Decay rates of autumn and spring leaf litter in a stream and effects on growth of a detritivore

SUMMARY. 1. Differences in decay rates of autumn and spring balsam poplar (Populus balsamifera L.) leaf litter input to a stream and their effects on a lotic detritivore Tipula commiscibilis Diane were investigated.
2. Autumnal leaf litter decay rates were significantly greater than spring decay rates despite higher initial quality of spring leaves. Reduced spring/summer decomposition rates were the result of decreased microbial activity and biomass, and significantly lower numbers, kinds and biomass of macroinvertebrate detritivores.
3. Growth of the detritivore Tipula commiscibilis was significantly lower when fed spring leaves indicating that they were a poorer quality food source than autumn leaves.
4. Lower numbers of detritivores coupled with reduced leaf quality resulted in lower leaf litter decay rates characteristic of spring/summer.     

Effects of drying regime on microbial colonization and shredder preference in seasonal woodland wetlands

1. Energy budgets of wetlands in temperate deciduous forests are dominated by terrestrially derived leaf litter that decays under different drying conditions depending on autumn precipitation. We compared decay rates and microbial colonization of maple leaves under different inundation schedules in a field experiment, and then conducted a laboratory study on shredder preference. In the field, litter bags either remained submerged (permanent), were moved to a dried part of the basin once and then returned (semi-permanent), or were alternated between wet and dry conditions for 8 weeks (temporary).
2. There was no difference in decay rates among treatments, but leaves incubated under permanent and semi-permanent conditions had higher fungal and bacterial biomass, and lower C : N ratios than those incubated under alternating drying and wetting conditions.
3. To determine the effects of these differences in litter nutritional quality on shredder preference, we conducted a laboratory preference test with larvae of leaf-shredding caddisflies that inhabit the wetland. Caddisflies spent twice as much time foraging on permanent and semi-permanent litter than on litter incubated under temporary conditions.
4. There is considerable variation among previous studies in how basin drying affects litter breakdown in wetlands, and no previous information on shredder preference. We found that frequent drying in a shallow wetland reduces the nutritional quality of leaf litter (lower microbial biomass and nitrogen content), and therefore preference by invertebrate shredders. These results suggest that inter-annual shifts in drying regime should alter detritus processing rates, and hence the mobilization of the energy and nutrients in leaf litter to the wetland food web.     

南亚热带两种优势树种叶凋落物分解对模拟酸雨的响应

通过对模拟酸雨处理下鼎湖山季风常绿阔叶林优势树种锥栗(Castanopsis chinensis)和木荷(Schima superba)叶凋落物分解的研究,试图探讨南亚热带区域日益严重的酸雨对森林凋落物分解的影响规律以及可能的机制。试验应用分解袋法进行,并设计4个模拟酸雨强度处理:CK(p H值4.5左右的天然湖水)、T1(p H值4.0)、T2(p H值3.5)和T3(p H值3.0)。21个月的分解结果表明,模拟酸雨抑制了两种优势树种叶凋落物的分解。CK、T1、T2和T3 4个处理下的分解速率常数k值分别为:锥栗(1.18、0.93、0.94和0.86)和木荷(1.10、0.97、0.88和0.94),与CK相比,k值在T1、T2和T3均有下降的趋势。同时,模拟酸雨下两种优势树种叶凋落物的质量残留率均为:T3T2T1CK。酸雨对凋落物分解的抑制作用可能与酸雨胁迫下土壤酸化从而导致土壤微生物活性下降有关。     

Experimental evidence quantifying the role of benthic invertebrates in organic matter dynamics of headwater streams

SUMMARY. 1. The insecticide methoxychlor was applied seasonally to one of three small headwater streams in the southern Appalachian Mountains in North Carolina, U.S.A. The initial application caused massive invertebrate drift (>1,000,000 organisms/week) and resulted in a community with few shredders and reduced abundances of most insect taxa.
2. Bacterial densities and microbial respiration rates were not affected by treatment.
3. Disruption of the invertebrate community resulted in significant reductions in leaf litter processing rates (50–74% reduction depending on leaf species) and in the amount of leaf litter processed annually (reduction of 25–28%).
4. Reductions in leaf litter processing rates resulted in significant reductions in fine particulate organic matter (FPOM) export. Declines in both concentration and total export were detectable within 1 week of treatment. Annual FPOM export was reduced to 33% of pretreatment levels. Alteration to the invertebrate community had a much greater effect on FPOM export than a severe (50–200 year) drought.
6. Course particulate organic matter (CPOM) export was not significantly influenced by treatment but was influenced by hydrologic differences among years.     

Litter breakdown in streams of the Agüera catchment: influence of dissolved nutrients and land use

1. The breakdown of oak ( Quercus robur L.), chestnut ( Castanea sativa Miller) and eucalypt ( Eucalyptus globulus Labill.) litter enclosed in 5-mm mesh bags was compared between first-order headwaters (two with native riparian forest and two with eucalypt plantations) and a third-order reach of Agüera stream. Weight loss and dynamics of phosphorus and nitrogen in litter were studied for a period of 155 days.
2. Among the different sites, processing rates ranged from 0.0045 to 0.0080 day^–1 for chestnut leaf litter, from 0.0036 to 0.0051 day^–1 for oak, and from 0.0027 to 0.0158 day^–1 for eucalypt.
3. The availability of nutrients in water clearly influenced nitrogen and phosphorus dynamics in litter. In headwater reaches, net immobilization was not observed and losses of phosphorus and nitrogen followed mass loss. However, there was an enrichment of litter at the low reach, where influence of human settlements—located upstream—could lead to a greater availability of phosphorus in water.
4. The enhancement of litter decay by the exogenous nutrient supply depended on leaf quality, as only the processing rate of eucalypt increased at the nutrient-rich site.
5. The processing rates differed little among headwaters, suggesting that riparian forest type, i.e. deciduous forest v eucalypt plantations, did not affect litter decay in the stream.     

Litter breakdown in streams of the Agüera catchment: influence of dissolved nutrients and land use

1. The breakdown of oak ( Quercus robur L.), chestnut ( Castanea sativa Miller) and eucalypt ( Eucalyptus globulus Labill.) litter enclosed in 5-mm mesh bags was compared between first-order headwaters (two with native riparian forest and two with eucalypt plantations) and a third-order reach of Agüera stream. Weight loss and dynamics of phosphorus and nitrogen in litter were studied for a period of 155 days.
2. Among the different sites, processing rates ranged from 0.0045 to 0.0080 day^–1 for chestnut leaf litter, from 0.0036 to 0.0051 day^–1 for oak, and from 0.0027 to 0.0158 day^–1 for eucalypt.
3. The availability of nutrients in water clearly influenced nitrogen and phosphorus dynamics in litter. In headwater reaches, net immobilization was not observed and losses of phosphorus and nitrogen followed mass loss. However, there was an enrichment of litter at the low reach, where influence of human settlements—located upstream—could lead to a greater availability of phosphorus in water.
4. The enhancement of litter decay by the exogenous nutrient supply depended on leaf quality, as only the processing rate of eucalypt increased at the nutrient-rich site.
5. The processing rates differed little among headwaters, suggesting that riparian forest type, i.e. deciduous forest v eucalypt plantations, did not affect litter decay in the stream.     

Microbial growth on polluted leaf and litter leachates: Effects on leachate pH and sulphate concentration

Summary Leaves and litter ofAcer pseudoplatanus L. (sycamore) were collected from woodland downwind of a coking works and from a site close to the industrial city of Sheffield and leached with water or simulated acid rain (pH 4.0). The effects of microbial growth on litter leachate, pH and sulphate concentration were determined by (a) allowing the indigenous litter flora to develop or (b) inoculating the fungusAureobasidium pullulans into sterilized leachates amended with synthetic aphid honeydew as carbon source. In both experiments microbial growth generally increased the pH and sulphate concentration of the leachates, independent of the origin of the litter or the leaching agent. The growth ofA. pullulans however, generally decreased the pH and sulphate content of sycamore leaf leachates. We conclude that the microbial mineralization of organic sulphur in deciduous litter leachate can act as a source of sulphate which could enhance cation leaching in atmospheric-pulluted soils.     

Differences in processing dynamics of fresh and dried leaf litter in a stream ecosystem

SUMMARY. 1. Although the bulk of litter input to stream ecosystems is in the form of fresh leaves, current understanding of organic matter processing is largely founded on experimental studies made with pre-dried leaves. This paradox points to the critical need for evaluating to what extent those experiments with dried leaves reflect natural litter decomposition.
2. The mass loss rates, patterns of mass loss, and chemical changes during processing of fresh leaf litter were compared with air-dried leaf litter in a stream ecosystem.
3. Although overall mass loss rates were similar between treatments ( k = 0.0213 day⁻¹ and 0.0206 day⁻¹), fresh leaves lost mass at a constant rate, whereas the decay of dried leaves proceeded in two distinct phases. Soluble organic carbon, phosphorus, and potassium were rapidly leached from dried litter, but were largely retained in fresh material for more than a week. Kinetics of concentrations of cellulose and changes in amounts of lignin remaining per leaf pack revealed further differences in decomposition dynamics between treatments, apparently related, either directly or indirectly, to differences in leaching behaviour.
4. Dynamics of nitrogen and protein contents were similar between treatments, indicating that microbial colonization was not greatly delayed on fresh leaves.
5. It is concluded that the retention of labile carbon and nutrients in fresh leaf litter facilitates their utilization by leaf-associated micro-organisms and invertebrates, resulting in an increased importance of biotic processes relative to physical processes such as leaching.
6. At the ecosystem level, retention of carbon and nutrients in streams would be increased, allowing greater overall productivity. Conversely, the availability of labile organic carbon would be reduced in compartments such as the epilithon, fine sediments, and the water column.     

模拟酸雨对红壤区茶树器官氮磷含量及其化学计量比的影响

以红壤区25年生茶园为对象,开展pH 4.5、pH 3.5、pH 2.5及自来水(对照)4种强度模拟酸雨处理原位试验,于处理第3年收集茶树体不同功能根系和不同年龄枝、叶,测定氮(N)、磷(P)含量,并计算化学计量比和酸雨响应敏感度.结果表明: 土壤pH值、硝态N和有效P随酸雨强度增加而显著降低.吸收根N含量随酸雨强度增加而提高,pH 2.5处理下吸收根N含量与对照相比显著提高32.9%;储藏根P含量随酸雨强度增强而显著降低;同时酸雨处理显著提高吸收根N/P.新叶和老叶N、P含量对不同强度酸雨处理响应不敏感,但酸雨处理增加了老叶N/P,且在pH 3.5处理下达到显著水平.酸雨处理对枝条的影响与其年龄有关,新枝N含量和N/P在低强度酸雨(pH 4.5)处理下显著增加,而老枝N含量和N/P对酸雨处理响应不敏感.吸收根、新叶和新枝N含量对酸雨响应敏感度分别高于储藏根、老叶和老枝,而储藏根和叶片P含量对酸雨响应敏感度高于其他器官.茶树器官N含量对酸雨处理较为敏感,适度酸雨可增加幼嫩器官N含量和N/P,改变茶树体N、P的循环和平衡.     

The influence of the invasive shrub, Lonicera maackii, on leaf decomposition and microbial community dynamics

Amur honeysuckle (Lonicera maackii) is an exotic invasive shrub that is rapidly expanding into forests of eastern North America. This species forms a dense forest understory, alters tree regeneration, negatively affects herb-layer biodiversity, and alters ecosystem function. In a second-growth forest in central Kentucky, we examined the timing and production of leaf litter and compared litter chemistry, decay rates, and microbial community colonization of Amur honeysuckle to that of two native trees, white ash (Fraxinus americana) and hickory (Carya spp.). The distribution of Amur honeysuckle was clumped, allowing us to compare differences in decomposition under and away from Amur honeysuckle shrubs. Amur honeysuckle leaf litter had significantly higher nitrogen, lower C:N, and lower lignin than the other species, and decomposition rates were greater than 5×?faster. Despite the much higher rate of Amur honeysuckle decomposition compared with the native species (p?<?0.0001), decomposition of all species was significantly slower (p?=?0.0489) in sites located under Amur honeysuckle shrubs. Nitrogen concentration increased through time in decomposing Amur honeysuckle litter; however, total mass of N rapidly declined. We found the initial microbial community on leaf litter of Amur honeysuckle was distinct from two native species and although all microbial communities changed through time, the microbial community of Amur honeysuckle remained distinct from native communities. In summary, a distinct microbial community that may originate on Amur honeysuckle leaves prior to senescence could explain the rapid decay rates.     

酸雨和SO2作用下SOD酶活性与菠菜叶片损伤相关性的研究

The studies show that under exposure to SO₂ and simulated acid rain spinach leaf contains less chlorophyll, its MDAcontent and conductivity are increased, and leaf ageing is accelerated. The SODactivity is decreased with increasing pH, and lossed by 50% at pH3.6 for SO₂ and simulated acid rain and at pH2.8 for simulated acid rain. The SODactivity of spinach leaves exposed to simulated acid rain is higher than that to SO₂ and simulated acid rain.     

Effects of chronic chlorine exposure on litter processing in outdoor experimental streams

SUMMARY 1. The effects of chlorine on litter ( Potamogeton crispus L.) processing were examined using six outdoor experimental streams. Downstream portions of two streams were dosed at c . 10 μg l⁻¹ Total Residual Chlorine (TRC), one stream at 64 μgl¹, and one stream at 230μg l⁻¹. Two control streams were not dosed; upstream riffles of each stream served as instream controls.
2. Two 35 day litter breakdown (per cent AFDW remaining) experiments indicated significantly lower decay rates in the high dose riffle. No other concentration of chlorine significantly affected decay rate.
3. A third experiment, conducted in medium and high dose streams, indicated that high dose chlorine exposure reduced litter decomposition rates significantly, and reduced microbial colonization, microbial electron transport system activity, and microbial litter decomposition after 4 days but not after 11 days of exposure. The number of amphipod shredders colonizing litter bags was also reduced significantly with high chlorine dose.
4. A fourth experiment, after dosing was terminated, provided direct evidence that amphipod shredders were important in facilitating litter decomposition: litter bags stocked with amphipods had significantly higher decomposition rates than bags which excluded shredders.
5. Overall results indicate that the high dose (c. 230 μgl⁻¹ TRC) of chlorine reduced litter processing rates partly by reducing initial microbial conditioning, but primarily by reducing the colonization of amphipod shredders.     

Copper and zinc mixtures induce shifts in microbial communities and reduce leaf litter decomposition in streams

1. To assess the impact of metal mixtures on microbial decomposition of leaf litter, we exposed leaves previously immersed in a stream to environmentally realistic concentrations of copper (Cu) and zinc (Zn) (three levels), alone and in all possible combinations. The response of the microbial community was monitored after 10, 25 and 40 days of metal exposure by examining leaf mass loss, fungal and bacterial biomass, fungal reproduction and fungal and bacterial diversity.
2. Analysis of microbial diversity, assessed by denaturing gradient gel electrophoresis and identification of fungal spores, indicated that metal exposure altered the structure of fungal and bacterial communities on decomposing leaves.
3. Exposure to metal mixtures or to the highest Cu concentration significantly reduced leaf decomposition rates and fungal reproduction, but not fungal biomass. Bacterial biomass was strongly inhibited by all metal treatments.
4. The effects of Cu and Zn mixtures on microbial decomposition of leaf litter were mostly additive, because observed effects did not differ from those expected as the sum of single metal effects. However, antagonistic effects on bacterial biomass were found in all metal combinations and on fungal reproduction in metal combinations with the highest Cu concentrations, particularly at longer exposure times.     

天童国家森林公园常见植物凋落叶分解的研究

 选择天童地区常绿阔叶林及其退化群落常见植物种为对象,着重探讨分解速率和基质营养含量以及比表面积(Specific Leaf Area, SLA)的关系,并试图通过单独分解试验和混合分解试验的比较,从物种、功能群角度探讨凋落叶多样性和分解这一生态系统过程的关系,为深入研究常绿阔叶林常见植物种的营养策略、群落养分循环等奠定基础,也为植被恢复、森林生态系统管理提供理论依据。结果表明：所有凋落叶随时间进程失重率增大,但失重率并不与时间呈线性相关;凋落叶分解后N、P均发生了变化,大多数凋落叶在分解初期N、P均发生了积累,营养元素的释放和富集与凋落叶初始营养状况无明显的相关性。凋落叶的年分解系数与凋落叶中的初始N含量有较高的相关性,而与初始P含量则无显著的相关性;凋落叶的分解速率与成熟叶的面积无相关性,而与其SLA有很强的相关性。通过模型分析,天童地区大多数常见树种凋落叶分解95%需1～4年,平均是2.54年;分解率最高的物种为山鸡椒(Litsea cubeba),其值为6.280,最低的为黄丹木姜子(Litsea elongata),其值为0.558。凋落物混合对分解有很大的影响,虽在初期对分解有阻碍作用,但长期是促进的。若不考虑功能群差异,则可得出多样性的增加有利于分解的结论。功能群数目的增加在凋落物分解前期对分解起促进作用,但这种作用随分解的进展逐渐减小。混合物种的特性往往是决定分解过程的最重要的因素。     

Effects of simulated acid rain on oilseed rape (Brassica napus) physiological characteristics at flowering stage and yield

A pot experiment was conducted to study the effects of different acidity simulated acid rain on the physiological characteristics at flowering stage and yield of oilseed rape (B. napus cv. Qinyou 9). Comparing with the control (pH 6.0), weak acidity (pH = 4.0-5.0) simulated acid rain stimulated the rape growth to some extent, but had less effects on the plant biomass, leaf chlorophyll content, photosynthetic characteristics, and yield. With the further increase of acid rain acidity, the plant biomass, leaf chlorophyll content, photosynthetic rate, antioxidative enzyme activities, and non-enzyme antioxidant contents all decreased gradually, while the leaf malonyldialdehyde (MDA) content and relative conductivity increased significantly. As the results, the pod number per plant, seed number per pod, seed weight, and actual yield decreased. However, different yield components showed different sensitivity to simulated acid rain. With the increasing acidity of simulated acid rain, the pod number per plant and the seed number per pod decreased significantly, while the seed weight was less affected.     

Effects of burial on leaf litter quality,microbial conditioning and palatability to three shredder taxa

Summary 1. Heterotrophic microorganisms are crucial for mineralising leaf litter and rendering it more palatable to leaf‐shredding invertebrates. A substantial part of leaf litter entering running waters may be buried in the streambed and thus be exposed to the constraining conditions prevailing in the hyporheic zone. The fate of this buried organic matter and particularly the role of microbial conditioning in this habitat remain largely unexplored. 2. The aim of this study was to determine how the location of leaf litter within the streambed (i.e. at the surface or buried), as well as the leaf litter burial history, may affect the leaf‐associated aquatic hyphomycete communities and therefore leaf consumption by invertebrate detritivores. We tested the hypotheses that (i) burial of leaf litter would result in lower decomposition rates associated with changes in microbial assemblages compared with leaf litter at the surface and (ii) altered microbial conditioning of buried leaf litter would lead to decreased quality and palatability to their consumers, translating into lower growth rates of detritivores. 3. These hypotheses were tested experimentally in a second‐order stream where leaf‐associated microbial communities, as well as leaf litter decomposition rates, elemental composition and toughness, were compared across controlled treatments differing by their location within the streambed. We examined the effects of the diverse conditioning treatments on decaying leaf palatability to consumers through feeding trials on three shredder taxa including a freshwater amphipod, of which we also determined the growth rate. 4. Microbial leaf litter decomposition, fungal biomass and sporulation rates were reduced when leaf litter was buried in the hyporheic zone. While the total species richness of fungal assemblages was similar among treatments, the composition of fungal assemblages was affected by leaf litter burial in sediment. 5. Leaf litter burial markedly affected the food quality (especially P content) of leaf material, probably due to the changes in microbial conditioning. Leaf litter palatability to shredders was highest for leaves exposed at the sediment surface and tended to be negatively related to leaf litter toughness and C/P ratio. In addition, burial of leaf litter led to lower amphipod growth rates, which were positively correlated with leaf litter P content. 6. These results emphasise the importance of leaf colonisation by aquatic fungi in the hyporheic zone of headwater streams, where fungal conditioning of leaf litter appears particularly critical for nutrient and energy transfer to higher trophic levels.     

The influence of elevated nitrate concentration on rate of leaf decomposition in a stream

SUMMARY.

• 1 Leaf decomposition was compared in two streams at the Coweeta Hydrologic Laboratory, North Carolina. U.S.A. One stream drains an undisturbed hardwood watershed, while the other drains a successional watershed subject to an insect outbreak. The successional watershed has elevated nitrate concentrations in the streamwater.
• 2 Both black locust (Robinia pseudo-acacia) and sweet birch (Betula lenta) leaf litter decomposed 2.8 times more rapidly in the stream with high nitrate concentrations.
• 3 The more rapid decay rates appeared to be partly due to accelerated microbial processing in response to nitrate enrichment, because microbial biomass (as ATP) was higher in the nitrate-enriched stream.
• 4 At each point in time, nitrogen and phosphorus content of the litter was lower in the high nitrate stream; however, there was no significant difference in nitrogen or phosphorus content at the same state of leaf decay in the two streams.

     

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.     

Rapid nutrient cycling in leaf litter from invasive plants in Hawai’i

Physiological traits that contribute to the establishment and spread of invasive plant species could also have impacts on ecosystem processes. The traits prevalent in many invasive plants, such as high specific leaf areas, rapid growth rates, and elevated leaf nutrient concentrations, improve litter quality and should increase rates of decomposition and nutrient cycling. To test for these ecosystem impacts, we measured initial leaf litter properties, decomposition rates, and nutrient dynamics in 11 understory plants from the Hawaiian islands in control and nitrogen + phosphorus fertilized plots. These included five common native species, four of which were ferns, and six aggressive invasive species, including five angiosperms and one fern. We found a 50-fold variation in leaf litter decay rates, with natives decaying at rates of 0.2–2.3 year^–1 and invaders at 1.4–9.3 year^–1. This difference was driven by very low decomposition rates in native fern litter. Fertilization significantly increased the decay rates of leaf litter from two native and two invasive species. Most invasive litter types lost nitrogen and phosphorus more rapidly and in larger quantities than comparable native litter types. All litter types except three native ferns lost nitrogen after 100 days of decomposition, and all litter types except the most recalcitrant native ferns lost >50% of initial phosphorus by the end of the experiment (204–735 days). If invasive understory plants displace native species, nutrient cycling rates could increase dramatically due to rapid decomposition and nutrient release from invasive litter. Such changes are likely to cause a positive feedback to invasion in Hawaii because many invasive plants thrive on nutrient-rich soils.     

Spatial and temporal variation of microbial respiration rates in a blackwater stream

1. The extent of spatial and temporal variation of microbial respiration was determined in a first-order, sand-bottomed, blackwater stream on the coastal plain of south-eastern Virginia, U.S.A.
2. Annual mean respiration rates (as g O₂ m^–3 h^–1) differed significantly among substrata: leaf litter, 12.9; woody debris, 2.4; surface sediment, 0.8; hyporheic sediment, 0.4; water column, 0.003. Rates associated with wood were higher than those with leaves when expressed per unit surface area.
3. Highest respiration rates on leaves, wood and in the water column occurred during the summer, whereas rates in the sediments were greatest during the late autumn and winter. Water temperature, as well as particulate organic matter and nitrogen content of the substrata, was correlated positively with respiration rates.
4. A stepwise multiple regression showed that temperature and nitrogen content together explained 88% of the variation in respiration rates of leaves and wood. In contrast, particulate organic matter content and nitrogen content explained 89–90% of the variation in respiration in the sediments. Although water temperature was a significant factor in the sediment multiple regressions, its addition as an independent variable improved the regression models only slightly.
5. Annual mean respiration in the stream channel, based on the proportional amount of respiration occurring associated with each type of substratum during each month, was 1.1 kg O₂ m^–2 yr^–1. Seventy per cent of respiration in the stream occurred in the hyporheic zone, 8–13% occurred in the surface sediment, leaf litter or woody debris, and < 1% occurred in the water column. Approximately 16% of total detritus, or 40% of non-woody detritus, stored in the stream during the year was lost to microbial respiration.