Effects of macro-detritivores density on leaf detritus processing rate: a macrocosm experiment

The effect of macroinvertebrate detritivore density on the mass loss rates of leaf litter of Alnus glutinosa (alder) was assessed. Experimental freshwater macrocosms, with increasing densities of four species of macroinvertebrate detritivores belonging to two functional groups (shredders and scrapers), were set up outdoors. The litter bag technique was used to assess decomposition rates of alder leaves. Indirect effects of increasing density of macroinvertebrates on phytoplankton standing crop in the water column were investigated by analysing Chlorophyll a concentration. Decomposition rate increased as animal density increased, although a continuous increase in detritivores density resulted in a discrete, step-wise increase of the decomposition rates. Animal colonisation followed an exponential pattern in low-medium density treatments versus a typical `bell-shape' curve in high density treatments; animals started to leave the consumed patches when about 60% of the initial leaf mass was lost (35th day in high-density treatments). Diversity (Hs) of the simplified detritivore community decreased as decomposition proceeded, with a dominance of shredders during the last phase of decomposition. Faster decomposition rate of detritus in the benthic compartment lead to a higher microalgae standing crop in the water column emphasising the role of allochthonous detritus as a source of nutrients for algae primary production in coastal freshwater ecotones.     

Effect of sudden flow reduction on the decomposition of alder leaves (Alnus glutinosa [L.] Gaertn.) in a temperate lowland stream: a mesocosm study

Climate change leads to summer low flow conditions and premature litter input in lowland streams in Central Europe. This may cause a sudden reduction of flow and fragmentation into isolated pools of permanently flowing streams, with a simultaneous increase in the food supply for detrivores during summer months. We performed a mesocosm study to investigate shredder and microbial mediated litter decomposition under these conditions. Leaf litter was placed in a lowland stream with a natural flow regime (reference) and in a stream mesocosm with significant flow reduction (FR) and a representative density of macroinvertebrates and detritus. Physicochemical parameters, leaf mass loss, macroinvertebrate abundance and biomass, leaf-associated respiration, fungal sporulation, and biomass were measured at regular intervals for 6 weeks. Coarse and fine-mesh bags were used to include or exclude macroinvertebrate shredders. In the coarse-mesh bags, leaf mass loss was significantly lower in the FR system than in the reference regime. In the fine-mesh bags, leaf respiration, fungal sporulation, and biomass but not leaf mass losses were substantially lower with flow reduction. Chironomid larvae (Micropsectra spp.) appeared to effectively fragment leaf litter in fine-mesh bags. In the FR system, leaf respiration was higher in the coarse-than in the fine-mesh bags. Our results suggest that, in temperate lowland streams, premature litter input during or after a sudden fragmentation into isolated pools and a reduction of stream flow reduces direct shredder-mediated litter decomposition, but shredders may indirectly influence the decomposition process. Handling editor: B. Oertli     

Macroinvertebrate identity mediates the effects of litter quality and microbial conditioning on leaf litter recycling in temperate streams

Plant litter decomposition is an essential ecosystem function that contributes to carbon and nutrient cycling in streams. Aquatic shredders, mainly macroinvertebrates, can affect this process in various ways; they consume leaf litter, breaking it down into fragments and creating suitable habitats or resources for other organisms through the production of fine particulate organic matter (FPOM). However, measures of litter‐feeding traits across a wide range of aquatic macroinvertebrates are still rare. Here, we assessed the contributions of 11 species of freshwater macroinvertebrates to litter decomposition, by measuring consumption rate, FPOM production, and assimilation rate of highly decomposable (Alnus glutinosa) or poorly decomposable (Quercus robur) leaf litter types. In general, an increase in the quality of litter improved the litter consumption rate, and fungal conditioning of the leaf litter increased both the litter consumption rate and FPOM production. Macroinvertebrates specializing in leaf litter consumption also appeared to be the most sensitive to shifts in litter quality and the conditioning process. Contrary to expectations, the conditioning process did not increase the assimilation of low‐quality litter. There was a strong correlation between the relative consumption rate (RCR) of the two litter types, and the relative FPOM production (RFP) was strongly correlated to the RCR. These findings suggest a consistent relationship between RCR and macroinvertebrate identity that is not affected by litter quality, and that the RFP could be inferred from the RCR. The varying responses of the macroinvertebrate feeding traits to litter quality and the conditioning process suggest that the replacement of a shredder invertebrate species by another species could have major consequences for the decomposition process and the detritus‐based food web in streams. Further studies onto the importance of invertebrate identity and the effects of litter quality in a variety of freshwater ecosystems are needed to understand the whole ecosystem functioning and to predict its response to environmental changes.     

Does predation by the introduced rainbow trout cascade down to detritus and algae in a forested small stream in Patagonia?

Cascade effects of an exotic predator, the rainbow trout (Oncorhynchus mykiss), on periphyton and leaf litter were analysed in a headwater, forested stream of Andean Patagonia (Argentina). We conducted seasonal field sampling and two field experiments measuring leaf litter mass, periphyton biomass and macroinvertebrate biomass in relation to the presence and absence of rainbow trout. In the field survey, the presence of trout influenced resource mass: leaf litter (60% decrease in summer, P = 0.024) and periphyton (tenfold increase in chlorophyll a, P < 0.001) were affected, which were mediated by a decrease in the biomass of shredders (95% decrease in summer, P < 0.001) and scrapers (90% decrease, P < 0.001). There was an effect on leaf litter biomass only in the summer, whereas fish presence reduced periphyton biomass all year except in the winter. In the field experiments, we observed that leaf litter breakdown and periphyton development were effectively controlled by consumers in the absence of fish. In contrast, the presence of fish caused a release of herbivory and detritivory resulting in a significant increase in periphyton biomass (100% increase, P < 0.001) and a decrease in leaf litter decay (40% decrease, P < 0.001). Our results suggest that in low order streams and in the presence of visual predators, trophic cascades may operate both on detritus and algae, but with different timing.     

Riparian plant species loss alters trophic dynamics in detritus-based stream ecosystems

Riparian vegetation is closely connected to stream food webs through input of leaf detritus as a primary energy supply, and therefore, any alteration of plant diversity may influence aquatic ecosystem functioning. We measured leaf litter breakdown rate and associated biological parameters in mesh bags in eight headwater streams bordered either with mixed deciduous forest or with beech forest. The variety of leaf litter types in mixed forest results in higher food quality for large-particle invertebrate detritivores (‘shredders’) than in beech forest, which is dominated by a single leaf species of low quality. Breakdown rate of low quality (oak) leaf litter in coarse mesh bags was lower in beech forest streams than in mixed forest streams, a consequence of lower shredder biomass. In contrast, high quality (alder) leaf litter broke down at similar rates in both stream categories as a result of similar shredder biomass in coarse mesh bags. Microbial breakdown rate of oak and alder leaves, determined in fine mesh bags, did not differ between the stream categories. We found however aquatic hyphomycete species richness on leaf litter to positively co-vary with riparian plant species richness. Fungal species richness may enhance leaf litter breakdown rate through positive effects on resource quality for shredders. A feeding experiment established a positive relationship between fungal species richness per se and leaf litter consumption rate by an amphipod shredder (Gammarus fossarum). Our results show therefore that plant species richness may indirectly govern ecosystem functioning through complex trophic interactions. Integrating microbial diversity and trophic dynamics would considerably improve the prediction of the consequences of species loss.     

Conversion of leaf litter to secondary production by a shredding caddis-fly

Summary 1. The aim of this study was to estimate the amount of leaf litter ingested by the shredder caddis‐fly Sericostoma vittatum in a small stream in central Portugal. The study combined field data on population dynamics and laboratory experiments to determine the effect of temperature (9, 12, 15 and 18 °C), leaf species (Alnus glutinosa, Castanea sativa, Populus × canadensis and Quercus andegavensis) and animal mass on growth and consumption rates of the larvae. 2. Sericostoma vittatum had two overlapping cohorts, each of which needed about 1 year to complete development. Mean annual density and biomass were 115 individuals m^?2 and 83 mg m^?2, respectively. Secondary production was 0.44 g m^?2 year^?1 and production/biomass ratio was 4.9–5 year^?1. 3. Consumption rates of larvae increased with temperature up to the optimal temperature for growth which varied between 13.7 and 16.7 °C depending on the diet. 4. Consumption rate was positively related to larval mass but growth rate was negatively related with larval mass. Larvae fed on A. glutinosa and P. × canadensis had higher consumption and growth rates than those fed on C. sativa or Q. andegavensis. 5. Annual leaf litter consumption by S. vittatum was estimated as 14–22 g m^?2 depending on the diet. No relationship was observed between the amount of detritus consumed by the population of this caddis‐fly in the field and either water temperature, the stock of detritus on the stream bottom, or larval abundance. Instead, the temporal dynamics of leaf litter consumption by S. vittatum were controlled by its life history. 6. This study highlights the influence of factors such as animal size and water temperature on the invertebrate energetics. Models explaining how these variables affect invertebrate production efficiency may be very important for obtaining accurate estimates of the role of shredders in the energy flow across stream ecosystems.     

Resource–consumer diversity: testing the effects of leaf litter species diversity on stream macroinvertebrate communities

1. Understanding relationships between resource and consumer diversity is essential to predicting how changes in resource diversity might affect several trophic levels and overall ecosystem functioning. 2. We tested for the effects of leaf litter species diversity (i.e. litter mixing) on litter mass remaining and macroinvertebrate communities (taxon diversity, abundance and biomass) during breakdown in a detritus‐based headwater stream (North Carolina, U.S.A.). We used full‐factorial analyses of single‐ and mixed‐species litter from dominant riparian tree species with distinct leaf chemistries [red maple (Acer rubrum), tulip poplar (Liriodendron tulipifera), chestnut oak (Quercus prinus) and rhododendron (Rhododendron maximum)] to test for additivity (single‐species litter presence/absence effects) and non‐additivity (emergent effects of litter species interactions). 3. Significant non‐additive effects of litter mixing on litter mass remaining were explained by species composition, but not richness, and litter‐mixing effects were variable throughout breakdown. Specifically, small differences in observed versus expected litter mass remaining were measured on day 14; whereas observed litter mass remaining in mixed‐species leaf packs was significantly higher on day 70 and lower on day 118 than expected from data for single‐species leaf packs. 4. Litter mixing had non‐additive effects on macroinvertebrate community structure. The number of species in litter mixtures (two to four), but not litter species composition, was a significant predictor of the dominance of particular macroinvertebrates (i.e. indicator taxa) within mixed‐species packs. 5. In addition, the presence/absence of high‐ (L. tulipifera) and low‐quality (R. maximum) litter had additive effects on macroinvertebrate taxon richness, abundance and biomass. The presence of L. tulipifera litter had both positive (synergistic) and negative (antagonistic) effects on invertebrate taxon richness, that varied during breakdown but were not related to litter chemistry. In contrast, the presence/absence of L. tulipifera had a negative relationship with total macroinvertebrate biomass (due to low leaf mass remaining when L. tulipifera was present and higher condensed and hydrolysable tannins associated with leaf packs lacking L. tulipifera). Macroinvertebrate abundance was consistently lower when R. maximum was present, which was partially explained by litter chemistry [e.g., high concentrations of lignin, condensed tannins, hydrolysable tannins and total phenolics and high carbon to nutrient (N and P) ratios]. 6. The bottom‐up effects of litter species diversity on stream macroinvertebrates and litter breakdown are different, which suggests that structural attributes of macroinvertebrate communities may only partially explain the effects of litter‐mixing on organic matter processing in streams. In addition, stream macroinvertebrates colonising decomposing litter are influenced by resource diversity as well as resource availability. Broad‐scale shifts in riparian tree species composition will alter litter inputs to streams, and our results suggest that changes in the diversity and availability of terrestrial litter may alter stream food webs and organic matter processing.     

Trophic cascading effects of predatory fish on leaf litter processing in a Japanese stream

A manipulative field experiment to test for trophic cascading effects of predatory fish on detritus processing by benthic invertebrates was performed in stream channels running through a wetland forest in northern Japan. To control for fish effects on benthic invertebrates, two simple treatments (fish-present and fish-absent) were established for 4 weeks, with two common predatory fish, rainbow trout (Oncorhynchus mykiss) and freshwater sculpin (Cottus nozawae), being introduced into and excluded from stream cages. At the end of experiment, the biomass of the dominant detritivore, an amphipod (Jesogammarus jezoensis), was significantly less in the fish-present treatment (0.56 g m^–2 in dry mass on average) than that in the fish-absent treatment (1.32 g m^–2), there being no significant treatment effect evident for the second-dominant detritivore, coleopteran larvae (Optioservus kubotai). The loss of oak leaves (Quercus crispla) from litter bags in the fish-present treatment (0.31 g week^–1 in dry mass on average) was significantly less than in the fish-absent treatment (0.54 g week^–1). Predator-induced lower biomass and likely lowered foraging activities of the J. jezoensis were responsible for the suppression of litter processing efficiency. In contrast, the standing crop of fine particulate organic matter did not differ significantly between the treatments. The experimental results revealed that the predatory fish had an indirect but significant effect on leaf litter processing in the stream.     

Positive effect of shredders on microbial biomass and decomposition in stream microcosms

1. Animals play a major role in nutrient cycling via excretory processes. Although the positive indirect effects of grazers on periphytic algae are well understood, little is known about top‐down effects on decomposers of shredders living on leaf litter. 2. Nutrient cycling by shredders in oligotrophic forest streams may be important for the microbial‐detritus compartment at very small spatial scales (i.e. within the leaf packs in which shredders feed). We hypothesised that insect excretion may cause local nutrient enrichment, so that microorganism growth on leaves is stimulated. 3. We first tested the effect of increasing concentration of ammonium (+10, +20 and +40 μg NH₄⁺ L^?1) on fungal and bacterial biomass on leaf litter in a laboratory experiment. Then we performed two experiments to test the effect of the presence and feeding activity of shredder larvae. We used two species belonging to the trichopteran family Sericostomatidae: the Palaearctic Sericostoma vittatum and the Neotropical Myothrichia murina, to test the effect of these shredders on fungal and bacterial biomass and decomposition on leaves of Quercus robur and Nothofagus pumilio, respectively. All experiments were run in water with low ammonium concentrations (2.4 ± 0.34 to 14.47 ± 0.95 μg NH₄⁺ L^?1). 4. After 5 days of incubation, NH₄ concentrations were reduced to near‐ambient streamwater concentrations in all treatments with leaves. Fungal biomass was positively affected by increased ammonium concentration. On the other hand, bacteria abundance was similar in all treatments, both in terms of abundance (bacteria cells mg^?1 leaf DW) and biomass. However, there was a tendency towards larger mean cell size in treatments with 20 μg NH₄ L^?1. 5. In the experiment with S. vittatum, fungal biomass in the treatment with insects was more than twice that in the control after 15 days. Bacteria were not detected in treatments with insects, where hyphae were abundant, but they were abundant in treatments without larvae. In the decomposition experiment run with M. murina, leaf‐mass loss was significantly higher in treatments with larvae than in controls. 6. Our hypothesis of a positive effect of shredders on fungal biomass and decomposition was demonstrated. Insect excretion caused ammonium concentration to increase in the microcosms, contributing to microbial N uptake in leaf substrata, which resulted in structural and functional changes in community attributes. The positive effect of detritivores on microbes has been mostly neglected in stream nutrient‐cycling models; our findings suggest that this phenomenon may be of greater importance than expected in stream nutrient budgets.     

Annual production of leaf-decaying fungi in a woodland stream

1. Fungi are thought to be important mediators of energy flow in the detritus-based food webs of woodland streams. However, until recently, quantitative methods to assess their contribution have been lacking. Growth rates of leaf-decaying fungi can be estimated from rates of acetate incorporation into ergosterol which, together with estimates of fungal biomass from ergosterol concentrations, enables calculation of fungal production. In this study, I used this method to estimate total production of leaf-decaying fungi over an annual cycle in a small woodland stream, Walker Branch, Tennessee, U.S.A. To calculate fungal biomass and production on an areal basis, I determined the amount of leaf litter occurring in the stream by sampling transects randomly selected in each of ten 10-m sections every 20–50 days. Subsamples of leaves chosen from five of the transects were used to determine ergosterol concentrations and in situ rates of acetate incorporation into ergosterol. 2. Leaf litter, fungal biomass m^–2, and fungal production m^–2 were highly seasonal. Leaf litter ranged from 249 g m^–2 in November to less than 5 g m^–2 during the summer. Fungal biomass as percentage of leaf litter ranged from 4.4 to 8.8% during the year, but on an areal basis ranged from 11 to 13 g m^–2 during November to January to 0.25 g m^–2 in June, primarily due to the seasonal variation in amount of leaf litter present. Fungal growth rates averaged 2.6% day^–1 (0.9–7.0% day^–1) during the year. Daily production of leaf-decaying fungi ranged from 0.49 g m^–2 in November, when the amount of leaf litter was at its maximum, to 0.006 g m^–2 during the summer when the amount of leaf litter was low. Annual production of leaf-decaying fungi was 34 g m^–2, with an annual production to biomass ratio (P/B) of 8.2. 3. Fungal spore concentrations in the stream were also seasonal and were correlated with amount of leaf litter m^–2 and fungal biomass m^–2. Spore concentrations varied between one and four spores ml^–1 throughout most of the year, but increased to eighteen spores ml^–1 shortly after the greatest amount of leaf litter was present in the stream during November.     

Effects of litter patch area on macroinvertebrate assemblage structure and leaf breakdown in Central Amazonian streams

Leaf breakdown in streams is affected by several factors, such as leaf characteristics, water chemistry, microbial activity, and abundance of shredders. In turn, shredders may be resource-controlled. We hypothesized that the size of litter patches affects leaf breakdown, because large patches should be stable over time and therefore harbor high densities of shredders. We selected litter patches (area 0.25–10 m²) in 10 pools of three first-order streams (Manaus, Brazil). We installed 10 leaf packs of Mabea speciosa (Euphorbiaceae) in each patch, and sampled one after 1 day and three after 5, 19, and 28 days. The leaf packs were quickly colonized by the shredding caddisflies Triplectides and Phylloicus. The leaf breakdown rate (mean k = 0.026 ± 0.0015 SE) was high and similar to values reported for other tropical and temperate streams, although much higher than values reported for the adjacent Cerrado biome. Assemblage composition varied over time, but was not related to the size of litter patches. Contrary to our hypothesis, litter patch area did not affect breakdown rates (r ² = 0.012, P = 0.766) or abundance of shredders after 5, 19, and 28 days (r ² < 0.243, P > 0.147). We found, however, a positive relationship between the abundance of tropical shredders and leaf breakdown after 19 days (r ² = 0.572, P = 0.011), suggesting that shredders play an important role in leaf breakdown in these headwater streams. Our study indicates that leaf breakdown rates in tropical streams are variable and can be as high as those of temperate streams.     

Temporal niches of shredders in lake littorals with possible implications on ecosystem functioning

Temporal growth separation of shredders is known in streams but has not been reported from lakes. In the present study, temporal niche/trait differentiations among shredders in lakes were investigated. We sampled quantitatively three lakes in SE Sweden over a period of 18 months. Dry weight and number of individuals of the collected shredders were measured monthly. Standing stock of detritus types was also monitored. The same 10 species of lentic shredders were found in each lake, one isopod and larvae of nine trichopterans. Functionally, the shredders could be categorized into two main groups; winter and summer growing species. However, also within these groups, temporal differences in growth pattern existed. The main input of detritus occurred during the autumnal leaf fall and a majority of winter shredders had the start of their lives tied to this period. A succession in loss of detritus types was evident with easily degraded matter disappearing first followed by more resistant matter. Shredder species richness, shredder biomass per m² and the ratio coarse/fine detritus all reached its annual low in late summer. We propose a temporal link between the shredder groups and the organic matter subject to decomposition; the successive palatability of coarse detritus is likely to make a temporally separated community of shredders efficient in terms of decomposition. We believe that a temporal differentiation per se is sufficient to conclude that different impact on ecosystem function exists among shredders. Additionally we discuss impacts of differences in abundance and shredding capacity among the species.     

Aquatic macroinvertebrate community structure of aNymphoides peltata-dominated and macrophyte-free site in an oxbow lake

The aquatic macroinvertebrates in two freshwater biotopes,viz. aNymphoides peltata-dominated site and a macrophyte-free site, were studied quantitatively in a shallow alkaline oxbow lake of the river Waal, the main branch of the river Rhine in The Netherlands. The research comprised the analysis of water, sediment and macrophyte samples.In the macrophyte-free site Oligochaeta and Nematocera, particularly of the collector gatherer functional feeding group, dominated the prevailing benthic community. The total macroinvertebrate biomass ranged here from 0.3 to 0.9 g ash-free dry weight per m² of biotope.Species richness, densities, and biomass of macroinvertebrates were considerably higher in the biotope dominated byNymphoides peltata. Many taxa were found associated with the aboveground macrophyte. The sediment compartment, however, contributed most to the total density and biomass of macroinvertebrates. Nematocera and Oligochaeta were the most abundant fauna groups, whereas the largest share in total biomass was provided by clams (Mollusca). The biomass of the total macroinvertebrate community in theNymphoides-dominated site ranged from 6.2 to 7.5 g ash-free dry weight per m² of biotope. The biomass of the aboveground phytophilous fauna ranged from 0.1 to 0.6 g ash-free dry weight per m² of biotope. In September, when theNymphoides peltata vegetation was in its senescent phase, the largest numbers and the highest biomass of phytophilous macroinvertebrates were observed. The contribution of the shredder functional feeding group was high in this period. This, and the overall high abundance of fauna with a detritivorous mode of life, indicates the importance of macrophyte detritus as input to food chains.     

氮添加对中亚热带杜鹃灌丛凋落物生产和叶分解的影响

凋落物的生产和分解是生态系统养分循环的重要过程,受到大气氮沉降的深刻影响。但目前相关研究主要集中于森林和草地生态系统,氮沉降对灌丛生态系统凋落物养分归还的影响规律尚不清楚。因此选择亚热带分布广泛的杜鹃灌丛为研究对象,进行了为期两年的模拟氮沉降试验。试验设置4个处理:对照(CK, 0 g m-2 a-1)、低氮(LN, 2 g m-2 a-1)、中氮(MN, 5 g m-2 a-1)和高氮(HN, 10 g m-2 a-1)。结果显示:CK、LN、MN和HN 4种处理下,群落年平均凋落物量分别为(1936.54±358.9)、(2541.89±112.5)、(2342.97±519.8)、(2087.22±391.8) kg/hm²,LN、MN和HN处理样地的凋落量分别比对照样地高出32.68%、21.16%和7.93%;凋落叶、花果、凋落枝和其他组分占总凋落量的比例分别为75.75%、15.09%、7.70%和1.45%,不同浓度氮处理下各组分的凋落量均高于对照样地;凋落物组分表现出明显的季节动态:凋落叶在10—11月份达到峰值,凋落枝在10月份达到峰值,花果凋落物则在5月份凋落量最高,不同氮处理下凋落物的季节动态基本一致;白檀凋落叶分解速率显著高于杜鹃,二者分解95%所需时间分别为5.08—11.11 a和7.69—17.65 a,施氮使白檀凋落叶分解周期比对照样地缩短18.18%—54.28%;凋落叶分解过程中,N元素表现为富集-释放模式,P元素表现为富集模式。研究表明,氮添加能够促进群落中白檀凋落叶分解及N、P元素的释放,说明施氮可以调节凋落叶养分释放模式,对灌丛生态系统的养分循环具有调控作用。     

Macroinvertebrate assemblages in perennial headwater streams of the Coastal Mountain range of Washington,U.S.A.

We studied headwater streams in 4 watersheds of Washington's Coastal Mountain region from June to August 1998 to establish macroinvertebrate reference conditions and describe variation in macroinvertebrate assemblage structure among stream orders and among substrates. Macroinvertebrates were sampled with mesh baskets (30 × 30 cm) filled with equal volumes of wood (1.5 l) and cobble (1.5 l) that were installed into fifteen 1st-order, six 2nd-order, and three 3rd-order streams. Low taxa richness and low macroinvertebrate densities were found in all streams. Crayfish dominated (92.7%) biomass estimates, with shredders dominating the non-crayfish component of the biomass. The importance of shredders declined from 1st- to 3rd-order streams. An abundance of wood and a lack of algae and non-wood based detritus in the 1st-order streams led us to suspect that food webs were wood based. We tested this hypothesis by comparing macroinvertebrate assemblages in substrate baskets filled with equal volumes (3 l) of naturally conditioned (1) wood, (2) cobble, or (3) wood and cobble (1.5 l of each). Macroinvertebrate richness was higher in wood-only and mixed baskets than the cobble-only baskets (p = 0.0118), and macroinvertebrate biomass was higher in mixed than cobble-only baskets (p = 0.044).     

Effects of subsidy quality on reciprocal subsidies: how leaf litter species changes frog biomass export

Spatial subsidies are resources transferred from one ecosystem to another and which can greatly affect recipient systems. Increased subsidy quantity is known to increase these effects, but subsidy quality is likely also important. We examined the effects of leaf litter quality (varying in nutrient and tannin content) in pond mesocosms on gray treefrog (Hyla versicolor) biomass export, as well as water quality and ecosystem processes. We used litter from three different tree species native to Missouri [white oak (Quercus alba), northern red oak (Quercus rubra), and sugar maple (Acer saccharum)], one non-native tree [white pine (Pinus strobus)], and a common aquatic grass [prairie cordgrass (Spartina pectinata)]. We found that leaf litter species affected almost every variable we measured. Gray treefrog biomass export was greatest in mesocosms with grass litter and lowest with white oak litter. Differences in biomass export were affected by high tannin concentrations (or possibly the correlated variable, dissolved oxygen) via their effects on survival, and by primary production, which altered mean body mass. Effects of litter species could often be traced back to the characteristics of the litter itself: leaf nitrogen, phosphorus, and tannin content, which highlights the importance of plant functional traits in affecting aquatic ecosystems. This work and others stress that changes in forest species composition could greatly influence aquatic systems and aquatic–terrestrial linkages.     

Forest floor biomass,litter fall and nutrient return in Central Himalayan oak forests

The seasonal dynamics of forest floor biomass, pattern of litter fall and nutrient return in Central Himalayan oak forests are described. Fresh and partially decomposed litter layers occur throughout the whole year in addition to herbaceous vegetation. The highest leaf litter value is found in April and May and the minimum in September. Partially and largely decomposed litter tended to increase from January to May with a slight decline in June. The wood litter peaked in March and April. The relative contribution of partially decomposed litter to the forest floor remains greatest the year round. The maximum herbaceous vegetation development was found in September with a total annual net production of 104.3 g m^-2yr^-1. The total calculated input of litter was 480.8 g m^-2yr^-1. About 68% of the forest floor was replaced each year with a subsequent turnover time of 1.47 yr. The total annual input of litter ranged from 664 (Quercus floribunda site) –952 g m^-2 (Q. lanuginosa site), of which tree, shrub and herbaceous litter accounted for respectively 72.0–86.3%, 6.4 – 19.4% and 5.2 – 8.6%. The annual nutrient return through litter fall amounted to (kg ha^-1) 178.0 – 291.0 N, 10.0 – 26.9 P, 176.8 – 301.6 Ca, 43.9 – 64.1 K and 3.98 – 6.45 Na. The tree litter showed an annual replacement of 66.0 – 70.0%, for different nutrients the range was 64 and 84%.     

Respiration and annual fungal production associated with decomposing leaf litter in two streams

1. We compared fungal biomass, production and microbial respiration associated with decomposing leaves in one softwater stream (Payne Creek) and one hardwater stream (Lindsey Spring Branch). 2. Both streams received similar annual leaf litter fall (478–492 g m^?2), but Lindsey Spring Branch had higher average monthly standing crop of leaf litter (69 ± 24 g m^?2; mean ± SE) than Payne Creek (39 ± 9 g m^?2). 3. Leaves sampled from Lindsey Spring Branch contained a higher mean concentration of fungal biomass (71 ± 11 mg g^?1) than those from Payne Creek (54 ± 8 mg g^?1). Maximum spore concentrations in the water of Lindsay Spring Branch were also higher than those in Payne Creek. These results agreed with litterbag studies of red maple (Acer rubrum) leaves, which decomposed faster (decay rate of 0.014 versus 0.004 day^?1), exhibited higher maximum fungal biomass and had higher rates of fungal sporulation in Lindsey Spring Branch than in Payne Creek. 4. Rates of fungal production and respiration per g leaf were similar in the two streams, although rates of fungal production and respiration per square metre were higher in Lindsey Spring Branch than in Payne Creek because of the differences in leaf litter standing crop. 5. Annual fungal production was 16 ± 6 g m^?2 (mean ± 95% CI) in Payne Creek and 46 ± 25 g m^?2 in Lindsey Spring Branch. Measurements were taken through the autumn of 2 years to obtain an indication of inter‐year variability. Fungal production during October to January of the 2 years varied between 3 and 6 g m^?2 in Payne Creek and 7–27 g m^?2 in Lindsey Spring Branch. 6. Partial organic matter budgets constructed for both streams indicated that 3 ± 1% of leaf litter fall went into fungal production and 7 ± 2% was lost as respiration in Payne Creek. In Lindsey Spring Branch, fungal production accounted for 10 ± 5% of leaf litter fall and microbial respiration for 13 ± 9%.     

The soil fauna of a beech forest on limestone: trophic structure and energy budget

Summary The soil fauna of a mull beech forest on lime-stone in southern Lower Saxony (West Germany) was sampled quantitatively. Biomass estimates, trophic characteristics, and measurement and calculation of the energetic parameters of the constituent animal populations were used to construct an energy budget of the total heterotrophic subsystem of the forest. Mean annual zoomass amounted to about 15 g d wt m^–2; earthworms (about 10 g d wt m^–2) and other groups of the macrofauna were dominant. Protozoa constituted about 1.5 g d wt m^–2. Relative distribution of zoomass among the trophic categories was 50% macrosaprophages, 30% microsaprophages, 12% microphytophages, and 4% zoophages. Total annual consumption rate of the saprophagous and microphytophagous soil fauna (6328 and 4096 kJ m^–2 yr^–1, respectively) was of the same order of magnitude as annual litter fall (canopy leaves 6124 kJ m^–2 yr^–1, flowers and fruits 944 kJ m^–2 yr^–1, herbs 1839 kJ m^–2 yr^–1, fine woody material 870 kJ m^–2 yr^–1, tree roots 3404 kJ m^–2 yr^–1, without coarse woody litter). Primary decomposers (macrosaprophages) were the key group for litter comminution and translocation onto and into the soil, thus contributing to the high decomposition rate (k=0.8) for leaf litter. Consumption rates of the other trophic groups were (values as kJ m^–2 yr^–1): bacteriophages 2954, micromycophages 416, zoophages 153. Grazing pressure of macrophytophages (including rhizophages) was low. Faeces input from the canopy layer was not significant. Grazing pressure on soil microflora almost equalled microbial biomass; hence, a large fraction of microbial production is channelled into the animal component. Predator pressure on soil animals is high, as a comparison between consumption rates by zoophages and production by potential prey — mainly microsaprophages, microphytophages and zoophages — demonstrated. Soil animals contributed only about 11% to heterotrophic respiration. However, there is evidence that animals are important driving variables for matter and energy transfer: key processes are the transformation of dead organic material and grazing on the microflora. It is hypothesized that the soil macrosaprophages are donor-limited.     

Community structure of Trichoptera in a mountain stream: spatial patterns of production and functional organization

SUMMARY. 1. Annual production was estimated for Trichoptera occurring in each of three distinct habitats of a mountain stream: bedrock-outcrops, riffles and pools. Production was greatest on bedrock-outcrops (2608 mg ash-free dry weight m^?2), followed by riffles (1038) and pools (950). 2. Annual production in bedrock-outcrops and pools was dominated by single functional groups, with collector-filterers and shredders contributing 73% and 75% of the annual production, respectively. Production in riffles was due primarily to shredders (46%), followed by collector-filterers (27%). 3. Taking account of the amount of stream area occupied by each habitat type, total annual production was estimated at 1336 mg AFDW m^?2. 53% of this production was attributable to four taxa: Parapsyche cardis Ross (25%), Pycnopsyche gentilis (MacLachlan) (10%), Neophylax mitchelli Carpenter (9%) and Rhyacophila nigrita Banks (9%). 4. Habitat-weighted production was distributed among functional groups as follows: collector-filterers (41%), shredders (29%), engulfing-predators (15%), scrapers (13%) and collector-gatherers (2%). 5. The distinct taxonomic and functional structures of trichopteran sub-communities were shaped by the distinct physical characteristics of their principal habitats. Bedrock-outcrops were characterized by low roughness and high current and were sites of low deposition or organic matter; thus the predominance of collector-filterers. In contrast, the other habitats of greater roughness (riffles) and/or lower current (pools) were sites of deposition of food (e.g. leaf litter) and greatest shredder production. 6. By distinguishing discrete mesoscale habitats, each with a functionally distinct caddisfly sub-community, we speculate that small mountain streams provided the diverse physical templates essential for the evolution of the major feeding tactics (e.g. scraping, shredding, filter-feeding) of the. Trichoptera.