Habitat structure and fish predation: effects on invertebrate colonisation and breakdown of stream leaf packs

1. We investigated the effects of two features of leaf‐pack habitat structure (i.e. mass of a leaf pack and surface area of leaves comprising a leaf pack) and fish predation on colonisation of shredders and leaf breakdown rates in a coldwater stream. Packs were constructed of red maple (Acer rubrum) leaves. 2. A 2 × 3 × 3 factorial experiment was used to manipulate fish predation (exclusion and control cage), leaf‐pack mass (1, 3 and 5 g dry mass) and leaf surface area (small: approx. 17.9 cm², medium: approx. 34.6 cm², large: approx. 65.6 cm²). Exclusion cages had mesh on all sides, whereas control cages lacked mesh on two sides to provide access to fish. 3. Common shredders were Gammarus pseudolimnaeus, Pycnopsyche and Lepidostoma. Shredder biomass per leaf pack increased with the mass of a leaf pack (P < 0.001), but biomass per unit mass of leaf pack did not differ with leaf‐pack mass (P = 0.506). Shredder densities did not respond to the exclusion of fish (P > 0.7) or leaf surface area (P > 0.7), and interactions among treatment factors were not significant (P > 0.2). 4. Breakdown rates were lower for leaf packs comprised of small leaves (P < 0.001) and leaf packs with high mass (P = 0.001). Excluding fish did not significantly affect leaf breakdown rates (P = 0.293), and interactions among treatment factors were not significant (P > 0.3). Breakdown rates were highest when packs consisted of few leaves (i.e. leaf packs with large leaves and low mass) and were colonised by many shredders. 5. Fish predation was not an important factor controlling shredder densities in leaf packs over the spatiotemporal scale of our experiment. Nevertheless, we found shredder colonisation was proportional to leaf‐pack mass and breakdown rates were affected by leaf‐pack size (i.e. number of leaves in a pack). We suspect that fragmentation is the primary mechanism causing the breakdown rates to be dependent on leaf‐pack size.     

Leaf decomposition in an experimentally acidified stream channel

Decomposition of Alnus rugosa and Myrica Gale leaves immersed in artificial stream channels fed by a small headwater creek was followed over a three month period. At the end of experiment, remaining weights of both leaf types confined in litter bags were significantly higher after immersion in experimentally acidified water (pH 4.0) than when immersed in control water (pH 6.2–7.0). For both types of leaves and for all sampling times, there was generally no difference in the C:N ratios between leaves in acidified and those in control water. In control water, oxygen uptake by microorganism on A. rugosa leaves was significantly higher after 46 days of immersion, whereas differences between treatments appeared only after 69 days for M. Gale leaves. Transfer of A. rugosa leaves from acid to control water led to a rapid increase in microbial activity; this increased activity was reflected in a fast weight loss of the leaves. For both leaf types, total numbers of macroinvertebrates were usually higher in litter bags immersed in control water. Macroinvertebrates colonizing the litter bags were mainly collector-gatherers: Chironomidae were numerically dominant in control leaf packs whereas Oligochaeta dominated in acid leaf packs. Macroinvertebrate biomass in M. Gale litter was higher in control than in acidified water, which contrasted with macroinvertebrate biomass in A. rugosa leaf packs which was not significantly different between treatments. Macroinvertebrate contribution to the breakdown of leaf litter was thus considered less important than the microbial contribution. This study demonstrated that decomposition of leaf litter in acidic headwater streams can be seriously reduced, mainly as a result of a lower microbial activity.     

Nutrient addition accelerates leaf breakdown in an alpine springbrook

This study assessed the effect of nutrient enrichment on organic matter breakdown in an alpine springbrook, using alder leaf packs to which phosphorus and nitrogen were added in the form of slow-release fertilizer briquettes. The breakdown of leaf packs with nutrients added (k=0.0284 day^–1) was significantly faster than that of unfertilized packs (k=0.0137 day^–1), resulting in a 30% higher mass loss after 42 days. Unfertilized leaves enclosed in fine-mesh bags broke down at an even slower rate (k=0.0062 day^–1). Phosphorus and nitrogen concentrations were initially higher in leaf packs with nutrients added, but this difference disappeared within 3 weeks. Fungal biomass developing in decomposing leaves was substantial (c. 55 mg dry mass per 1 g leaf dry mass) although similar between fertilized and unfertilized packs, as was the sporulation activity of aquatic hyphomycetes. There was a significantly greater number and higher biomass of macroinvertebrates (shredding nemourid stoneflies in particular) on the fertilized packs, suggesting that the increased leaf mass loss was brought about by shredder feeding. Received: 11 March 1999 / Accepted: 6 September 1999     

Bioturbation by a dominant detritivore in a headwater stream: litter excavation and effects on community structure

Bioturbation can affect community structure by influencing resource distribution and habitat heterogeneity. Bioturbation by detritivores in small headwater streams could affect community structure by reintroducing buried detrital resources into the food web and could also affect the distribution of various taxa on detritus. We evaluated the ability of the caddisfly Pycnopsyche gentilis to uncover experimentally buried leaves in a headwater stream. Packs of leaves were placed in enclosures and covered with a known volume of sediment. We added 0, 3 or 6 large Pycnopsyche to the enclosures which were permeable to most other invertebrate taxa. Leaf packs were sampled after 23 days and leaf pack mass, the amount of sediment covering the leaf packs, and macro‐ and microinvertebrate densities on leaf packs were quantified. There was a significant negative relationship between Pycnopsyche density and leaf pack mass. Pycnopsyche also reduced the volume of sediment covering leaf packs. Pycnopsyche had complex effects on the abundance of invertebrate taxa associated with the leaves. Some taxa exhibited their highest abundance in the 3 Pycnopsyche treatment while others exhibited non‐significant increases as Pycnopsyche density increased. These results suggest that the beneficial effects of Pycnopsyche (e.g. uncovering leaves which increases the availability of habitat and food) outweigh any negative effects (e.g. disturbance, encounter competition) of the caddisfly when it is present at lower densities. However, the negative impacts of Pycnopsyche appear to outweigh the positive effects via sediment removal at higher caddisfly densities for some taxa. Our results suggest that bioturbating organisms in streams have the potential to reintroduce organic matter to detrital food webs and affect the distribution and abundance of benthic taxa associated with organic matter.     

Subsurface hydrology and degree of burial affect mass loss and invertebrate colonisation of leaves in a woodland stream

SUMMARY 1. In deciduous forest streams, fallen leaves form a large component of the total organic matter budget, and many leaves become buried within stream sediments. We examined the processing of buried leaves as compared with those at the surface, and the influence of subsurface hydrology on processing rates.
2. Leaf packs were secured on the streambed surface or buried 10 cm deep in upwelling and downwelling reaches of a second-order stream in Michigan, U.S.A. Mass loss and invertebrate colonisation were measured from October to February.
3. Leaves buried in upwelling reaches lost mass more slowly (exponential decay coefficient, k =−0.0097) than did leaves from the other treatments (buried downwelling: −0.017; surface upwelling: −0.022; surface downwelling: −0.021).
4. Initially, more invertebrates colonised surface leaf packs than buried packs. During the remainder of the study, however, hydrology had a greater effect on invertebrate abundance than did burial, as more invertebrates were found in packs in downwelling reaches than in upwelling reaches.
5. Local subsurface hydrology and degree of burial, factors rarely considered in studies of detritus processing, can significantly influence mass loss and invertebrate colonisation of fallen leaves in streams. Furthermore, because of slower processing, subsurface zones may function as organic matter reservoirs that gradually 'spiral' carbon to downstream subsurface and surface habitats.     

The effect of leaf pack composition on processing: A comparison of mixed and single species packs

The effect of leaf species composition on decomposition patterns was examined in a coastal plain stream. Red maple leaves (Acer rubrum) decomposed at the same rate separately or when mixed with cypress leaves (Taxodium distichum). Cypress addition increased structural integrity but its effects differed between sites with different hydrologic regimes. Invertebrate communities varied slightly between mixed and single species packs, however invertebrates did not appear to be the primary agent of decomposition. Mixed species packs may be an alternative method to fine mesh bags for studying processing of small, narrow leaves in a more realistic manner.     

Litter retention in Tasmanian headwater streams after clear-fell logging

Clear-fell logging around small headwater streams in Tasmanian wet eucalypt forests was predicted to affect both the retention of leaf litter and the composition and size of leaf packs. Retention structures were surveyed in six natural streams and six streams in forest regenerated 3–5 years after clear-fell and burn logging. Logged streams had more wood, but retained less leaves than natural streams, and consequently had fewer and smaller leaf packs. Leaf packs from natural streams contained 200% more leaves, bark and twigs than packs from logged streams. The effect of buoyancy on leaf retention was assessed with release and recapture of marked Eucalyptus obliqua and Nothofagus cunninghamii leaves. Eucalypt leaves were more likely to be trapped by retention structures on the bed of the stream, while smaller, more buoyant N. cunninghamii leaves were mainly trapped by leaf packs. Leaf packs in natural streams were formed on a matrix of small twigs and long strips of bark, shed from the upper branches of mature stringybark eucalypts, while leaf retention was reduced in logged streams because there are no mature trees to provide effective retention structures. Changes to the channel form increase both discharge and sedimentation. These factors have strong implications for downstream nutrient processing and riverine food webs.     

Patterns of macroinvertebrate colonization on fresh and senescent alder leaves in two Michigan streams

SUMMARY.

• 1 Communities of invertebrates colonizing senescent autumn and fresh summer alder leaves (Alnus rugosa) were compared. Leaf packs for each treatment were placed in two hardwater streams in the Upper Peninsula of Michigan in late summer and early autumn. One stream has a cobble-bottom and the other a sand-bottom and both receive fresh leaf inputs by beaver fellings.
• 2 Fresh leaf packs remained intact after 26 days immersion, but thereafter were processed faster than were the autumn leaf packs in both streams.
• 3 In the cobble-bottom stream taxon richness (S), numbers of individuals and biomass were higher on fresh than on autumn leaves.
• 4 Fresh leaves in the sand-bottom stream supported a more diverse (H'), richer (S) and more equitably distributed (J') insect fauna than did the autumn leaves.
• 5 We discuss the simultaneous lack of fresh leaf loss and the presence of more complex insect communities on those leaves during the first 26 days of the study. Invertebrates in both mid-latitude heterotrophic streams and in tropical lowland wet forest streams may rely on fresh leaf inputs, which have received little attention.

     

Anatomical Injury Induced by the Eriophyid Mite Aceria anthocoptes on the Leaves of Cirsium arvense

Anatomical injury of the leaves of the invasive species, Cirsium arvense (L.) Scop., caused by the eriophyid mite Aceria anthocoptes (Nal.), which is the only eriophyid mite that has been recorded on C. arvense worldwide, is described. The injury induced by the mite feeding on the leaves of C. arvense results in visible russeting and bronzing of the leaves. Other conspicuous deformations are folding and distortion of the leaf blade and curling of leaf edge, as well as gradual drying of leaves. The anatomical injury of the mature leaves of field-collected plants was limited to the epidermis of the lower leaf surface. However, on young leaves of experimentally infested plants, rust mite injuries extend to epidermal cells on both leaf surfaces and to those of deeper mesophyll layers. On these leaves, lesions on the lower leaf surface even affected the phloem of the vascular bundles. Leaf damage induced by A. anthocoptes is discussed with regard to the mite’s potential as a biological control agent of C. arvense.     

Leaf breakdown in a regulated desert river: Colorado River, Arizona, U.S.A.

We compared processing rates (k _d) for leaves of the native willow (Salix exigua Nutt.) and cottonwood (Populus fremontii Wats.) to those of the non-native salt cedar (Tamarix chinensis Lour.) in the regulated Colorado River, U.S.A. Leaf packs of each species were incubated at Lees Ferry, approximately 26 km below Glen Canyon Dam, Arizona. Leaf packs were processed at 2, 21, 46, 84 and 142-d intervals. Water temperatures remained relatively constant (10 °C, SE ± 1 °C) during the study. There were significant differences in processing rates between species, with P. fremontii showing the fastest breakdown. After 142 d, only 20% of the P. fremontii leaf mass remained, whereas 30% and 52% of leaf masses remained for T. chinensis and S. exigua, respectively. The k _d value for P. fremontii was 0.0062 compared to 0.0049 and 0.0038 for T. chinensis and S. exigua, respectively. Invertebrate colonization was not significantly different between native and non-native plant species with oligochaetes the most abundant animal colonizing the leaf packs. Dual stable isotope analysis showed that leaf material was not the primary food for invertebrates associated with leaf packs. Processing rates for all leaf types were slow in the regulated Colorado River compared to rates reported in many other systems. This is likely due to the lack of caddisfly and stonefly shredders and perhaps slow metabolic rates by microbes.     

Leaf beetle grazing does not induce willow trichome defence in the coppicing willow Salix viminalis

Abstract 1 Willows are frequently attacked and defoliated by adult leaf beetles (Phratora vulgatissima L.) early in the season and the plants are then attacked again when new larvae emerge. The native willow Salix cinerea has previously been shown to respond to adult grazing by producing new leaves with an increased trichome density. Subsequent larval feeding was reduced on new leaves. This type of induced plant response may reduce insect damage and could potentially be utilized for plant protection in agricultural systems. 2 Here, we investigated if the willow species most commonly used for biomass production in short rotation coppice, Salix viminalis, also responds to adult beetle grazing by increasing trichome density. Larval performance and feeding behaviour on plants previously exposed to adult beetles was compared with that on undefoliated control plants in a greenhouse. 3 We found an overall decrease in trichome density within all the plants (i.e. trichome density was lower on new leaves compared to that for older basal leaves on S. viminalis). However, leaves of beetle defoliated plants had a higher trichome density compared to control plants. Larval growth and feeding was not affected by this difference between treatments. Larvae appeared to remove trichomes when feeding on S. viminalis, a behaviour that might explain the lack of difference between treatments.     

Freshwater crabs (Decapoda: Pseudothelphusidae) increase rates of leaf breakdown in a neotropical headwater stream

1. Freshwater crabs are the largest macroconsumers in many neotropical headwater streams, but few studies have examined their roles in ecosystem processes such as leaf litter breakdown. As omnivorous macroconsumers, freshwater crabs affect multiple trophic levels. They may directly increase leaf breakdown through fragmentation and consumption or indirectly decrease breakdown by consuming other macroinvertebrates, including shredders and detritivores.
2. In a headwater stream in Monteverde, Costa Rica, we conducted an in-stream experiment with 40 enclosures to quantify the effects of pseudothelphusid crabs on both leaf breakdown and macroinvertebrate colonisation of leaves. Half of the enclosures were randomly selected to contain two crabs (mean carapace width = 30 mm) and half were controls without crabs. We sampled mixed leaf packs from the enclosures on days 11, 19, 28, 34, and 42. We found the leaves of one species (Koanophyllon pittieri) almost completely decomposed by day 28 in both treatments (crab versus no crab). The other two leaf species (Meliosma idiopoda, Quercus brenesii) composed the remaining leaf mass at the end of the experiment.
3. At 42 days, enclosures with crabs had faster rates of leaf breakdown than those without crabs (with crabs: k = −0.020; without crabs: k = −0.016; p = 0.034). This suggests that the magnitude of direct leaf breakdown by crabs, due to fragmentation, consumption, or manipulation of leaves, was greater than any indirect effects on leaf breakdown via crab consumption of other leaf-consuming species.
4. Macroinvertebrate composition based on taxa abundances or biomasses did not significantly differ between treatments (ANOSIM; p = 0.73 and p = 0.65, respectively). Shredder and detritivore abundances and biomasses increased significantly through time (ANOVA; p ≤ 0.001), but there was no evidence of an effect of crab presence (p > 0.2), nor were there significant interactions between crab presence and time (p > 0.3).
5. This is one of the first studies to quantify the effects of pseudothelphusid freshwater crabs on leaf breakdown rates. Our results suggest that these crabs can play a significant role in detrital processing in neotropical headwater streams. This study has also demonstrated that short-term enclosure experiments are useful in measuring in-stream effects of crab activity on leaf breakdown.

     

Slow larval growth on a suboptimal willow results in high predation mortality in the leaf beetle Galerucella lineola

The slow growth/high mortality hypothesis predicts that herbivorous insects feeding on suboptimal host plants are subjected to higher predation mortality owing to the longer time spent in the vulnerable juvenile stages compared with conspecifics feeding on optimal plants. We tested this hypothesis for the willow-feeding leaf beetle Galerucella lineola raised on one suitable (Salix viminalis) and one unsuitable (S. dasyclados) willow species as well as on plants from an interspecific cross between the two species. Cohorts of larvae raised on caged plants (protected from enemies) and uncaged plants (exposed to enemies) were monitored daily throughout larval development in two consecutive years. Larvae raised on S. viminalis developed faster, grew larger and survived better than those raised on S. dasyclados. The suitability of the hybrid plants was intermediate to that of their parents. Our results strongly support the slow growth/high mortality hypothesis. In both years, total predation during the larval period was higher on S. dasyclados than on S. viminalis. Furthermore, the daily predation rate (i.e. the proportion of larvae preyed upon per day) was higher on S. dasyclados than on S. viminalis. When hybrid plants were included in the analysis total predation was positively correlated with both larval development time and daily predation rate. We suggest that high predation on beetles on low-quality plants is the combined result of their longer development time and elevated daily predation rate. The results are discussed in relation to the evolution of host plant selection and the paradox of sublethal plant defenses.     

Does nitrogen become limiting under high‐P conditions in detritus‐based tropical streams?

1. We examined effects of nutrients on leaf breakdown in interior forest streams at La Selva Biological Station, Costa Rica. We tested the hypothesis that dissolved inorganic nitrogen (DIN) becomes limiting when ambient phosphorus (P) concentration is high. We also compared the breakdown of relatively ‘low quality’ leaves (lower C : N, Trema integerrima) with that of ‘higher quality’ leaves (higher C : N, Ficus insipida) in a high‐P stream. 2. Litterbags were incubated in two streams: one enriched experimentally with P [target concentration 200 μg soluble reactive phosphorus (SRP) L^?1] and one control (naturally low P concentration approximately 10 μg SRP L^?1). Ammonium enrichment was achieved by adding fertiliser upstream of half of the litterbags in each stream. 3. Phosphorus addition stimulated leaf breakdown, microbial respiration, ergosterol and leaf %P. Leaf breakdown rate was consistent with those in La Selva streams with naturally high P concentration. 4. Nitrogen (N) addition had no effect on leaf breakdown, microbial respiration, ergosterol or leaf chemistry in either the P‐enriched or the reference stream, in spite of low N : P ratios. We conclude that N is probably not limiting in streams at La Selva that are naturally high in P. This may be due to moderately high ambient N concentration (>200 μg DIN L^?1) prevailing throughout the year. 5. The species with a lower C : N decomposed more rapidly and supported higher microbial activity than that with a higher C : N. Subtle differences in leaf N content, as well as dissolved P concentration, may be important in determining microbial colonisation and subsequent leaf breakdown.     

Annelid sperm and fertilization biology

This paper provides data on fine particulate organic matter (FPOM) and macroinvertebrates associated to natural and artificial leaf packs in a small woodland stream (Schlaube, Brandenburg). Macroinvertebrate colonisation and the dynamics of FPOM were studied in oven-dried alder leaf packs, air-dried alder leaf packs and packs with artificial leafshaped substrate exposed in the stream during a 68-day period. The importance of FPOM as a potential food source for macroinvertebrates especially in artificial leaf packs was evaluated. Changes in the quantity as well as in the chemical composition of the accumulating FPOM (>63 and <63 μm) was determined using soluble carbohydrates, proteins and chlorophyll a as parameters of the nutritional quality. Mass loss and the chemical changes of alder leaves during the decompositional process were also described. The loss of soluble carbohydrates due to leaching was more rapid in oven-dried alder leaf packs than in air-dried ones. After 3 days of leaf pack exposure weight loss of oven-dried and air-dried leaf packs was nearly comparable, as the similar decay coefficients, k = 0.0228 (oven-dried leaf packs) and k = 0.0214 (air-dried leaf packs), respectively, show. The amount of FPOM per unit leaf area constantly increased in artificial packs, although it remained below that of alder leaf packs at all sampling dates. The nutritional quality of FPOM <63 μm was constantly greater than that of FPOM >63 μm and decreased in both size-fractions with length of exposure. Referring to leaf area the abundance of macroinvertebrates continually increased in all packs till the end of exposure, whereas the numbers in artificial packs remained below that in alder leaf packs. The taxonomic composition of all treatments was very similar with Gammarus pulex being the most abundant taxon in all packs until day 42, while afterwards the caddis fly genus Hydropsyche gained in importance. The amphipod Gammarus pulex in general did not show a preference for air-dried alder leaf packs compared to oven-dried alder leaf and artificial packs. Corresponding dynamics of macroinvertebrate colonisation and FPOM content in artificial packs support the hypothesis that FPOM functions not only as an important food source for macroinvertebrates including gammarideans but also as a control mechanism of macroinvertebrate abundance in stream habitats. Even if the accumulation of FPOM and drifting macroinvertebrates might be influenced by the same abiotic factor (e.g. by reduction in stream velocity inside the packs) it is quite unlikely that only physical properties caused the invertebrates to stay.     

The importance of crayfish in the breakdown of rhododendron leaf litter

1. Rhododendron (Rhododendron maximum) is a common evergreen shrub in riparian areas of the southern Appalachians, where its leaves can comprise a large proportion of leaf litter in streams. However, they are relatively refractory and generally considered a low quality food resource for detritivores. 2. Our objective was to assess whether macroconsumers [primarily crayfish (Cambarus bartonii)] influence rhododendron leaf breakdown in a forested southern Appalachian stream in both summer (when leaves other than rhododendron are relatively scarce) and autumn (when other leaves are relatively abundant). We conducted two leaf decay experiments, one in summer and one in autumn, using pre‐conditioned leaves. Macroconsumers were excluded from the benthos of a fourth‐order stream using electric ‘fences’; we predicted that excluding macroconsumers would reduce the decay rate of rhododendron leaves in both summer and autumn. 3. In both experiments, breakdown rate was lower in exclusion treatments. Macroconsumers accounted for approximately 33 and 54% of rhododendron decay in summer and autumn, respectively. We attribute this effect to direct shredding of rhododendron by crayfish. Biomass of insect shredders, insect predators and fungi did not differ between control and exclusion treatments, indicating that insectivorous sculpins (Cottus bairdi) had no effect on rhododendron decay and that omnivorous crayfish did not exert an indirect effect via alteration of insect or fungal biomass. 4. The influence of shredding insects varied between summer and autumn. In summer, when other, more palatable leaf types were not available, rhododendron leaf packs appeared to provide ‘resource islands’ for insect shredders. There was a significant inverse relationship between insect shredders and leaf pack mass in the summer exclusion treatment: insects were the only organisms eating leaves in this treatment and, as shredder biomass increased, remaining leaf pack mass decreased. In the control treatment, however, we did not see this relationship; here, the effect of insect shredders was presumably swamped by the impact of crayfish. In autumn, when other leaves were abundant, insect shredder biomass in rhododendron leaf packs was less than one‐third of summer values. 5. Even at low density (approximately 2 m^–2) crayfish were able to influence an ecosystem process such as leaf decay in both summer and autumn. Given the threatened status of many crayfish species in the United States, this finding is especially relevant. Even small alterations in crayfish assemblages, whether via loss of native species and/or introduction of exotic species, may have significant repercussions for ecosystem function.     

Decomposition of Eucalyptus globulus leaves and three native leaf species (Alnus glutinosa, Castanea sativa and Quercus faginea) in a Portuguese low order stream

Leaf decomposition of the exotic evergreen Eucalyptus globulus (eucalyptus), and three native deciduous tree species, Alnus glutinosa (alder), Castanea sativa (chestnut) and Quercus faginea (oak), was compared in a second order stream in Central Portugal. Changes in dry weight, nitrogen and polyphenolic compounds and microbial colonization were periodically assessed for three months.Negative exponential curves fit the leaf weight loss with time for all leaf species. Mass loss rate was in the order alder (K = 0.0161) > chestnut (K = 0.0079) > eucalyptus (K = 0.0068) > oak (K = 0.0037). Microbial colonization followed the same pattern as breakdown rates. Evidence of fungal colonization was observed in alder after 3 days in the stream, whereas it took 21 days in oak leaves to have fungal colonization. Fungal diversity was leaf species-dependent and increased with time. In all cases, percent nitrogen per unit leaf weight increased, at least, at the initial stages of decay while soluble polyphenolics (expressed as percentage per unit leaf weight) decreased rapidly in the first month of leaves immersion.Intrinsic factors such as nitrogen and polyphenolic content may explain differences in leaf decomposition. The possible incorporation of eucalyptus litter into secondary production in a reasonable time span is suggested, although community balance and structure might be affected by differences in allochthonous patterns determined by eucalyptus monocultures.     

Does leaf quality mediate the stimulation of leaf breakdown by phosphorus in Neotropical streams?

1. Lowland tropical streams have a chemically diverse detrital resource base, where leaf quality could potentially alter the effect of high nutrient concentrations on leaf breakdown. This has important implications given the extent and magnitude of anthropogenic nutrient loading to the environment. 2. Here, we examine if leaf quality (as determined by concentrations of cellulose, lignin and tannins) mediates the effects of high ambient phosphorus (P) concentration on leaf breakdown in streams of lowland Costa Rica. We hypothesised that P would have a stronger effect on microbial and insect processing of high‐ than of low‐quality leaves. 3. We selected three species that represented extremes of quality as measured in leaves of eight common riparian species. Species selected were, from high‐ to low‐quality: Trema integerrima > Castilla elastica > Zygia longifolia. We incubated single‐species leaf packs in five streams that had natural differences in ambient P concentration (10–140 μg soluble reactive phosphorus (SRP) L^?1), because of variable inputs of solute‐rich groundwater and also in a stream that was experimentally enriched with P (approximately 200 μg SRP L^?1). 4. The breakdown rate of all three species varied among the six streams: T. integerrima (k‐values range: 0.0451–0.129 day^?1); C. elastica (k‐values range: 0.0064–0.021 day^?1); and Z. longifolia (k‐values range: 0.002–0.008 day^?1). Both ambient P concentration and flow velocity had significant effects on the breakdown rate of the three species. 5. Results supported our initial hypothesis that litter quality mediates the effect of high ambient P concentration on leaf processing by microbes and insects. The response of microbial respiration, fungal biomass and invertebrate density to high ambient P concentration was greater in Trema (high quality) than in Castilla or Zygia (low quality). Variation in flow velocity, however, confounded our ability to determine the magnitude of stimulation of breakdown rate by P. 6. Cellulose and lignin appeared to be the most important factors in determining the magnitude of P‐stimulation. Surprisingly, leaf secondary compounds did not have an effect. This contradicts predictions made by other researchers, regarding the key role of plant secondary compounds in affecting leaf breakdown in tropical streams.     

Leaf breakdown and associated macroinvertebrates in alpine glacial streams

1. The relationship between macroinvertebrate assemblages and the breakdown of alder [Alnus viridis (Chaix), Dc.] leaves was examined by exposing leaf packs in four streams in an alpine glacial floodplain over 8 months. Although glacially fed, the four sites (pro-glacial, glacial lake outlet, main channel, and a side-channel with a mix of water sources) differed physically and contained different benthic communities.
2. Leaf breakdown and associated fungal properties differed widely among sites. Leaf decay rate varied by an order of magnitude ( k ranged from 0.0029 to 0.0305 day^–1), and was fastest at the lake outlet (< 20% leaf mass remaining by day 45) and slowest at the pro-glacial site (> 75% remaining on day 45). Rapid processing at the lake outlet was because of the presence of Acrophylax zerberus Brauer, a shredding caddisfly.
3. There were few macroinvertebrate taxa at the pro-glacial site (two to four taxa present in packs) and leaf breakdown was attributed primarily to micro-organisms. Leuctra abundance in leaf packs was strongly correlated with fungal biomass but not with the sporulation activity of any specific aquatic hyphomycete. Other taxa, such as Baetis and chironomids, showed no relationship with any leaf characteristic, suggesting that leaf packs were used mainly as a habitat and not as a food resource.
4. The predatory stonefly Isoperla was significantly associated with the abundance of macroinvertebrate prey ( Baetis , Chironomidae and Leuctra ) in leaf packs at the main and side-channel sites. The results indicate that leaf breakdown can vary widely in alpine lotic environments, reflecting site-specific differences in habitat characteristics, and in macroinvertebrate and fungal composition.     

巨桉不同状态叶片浸提液对萝卜幼苗形态和抗性生理特性的影响

以珍珠岩作为基质,选择4年生巨桉(Eucalyptus grandis)嫩叶(T1)、老叶(T2)、表层凋落叶(T3)、腐解凋落叶(T4)4种状态的叶片,每种状态叶片设置3个浸提液浓度水平[分别称取风干叶片30g、15g和7.5g加入900mL蒸馏水进行浸提,以蒸馏水为对照(CK)],采用水培法研究了不同状态叶片浸提液对萝卜(Raphanus sativus)幼苗形态生长和抗性生理特性的影响。结果显示:(1)巨桉不同状态叶片浸提液显著抑制了萝卜幼苗的根长,其中嫩叶的抑制作用最强,腐解凋落叶抑制作用最弱。(2)各状态叶片浸提液处理后萝卜幼苗中过氧化氢酶(CAT)和过氧化物酶(POD)的活性均呈现升高趋势,嫩叶各浓度处理以及其他状态叶片的高浓度处理下超氧化物歧化酶(SOD)活性升高,而其余浓度处理的SOD活性降低。(3)各状态叶片浸提液处理萝卜幼苗的丙二醛(MDA)含量在低浓度处理时低于CK,其余处理下则高于CK。(4)嫩叶各浓度处理萝卜幼苗的可溶性糖(SS)含量显著高于CK,且随着老叶和表层凋落叶浸提液浓度的升高,幼苗SS含量先升后降,腐解凋落叶各浓度处理下则呈渐增的趋势;而可溶性蛋白(SP)含量则随浸提液浓度的增加而升高,且T2和T3两种状态叶片的各浓度处理与CK差异显著。研究表明,巨桉不同状态叶片浸提液对萝卜幼苗生长和抗性生理产生了强烈的抑制作用,其中以嫩叶最强,老叶和表层凋落叶次之,腐解凋落叶最弱。