian canopy was not directly affected by the fire, in that it did not burn, a heavy, aseasonal leaf-fall occurred shortly afterwards. The following summer, litter-fall was less than half that of the pre-burn summer.
• 3 Standing stocks of BOM were significantly higher in autumn than in winter in the pre-burn year and were inversely related to discharge. Despite the heavy post-burn leaf-fall and low litter-fall during the post-burn summer, there was no significant difference between pre- and post-burn BOM standing stocks.
• 4 Proportions and quantities of fine benthic organic matter (FBOM) in the soft BOM fraction were significantly higher in the post-burn spring, and monthly accumulation of ultra-fine benthic organic matter (UBOM) was also significantly higher in the post-burn spring and summer. These results may reflect accelerated decay rates of BOM in response to enhanced post-burn nitrate concentrations in stream water.
• 5 Export of CPOM was low in comparison to FPOM and particularly to UPOM, and the stream appears to be highly retentive of CPOM.
• 6 The natural resilience of the riparian vegetation minimizes the potentially disturbing effects of fire on the stream environment. As a result, the prescribed burn had a less than expected effect on both standing stocks of BOM and the stream environment in general.

     

Influence of largemouth bass,Micropterus salmoides,on abundance and habitat selection of Cape galaxias,Galaxias zebratus,in a mountain stream in the Cape Floristic Region,South Africa

Predatory alien fishes have been widely introduced into streams in the Cape Floristic Region (CFR), South Africa, but little is known about their effect on native fishes. Results from this 2006 study suggest that the presence of alien predatory largemouth bass, Micropterus salmoides, may have influenced abundance and habitat selection of the native Galaxias zebratus at one location in a small CFR mountain stream. Numbers of adults, but not of juveniles, were significantly lower where M. salmoides was present, suggesting a size-specific influence on G. zebratus abundance. Because other studies have found the influence of a predator to be affected by prey size and the diel activity of predator and prey, we measured microhabitat use by adult and juvenile G. zebratus by day and by night. Galaxias zebratus selected deeper, faster-flowing microhabitats more strongly where M. salmoides was present than where it was absent. This suggests that G. zebratus adjusts its habitat use in the presence of M. salmoides, although differences in available habitat, or in interactions with other indigenous species, could be partly responsible for the observed differences. In-stream vegetation presence was strongly positively related to depth and strongly negatively related to velocity at positions where G. zebratus was observed, suggesting that the deeper, slower-flowing microhabitats occupied by G. zebratus were structurally more complex than the shallower, faster-flowing ones.     

Benthic detritus dynamics in a mountain stream

The dynamics of sedimentary detritus was examined over a 17-month period in a third-order mountain stream in Colorado. Autumnal and vernal peaks in total detritus standing crop (197 g and 165 g AFDW m⁻², respectively) were associated with leaf abscission and ice out. The flushing action of snowmelt runoff rapidly reduced benthic detritus to 14 g AFDW m⁻². Detrital size classes differentially contributed to temporal fluctuations in total detritus. The autumnal maximum was largely comprised of coarse particulate organic matter (CPOM), primarily leaf litter. The large increase following ice out was mainly due to fine detritus (FPOM, 0.45–75 μm) which accounted for > 50% of benthic detritus during 9 months of the year. Evidence suggests that changes in standing crop and particle size distribution during the extended period of ice cover (November-March) resulted from in situ processing rather than transport. Despite major differences in climate, geochemistry and hydrology, the Colorado stream exhibited patterns of detrital dynamics remarkably similar to an eastern deciduous forest stream.     

Leaf-pack dynamics in a southern African mountain stream

SUMMARY 1. The occurrence, composition and invertebrate fauna of naturally-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-flowing 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 from riparian trees. Packs were smallest (minimum dry mass 17 g, minimum volume 1.7–10^?5 m³) in winter when discharge was high, and largest (maximum dry mass 191 g, maximum volume 4.2–10^?3 m³) in spring when discharge decreased and leaf fall from the evergreen riparian trees began. Through the year the packs covered a mean 0.41 % of the stream bed and had a mean abundance of 0.46 packs m^?2 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,     

Colonization of leaf patches at topographically different locations by insect shredders in a small mountain stream

We examined physical constraints on the colonization of leaf patches by shredder individuals by comparing the colonizations of artificially standardized leaf patches placed at different locations within a stream reach (i.e., riffles, middles and edges of pools). Stonefly taxa (Nemoura, Protonemura) colonized riffle patches 2–10 times more often than pool (middle, edge) patches, whereas caddisfly taxa (two species of Lepidostoma, Nothopsyche) almost exclusively colonized pool patches. Colonization also differed between the middle and edge patches in pools for most taxa; it was 2–5 times greater in edge patches for Nemoura and in middle patches for Lepidostoma. The abilities of species to cope with low oxygen circulation and high shear stress appear to determine differences in colonization between riffle and pool patches, whereas species-specific dispersion behavior (e.g., return time from drift) may differentiate colonization between middle and edge patches in pools. Our results suggest that changes in leaf distribution within a reach can affect the suitability of stream reaches in terms of food acquisition for shredder individuals.     

Interactions among nutrients, algae and invertebrates in a New Zealand mountain stream

SUMMARY. 1. Colonization of nutrient-diffusing substrata by periphyton and invertebrates was investigated at forested and open sites in a small, mountain stream and a spring in the South Island of New Zealand.
2. Substrata had colonization surfaces made from 100μm mesh plankton netting that enabled algal assemblages to be removed intact for scanning electron microscopy. They also allowed small volumes of solvent to be used for the extraction of photosynthetic pigments.
3. At all sites, periphyton assemblages were dominated by species of Achnanthes, Cocconeis and Gomphonema , and except in the forest in winter, periphyton biomass was always greater on enriched (N + P added) than control substrata.
4. Invertebrates colonizing diffusion substrata were principally larval Chironomidae (Orthocladiinac). No larvae were present in winter, but in three spring and summer trials mean larval densities were higher on nutrient-enriched than control substrata at all sites.
5. The inclusion of an insecticide Malathion in diffusion substrata, reduced insect colonization at open and forested sites. After 28 days, no concurrent increases in algal pigment concentration were observed on nutrienl-enriched or control substrata at the forested site. However, pigment concentrations were higher on substrata incorporating Malathion at the open site suggesting that algal standing crop was depressed by the activities of grazers.     

Effects of stream-crossing by a pipeline on the benthic macroinvertebrate communities of a small mountain stream

Aquatic environmental impact associated with stream-crossing by a pipeline was monitored at Archibald Creek, B.C. for two years. Water chemistry and benthic macroinvertebrates were used as monitoring tools. Results indicated that impacts arising from stream-crossing were short-term and non-residual.Funded by a contract from Canadian Arctic Gas Study Limited, calgary, to Aquatic Environments Limited, Calgary.Aquatic Environments Limited, Calgary.Aquatic Environments Limited, Calgary.     

Trophic status of small mammals in the semi-arid Karoo, South Africa

The feeding habits of a community of small mammals from the semi-arid Karoo, South Africa were analysed by microscopic examination of the stomach contents of animals caught during a 13-month snap-trapping study. The community comprised eight rodent species ( Gerbillurus paeba, Mus minutoides, Rhabdomys pumilio, Otomys unisulcatus, Saccostomus campestris, Desmodillus auricularis, Malacothrix typica, Mastomys natalensis ) and a single macroscelid elephant shrew ( Macroscelides proboscideus. ) The rodents were all predominantly herbivorous, while the elephant shrew ate mainly insects. No granivores or omnivores were represented in this community. The results presented here (including the first analysis of the diet of M. typica ) indicate a greater emphasis on herbivory than previously described for these species. Limited dietary overlap was recorded between these species, although some instances of potential dietary competition that deserve further study were identified. The paucity of granivorous small mammals in the semi-arid Karoo contrasts the patterns of trophic specialization for North American and Israeli semi-arid communities, but is similar to that of South American, Australian and other southern African semi-arid communities.     

Life-history variations in the fluvial sculpin, Cottus nozawae (Cottidae), along the course of a small mountain stream

Life-history variations in male and female fluvial sculpins, Cottus nozawae, were studied in a small mountain stream in Hokkaido, Japan, primarily by using capture-mark-recapture methods. At three study areas established along the stream course, the majority of marked sculpins were recaptured in their original location over one or more years, indicating their long-term occupation of each restricted habitat area. Sculpin densities increased toward the upstream habitats, whereas individual growth rates were more rapid downstream. In both sexes, sculpins distributed downstream matured at a larger body size and later in life than upstream sculpins, clearly demonstrating a clinal variation in these respects. A comparison of life-history variations in C. nozawae with those in amphidromous C. hangiongensis suggests that intrapopulational life-history variations in the former might be environmentally induced, and that one of the most important determinants for the variations in Cottus species might be population density.     

Resource utilization, overlap and temporal dynamics in a guild of mountain stream insects

SUMMARY 1. Resource utilization was quantified for six mayfly (Ephemeroptera) and one caddis (Trichoptera) species comprising a lotic scraper/collector-gatherer guild across three niche dimensions (temporal, trophic and spatial). Based on trophic differences and inferred microspatial utilization, the members of this guild separated into two groups: (1) cryptic detritivores and (2) exposed algivores.
2. Each species demonstrated a slow seasonal univoltine life cycle except for Epeorus longimanus (Eaton) and Baetis iricaudatus (Dodds) which were fast seasonal univoltine and multivoltine, respectively.
3. Temporal sequencing of periods of peak resource utilization were not demonstrated by the members of this guild. A null analysis indicated that periods of peak resource utilization were aggregated.     

The influence of different litter bag designs on the breakdown of leaf material in a small mountain stream

Leaf breakdown of two riparian tree species, Cunonia capensis L. and Ilex mitis (L.) Radlk. was investigated in vitro at Window Stream, Table Mountain, using three different designs of litter bag. Breakdown of Cunonia and Ilex in coarse-mesh (5 mm) litter bags was very rapid (respectively 14.79 and 13.93% loss d^–1), and was significantly greater than the loss of leaf material of 1% d^–1 for both species from fine-mesh bags (180 µm). Differences recorded between fine-mesh and composite-mesh bags (180 µm mesh with 5 mm mesh top) represented macro-invertebrate ingestion, and at t = 28 d, amounted to 67.57% material loss in Cunonia and 62.58% in Ilex. The losses due to microbial activity and leaching, 31.28% in Cunonia and 29.17% in Ilex were not significantly different. Weight loss of Cunonia in coarse-mesh bags (14.79% loss d^–1) and in composite-mesh bags (13.93% loss d^–1) did not differ, but this was not the case for Ilex, where a significantly higher rate of loss in coarse-mesh bags (13.93% loss d^–1) than in composite-mesh bags (7.69% loss d^–1) was observed. This difference was used to quantify fragmentation losses. It was concluded that future leaf breakdown experiments in mountain streams must take cognisance of differential fragmentation losses before inferences can be made as to both invertebrate feeding preferences and biological decomposition of leaves.     

Aquatic insect faunas and communities of a mountain stream in Sapa Highland,northern Vietnam

Aquatic insect communities were investigated from the Muonghoa Stream in the Sapa Highland (highest peak 3,143 m), a subtropical mountain stream in northern Vietnam. Field investigations for quantitative (Surber net 50 cm × 50 cm, mesh size 0.2 mm, riffle and pool/run) and qualitative (hand net, mesh size 1 mm) sampling were conducted at nine sites along the watercourse between 27 November and 2 December 2005. As a result, a total of 216 species (the majority of them undescribed) belonging to 139 genera, 61 families, and nine orders were recognized: 53 Ephemeroptera species (24.5%), nine Odonata species (4.2%), 15 Plecoptera species (6.9%), seven Hemiptera species (3.2%), 35 Coleoptera species (16.2%), one Megaloptera species (0.5%), 29 Diptera species (13.4%), 66 Trichoptera species (30.6%), and one Lepidoptera species (0.5%). Trichoptera, Ephemeroptera, and Coleoptera represented the major aquatic insect groups with regard to taxonomic and individual richness, whereas Hemiptera and Odonata were relatively less diverse and abundant than in studies of other tropical Southeast Asian streams. The dominance, richness, and diversity indices (H′) fell within the following ranges [mean ± standard deviation (SD)]: 0.18–0.76 (0.42 ± 0.19), 4.13–9.19 (7.06 ± 1.45), and 1.61–3.22 (2.67 ± 0.55), respectively. Riffle habitats generally yielded numbers of aquatic insect species and individuals approximately twice that sampled in pool/run habitats. Shredders were relatively larger in proportion within the headwater reach, whereas scrapers and collector-gatherers were more abundant in the middle and lower stream reaches. This functional feeding group composition is characteristic of temperate streams in East Asia. The results of detrended correspondence analysis and Bray–Curtis cluster analysis indicated that aquatic insect compositions at the sampling sites were very reflective of the reach characteristics, which evidence gradual changes with altitude and stream order along the stream watercourse. This is the first comprehensive investigation of aquatic insects in highland Southeast Asian regions.     

Differences in patches of retention among leaves,woods and small litter particles in a headwater stream: the importance of particle morphology

The location of retention in the channel can influence invertebrate assemblage and breakdown processes associated with a litter particle in heterogeneous streams. We previously identified four types of litter patches that formed on riffles or different locations in pools (middle, edge, alcove), and demonstrated that middle patches had higher litter processing rates than the other patches. In this study, we examined differences in retention on the four patch types among leaves, woody materials and small litter particles, and among leaves of different sizes, by sampling natural and manipulated litter particles that were newly retained. Proportionally more woody materials, leaf pieces (16–50 mm) and particulate organic matter (1–16 mm) than leaves (>50 mm) were retained on middle patches, while proportionally more leaves than the other litter particles were retained on riffle and edge patches. The retention pattern of leaf species with different leaf sizes and a released experiment of leaf particles revealed that proportionally more leaf particles with smaller sizes were retained on middle patches. The flexibility, shape and dryness of litter particles also seem to affect the location of retention in the channel. These results suggest that the morphology of litter particles have the potential to affect the biological use and breakdown of litter particles through determining the location of retention within the channel. The size of leaves and processes that alter leaf size may have important roles on the breakdown and utilisation by invertebrates in these heterogeneous streams.     

Extracellular phosphatase activity in sediments of the Breitenbach,a Central European mountain stream

Activity of extracellular phosphatases (phosphomonoesterases) was measured in sandy streambed sediments of the Breitenbach, a small unpolluted upland stream in Central Germany. Fluorigenic 4-methylumbelliferyl phosphate served as a model substrate. Experiments were conducted using sediment cores in a laboratory simulation of diffuse groundwater discharge through the stream bed, a natural process occurring in the Breitenbach as well as many other streams.Streambed sediments contained high levels of particulate phosphorus, but concentrations of dissolved phosphorus in the interstitial water were 3 to 4 orders of magnitude lower. These interstitial concentrations were similar to those in the stream and groundwater. Extracellular phosphatase activity was high in the streambed sediments. These enzymes probably contribute significantly to the flux of phosphorus in sediment by hydrolyzing phosphomonoesters, making free phosphate available to the sediment microorganisms.Factors influencing the kinetic parameters V _max (maximum activity) and apparent K _m (enzyme affinity) of phosphatase were discharge rates of water through the sediment, water quality (ground- or stream water), and substrate (phosphomonoesters) as well as dissolved ortho-phosphate concentrations. Enzymes are supposed to be effective at limiting substrate concentrations, where, in this study, changes in discharge rates had little influence on rates of hydrolysis. Higher V _max and lower K _m values were found during percolation of groundwater through the sediment cores, compared with stream water. This indicates that rates of hydrolysis were higher with groundwater, both at substrate limitation and at substrate saturation. This was probably a consequence of the lower levels of dissolved ortho-phosphate in the groundwater.     

Ecological correlates of small mammal community structure in the semi-arid Karoo, South Africa

The understanding of the determinants of small mammal community structure in arid and semiarid ecosystems is of importance, both in the light of the role that small mammals play, and the impact of livestock grazing on the flora of these systems. In a study aimed at identifying these determinants, small mammal assemblages and environmental features were quantified at six localities (a gradient of floristic structure, with constant annual rainfall) across the southern Karoo, South Africa. Stepwise variable regression indicated that small mammal diversity was correlated with plant and rock cover, as well as plant cover and horizontal foliage diversity at intermediate heights (40–60 cm). Initially, small mammal diversity increased with increasing plant cover, but decreased at cover levels greater than 30%. This relationship is similar to that found in other desert systems, although the peak in diversity found here is at higher levels of plant cover than found previously. I suggest that this higher peak may be owing to the lack of reliance on granivory by these animals, which are relatively omnivorous. This model may therefore explain the conflicting reports on the impact of livestock grazing on desert small mammals, with small mammal diversity decreasing with grazing below the peak, and increasing with grazing above the peak.