Snag Surface Research Articles

Effect of non-standard SnAg surface finishes on properties of solder joints

In this study, the effects of nonstandard near-eutectic SnAg surfaces on the wettability, intermetallic compound growth and mechanical properties of solder joints were investigated. Four types of surface finishes based on near-eutectic SnAg alloys with different compositions and numbers of layers were analyzed. Design/methodology/approachThe effect of surface-finishes SnAg5NiP and SnAg7NiP, which were prepared with and without a Ni-P diffusion barrier, was tested. The Ni-P layer was produced either chemically or electrochemically. The effect of the SnAg surface finish on wettability, formation, and growth of intermetallic compound in the solder joint was analyzed. We further explored the impact of different PCB finishes on the quality of soldered joints. This investigation involved a comparative analysis of the behavior of SnAg finishes in comparison to pure copper, as well as the commonly employed HASL, ENIG, and ImSn finishes. FindingsIt was found that SnAg surface finishes with a Ni-P diffusion barrier are wetted by molten solder faster than surface finishes without a diffusion barrier. The mechanical properties of the joints on both surfaces are comparable to those on ENIG. SnAg that include a chemically created Ni-P diffusion layer is characterized by the formation of more thermally stable ternary intermetallic compounds based on (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4. On the contrary, the electrochemically formed Ni layer supports the increase in the rate of intermetallic compound formation, which affects the properties of the soldered joints. OriginalityNonstandard near-eutectic SnAg finishes, which contains a Ni-P diffusion layer can be used as promising replacement of ENIG or other standard finishes.

Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy

Molecular materials enable a vast variety of functionalities for novel electronic and spintronic devices. The unique possibility to alter or substitute organic molecules or metallic substrates offers the opportunity to modify and optimize interfacial properties for almost any desired field of application. For this reason, we extend the successful approach to control molecular interfaces by surface alloying. We present a comprehensive characterization of the structural and electronic properties of the interface formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown on an Ag(111) single crystal surface. We monitor the changes of adsorption height of the surface alloy atoms and electronic valence band structure upon adsorption of one layer of PTCDA using the normal incidence x-ray standing wave technique in combination with momentum-resolved photoelectron spectroscopy. We find that the vertical buckling and the surface band structure of the SnAg$_2$ surface alloy is not altered by the adsorption of one layer of PTCDA, in contrast to our recent study of PTCDA on a PbAg$_2$ surface alloy [Phys. Rev. Lett. 117, 096805 (2016)] . In addition, the vertical adsorption geometry of PTCDA and the interfacial energy level alignment indicate the absence of any chemical interaction between the molecule and the surface alloy. We attribute the different interactions at these PTCDA/surface alloy interfaces to the presence or absence of local $\sigma$-bonds between the PTCDA oxygen atoms and the surface atoms. Combining our findings with results from literature, we are able to propose an empiric rule for engineering the surface band structure of alloys by adsorption of organic molecules.

Growth of thin Sn films on Ag(111) studied with low-energy electron diffraction and X-ray photoelectron spectroscopy

Thin Sn films on Ag(111) grown at different Sn coverage and substrate temperature were characterized with low-energy electron diffraction and x-ray photoelectron spectra. The results show that surface alloying of Sn atoms on Ag(111) occurs at a temperature as low as 96 K, indicating a strong tendency to form a surface alloy between Sn and Ag. During the alloying, Sn atoms substitute Ag atoms in random locations, resulting in a disordered Sn/Ag(111) surface. The alloy surface becomes completely ordered with a (√3 × √3) R30° reconstruction (the √3 phase) only when it is annealed above 453 K, but annealing at 403 K converts all Sn atoms on Ag(111) into the alloy state; the surface coverage of Sn is no more than 1/3 monolayer with excess Sn diffusing into the Ag(111) substrate. These observations indicate that the √3 phase with the Sn-Ag surface alloy is the only ordered phase obtainable on Sn/Ag(111) once the Sn-Ag surface alloy is formed. It is consequently impracticable to grow non-alloy ordered phases, such as stanene, on Ag(111) epitaxially at a temperature above 96 K, showing evident discrepancies with the conclusions of the previous computational study.

Promotion of Ternary Pt–Sn–Ag Catalysts toward Ethanol Oxidation Reaction: Revealing Electronic and Structural Effects of Additive Metals

The use of a computation-guided method and the discovered structure–property relationship would establish a rational strategy to aid the development of ethanol oxidation reaction (EOR) catalysts for possible commercialization of direct ethanol fuel cells. Here, we investigate the promotion roles of additive metals in ternary Pt–Sn–Ag catalysts toward EOR via a combination of density functional theory calculation and experimental evidence. By calculating the EOR energetics, the promotion roles of Sn and Ag were revealed from the viewpoints of electronic and structural effects, respectively: (1) The addition of Sn and Ag on Pt essentially reduce the reaction energy and activation barrier of the second two-electron transfer process of EOR, facilitating the oxidation of acetaldehyde to acetic acid; (2) a homogeneous Pt–Sn–Ag surface configuration strengthens the adsorption energy of ethanol, thus improving the activity for ethanol oxidizing to acetaldehyde. Experimentally, various Pt–Sn–Ag nanorod catalysts ...

Biomass Turnover Rates of Snag Chironomid Larvae in a Second Order Stream of Middle China

ABSTRACT During June 2003 to June 2004, annual production and Production/Biomass (P/B) ratios were estimated for the larval chironomid assemblage found on the submerged branches and leaves (snag) of Heizhuchong Stream, a second order river of Hanjiang River basin, China. Temperature-specific growth rate equations from field growth studies were applied to the biomass values from quantitative field samples to obtain mean daily production on a monthly basis throughout the year. The annual production of the filtering collector Rheotanytarsus spp. was 32.1g'm−2'y −1 for snag surface area, and productions for the gathering collectors Polypedilum spp. and Rheocricotopus spp. were 10.9 g'm−2'y−1 and g'm−2'y−1, respectively. These production estimates are among the highest ones ever reported for chironomids in freshwater ecosystems, mainly owing to extremely high P/B ratios. Annual PB ratios were 177 for Rheocricotopus spp., 261 for Polypedilum spp. and 194 for Rheotanytarsus spp.

Food web quantification using secondary production analysis: predaceous invertebrates of the snag habitat in a subtropical river

1. Secondary production was estimated for Plecoptera, Odonata and Megaloptera (mostly large predators) occurring on the snag habitat of a subtropical blackwater river in the southeastern U.S.A. Coastal Plain for 2 years. Production estimates and gut analyses were used in estimating species‐specific ingestion to construct a quantitative food web of the predator portion of the invertebrate assemblage. Neither basal resources (e.g. detritus) nor predaceous vertebrates (e.g. fishes) were considered in this analysis. A discharge‐specific model of snag‐habitat availability was used to convert values per m2 of snag surface to values per m2 of river bed. 2. These three orders included the major large predators on the snag habitat, as well as two detritivorous stoneflies. The major predators were the hellgrammite (Corydaluscornutus), five perlid stoneflies (Paragnetinakansensis, Perlestaplacida, Neoperlaclymene, Acroneuriaevoluta and Acroneuriaabnormis) and two dragonflies (Neurocorduliamolesta and Boyeriavinosa). The detritivores were Pteronarcysdorsata and Taeniopteryxlita. 3. Total predator production was high, but varied from only 7.1 to 7.4 g dry mass (DM) m−2 y−1 of snag surface (2.4–2.7 g DM m−2 y−1 of river bed) over two years. Corydalus was the largest predator and had the highest production (2.8–3.1 g DM m−2 of snag surface). The most productive stoneflies were Perlesta (0.7–1.0 g DM m−2 of snag surface) and Paragnetina (1.0–1.3 g DM m−2 of snag surface). The most productive dragonfly was Neurocordulia (0.7–1.9 g DM m−2 of snag surface). Production of the non‐predaceous stoneflies was 1.0–2.3 g DM m−2 of snag surface. Production values per m2 of river bed were 2–3.5 times lower than the values per m2 snag surface. 4. Measurement of ingestion fluxes within the predator portion of the food web showed that predaceous invertebrates were primarily supported by chironomid and mayfly prey. However, the greatest consumption of chironomids and mayflies was by omnivorous hydropsychid caddisflies, which had a considerably higher production than the larger predators. There was a hierarchy of feeding with Corydalus as top predator consuming all other groups, followed in order by dragonflies, stoneflies and hydropsychids. Although the feeding hierarchy suggested the presence of four predatory trophic levels within the invertebrate assemblage, calculations of trophic position indicated there were less than two. With primary consumers (e.g. midges) having a trophic position of 2, Corydalus had a trophic position of only 3.5. 5. A relatively high fraction of invertebrate production was consumed by predaceous invertebrates, ranging from 9 to >100% for various primary consumer groups, with total consumption representing 52% of total production. Because these estimates do not include vertebrate consumption or emergence, it means that a high fraction of larval mortality is due to predation.

PRODUCTION DYNAMICS OF RIVERINE CHIRONOMIDS: EXTREMELY HIGH BIOMASS TURNOVER RATES OF PRIMARY CONSUMERS

A critical step in understanding food webs and trophic dynamics of communities is quantification of the role of the primary consumers, and a major aspect of such quantification is determination of their production. Annual production and biomass turnover were estimated for the larval chironomid (midge) assemblage found on the submerged woody (snag) habitat of a Coastal Plain blackwater river. Temperature-specific growth rate equations, generated from field growth studies, were applied to the biomass values from quantitative field samples to obtain mean daily production on a monthly basis throughout the year. The most productive genera were the filtering collectors, Rheotanytarsus (31.1 g dry mass·m−2·yr−1 for snag surface area) and the gathering collectors, Polypedilum (11.3 g·m−2·yr−1) and Rheocricotopus (9.8 g·m−2·yr−1). Total production was 65.4 g·m−2·yr−1 based on summing values of individual taxa, and 69.9 g·m−2·yr−1 when applying a family-level equation to biomass of all taxa. When converted to production per square meter of river channel bottom, values were about one-third of the snag surface area estimates (20.1 g·m−2·yr−1). Total chironomid production was somewhat higher in a second sampling year (81.9 g·m−2·yr−1 for snag surfaces and 26.8 g·m−2·yr−1 for channel bottom). Production was relatively high throughout both years but tended to be lowest in winter with peaks in either summer or fall. These production estimates are among the highest ever reported for chironomids in freshwater systems, primarily owing to extremely high biomass turnover. Annual production/biomass (P/B) values were 158 for Rheocricotopus, 258 for Polypedilum, 196 for Rheotanytarsus, and 202 for total Chironomidae. For Polypedilum, such high annual P/B represents a biomass turnover rate of almost once per day. The P/B values are among the highest estimated for aquatic metazoans (including zooplankton) and are similar to or exceed turnover rates of microbes in many aquatic ecosystems. Such high biomass turnover may be more widespread among freshwater benthic invertebrates than is commonly supposed and may have profound implications for trophic dynamics.

Production Dynamics of Riverine Chironomids: Extremely High Biomass Turnover Rates of Primary Consumers

A critical step in understanding food webs and trophic dynamics of communities is quantification of the role of the primary consumers, and a major aspect of such quantification is determination of their production. Annual production and biomass turnover were estimated for the larval chironomid (midge) assemblage found on the submerged woody (snag) habitat of a Coastal Plain blackwater river. Temperature-specific growth rate equations, generated from field growth studies, were applied to the biomass values from quantitative field samples to obtain mean daily production on a monthly basis throughout the year. The most productive genera were the filtering collectors, Rheotanytarsus (31.1 g dry mass·m−2·yr−1 for snag surface area) and the gathering collectors, Polypedilum (11.3 g·m−2·yr−1) and Rheocricotopus (9.8 g·m−2·yr−1). Total production was 65.4 g·m−2·yr−1 based on summing values of individual taxa, and 69.9 g·m−2·yr−1 when applying a family-level equation to biomass of all taxa. When converted to production per square meter of river channel bottom, values were about one-third of the snag surface area estimates (20.1 g·m−2·yr−1). Total chironomid production was somewhat higher in a second sampling year (81.9 g·m−2·yr−1 for snag surfaces and 26.8 g·m−2·yr−1 for channel bottom). Production was relatively high throughout both years but tended to be lowest in winter with peaks in either summer or fall. These production estimates are among the highest ever reported for chironomids in freshwater systems, primarily owing to extremely high biomass turnover. Annual production/biomass (P/B) values were 158 for Rheocricotopus, 258 for Polypedilum, 196 for Rheotanytarsus, and 202 for total Chironomidae. For Polypedilum, such high annual P/B represents a biomass turnover rate of almost once per day. The P/B values are among the highest estimated for aquatic metazoans (including zooplankton) and are similar to or exceed turnover rates of microbes in many aquatic ecosystems. Such high biomass turnover may be more widespread among freshwater benthic invertebrates than is commonly supposed and may have profound implications for trophic dynamics.

TROPHIC BASIS OF PRODUCTION AMONG RIVERINE CADDISFLIES: IMPLICATIONS FOR FOOD WEB ANALYSIS

We determined the trophic basis of production and quantified the food web of caddisfly larvae from the submerged woody (snag) habitat of a Coastal Plain blackwater river (Ogeechee River). Production was dominated by three net-spinning taxa (Cheumatopsyche spp., Hydropsyche rossi, and Chimarra moselyi), comprising 97–98% of the total among at least 14 trichopteran species. Annual production (as dry mass) was among the highest estimates reported for caddisflies, ranging from 43.5 to 63.9 g/m2 of snag surface (or from 14.2 to 25.5 g/m2 of channel bottom) in two consecutive years. Although all taxa were present throughout the year, C. moselyi had much higher production in summer, H. rossi was highest from late summer to winter, and Cheumatopsyche spp. showed no pattern. Ingestion was determined from production estimates, bioenergetic efficiencies, and quantitative gut analyses. The two macrofiltering taxa (Cheumatopsyche spp. and H. rossi) were omnivorous, with 50.6 and 64.4% of their production, respectively, due to eating animals. Somewhat lower amounts of their production (40.7 and 23.5%) were due to ingestion of amorphous detritus. Production of the microfiltering species Chimarra moselyi was primarily due to eating amorphous detritus (91%). Removal of amorphous detritus from the system by the major taxa was highest in summer, and diatom removal was highest in fall, whereas animals were eaten consistently throughout the year. A quantitative food web showed that while the linkages among nine taxa were complex, the ingestion pathways were dominated by amorphous detritus (total consumption = 62.5 g·m−2·yr−1) and animal prey (22.3 g·m−2·yr−1), with >99% of food resources being consumed by the three dominant filter-feeding taxa. A connectivity food web was misleading because it implied equivalence of all food resources and consumers when great variation in strength of linkages actually existed. These comparisons provide a strong argument for considering energy flow as a measure of linkage strength when evaluating food webs, and they cast doubt on the usefulness of oversimplified connectivity webs as a basis for food web theory.

Trophic Basis of Production Among Riverine Caddisflies: Implications for Food Web Analysis

We determined the trophic basis of production and quantified the food web of caddisfly larvae from the submerged woody (snag) habitat of a Coastal Plain blackwater river (Ogeechee River). Production was dominated by three net-spinning taxa (Cheumatopsyche spp., Hydropsyche rossi, and Chimarra moselyi), comprising 97–98% of the total among at least 14 trichopteran species. Annual production (as dry mass) was among the highest estimates reported for caddisflies, ranging from 43.5 to 63.9 g/m2 of snag surface (or from 14.2 to 25.5 g/m2 of channel bottom) in two consecutive years. Although all taxa were present throughout the year, C. moselyi had much higher production in summer, H. rossi was highest from late summer to winter, and Cheumatopsyche spp. showed no pattern. Ingestion was determined from production estimates, bioenergetic efficiencies, and quantitative gut analyses. The two macrofiltering taxa (Cheumatopsyche spp. and H. rossi) were omnivorous, with 50.6 and 64.4% of their production, respectively, due to eating animals. Somewhat lower amounts of their production (40.7 and 23.5%) were due to ingestion of amorphous detritus. Production of the microfiltering species Chimarra moselyi was primarily due to eating amorphous detritus (91%). Removal of amorphous detritus from the system by the major taxa was highest in summer, and diatom removal was highest in fall, whereas animals were eaten consistently throughout the year. A quantitative food web showed that while the linkages among nine taxa were complex, the ingestion pathways were dominated by amorphous detritus (total consumption = 62.5 g·m−2·yr−1) and animal prey (22.3 g·m−2·yr−1), with >99% of food resources being consumed by the three dominant filter-feeding taxa. A connectivity food web was misleading because it implied equivalence of all food resources and consumers when great variation in strength of linkages actually existed. These comparisons provide a strong argument for considering energy flow as a measure of linkage strength when evaluating food webs, and they cast doubt on the usefulness of oversimplified connectivity webs as a basis for food web theory.

Production Dynamics and Resource Utilization of Snag‐Dwelling Mayflies in a Blackwater River

We quantified production dynamics, determined the trophic basis of production, and assessed the overlap in resource utilization within a diverse assemblage of mayfly larvae from the submerged woody (snag) habitat of a Coastal Plain blackwater river (Ogeechee River). Total annual production (as dry mass) was among the highest reported for mayflies, ranging from 20.8 to 42.2 g/m2 of snag surface (7.4 to 12.3 g/m2 of channel bottom) in two consecutive years. Relatively little change occurred between years for four out of six families. Biomass turnover rates (annual production/biomass) were generally high, ranging from 8 (single generation per year) to 96 (multiple generations per year) for individual taxa. Distinct types of temporal production patterns occurred: (1) concentration in winter (Ephemerella argo, E. dorothea, Eurylophella sp., and Isonychia), (2) concentration in summer (Baetis ephippiatus, Tricorythodes sp., Caenis spp.) or (3) spread throughout the year and peaking in summer (Baetis intercalaris, Stenonema modestum, S. integrum, S. exiguum, and Hexagenia sp.). Temporal overlap of production, using the proportional similarity index, was highest among closely related taxa (>0.60), with the least overlap (<0.15) among taxa in different families. Overlap based upon the amount that various food types contribute to production was high among all species, suggesting even less resource partitioning for food than for time. Of the annual food consumption (as dry mass) by mayflies per unit snag surface area (423 g°m—2°yr—1), °87% consisted of amorphous detritus that is rapidly replenished on snags from floodplain—derived seston. This food source accounted for °70% of total mayfly production. We suggest that a fluctuating habitat (snags), a continuously replenished food supply, high biomass turnover, and high drift densities help perpetuate a persistent, resilient, and diverse mayfly assemblage with little resource partitioning.

Quantification of submerged wood in a lowland Australian stream system

SUMMARY1. The importance of submerged wood (snags) as macroinvertebrate habitat was evaluated in the Pranjip‐Creightons Creek system, a lowland stream system in northern Victoria. Snag surface area and biomass were measured at ten sites along the system. The first four upstream sites, located in the foothills of the Strathbogie Ranges, and the next three sites, on the northern Victorian riverine plain, were affected by streambank erosion and high sediment loads and contained little instream wood. A further three sites (Sites 8, 9 and 10) downstream on the riverine plain were not as affected by erosion and possessed extensive stands of riparian river redgum, Eucalyptus camaldulensis, which contributed large amounts of wood to the stream channel.2. Wood quantities at Site 8 were less than at Sites 9 and 10 downstream where the density of riparian redgum was greater. At Sites 9 and 10, snag surface area per m2 of stream bed was 0.57–0.92m2 and 0.38–0.71m2 depending on discharge. Total snag biomass in the stream channel at the same sites was 26 and 41kg m−2, respectively. Redgum was important to macroinvertebrates as habitat, at one site contributing 25% of total macroinvertebrate densities and over 30% of total macroinvertebrate biomass m−2 of stream bed.3. Estimations of nitrogen content and C:N ratios of decayed redgum were carried out to provide information on its putative nutritional quality to xylophagous macroinvertebrates. Decayed redgum wood has a comparatively high N content and therefore a low C:N ratio, but appeared to be unpalatable to most macroinvertebrates. Only two macroinvertebrate species, the chironomid larvae Stenochironomus sp. and Dicrotendipes sp., were found to consume decayed redgum.

Life Histories and Abundance Patterns of Snag-Dwelling Mayflies in a Blackwater Coastal Plain River

A diverse mayfly community (28 species) was found in the Ogeechee River, a blackwater river in the Georgia Coastal Plain. Quantitative samples of snag habitat, where most species (22) were found, were collected at 2-4 wk intervals in 1982 to investigate life history characteristics, and at monthly intervals in 1983 to determine consistency of seasonal and annual abundance patterns. Baetidae and Heptageniidae were abundant throughout each year with average densities of 1553-5981 / m² (1982-1983) and 1498-1828 / m² of snag surface, respectively. Field data for the most common species of Baetidae (Baetis ephippiatus, B. intercalaris) and Heptageniidae (Stenonema exiguum, S. integrum, S. modestum) showed no recognizable pattern of cohort development. This developmental asynchrony and long emergence periods for S. exiguum and S. modestum are consistent with previously published growth studies indicating multivoltinism. Ephemerellidae (551-719/m²), Oligoneuriidae (206-469/m²), Tricorythidae (721-1599/m²) and Caenidae (114-110/m²) were also commonly found on snags, but were seasonal in abundance. The major species of Ephemerellidae (Ephemerella argo, E. dorothea, Eurylophella sp.) showed distinct single cohorts developing primarily during winter/spring. Isonychia spp. (Oligoneuriidae) also showed a well-defined winter cohort, but had as many as two weakly defined cohorts during warm months. Caenis spp. (Caenidae) and Tricorythodes sp. (Tricorythidae) occurred primarily in summer months, during which time they were able to pass through multiple generations, as substantiated by either long emergence periods or independent growth studies. Major differences in life histories occurred between rather than within families. Relative abundance and seasonality at the family level were very consistent between years. The high degree of life history overlap in this diverse mayfly assemblage raises questions about the relationship between phenology and community structure, as well as latitudinal patterns of species richness.

Modelling black fly production dynamics in blackwater streams

SUMMARY. 1. Two predictive models were employed along with intensive field sampling to estimate production of black flies (Simulium spp.) on snags (submerged wood) in three blackwater streams on the Georgia Coastal Plain of the southeastern U.S.A. One model predicts daily growth rate from temperature and hydrograph pattern; the other predicts habitat abundance (of snags) from river height.2. In the sixth order Ogeechee River, annual production was twice as high in 1982 (7.1 g dry mass [=DM] m−2 of snag surface) as in 1983 (3.6 g DM m−2). When converted to production per m2 of river bottom, values were 35–40% of the snag surface estimates. Annual production was much lower in fourth order Black Creek (1982, 1.3 g DM m−2 of snag surface) and much higher in the sixth order Satilla River (1975, 15.6–40.0 g DM m−2).3. There was a distinct bimodal pattern of black fly production in the Ogeechee River in both years, with peaks occurring in winter and summer. Similar bimodal patterns of production were found in Black Creek and in the Satilla River. Although there appears to be an intrinsic component to the bimodal pattern, production peaks (growth rate and biomass) appear to be associated with initial stages of flooding.4. Annual production/biomass ratios (37–85) are the highest reported for black fly populations. The variation of annual P/B ratios among sites was more strongly dependent on the temporal distribution of standing stock biomass than on differences in growth rates. Variation in production among sites appears to be due to differences in current velocity, hydro‐graph variability, and abundance of coexisting consumers.

Quantification of Wood Habitat in Subtropical Coastal Plain Streams

To assess the importance of woody debris in two relatively unaltered Coastal Plain streams in the southeastern United States, a line intersect technique, developed by foresters, was used to estimate volume, mass, surface area, and spatial distribution. The ash-free dry mass of in-channei woody material was 6.5 kg/m2 of stream channel bottom in the sixth-order Ogeechee River and 5.0 kg/m2 in the fourth-order Black Creek. Most wood is located near the erosional bank in these meandering streams. These wood mass estimates are much higher than expected for middle-order streams and are similar to those from several small headwater streams in other regions. Due to their very low slopes (<0.02%), these streams appear to have insufficient stream power to move large woody material. Snag, or woody, habitat is the major stable substrate in these sandy-bottomed streams and is a site of high invertebrate diversity and productivity. In-channel snag surface area per square metre of channel bottom was 0.249–0.433 m2 in the Ogeechee and 0.191–0.379 m2 in Black Creek, depending on the hydrograph stage. With invertebrate biomass of 6.6 g dry mass/m2 of snag surface, this results in an invertebrate biomass of at least 1.88 g/m2 of channel bottom. Wood is also important to fishes, providing a rich source of invertebrate food, habitat, and cover. In addition to its obvious biological role, wood enhances the ability of a stream to process and conserve nutrient and energy inputs and has a major influence on the hydrodynamic behavior of the river. The quantification of wood habitat seems mandatory to assess past or potential impacts of snag removal on ecosystem processes in low-gradient streams.

Life cycles, microdistribution and production dynamics of six species of net‐spinning caddisflies in a large southeastern (U.S.A.) river

Hydropsyche incommoda. H. rossi, Cheumalopsyche pasella, Mocronema Carolina. Chimarra moslyi, and Neureclipsis crepuscularis were studied on woody substrates (snags) in the lower Savannah River. Distribution of most species biomass was positively correlated with attached macrophytes on snags at medium (25–50 cm s‐1) current velocities. All species were bivoltine at the study site with the exception of M. Carolina which was univoltine. Annual and cohort production estimates (Hynes and removal‐summation methods) were obtained for each species at three velocity ranges. Total annual caddisfly production calculated by the Hynes method was 12.6, 41.4, and 22.7 g (ash free dry wt m‐2 of snag surface area at low, medium and high velocities, respectively; 9.6, 34.4, and 19.9 g with removal‐summation; and, 11.5, 36.1 and 21.1 g when the Hynes method was applied to discrete cohorts. Higher cohort production was attained by the overwintering generation because of higher terminal instar body weights. Dissolved organic carbon comprised most of the total organic load transported by the river; however, abundant paniculate organic food was also apparently available for filler feeders. Organic and inorganic seston concentrations were highest in winter months and lowest in autumn. Over 90 percent of the seston (by weight) was <105 μm. It is suggested that the variation in catchnet mesh size, coupled with spatial heterogeneity of the snag habitat, allows the caddisflies to partition space, which is apparently the limited resource.