A Quantitative Food Web Model for the Macroinvertebrate Community of a Northern German Lowland Stream

Trophic interactions and cycling of organic carbon within the macroinvertebrate community of a Northern German lowland stream were analyzed based on a compartment model. The network model describes the structure of the food web quantifying biomass, production, and consumption of their elements, of the entire system and between trophic levels. System primary production is 153.7 g C m⁻² yr⁻¹ and invertebrate production 53.3 g C m⁻² yr⁻¹. Invertebrate consumption amounts to 702.6 g C m⁻² yr⁻¹. Main flows are identified between trophic level 1 and 2 and are connected with highly productive compartments. ‘Anodonta and Pseudanodonta’ and Dreissena polymorpha show the highest consumption of all groups with 269.9 g C m⁻² yr⁻¹ and 114.1 g C m⁻² yr⁻¹, respectively. System consumption is highest on the import from the upstream lake with 532.5 g C m⁻² yr⁻¹, sediment detritus with 135.5 g C m⁻² yr⁻¹, and primary producers with 25.7 g C m⁻² yr⁻¹. The lowest predation pressure is observed for Bivalvia with an ecotrophic efficiency of <10% and highest for Chironomidae with 91%. Approximately 20% of organic matter entering the detritus pool are recycled to the living groups of the system. Transfer efficiencies between discrete trophic levels are generally low except for transfer of detrital material between level I and II.     

The effects of predation and detritus on the structure of a stream insect community: a field test

Summary The role of predator density and plant detritus in determining patterns of insect abundance was tested in an Appalachian stream community (Reeds Creek, Pendleton Co, West Virginia). Insect colonization was followed over a 21-day period in field enclosures containing different densities of sculpins (Cottus bairdi and Cottus girardi). Sculpins caused no significant reductions in prey abundance; however, the combined effect of a guild of vertebrate predators caused a significant depression in the abundance of Chironomidae and the stonefly Leuctra. Since Chironomidae comprised approximately 85% of the total benthic fauna, vertebrate predation had an important role in the overall structure of the invertebrate community.Total insect abundance and diversity were correlated to the presence of plant detritus. However, this relationship was taxon specific, as abundance was highly correlated to detritus for only selected insect taxa. In general, abundance was best correlated with coarse particulate organic matter (CPOM), compared to fine particulate organic matter (FPOM) or total detritus (FPOM+CPOM+Whole leaf organic matter (LVOM)).This experiment illustrates that the influence of various potential structuring mechanisms is highly taxon dependent.     

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.     

Macroinvertebrate distribution and production on the floodplains of two lowland headwater streams

SUMMARY.

• 1 Spatial and temporal distribution, abundance and production of the floodplain macroinvertebrate communities of two low-gradient headwater streams in Virginia, U.S.A., were studied over 1 year.
• 2 Spatial and temporal distribution patterns of invertebrates were affected by inundation patterns. Numbers, biomass and production were higher in low than high floodplain areas and on a floodplain continuously inundated over 9 months compared with a floodplain completely flooded only occasionally during storms.
• 3 The predominant species in terms of numbers and production on both floodplains was the harpacticoid copepod Attheyella illinoisensis Forbes. Other species with relatively high production were the sphaeriid clam Pisidium sp., the leptophlebiid mayfly Leptophlebia sp., the isopod Caecidotea racovitzai (Williams), and the chironomids Paratendipes sp. and Polypedilum spp.
• 4 Annual invertebrate production (dry weight) on the floodplain continuously inundated over 9 months was 6.1 g m^?2; production on the periodically inundated floodplain was 1.7 g m^?2. Collector-gatherers accounted for 58–61%, and predators 19–25%, of the production.
• 5 Annual invertebrate production on the floodplains, calculated on the basis of a linear metre of channel length, was 84–490 gm^?1, or about one to two orders of magnitude greater than probable production in the channels, suggesting the significance of floodplain invertebrates to stream system trophic dynamics.

     

Predation and drift of lotic macroinvertebrates during colonization

Summary A field experiment was carried out to determine the effect of an invertebrate predator on the colonization and drift of benthic macroinvertebrates in experimental stream channels. Lotic invertebrates colonized four replicate channels: two controls with no predators, and two channels with low densities (2.8 m^–2) of predatory stonefly nymphs, Doroneuria baumanni (Perlidae). Immigration rates were measured at the inflow of two other channels. Drift rates of invertebrates immigrating to and emigrating from channels were measured daily, and benthic samples were collected every five days. Over a 25-day colonization period, benthic densities of Baetis nymphs and larval Chironomidae were reduced by D. baumanni. Colonization curves were fit with a power function and significantly different colonization rates were indicated for both Baetis and chironomids in predation and control channels. A predator-induced drift response was exhibited by Baetis only and this response was size-dependent. In the presence of D. baumanni, large Baetis drifted more frequently than small nymphs and, correspondingly, small nymphs were more frequent in the benthos. Net predator impacts on invertebrate densities in channel substrates were partitioned into predator-induced drift and prey consumption. These estimates suggest that predator avoidance by Baetis is a prominent mechanism causing density reductions in the presence of predators. Reductions in the density of Chironomidae, however, were attributed to prey consumption only. A rainstorm during the experiment demonstrated that stream flow disruptions can override the influence of predators on benthic invertebrates, at least temporarily, and re-set benthic densities.     

Distribution and production of Chironomidae (Diptera) in the lower course of the Grabia River (Central Poland)

SUMMARY.

• 1 A fifth-order section of the Grabia River was investigated over two years: 1984/85 (low water level) and 1985/86 (high spate). Chironomidae (thirty-nine species) dominated the macrobenthos in both years.
• 2 In 1984/85 (mean annual water temperature 10.2°C), the number of Chironomidae species in coarse sediment was highest in spring (thirty-three species) while in sand it was highest in summer (fifteen species). A decrease in species richness was recorded in autumn and winter at each of the established sites. The estimated Chironomidae production for the whole river section was 5.37 g dry wt m^?2 yr^?1, but it differed widely amongst the study sites, ranging from 3.75 to 12.07 g m^?2 yr^?l.
• 3 In 1984/85 (mean annual temperature 8.1°C) there occurred a summer spate which was more intense than any recorded over the previous 20 years. This substantially reduced the number of species, the greatest reduction being in the coarse sediment, which was buried with sand during the spate. Production for the whole section decreased to 1.17 g m^?2 yr^?1, and the variability amongst sites ranged from 1.04 to 1.45 g m^?2 yr^?1.

     

Export of invertebrates and detritus from fishless headwater streams in southeastern Alaska: implications for downstream salmonid production

1. We examined the export of invertebrates (aquatic and terrestrial) and coarse organic detritus from forested headwaters to aquatic habitats downstream in the coastal mountains of southeast Alaska, U.S.A. Fifty‐two small streams (mean discharge range: 1.2–3.6 L s^?1), representing a geographic range throughout southeast Alaska, were sampled with 250‐μm nets either seasonally (April, July, September) or every 2 weeks throughout the year. Samples were used to assess the potential subsidy of energy from fishless headwaters to downstream systems containing fish. 2. Invertebrates of aquatic and terrestrial origin were both captured, with aquatic taxa making up 65–92% of the total. Baetidae, Chironomidae and Ostracoda were most numerous of the aquatic taxa (34, 16 and 8%, respectively), although Coleoptera (mostly Amphizoidae) contributed the greatest biomass (30%). Mites (Acarina) were the most numerous terrestrial taxon, while terrestrial Coleoptera accounted for most of the terrestrial invertebrate biomass. 3. Invertebrates and detritus were exported from headwaters throughout the year, averaging 163 mg invertebrate dry mass stream^?1 day^?1 and 10.4 g detritus stream^?1 day^?1, respectively. The amount of export was highly variable among streams and seasons (5–6000 individuals stream^?1 day^?1 and <1–22 individuals m^?3 water; <1–286 g detritus stream^?1 day^?1 and <0.1–1.7 g detritus m^?3 water). Delivery of invertebrates from headwaters to habitats with fish was estimated at 0.44 g dry mass m^?2 year^?1. We estimate that every kilometre of salmonid‐bearing stream could receive enough energy (prey and detritus) from fishless headwaters to support 100–2000 young‐of‐the‐year (YOY) salmonids. These results illustrate that headwaters are source areas of aquatic and terrestrial invertebrates and detritus, linking upland ecosystems with habitats lower in the catchment.     

Resource dynamics and detritivore production in an acid stream

1. Life history patterns and production of eight shredder‐detritivore species were studied in relation to the detritus dynamics of a small acidic stream in England. Mean annual detritus inputs (direct and lateral sources combined) were approximately 400 g m^?2 year^?1 and showed significant seasonal and annual variation. 2. Detritus standing stock did not increase significantly during times of high input, reflecting low retention efficiency. However, the mean detritus standing stock was relatively large (108 g m^?2) reflecting a slow decomposition rate typical of acid streams. 3. Four species were univoltine with highly synchronous patterns of emergence and recruitment (Leuctra inermis, Leuctra hippopus, Capnia vidua and Amphinemura sulcicollis). Two species were univoltine with extended patterns of emergence and recruitment (Nemoura cinerea, Potamophylax cingulatus). Leuctra nigra was apparently semivoltine, while Protonemura meyeri showed two successive cohorts in the second year of the study, suggesting either bivoltinism or cohort splitting. 3. Secondary production of the dominant shredders was 1.67 g m^?2 year^?1 in 1997 and 1.99 g m^?2 year^?1 in 1998, which is low compared with other small European streams. This was probably because of an impoverished invertebrate community and poor food quality associated with acid conditions. Food availability probably did not account for the low production as the detritus standing stock far exceeded the estimated shredder ingestion of 42–50 g m^?2 year^?1. 4. Despite low overall shredder production, species‐specific production was high, possibly because of competitive release in this species‐poor acid stream. Periods of high production and growth showed no relationship with detritus availability but were closely related to life history.     

ORGANIC MATTER DYNAMICS IN FOUR SEASONALLY FLOODED FOREST COMMUNITIES OF THE DISMAL SWAMP

Budgets of organic matter dynamics for plant communities of the Great Dismal Swamp were developed to summarize an extensive data base, determine patterns of biomass allocation, transfer and accumulation, and make comparisons with other forested wetlands. Aboveground net primary production on the flooded sites (1,050–1,176 g m^-2 yr^-1) was significantly greater than on a rarely flooded site (831 g m^-2 yr^-1). Estimates of belowground net primary production were comparable to aboveground production on flooded sites (824–1,221 gm^-2 yr^-1). However, productivity was nearly three times greater belowground than aboveground on the rarely flooded site (2,256 g m^-2 yr^-1). Aboveground productivity in Dismal Swamp forests is relatively high compared to other forested wetlands. This is attributed to the timing and periodic nature of flood events. Fine root turnover is shown to be an important source of soil organic matter. Estimates indicate that roots contribute about 60% of the annual increment to soil organic matter. Leaflitter contributes 6–28% and wood debris contributes 5–15%. Comparisons with other forested wetlands suggest that detritus accounts for greater than half of the total organic matter (living + dead) in many wetland systems.     

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.     

Life cycle and production of Chironomidae (Diptera) from Lake Banyoles (NE Spain)

1. The life cycles and annual production of the eight most abundant species of chironomids (Prodadius cf. choreus, Tanypus punctipennis, Chironomus bernensis, Chironomus gr. plumosus, Cladopelma virescens, Microchironomus tencr, Tanytarsus gr. lestagei, and Cladotanytarsus atridorsum) were studied from sublittoral and profundal samples taken monthly in Lake Banyoles during 1987 at five sampling stations (depths ranging from 5 to 20 m). 2. The number of generations per year deduced from instar-frequency data varied from one to four, depending on the species, lake basin and depth. Annual temperature range, dissolved oxygen in the stratified period and presence of sulphide are the key factors that may explain the differences in the number of generations. 3. Production estimates were calculated using the size-frequency (SF) method corrected for the number of generations (SFG), and the increment-summation method (IS) when cohorts could be clearly deduced. 4. Production calculated with the SFG method gave results which were comparable with those of the IS method using smoothed-survivorship curves in the three species for which the use of the IS method was possible (C. virescens, M. tener and C. atridorsum). Using these methods production was estimated to range from 23–70 mg AFDW (ash-free dry weight) m^?2 yr^?1 at 12 m to 74–275 mg AFDW m^?2 yr^?1 in the sublittoral zone of the lake (5-m depth). 5. Calculation of production for the other five species using the size-frequency method with the number of generations (SFG) deduced from monthly instar-frequency data gave values ranging from 12 mg AFDW m^?2 yr^?1 (Chironomus bernensis at 20 m depth) to 3.75 g AFDW m^?2 yr^?1(Prodadius cf. choreus at 12 m). 6. Total chironomid production (with the SFG method) varied from 0.8 to 5 g AFDW m^?2 yr^?1 in the profundal and sublittoral, respectively. At each sampling station two species groups accounted for most of the production: Prodadius cf. choreus and Chironomus spp. Annual production/biomass ratio (P/B) varied from very high values for Prodadius (between 11 and 27, as four generations completed each year) to very low values for Chironomus gr. plumosus (2.20), which completed only one generation each year. The annual production of P. cf. choreus in Lake Banyoles is higher than any reported in the literature due to the completion of four generations and to the high densities of this species.     

Bryophytes as invertebrate habitat in two New Zealand alpine streams

SUMMARY. 1. Quantitative samples of lotic benthic invertebrates and associated organic material were taken monthly for 18 months from bryophyte and gravel habitats in two New Zealand alpine streams in Arthur's Pass National Park, South Island; an unshaded site above the tree-line and a shaded stream flowing through mountain beech forest. This was to assess the ecological role of bryophytes in these habitats and to determine the relative effects on the fauna of algal and detrital resources. 2. Bryophytes were confined to stable bedrock substrata in shallow (mean depth of 4.5 and 10.5 cm), fast water (mean velocity of 0.21 ms^?1 and 0.30ms^?1) at the unshaded and shaded sites. Mean ash-free dry weight (AFDW) of organic material from bryophyte areas was similar at the unshaded and shaded sites (335.6 and 272.4 gm^?2). Low stream-bed stability of the gravel areas resulted in poor retention of organic matter, with average AFDW of 59.1 and 77.9 gm^?2 at the unshaded and shaded sites, respectively. Periphyton biomass, however, was much higher at the unshaded site. 3. Within bryophytes, water velocity is reduced and large quantities of periphyton and detritus accumulate. This explains the greatly enhanced invertebrate densities within bryophytes (218400 and 53500 individuals m^?2), compared with gravel areas (20900 and 7400 individuals m^?2) in the unshaded and shaded sites, respectively. The higher invertebrate densities at the unshaded site reflect greater periphyton biomass there. 4. Invertebrate densities followed clear seasonal patterns, attributable to changes in chironomid densities. These reflected larval chironomid hatching following adult oviposition. This illustrates the importance of these plants as oviposition sites, and as protective nurseries for small larvae where they can obtain shelter during floods and consume the abundant food sources available to them. 5. DECORANA ordination revealed taxonomically distinct invertebrate communities in each stream, and in bryophytes and gravels. Several taxa appeared restricted to either the unshaded site (e.g. Acroperla spiniger and Hydrobiosis silvicola) or the shaded site (e.g. Zelolessica     

Net primary production and net ecosystem production of a boreal black spruce wildfire chronosequence

Net primary production (NPP) was measured in seven black spruce (Picea mariana (Mill.) BSP)‐dominated sites comprising a boreal forest chronosequence near Thompson, Man., Canada. The sites burned between 1998 and 1850, and each contained separate well‐ and poorly drained stands. All components of NPP were measured, most for 3 consecutive years. Total NPP was low (50–100 g C m^?2 yr^?1) immediately after fire, highest 12–20 years after fire (332 and 521 g C m^?2 yr^?1 in the dry and wet stands, respectively) but 50% lower than this in the oldest stands. Tree NPP was highest 37 years after fire but 16–39% lower in older stands, and was dominated by deciduous seedlings in the young stands and by black spruce trees (>85%) in the older stands. The chronosequence was unreplicated but these results were consistent with 14 secondary sites sampled across the landscape. Bryophytes comprised a large percentage of aboveground NPP in the poorly drained stands, while belowground NPP was 0–40% of total NPP. Interannual NPP variability was greater in the youngest stands, the poorly drained stands, and for understory and detritus production. Net ecosystem production (NEP), calculated using heterotrophic soil and woody debris respiration data from previous studies in this chronosequence, implied that the youngest stands were moderate C sources (roughly, 100 g C m^?2 yr^?1), the middle‐aged stands relatively strong sinks (100–300 g C m^?2 yr^?1), and the oldest stands about neutral with respect to the atmosphere. The ecosystem approach employed in this study provided realistic estimates of chronosequence NPP and NEP, demonstrated the profound impact of wildfire on forest–atmosphere C exchange, and emphasized the need to account for soil drainage, bryophyte production, and species succession when modeling boreal forest C fluxes.     

Effects of organic matter availability on the life history and production of a top vertebrate predator (Plethodontidae: Gyrinophilus palleucus) in two cave streams

1. Surface ecosystems provide the primary source of organic matter to many cave communities. Variation in the strength of connectivity to the surface suggests that some caves may be more resource‐limited than others. To test this, we examined diet, prey availability and production of an obligate cave salamander Gyrinophilus palleucus (Plethodontidae), a top predator, in two south‐eastern U.S.A. caves with different levels of organic matter (Tony Sinks cave, 165 g AFDM m^?2; Bluff River cave, 62 g AFDM m^?2). 2. We quantified density, biomass, growth rate, production and diet of G. palleucus monthly for 21 months. Diet composition, differences in prey communities and seasonal patterns in prey consumption were also analysed. 3. Salamander density, biomass and secondary production were significantly greater in the high organic matter cave (0.10 m^?2, 0.18 g AFDM m^?2, 0.12 g AFDM m^?2 year^?1) than in the low organic matter cave (0.03 m^?2, 0.03 g AFDM m^?2, 0.01 g AFDM m^?2 year^?1). Although growth rates were not statistically different between the two cave salamander populations, low recaptures probably influenced this result. 4. Isopoda prey were the major contributor to salamander production in the high organic matter cave (69%). In the low organic matter cave, production was provided by isopods (41%) and oligochaetes (20%). The lower number of prey taxa contributing to salamander production in the high organic matter cave suggests the ability to forage more selectively. 5. The differences in foraging strategy, density, biomass and secondary production were probably related to differences in the strength of surface connectivity, which controls organic matter supply. Links between basal resource level and top predator performance show the importance of bottom‐up limitation in the food webs of caves and other detritus‐based ecosystems.     

Longitudinal patterns of production, food consumption, and seston utilization by net-spinning caddisflies (Trichoptera) in a southern Appalachian stream (USA)

Larval production of ten species of Hydropsychidae and Philipotamidae was studied at six stations along 6.4 km of a southern Appalachian stream, encompassing stream orders 1–4 and a 600 m elevation change. Species-specific production estimates ranged from 23–983 mg AFDM m^?2 yr^?1 These low values are attributed to the paucity of nutrients in these undisturbed headwater streams which reduces detrital food quality, algal growth, and production of smaller invertebrates eaten by hydropsychids. Animal food supported the majority of hydropsychid production (72%); philopotamids relied primarily on fine detritus (80%) and diatoms (15%). The contribution of animal food to caddisfly production decreased downstream, while the relative importance of filamentous algae and diatoms increased. These changes reflect the downstream decline of more carnivorous species, as well as increased primary production which accompanies the shift in lotic community metabolism from heterotrophy towards autotrophy with increasing stream order. Net-spinning caddisflies had a minor impact on seston quantity, consuming only 0.0003%-0.005% of the total seston (including invertebrate drift) passing over a m² of substrate annually. In contrast, the percentage of invertebrate drift consumed was, on the average, > 400 × higher than total seston consumption. These insects influence seston quality rather than quantity. The percentage of total seston and animal drift consumed declined downstream, indicating that turnover lengths of these materials increase with stream order. Longer turnover lengths or “spirals” result from changes in the physical characteristics of the stream, i.e., increasing discharge and stream power and decreasing numbers of retention devices (i.e., organic debris dams), which increase the downstream transport velocity of seston. Higher transport velocities reduce the rates at which these filter feeding caddisflies can process the organic inputs to a given reach of stream. Small streams (orders 1–3) appear to be most responsible for efficient processing of a stream's energy inputs.     

A comparison of Acroneuria lycorias (Plecoptera) production and growth in northern Michigan hard- and soft-water streams

1. Density, biomass, production and growth of a predaceous stonefly, Acroneuria lycorias, were compared between fourth-order hard- and soft-water streams in Michigan's upper peninsula, U.S.A. 2. Mean densities, estimated from Hess samples, were higher (100 ± 17 individuals m^?2) at the hard-water site than at the soft-water site (40 ± 9 ind. m^?2). Mean dry weight biomass was 4.9 times greater at the hard-water site. 3. Mean annual production, calculated using the size frequency method, was 5.0 times greater at the hard-water site (2.18 ± 0.44 g dry weight m^?2yr^?1) than at the soft-water site (0.43 ± 0.02g dry weight m^?2yr^?1). Annual production/mean biomass ratios were similar between sites. 4. Monthly growth rates of naturally occurring nymphs of paired cohorts were similar in both streams. Individual growth rates were similar for nymphs reared in artificial streams at high and low water hardnesses with unlimited food and space. 5. Stonefly production and growth rates were influenced more by indirect physical, biological, or habitat factors than by streamwater cation concentrations.     

The relation between invertebrate drift and two primary controls,discharge and benthic densities,in a large regulated river

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LITTER NUTRIENT CONTENT AND PRODUCTION IN THE GREAT DISMAL SWAMP

Litter production was studied in the Great Dismal Swamp, Virginia in four plant communities which differ primarily in species composition and flooding regime. Greatest leaf deposition occured in the more flooded communities, maple-gum (Acer-Nyssa) with 536 gm^-2 yr^-1 and cypress (Taxodium distichum (L.) Richard) with 528 gm^-2 yr^-1, followed by the cedar (Chamaecyparis thyoides (L.) BSP) and mixed hardwood (Quercus-Acer-Nyssa-Liquidambar) communities, with 506 gm^-2 yr^-1 and 455 gm^-2 yr^-1, respectively. Apparently periodic flooding promotes production. Peaks occurring in October and November corresponded to autumn leaf fall, while peaks in January and May were due to some leaf litter combined with a large amount of woody litter. Litter nutrient concentrations were higher, except for Ca, in the most abundant species (cypress and water gum) in the frequently flooded cypress community. Higher leaf fall rates and litter nutrient concentrations resulted in greater nutrient deposition in the cypress and maple-gum communities.     

A comparative study of above-ground productivity of dominant U.S. Gulf Coast marsh species

Above-ground productivity of dominant freshwater, brackish, and salt-marsh species from the U.S. Gulf Coast was evaluated using both gas exchange techniques and harvest methods. Both techniques showed significant differences in productivity among the study species which represent major components of their respective communities. Estimates of net aerial primary productivity using the harvest method yielded 3683 g dw (dry weight) m^?2yr^?1 for Spartina alterniflora (tall), 2008 g dw m^?2yr^?1 for S. alterniflora (short), 3677 g dw m^?yr^?1 for S. patens and 1641 g dwm^?yr^?1 for Panicum hemitomon. Carbon balance estimated from gas exchange calculation yielded values approximately equivalent to a biomass accumulation of 6024 g dw m^?2yr^?1 for S. alterniflora (tall), 3047 g dw m^?yr^?1 for S. alterniflora (short), 5702 g dw m^?yr^?1 for S. patens, and 2912 g dm^?yr^?1 for P. hemitomon. The net aerial primary production was estimated to be approximately 61% of total productivity in S. alterniflora (tall-form) and 66%o of total productivity in short-form, 64% in S. patens and 56%) in P. hemitomon. The assimilation data also indicated that Spartina alterniflora and S. patens continue carbon fixation throughout the year while assimilation in Panicum hemitomon is absent due to lack of live leaves during the winter. Various aspects of harvest and gas exchange techniques are discussed.     

Production estimates of the dominant taxa of Chironomidae (Diptera) in the modified,River Widawka and the natural,River Grabia,Central Poland

Production and community composition of selected taxa of Chironomidae were estimated in cross-sections of the lower course of two lowland rivers, the Widawka and the Grabia. At the selected site, the Grabia has not changed its morphometry for many years, whereas the discharge of the Widawka has increased (mainly due to inputs of coal mine water) by 75% in comparison with its hypothetical natural discharge. In the Widawka, the production value, for 76.7% of total Chironomidae abundance, amounted to 12.45 g dry wt m^–2 yr^–1, while in the Grabia it amounted to 11.91 g dry wt m^–2 yr^–1 The annual P : B ranged from 14.5 for large-sized species (Chironomus in the Widawka) to 82.0 for small-sized species (Eukiefferiella in the Grabia). The similar production values estimated for both rivers, despite a three times higher density of Chironomidae in the Grabia, is noteworthy.