Seasonal Changes in the Quantity and Nitrogen Content of Coarse Detritus in a Southern Ontario Stream

Seasonal changes in standing crop of coarse, benthic detritus at five locations across a second-order stream were studied between May 1971 and May 1972. Standing crop near the banks varied seasonally with highest quantities in the autumn and winter, whereas near midstream seasonal changes in standing crop were negligible. Mean annual standing crop was 747 g/m² (ash-free dry weight) near the banks and 225 g/m² (AFDW) at midstream. Deposition experiments showed that standing crops of detritus came to equilibrium in 2–3 days at midstream and in about one week near the banks. Nitrogen content of coarse detritus, which ranged between 0.65 and 3.51% (AFDW), did not vary seasonally but was significantly greater near the banks than at midstream.     

Fungal and Bacterial Colonisation of Salix pedicellataLeaves Decaying in Permanent and Intermittent Streams in Eastern Morocco

Leaf degradation was investigated at four sites in the Zegzel/Cherraa river system (Oueds). Two sites (Upper Zegzel and Lower Zegzel) carry water throughout the year and two (Upstream and Downstream Cherraa) are dry for 5–7 months each year. The dynamics of leaf weight loss and microorganisms associated with Salix pedicellata leaves decaying at the four sites were compared for the first time during the same period over this permanent and intermittent system. Overall decay rates of leaves were significantly higher in the permanent Zegzel stream sections (k = 0.0094 d^–1 upstream and 0.0056 downstream) than in the intermittently flowing Cherraa sites (k = 0.0046 upstream, and 0.0036 downstream). In the latter, decay was much slower during dry than during wet periods (Upstream Cherraa: k = 0.0028 and 0.0446, respectively; Downstream Cherraa, k = 0.0008 and 0.0357, respectively). Similar gradients from permanent to intermittent sites were observed in numbers of bacteria per unit area or per weight of decaying leaves (direct counts by epifluorescence microscopy), in numbers of fungal species and in sporulation rates, from leaves recovered at the four sites. Ten hyphomycete species were new for Morocco.     

Seasonal dynamics of macroinvertebrate assemblages in the benthos and associated with detritus packs in two low-order streams with different riparian vegetation

• 1 The seasonal dynamics of the benthic macroinvertebrate assemblage, and the subset of this assemblage colonising naturally formed detritus accumulations, was investigated in two streams in south‐west Ireland, one draining a conifer plantation (Streamhill West) and the other with deciduous riparian vegetation (Glenfinish). The streams differed in the quantity, quality and diversity of allochthonous detritus and in hydrochemistry, the conifer stream being more acid at high discharge. We expected the macroinvertebrate assemblage colonising detritus to differ in the two streams, due to differences in the diversity and quantity of detrital inputs.
• 2 Benthic density and taxon richness did not differ between the two streams, but the density of shredders was greater in the conifer stream, where there was a greater mass of benthic detritus. There was a significant positive correlation between shredder density and detritus biomass in both streams over the study period.
• 3 Detritus packs in the deciduous stream were colonised by a greater number of macroinvertebrates and taxa than in the conifer stream, but packs in both streams had a similar abundance of shredders. The relative abundance of taxa colonising detritus packs was almost always significantly different to that found in the source pool of the benthos.
• 4 Correspondence analysis illustrated that there were distinct faunal differences between the two streams overall and seasonally within each stream. Differences between the streams were related to species tolerances to acid episodes in the conifer stream. Canonical correspondence analysis demonstrated a distinct seasonal pattern in the detrital composition of the packs and a corresponding seasonal pattern in the structure of the detritus pack macroinvertebrate assemblage.
• 5 Within‐stream seasonal variation both in benthic and detritus pack assemblages and in detrital inputs was of similar magnitude to the between‐stream variation. The conifer stream received less and poorer quality detritus than the deciduous stream, yet it retained more detritus and had more shredders in the benthos. This apparent contradiction may be explained by the influence of hydrochemistry (during spate events) on the shredder assemblage, by differences in riparian vegetation between the two streams, and possibly by the ability of some taxa to exhibit more generalist feeding habits and thus supplement their diets in the absence of high quality detritus.

     

Leachates of Eucalyptus globulus in Intermittent Streams Affect Water Parameters and Invertebrates

Low order streams running through Eucalyptus globulus plantations in Central Portugal are frequently reduced to isolated permanent or temporary summer pools with darkly stained water due to leaf leachates. Here we assess the toxicity of such leachates to the shredder Sericostoma vittatum. Leachates resulted in deoxygenated and more acid water, and increased its phenolic content and conductivity. S. vittatum exposed to high concentrations of leachates failed to grow and died within 30 days even in the presence of high quality food. Larvae in water with half the leachate concentration, consumed less leaf material, had lower growth rates, did not pupate and died within 100 days. Eucalypt leachates per se may affect the viability and ecology of macroinvertebrates in Portuguese streams. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Drought Frequency and Other Reach Characteristics on Invertebrate Communities and Litter Breakdown in the Intermittent Mediterranean River Pula (Sardinia,Italy)

Benthic invertebrate communities were sampled in spring and fall, and breakdown of reed litter was measured in winter, spring, summer and fall, at 23 sites in an intermittent Mediterranean river basin (Pula, Sardinia, Italy), and their variations were related to drought frequency and other reach characteristics (altitude, order, stream channel width and depth, vegetation cover, sediment type, organic content, and water quality). 78% of the sites were affected by drought events, mainly in summer and fall. Benthic taxonomic richness was higher and most taxa were more widely distributed in spring than in fall. Canonical correspondence analysis showed that measured environmental variables accounted for 72% of the variance in invertebrate community structure. Reed decay rates varied among seasons, and during the dry season among stream orders as well. Drought events slowed down breakdown and explained 44% of spatial variability in decay rates (Stepwise multiple regression analysis), an additional 8% being explained by water chemistry. Results suggest that drought events affect ecosystem functioning (litter breakdown) more than structure (invertebrate communities). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Deforestation alters the resource base and biomass of endemic stream insects in eastern Madagascar

1. Rainforest streams in eastern Madagascar have species‐rich and diverse endemic insect communities, while streams in deforested areas have relatively depauperate assemblages dominated by collector‐gatherer taxa. We sampled a suite of benthic insects and their food resources in three primary rainforest streams within Ranomafana National Park in eastern Madagascar and three agriculture streams in the park's deforested peripheral zone. We analysed gut contents and combined biomass and stable isotope data to examine stream community responses to deforestation in the region, which is a threatened and globally important hotspot for freshwater biodiversity. 2. Gut analyses showed that most taxa depended largely on amorphous detritus, obtained either from biofilms (collector‐gatherers) or from seston (microfilterers). Despite different resource availability in forest versus agriculture streams, diets of each taxon did not differ between stream types, suggesting inflexible feeding modes. Carbon sources for forest stream insects were difficult to discern using δ¹³C. However, in agriculture streams dependence on terrestrial carbon sources was low relative to algal sources. Most insect taxa with δ¹³C similar to terrestrial carbon sources (e.g. the stonefly Madenemura, the caddisfly Chimarra sp. and Simulium blackflies) were absent or present at lower biomass in agriculture streams relative to forest streams. Conversely, collector‐gatherers (Afroptilum mayflies) relied on algal carbon sources and had much higher biomass in agriculture streams. 3. Our analyses indicate that a few collector‐gatherer species (mostly Ephemeroptera) can take advantage of increased primary production in biofilms and consequently dominate biomass in streams affected by deforestation. In contrast, many forest stream insects (especially those in the orders Plecoptera, Trichoptera and Diptera) depend on terrestrial carbon sources (i.e. seston and leaf litter), are unable to track resource availability and consequently decline in streams draining deforested landscapes. These forest‐specialists are often micro‐endemic and particularly vulnerable to deforestation. 4. The use of consumer biomass data in stable isotope research can help detect population‐level responses to shifts in basal resources caused by anthropogenic change. We also suggest that restoration of vegetated riparian zones in eastern Madagascar and elsewhere could mitigate the deleterious effects of deforestation on sensitive, endemic stream taxa that are dependent on terrestrial carbon sources.     

Managing and Rehabilitating Ecosystem Processes in Regional Urban Streams in Australia

Urbanization is acknowledged as one of the most severe threats to stream health, spawning recent research efforts into methods to ameliorate these negative impacts. Attention has focused on streams in densely-populated cities but less populous regional urban centres can be equally prone to some of the same threats yet might not meet the conventional definitions of urban. Several recent reviews have identified the changes to streams that occur during urbanization but they note that few ecological studies have explored ecosystem-level responses, typically focusing instead on state variables such as invertebrate abundance. In many regional urban streams, changes to the extent of impervious drainage have implications for their hydrology and channel morphology but the influence of these changes on fundamental ecosystem processes of leaf litter breakdown and transport compared with those in nearby rural streams are poorly known. The widespread practice of planting exotic trees along riparian zones and street margins draining into urban streams further exacerbates the disruption of natural organic matter dynamics. The combination of seasonal leaf fall by exotic species and the altered drainage patterns through urbanization in Armidale, a regional town in New South Wales, Australia, resulted in contrasting patterns of benthic organic matter storage over 18 months compared to nearby reference and rural streams. Macroinvertebrate detritivore densities were low in the urban stream, implying disruption of the usual biological pathways of leaf breakdown. Understanding the interactions of hydrology, drainage pattern, leaf input and biological attributes of a stream is crucial for managers trying to restore stream ecosystem services without incurring public concern about the appearance of regional urban streams.     

Flow extremes and benthic organic matter shape the metabolism of a headwater Mediterranean stream

1. Single‐station diel oxygen curves were used to monitor the oxygen metabolism of an intermittent, forested third‐order stream (Fuirosos) in the Mediterranean area, over a period of 22 months. Ecosystem respiration (ER) and gross primary production (GPP) were estimated and related to organic matter inputs and photosynthetically active radiation (PAR) in order to understand the effect of the riparian forest on stream metabolism. 2. Annual ER was 1690 g O₂ m^?2 year^?1 and annual GPP was 275 g O₂ m^?2 year^?1. Fuirosos was therefore a heterotrophic stream, with P : R ratios averaging 0.16. 3. GPP rates were relatively low, ranging from 0.05 to 1.9 g O₂ m^?2 day^?1. The maximum values of GPP occurred during a few weeks in spring, and ended when the riparian canopy was fully closed. The phenology of the riparian vegetation was an important determinant of light availability, and consequently, of GPP. 4. On a daily scale, light and temperature were the most important factors governing the shape of photosynthesis–irradiance (P–I) curves. Several patterns could be generalised in the P–I relationships. Hysteresis‐type curves were characteristic of late autumn and winter. Light saturation responses (that occurred at irradiances higher than 90 μE m^?2 s^?1) were characteristic of early spring. Linear responses occurred during late spring, summer and early autumn when there was no evidence of light saturation. 5. Rates of ER were high when compared with analogous streams, ranging from 0.4 to 32 g O₂ m^?2 day^?1. ER was highest in autumn 2001, when organic matter accumulations on the streambed were extremely high. By contrast, the higher discharge in autumn 2002 prevented these accumulations and caused lower ER. The Mediterranean climate, and in its effect the hydrological regime, were mainly responsible for the temporal variation in benthic organic matter, and consequently of ER.     

The impacts of habitat disturbance on adult and larval dragonflies (Odonata) in rainforest streams in Sabah,Malaysian Borneo

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Spatial and temporal patterns of nitrogen concentrations in pristine and agriculturally-influenced prairie streams

Long-term data on nitrogen chemistry of streams draining Konza Prairie Biological Station (Konza), Kansas were analyzed to assess spatial and temporal patterns and examine the influence of agricultural activity on these patterns. Upland watersheds of Konza are predominantly tallgrass prairies, but agricultural fields and riparian forests border the lower reaches of the streams. We have up to 11 years of data in the relatively pristine upland reaches and 4 years of data on wells and downstream reaches influenced by fertilized croplands. Seasonal and spatial patterns in total nitrogen (TN) concentrations were driven largely by changes in the nitrate (NO₃ ^–) concentrations. A gradient of increasing NO₃ ^– concentrations occurred from pristine upland stream reaches to the more agriculturally-influenced lowland reaches. Nitrate concentrations varied seasonally and were negatively correlated with discharge in areas influenced by row-crop agriculture (p = 0.007). The NO₃ ^– concentrations of stream water in lowland reaches were lowest during times of high precipitation, when the relative influence of groundwater drainage is minimal and water in the channel is primarily derived from upland prairie reaches. The groundwater from cropland increased stream NO₃ ^– concentrations about four-fold during low-discharge periods, even though significant riparian forest corridors existed along most of the lower stream channel. The minimum NO₃ ^– concentrations in the agriculturally influenced reaches were greater than at any time in prairie reaches. Analysis of data before and after introduction of bison to four prairie watersheds revealed a 35% increase of TN concentrations (p < 0.05) in the stream water channels after the introduction of bison. These data suggest that natural processes such as bison grazing, variable discharge, and localized input of groundwater lead to variation in NO₃ ^– concentrations less than 100-fold in prairie streams. Row-crop agriculture can increase NO₃ ^– concentrations well over 100-fold relative to pristine systems, and the influence of this land use process over space and time overrides natural processes.     

Leaf Litter as a Source of Dissolved Organic Carbon in Streams

Dissolved organic carbon (DOC) is an abundant form of organic matter in stream ecosystems. Most research has focused on the watershed as the source of DOC in streams, but DOC also comes from leaching of organic matter stored in the stream channel. We used a whole-ecosystem experimental approach to assess the significance of leaching of organic matter in the channel as a source of DOC in a headwater stream. Inputs of leaf litter were excluded from a forested Appalachian headwater stream for 3 years. Stream-water concentration, export, and instream generation of DOC were reduced in the litter-excluded stream as compared with a nearby untreated reference stream. The proportion of high molecular weight (HMW) DOC (more than 10,000 daltons) in stream water was not altered by litter exclusion. Mean DOC concentration in stream water was directly related to benthic leaf-litter standing stock. Instream generation of DOC from leaf litter stored in the stream channel contributes approximately 30% of daily DOC exports in this forested headwater stream. This source of DOC is greatest during autumn and winter and least during spring and summer. It is higher during increasing discharge than during base flow. We conclude that elimination of litter inputs from a forested headwater stream has altered the biogeochemistry of DOC in this ecosystem. Received 2 September 1997; accepted 27 January 1998.     

Benthic organic matter and detritivorous macroinvertebrates in two intermittent streams in south-eastern Australia

Temporal changes in coarse (> 1 mm), fine (< 1 mm, > 250 m), and woody benthic organic matter (BOM), and densities of detritivores in pools and riffles were monitored at three sites on two intermittent streams (Werribee and Lerderderg Rivers) in Victoria, Australia during a drought year followed by a wetter year. Standing stocks of BOM peaked in both habitats during summer when discharge ceased and eucalypt leaf fall was greatest. During high winter and spring discharges, concentrations of BOM were low. Floods did not always scour BOM from the pools and riffles; after floods in October, standing stocks rose when BOM was imported from upstream or mobilized from the riparian zone. Densities of benthic detritivores, collector-gatherers, and shredders also varied seasonally, usually peaking in summer. Correlations between detritivore feeding group densities and amounts of putative food resource were habitat-specific. There were few significant correlations in depositional habitats, even after incorporating lag-times of two and four weeks into the analysis. However, detritivore densities in riffles, especially in the Lerderderg River, were strongly positively correlated with the amounts of BOM. Possibly, physico-chemical conditions in riffles are more conducive to litter conditioning and invertebrate colonization and breakdown of leaf material than are those in pools.     

Top‐Down Control of Reed Detritus Processing in a Lake Littoral Zone: Experimental Evidence of a Seasonal Compensation between Fish and Invertebrate Predation

We investigated whether predatory fish exert a top‐down control on reed leaf packs processing in a lake littoral zone through a trophic cascade. Exclosure experiments were repeated in summer and winter, under high and low natural fish abundance, respectively. Fish exclusion effects on detritus processing and fungal conditioning were consistent with trophic cascade predictions only in summer. In winter, however, results indicated that a trophic cascade was induced by predatory invertebrates. In both seasons, variations in detritivores abundance generally supported a cascade scenario, whereas several taxon‐specific departures occurred during the experimental periods. We conclude suggesting that predators may continuously regulate leaf detritus processing in lake littoral zones, through a seasonal shift in the relative contribution of fish and invertebrate predation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Leaf litter decomposition and macroinvertebrate communities in headwater streams draining pine and hardwood catchments

Benthic invertebrates, litter decomposition, andlitterbag invertebrates were examined in streamsdraining pine monoculture and undisturbed hardwoodcatchments at the Coweeta Hydrologic Laboratory in thesouthern Appalachian Mountains, USA. Bimonthlybenthic samples were collected from a stream draininga pine catchment at Coweeta during 1992, and comparedto previously collected (1989–1990) benthic data froma stream draining an adjacent hardwood catchment. Litter decomposition and litterbag invertebrates wereexamined by placing litterbags filled with pine ormaple litter in streams draining pine catchments andhardwood catchments during 1992–1993 and 1993–1994. Total benthic invertebrate abundance and biomass inthe pine stream was ca. 57% and 74% that of thehardwood stream, respectively. Shredder biomass wasalso lower in the pine stream but, as a result ofhigher Leuctra spp. abundance, shredderabundance was higher in the pine stream than thehardwood stream. Decomposition rates of both pine andred maple litter were significantly faster in pinestreams than adjacent hardwood streams (p<0.05). Total shredder abundance, biomass, and production weresimilar in maple bags from pine and hardwood streams. However, trichopteran shredder abundance and biomass,and production of some trichopteran taxa such asLepidostoma spp., were significantly higher in maplelitterbags from pine streams than hardwood streams(p<0.05). In contrast, plecopteran shredders(mainly Tallaperla sp.) were more important inmaple litterbags from hardwood streams. Shredderswere well represented in pine litterbags from pinestreams, but low shredder values were obtained frompine litterbags in hardwood streams. Resultssuggest conversion of hardwood forest to pinemonoculture influences taxonomic composition of streaminvertebrates and litter decomposition dynamics. Although the impact of this landscape-leveldisturbance on invertebrate shredder communitiesappeared somewhat subtle, significant differences indecomposition dynamics indicate vital ecosystem-levelprocesses are altered in streams draining pinecatchments.     

Comparing Effects of Nutrients on Algal Biomass in Streams in Two Regions with Different Disturbance Regimes and with Applications for Developing Nutrient Criteria

Responses of stream algal biomass to nutrient enrichment were studied in two regions where differences in hydrologic variability cause great differences in herbivory. Around northwestern Kentucky (KY) hydrologic variability constrains invertebrate biomass and their effects on algae, but hydrologic stability in Michigan (MI) streams permits accrual of high herbivore densities and herbivory of benthic algae. Multiple indicators of algal biomass and nutrient availability were measured in 104 streams with repeated sampling at each site over a 2−month period. Many measures of algal biomass and nutrient availability were positively correlated in both regions, however the amount of variation explained varied with measures of biomass and nutrient concentration and with region. Indicators of diatom biomass were higher in KY than MI, but were not related to nutrient concentrations in either region. Chl a and % area of substratum covered by Cladophora were positively correlated to nutrient concentrations in both regions. Cladophora responded significantly more to nutrients in MI than KY. Total phosphorus (TP) and total nitrogen (TN) explained similar amounts of variation in algal biomass, and not significantly more variation in biomass than dissolved nutrient concentrations. Low N:P ratios in the benthic algae indicated N as well as P may be limiting their accrual. Most observed responses in benthic algal biomass occurred in nutrient concentrations between 10 and 30 μg TP l⁻¹ and between 400 and 1000 μg TN l⁻¹.     

Drainage Size, Stream Intermittency, and Ecosystem Function in a Sonoran Desert Landscape

Understanding the interactions between terrestrial and aquatic ecosystems remains an important research focus in ecology. In arid landscapes, catchments are drained by a channel continuum that represents a potentially important driver of ecological pattern and process in the surrounding terrestrial environment. To better understand the role of drainage networks in arid landscapes, we determined how stream size influences the structure and productivity of riparian vegetation, and the accumulation of organic matter (OM) in soils beneath plants in an upper Sonoran Desert basin. Canopy volume of velvet mesquite (Prosopis velutina), as well as overall plant cover, increased along lateral upland–riparian gradients, and among riparian zones adjacent to increasingly larger streams. Foliar δ¹³C signatures for P. velutina suggested that landscape patterns in vegetation structure reflect increases in water availability along this arid stream continuum. Leaf litter and annual grass biomass production both increased with canopy volume, and total aboveground litter production ranged from 137 g m⁻² y⁻¹ in upland habitat to 446 g m⁻² y⁻¹ in the riparian zone of the perennial stream. OM accumulation in soils beneath P. velutina increased with canopy volume across a broad range of drainage sizes; however, in the riparian zone of larger streams, flooding further modified patterns of OM storage. Drainage networks represent important determinants of vegetation structure and function in upper Sonoran Desert basins, and the extent to which streams act as sources of plant-available water and/or agents of fluvial disturbance has implications for material storage in arid soils.     

Light absorption by phytoplankton and chromophoric dissolved organic matter in the drainage basin and estuary of the Neuse River, North Carolina (U.S.A.)

1. We examined the absorption of solar radiation by phytoplankton and chromophoric dissolved organic matter (CDOM) taking into account riparian shading in the rivers, reservoirs, swamps of the Neuse River Estuary and its drainage basin. 2. In the streams, CDOM typically absorbed 55 and 64% of photons in the spectral range of 400–700 nm (photosynthetically active radiation, PAR) and 500–600 nm, respectively. The large proportion of photons absorbed by CDOM indicates high potential for abiotic photochemial reactions in the 500–600 nm region. 3. Despite the high concentration of nutrients, phytoplankton contributed little (2%) to the total absorption of PAR in the streams. Small (<30 m wide) streams typically received only 7% of incident PAR that impinged onto the more exposed reservoirs and estuary. Riparian shading and the low contribution of phytoplankton to the total absorption resulted in conditions where phytoplankton absorbed nearly two orders of magnitude less PAR in the streams than in the estuary and reservoirs. 4. The results indicated that riparian shading and non‐algal absorbing components can significantly restrict phytoplankton production in nutrient‐rich streams with a high concentration of CDOM flowing throughout forested catchments.     

Estimation of Standing Stock and Decomposition Rates of Labile Organic Matter in Waters of Novorossiisk Bay,Black Sea

The annual dynamics of the decomposition rate, standing stock, and residence time of labile organic matter as an index of full self-purification were investigated in Novorossiisk Bay, Black Sea. The results are suggestive of fairly effective processes of biological self-purification in polluted waters of the bay. The decomposition rate was highest (0.3–0.7 mgO₂/l per day) during the summer, and it decreased by 4–8 times in winter. The residence time of labile organic matter was 97–104 days in winter and 8–11 days in summer. Oxygen consumption rates measured in different areas of the bay conformed to their trophic status and were not above the normal level for summer.     

Responses of microbial activity in hyporheic pore water to biogeochemical changes in a drying headwater stream

1. Microbial heterotrophic activity is a major driver of nutrient and organic matter processing in the hyporheic zone of headwater streams. Additionally, the hyporheic zone might provide refuge for microbes when surface flow ceases during drought events.
2. We investigated chemical (organic and inorganic nutrients) and microbiological parameters (bacterial cell concentration, live–dead ratios, and extracellular enzyme activities) of surface and interstitial pore water in a period of progressive surface‐hyporheic disconnection due to summer drying. The special situation of the chosen study reach, where groundwater mixing is impeded by the bedrock forming a natural channel filled with sediment, allowed as to study the transformation of these parameters along hyporheic flow paths.
3. The chemical composition of the hyporheic pore water reflected the connectivity with the surface water, as expressed in the availability of nitrate and oxygen. Conversely, microbiological parameters in all hyporheic locations were different from the surface waters, suggesting that the microbial activity in the water changes rapidly once the water enters the hyporheic zone. This feature was principally manifested in higher live–dead ratios and lower leucine aminopeptidase (an activity related to nitrogen acquisition) in the hyporheic pore waters.
4. Overall, bacterial cell concentration and extracellular enzyme activities increased along hyporheic flow paths, with a congruent decrease in inorganic nutrients and dissolved organic matter quantity and apparent molecular size.
5. Our findings show two important functions of the hyporheic zone during drought: (1) deeper (?50 cm) water‐saturated layers can act as a refuge for microbial activity; and (2) the hyporheic zone shows high rates of carbon and nitrogen turnover when water residence times are longer during drought. These rates might be even enhanced by an increase in living microbes in the remaining moist locations of the hyporheic zone.

     

Leaf Litter Decomposition of Three Riparian Tree Species and Associated Macroinvertebrates of Eswathu Oya,a Low Order Tropical Stream in Sri Lanka

Leaf decomposition, an important component of the organic matter dynamics in streams, has been widely examined in temperate regions but much less documented in tropical regions. We report here the first study of leaf decomposition in a Sri Lankan stream. The litterbag technique was used. Coarse (8 mm) and fine (100 µm) litterbags, that included or excluded macroinvertebrates respectively, were used to enclose leaves of three dominant riparian tree species: native Ochlandra stridula (bamboo), and the introduced Alstonia macrophylla and Hevea brasiliensis (rubber). A fourth set of litterbags contained a mixture of these three species. Leaf colonization by macroinvertebrates was highest in the early stages of the decomposition process on Hevea leaves but invertebrate densities declined later. The opposite colonization effect was observed on the native Ochlandra leaves: slow colonization with continuing low densities from the beginning to the end of the process. Decomposition of all three species was significantly faster in the coarse than in the fine mesh bags. Alstonia, which has the softest leaf tissue, was most rapidly decomposed while Ochlandra, with its tough structure, was the slowest. Among the invertebrates, insects were the most important leaf colonizing animals, with Diptera, Coleoptera and Trichoptera the most dominant. The invertebrate variety in the mixed bags was higher than in the single‐species leaf bags, where Chironomidae dominated the colonizing assemblages. This study has shown that toughness, indicated by the ‘specific weight of leaf tissue’, and the quality of the leaves was more important in determining breakdown rates than their origin. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)