Benthic Detritus Research Articles

The influence of plant community on chironomid secondary production in two wetland types: cypress-gum swamps and grass-sedge marshes

We sampled Chironomidae larvae (Diptera) diversity, density, biomass, and secondary production within three cypress-gum swamps and three grass-sedge marshes in Southwest Georgia, U.S.A. These wetlands flood seasonally, and were inundated from November 1998 through April 1999. Chironomid diversity was greatest in the grass-sedge marshes, although the same genera dominated both wetland types. Total mean biomass ranged from 449 to 1154 mg AFDM/m 2 in cypress-gum swamps and 268 to 2116 mg AFDM/m 2 in the grass-sedge marshes. Density of chironomids showed similar patterns of variation as biomass. Production ranged from 649 to 2000 mg AFDM/m 2 in cypress-gum swamps and 562 to 4853 mg AFDM/m 2 in grass-sedge marshes over a five month hydroperiod. Additionally, laboratory growth studies using cultured chironomids fed benthic detritus were conducted to further examine the influence of food quality on chironomid growth. The variation in chironomid density, biomass, and production among the wetlands seems to be attributable to an interaction among biotic and abiotic variables including hydroperiod, food quality, and chironomid species assemblage.

Seasonal availability of resources and habitat degradation for the western tree-hole mosquito, Aedes sierrensis.

The nutrient base of aquatic tree-hole communities is derived from leaf litter, benthic detritus, and water flowing down the tree trunk (stemflow water). Previous studies in eastern North America with the mosquito, Aedes triseriatus, have identified leaf litter as a major and stemflow water as a minor source of mosquito nutrition, but did not consider the role of the benthic detritus or how the aggregate or relative contribution of these sources of mosquito nutrition changed during the year. We use the leaf litter, benthic detritus, and stemflow water from tree holes in western Oregon (USA) to determine how these substrates affect mass at metamorphosis, biomass yield, and fitness (cohort replacement rate; R 0) of the mosquito, Aedes sierrensis, through both natural and simulated winters, the normal growing season for larvae in tree holes. We found that fresh leaf litter constitutes the major determinant of mosquito fitness by a factor of >15:1 over any other substrate taken directly from tree holes in nature. The other substrates, including the benthic detritus, individually make only a meager contribution to mosquito fitness but, when added to the leaf litter, can sustain yield and improve fitness at high, limiting larval densities. Nutritional quality of tree-hole substrates declines by >90% from early (fall) to late (spring) in the larval growing season. At both times of year, the coarse or fine detritus provide minor resources, and stemflow water provides no detectable contribution to mosquito nutrition. The resources in the litter are not transported during the year to the benthic detritus; rather, these resources are either exploited by mosquitoes when they first become available, or they deteriorate and become progressively more unavailable to them. Growth and development of A. sierrensis feeding on dried and reconstituted tree-hole contents during a 6-month simulated winter in the laboratory showed: (1) the same relative contributions of leaf litter, benthic detritus, and stemflow water to mosquito nutrition, (2) that the winter deterioration of substrate quality is a direct consequence of microbial decomposition, and (3) that pre-emptive competition from pre-existing A. sierrensis greatly increases substrate deterioration. We conclude that the progressive winter deterioration of larval resources in combination with the dry summers of western North America are the most likely environmental factors that limit species diversity in tree holes and that have selected for early recruitment (autumnal hatching) of A. sierrensis and for its univoltine life cycle from Mexico to Canada.

Seasonal dynamics of macroinvertebrate assemblages in the benthos and associated with detritus packs in two low-order streams with different riparian vegetation

Summary 1 The seasonal dynamics of the benthic macroinvertebrate assemblage, and the subset of this assemblage colonising naturally formed detritus accumulations, was investigated in two streams in south-west Ireland, one draining a conifer plantation (Streamhill West) and the other with deciduous riparian vegetation (Glenfinish). The streams differed in the quantity, quality and diversity of allochthonous detritus and in hydrochemistry, the conifer stream being more acid at high discharge. We expected the macroinvertebrate assemblage colonising detritus to differ in the two streams, due to differences in the diversity and quantity of detrital inputs. 2 Benthic density and taxon richness did not differ between the two streams, but the density of shredders was greater in the conifer stream, where there was a greater mass of benthic detritus. There was a significant positive correlation between shredder density and detritus biomass in both streams over the study period. 3 Detritus packs in the deciduous stream were colonised by a greater number of macroinvertebrates and taxa than in the conifer stream, but packs in both streams had a similar abundance of shredders. The relative abundance of taxa colonising detritus packs was almost always significantly different to that found in the source pool of the benthos. 4 Correspondence analysis illustrated that there were distinct faunal differences between the two streams overall and seasonally within each stream. Differences between the streams were related to species tolerances to acid episodes in the conifer stream. Canonical correspondence analysis demonstrated a distinct seasonal pattern in the detrital composition of the packs and a corresponding seasonal pattern in the structure of the detritus pack macroinvertebrate assemblage. 5 Within-stream seasonal variation both in benthic and detritus pack assemblages and in detrital inputs was of similar magnitude to the between-stream variation. The conifer stream received less and poorer quality detritus than the deciduous stream, yet it retained more detritus and had more shredders in the benthos. This apparent contradiction may be explained by the influence of hydrochemistry (during spate events) on the shredder assemblage, by differences in riparian vegetation between the two streams, and possibly by the ability of some taxa to exhibit more generalist feeding habits and thus supplement their diets in the absence of high quality detritus.

Effects of trophic status and wetland morphology, hydroperiod, and water chemistry on mercury concentrations in fish

We sampled fish and selected water chemistry variables (dissolved organic carbon, sulfate, and pH) in nine southeastern depression wetlands to determine relationships among wetland morphology (surface area and maximum depth), hydrology, water chemistry, and bioaccumulation of mercury (Hg) in fishes. We concentrated on three fish species representing the range of trophic levels occupied by fish in southeastern depression wetlands. Whole-body Hg concentrations were lowest in lake chubsucker (Erimyzon sucetta), a benthic detritivore, and highest in redfin pickerel (Esox americanus americanus), a top carnivore. However, variation in Hg concentrations among wetlands was greater than variation among species. Regression analyses indicated that maximum depth and hydroperiod accounted for significant portions of variation among wetlands in standardized lake chubsucker and redfin pickerel Hg concentrations. Maximum depth and dissolved organic carbon had a negative effect on standardized Hg concentrations in mud sunfish (Acantharchus pomotis). Path analysis confirmed the results of regression analyses, with maximum depth and hydroperiod having relatively large direct negative effects on Hg concentrations. Our results suggest that leaching of Hg from sediments during the drying and reflooding cycle and binding of Hg species by dissolved organic carbon in the water column are primary factors controlling the bioavailability of Hg in southeastern depression wetlands.

Influence of large woody debris on stream insect communities and benthic detritus

We examined the extent to which benthic detritus loadings and the functional feeding group structure of stream insect communities respond to channel modifications produced by experimental addition of large woody debris (LWD, entire logs) to Stony Creek, Virginia. Benthic detritus loadings per sample did not change after LWD additions, but large increases in pool habitats created by LWD increased net detritus by an estimated 27 kg (25%) in the 250 m of stream receiving LWD. A large increase in the proportional area of pool habitats may result in a dominance of collector-gatherers and corresponding decreases in shredders and scrapers. Functional feeding group community structure in pools was similar spatially and temporally. Riffles were spatially convergent, but differed temporally. Community structure was significantly different between pools and riffles. The results indicate possible large scale influences in overall community structure due to channel alterations by LWD, but little within-habitat change.

Mapping the intra-habitat variation of leaf mass loss rate in a brackish Mediterranean lake

To study patterns of spatio-temporal variation of both leaf breakdown and interactions between leaf-associated detritivores and fungi, we mapped the variation of the mass loss rate of Phragmites australis in a brackish Mediterranean lake. We carried out the study at 20 sampling points randomly chosen on a 400 m 2 grid on the bottom of Lake Alimini, using litterbags which con- tained reed leaves. The mass loss from litterbags after 30 d, leaf-colonizing animals, and the micro- bial activity on the reed litter were assessed seasonally over a year in each sampling site. Half of the litterbags were protected from animal colonisation and used to determine the mass loss due solely to microbial activity and abiotic factors. The benthic detritivores colonizing the remaining litterbags were collected and sorted by their functional group. Microbial respiration on protected and unpro- tected litterbags and the sediment organic content were measured at each sampling site and related to litter mass loss. Comparisons of mass loss and microbial oxygen consumption between protected and unprotected litterbags indicated that leaf processing was affected by the grazing activity of detri- tivores on microflora. Hydrological conditions and the potential litter distribution influenced detriti- vore occurrence which, in turn, controlled the litter processing. The observed between-site variance in the reed mass loss showed that, beside the rate, spatial variation is an important process feature in the habitat description. Geostatistics based on either the average values or the temporal variation of the mass loss rate provided 2 different, statistically validated lake topographies. However the latter was more convenient for outlining site differences within the lake. The extent of reed cover along the shore, as well as variable freshwater and nutrient inputs, determined the intra-habitat variability. We suggest an index (RL), integrating 3 critical factors for the allochthonous detritus processing (i.e. dis- tance, sinuosity, and plant cover of the shore), as an indirect estimate of the rate of detritus mass loss. This can facilitate the approach to the functional geography of aquatic ecosystems.

On the relative importance of pool morphology and woody debris to distributions of shrimp in a Puerto Rican headwater stream

In this paper, we report the sizes and distributional orientation of woody debris in a headwater rainforest stream in the Luquillo Experimental Forest (LEF), Puerto Rico. We also provide results of a 4-month study of a wood addition experiment designed to increase cover for benthic macroinvertebrates (freshwater shrimp). We added branch-sized woody debris to 20 pools in three streams. We trapped four species of freshwater shrimp (two species of benthic detritivores and two predatory shrimp species) during each of the 4 months following wood additions. An analysis of pool morphology (maximum depth, surface area and volume) provided a useful predictor of shrimp abundances. In general, numbers of shrimps increased with sizes of stream pools. A repeated measures ANOVA demonstrated no effect of woody debris additions on total numbers of shrimp per pool area. Two detritivore species (Atya lanipes, a filter feeder and Xiphocaris elongata, a shredder) decreased in abundance with increased woody debris and there was no statistical relationship between woody debris additions and predators ( Macrobrachium carcinus and M. crenulatum). Small woody debris additions may have altered flow velocities that were important to filter-feeding Atya at the microhabitat scale, although the overall velocities within pools were not altered by wood additions. Lower numbers of Atya and Xiphocaris in two of the three streams may result from the occurrence of two predaceous fishes (American eel and mountain mullet) and more predatory Macrobrachium in these streams. One likely interpretation of the results of this study is that the stream pools in these study reaches had sufficient habitat structure provided by numerous rock crevices (among large rocks and boulders) to provide refuge from predators. Addition of woody debris did not add significantly to the existing structure. These results may not apply to stream channels with sand and gravel substrata where crevices and undercut banks are lacking and where woody debris often plays a major role by providing structure and refuge.

Sediment resuspension rates, organic matter quality and food utilization by sea scallops (<I>Placopecten magellanicus</I>) on Georges Bank

Benthic detritus, bacteria, and settled phytoplankton are transported into the water column by resuspension, potentially providing a high quality food source to suspension feeders. Two aspects of resuspension must be considered in relation to food supplies for suspension feeders: the flux of particles from the sediments to the water column and its food value. Sediment resuspension rates on Georges Bank and the role of resuspended sediment in the diet of sea scallops (Placopecten magellanicus) were determined in laboratory flume experiments and shipboard feeding experiments, respectively. Resuspended carbon flux was estimated from flume bedload transport rates and the mass of organic carbon associated with the silt-clay fraction eroded from Georges Bank sediment during transport. A comparison of sand erosion thresholds with the frequency distribution of shear velocity estimated from field current meters indicated that tidal sediment resuspension will occur 62% of the time. Resuspended material had a carbon content of 4-8% and a C:N of 5-8. Rates of resuspension (33-229 mg C m -2 h -1 ) and settling rates indicate that resuspended sediment in a size range available to scallops (>5 pm) remains in suspension for periods of hours to days. Clearance rates of resuspended sediment by scallops were similar to those for water column particles, and filtration rates increased with increasing concentrations of resuspended material. Feeding experiments demonstrated that scallops absorbed organic matter from resuspended sediments with an efficiency of up to 40%. Therefore, in terms of particle retention, ingestion, and digestion, sea scallops are able to exploit resuspended organic matter from a continental shelf habitat. Furthermore, resuspension occurs with sufficient frequency, and resuspended sediment has long enough residence time in the water column to provide a consistent nutritional benefit to scallops.

Benthic Organic Matter Storage in Streams: Influence of Detrital Import and Export, Retention Mechanisms, and Climate

In lotic ecosystems, BOM is a major energy source for secondary production (Minshall 1967, Benke et al. 1984), influences nutrient cycles (Mulholland et al. 1985), and affects export of DOM and POM (Bilby and Likens 1980, Bilby 1981, Smock et al. 1989). Benthic detritus also influences channel stability and retention characteristics (Keller and Swanson 1979, Mosley 1981, Webster et al. 1994) and provides habitat for stream microorganisms, macroinvertebrates (Benke et al. 1984, Huryn and Wallace 1987), and fish (Angermeirer and Karr 1984, Elliott 1986). However, in spite of the great importance of BOM to stream ecosystem function, benthic detrital storage is one of the most poorly understood components of stream organic matter budgets (Cummins et al. 1983). Storage of BOM is governed by a broad spectrum of processes. At the scale of the river continuum, BOM tends to decrease downstream as channels become larger and riparian influences decline (Naiman and Sedell 1979, Minshall et al. 1983, Conners and Naiman 1984, Naiman et al. 1987). Individual reaches, however, are influenced not only by their position along a river continuum but also by local variations such as the character of riparian vegetation (Minshall et al. 1983, Gurtz et al. 1988) and floodplain size. Within reaches, BOM distribution is further affected by retention mechanisms, including debris dams and pools (Huryn and Wallace 1987, Smock et al. 1989, Trotter 1990, Jones and Smock 1991), channel characteristics like gradient, and interactions between main-channels and flood-

Origin and trophic importance of detritus-evidence from stable isotopes in the benthos of a small, temperate estuary.

Ratios of 13C/12C and 15N/14N were measured in dissolved inorganic carbon (DIC), marginal vegetation, benthic macrodetritus (diameter > 1 mm) and selected invertebrate consumers in the Gamtoos estuary, South Africa to: (1) trace the provenance of benthic detrital deposits, and (2) determine the extent to which three abundant species of macroinvertebrates utilise this resource. DIC was strongly depleted in 13C with average δ13C values (-9.5±0.5‰) being typical of limnetic waters. Benthic detrital particles (δ13C-24.1±0.3‰) originated mainly from marginal vegetation (δ13C-25.7±0.3‰), but their slightly elevated carbon ratio suggests additional input from 13C-rich sources-possibly C4 plants cultivated on the floodplain. Populations of the fossorial ghost shrimp Callianassa kraussi, the bentho-pelagic amphipod Grandidierella lignorum and the epifaunal crab Hymenosoma robiculare together account for 96% of total benthic biomass in the upper regions of this estuary. Marked differences in trophic niches were evident among these three consumer species. Ghost shrimp (δ13C -32.5±0.3‰) foraged by filter-feeding on fine suspended particulate organic matter (δ13C-31.2±0.5‰). Amphipods (δ13C-28.0±0.6‰) utilised some benthic detritus but fed mainly on suspended material. Only the relatively rare crabs (δ13C-23.8±1.5‰) appeared to utilise benthic detrital particles to any significant extent. In the benthic consumer community of the upper Gamtoos estuary, suspension feeders make up 98% of biomass and thus clearly dominate over deposit feeders. This can be traced to the low contribution of higher plants (c. 13%) to overall carbon production, and detritus originating from macrophytes is consequently relatively unimportant in supporting invertebrate secondary production in this particular system.

Do stream invertebrates accumulate aluminium at low pH conditions?

Acidified surface waters often show elevated aluminium (Al) levels, detrimental to fish and some invertebrates. Whether Al can accumulate in benthic invertebrates, with time and/or along the food chain, is not clear. To test this, benthic invertebrates, representing different functional feeding groups, were collected in spring from streams, with different acidity and Al concentrations. Weight-specific Al content was determined with an AAS. At localities with pH ≈ 4, high Al contents (≈ 1 mg inorg-Al g−1 af dw) were found in shredders and/or deposit feeders (Asellus aquaticus, Nemoura sp., and limnephilids), while the predator Isoperla grammatica contained only ≈ 0.3 mg Al g−1, and the “filtering predator” Plectrocnemia conspersa almost no Al. Also at pH ≈ 6 Nemoura sp. and limnephilids showed significantly higher Al contents than did the predators Isoperla grammatica and Rhyacophila nubila, Al concentrations of the animals were often higher at pH 4 than at pH 6. Thus, no evidence of any food chain accumulation (or biomagnification) of Al could be validated. Accordingly, this study gives no support that the high concentrations of Al in fish and birds are due to their feeding on benthic invertebrates at low pH conditions. It was also found that animals that inhabit and/or consume benthic detritus as food contain highest Al levels.

Influence of Season, Size, and Collection Site on Surface Area of Fine Particular Matter in Streams

Benthic detritus was collected from an unnamed stream in Holly Springs Experimental Forest, Mississippi, and from King's Creek. Kansas. Detritus was wet-sieved into five size classes from 1 mm down to 0.075 mm. Water and particles passing through the 0.075-mm sieve were vacuum-filtered through 0.45-μm membrane filters (Millipore); particles retained on these filters represent the sixth size class. Scanning electron microscopy revealed fine particulate organic matter (FPOM), fine particulate inorganic matter (FPIM), and fine particulate matter (FPM), i.e., aggregates of organic and inorganic particles. Many of these particles fell within the expected size ranges; however, we also detected numerous particles too small to have been retained by the sieves. Laboratory techniques were developed to separate the FPOM from the FPIM. Using nitrogen-adsorption techniques, surface-area measurements were made on the separated fractions as well as on samples that had been only wet-sieved. Comparisons of mean mass-specific surface areas (m²/g) revealed significantly lower values when inorganic material was left in the sample (FPM) or when the sample contained only inorganic material (FPIM). King's Creek samples had much greater mean surface area per unit mass than did Holly springs samples. When location and particle size class were considered together, location explained most (85%) of the variation in surface area. When season and size class were analyzed together, most of the variation (61%) was explained by size class. Organic content, size class, season of collection, and collection site are all important influences on FPM surface area and should be considered in studies of surface-related phenomena.

What sources of organic carbon drive food webs in billabongs? A study based on stable isotope analysis.

We used the stable isotopes of carbon and nitrogen to examine the food webs of three small flood-plain lakes (billabongs) in south-eastern Australia. With few exceptions, stable carbon isotope analysis could not be used to discriminate among the conspicuous potential sources of fringing, emergent or floating vegetation or benthic detritus. These primary sources showed little spatial or temporal variation in δ13C values, with means ranging from-28.5 to-26.8‰ in spring and-29.1 to-25.4‰ in late summer. Submerged vegetation had similar δ13C values to the above sources in spring but showed greater spatial variation and were less 13C-depleted, considerably so in some species, in late summer. Epiphytes and algae were 13C-depleted in spring compared with the other primary sources but became more 13C-enriched in late summer. Mean δ13C values for primary and secondary consumers were not only far more variable (-37.4 to-22.7‰) but in general were more negative than the potential food sources, particularly in spring. Using the combined information from stable carbon and nitrogen isotope analysis, we could narrow down the list of potential primary sources driving food webs in these billabongs. The freshwater crayfish (Cherax) was one of the few taxa that appeared to obtain its biomass carbon from detrital material. Gastropods and leptocerid caddis larvae on emergent or submerged vegetation obtained a mixture of carbon from epiphytes and macrophytes; in both taxa, epiphytes contributed more to biomass carbon than did the macrophytes. However, other common grazers and collector/gatherers sampled from macrophytes, e.g. baetid mayflies, chironomid larvae and atyid shrimps, were often too 13C-depleted even to have derived their biomass carbon solely from epiphytes. Many other primary consumers, including zooplankton, and mussels (Velesunio), and most of the secondary consumers, including water mites (Hydracarina), phantom midge larvae (Chaoborus) and fish, were also 13C-depleted. The enormous biomass of littoral and fringing vegetation could contribute to metazoan food webs in these billabongs only if an additional highly 13C-depleted source was consumed simultaneously. Methane released from billabong sediments could provide such a 13C-depleted carbon source that is re-introduced into metazoan food webs via the consumption of methanotrophic bacteria. Alternatively, food webs in these water bodies are largely driven by an unknown and inconspicuous 13C-depleted primary producer, such as planktonic Chlorophyta.

Benthic organic matter and detritivorous macroinvertebrates in two intermittent streams in south-eastern Australia

Temporal changes in coarse (> 1 mm), fine ( 250 μm), and woody benthic organic matter (BOM), and densities of detritivores in pools and riffles were monitored at three sites on two intermittent streams (Werribee and Lerderderg Rivers) in Victoria, Australia during a drought year followed by a wetter year. Standing stocks of BOM peaked in both habitats during summer when discharge ceased and eucalypt leaf fall was greatest. During high winter and spring discharges, concentrations of BOM were low. Floods did not always scour BOM from the pools and riffles; after floods in October, standing stocks rose when BOM was imported from upstream or mobilized from the riparian zone. Densities of benthic detritivores, collector-gatherers, and shredders also varied seasonally, usually peaking in summer. Correlations between detritivore feeding group densities and amounts of putative food resource were habitat-specific. There were few significant correlations in depositional habitats, even after incorporating lag-times of two and four weeks into the analysis. However, detritivore densities in riffles, especially in the Lerderderg River, were strongly positively correlated with the amounts of BOM. Possibly, physico-chemical conditions in riffles are more conducive to litter conditioning and invertebrate colonization and breakdown of leaf material than are those in pools.

Benthic predators and zooplanktonic prey: predation by Crangonyx shoemakeri (Crustacea; Amphipoda) on Daphnia obtusa (Crustacea; Cladocera)

Amphipods and cladocerans are common inhabitants of seasonal ponds throughout North America. Although amphipods are usually considered to be benthic detritivores, Crangonyx shoemakeri was found to capture and consume mosquito larvae and Daphnia obtusa. The rate of capture of Daphnia was independent of the amount of decomposing leaf matter available. However, this rate was dependent on prey density and the presence of light, indicating that these amphipods are visual predators.

The lethal and sublethal responses ofgammarus pulexto stress: Sensitivity and sources of variation in an in situ bioassay

The feeding rate and mortality of the freshwater amphipod Gammarus pulex L., a common benthic detritivore, were measured during four trials of an in situ bioassay. The Gammarus pulex were deployed in a factorial experiment designed to assess the effects of water quality (three stations), test population (two sources), and test operator (two laboratories). Stations were selected above and below a domestic sewage treatment works, a sewage treatment works processing domestic and industrial waste, a quarry, and a pulp mill. Feeding rate appeared to have potential as an indicator of water quality, but the source of the test population and the laboratory performing the measurements both significantly influenced the results.

Intrabiome Distributional Patterns of Lotic Macroinvertebrate Assemblages

Macroinvertebrates were collected at five sites along each of three rivers within the eastern deciduous forest biome in spring, summer, and autumn. Macroinvertebrate density at sites in mixed land use areas was intermediate between high levels at sites in farmlands and low levels in forests. Cluster analysis using mean density of the 24 dominant taxa revealed three groupings of river sites each of which was consistently associated with a land use type. Clusters were poorly associated with season of collection, site location along rivers, and with specific river drainage. Hydropsychidae and Leptophlebiidae characterized forested sites. The assemblage associated with mixed Sand use areas included Psephenidae, Baetidae, Tricorythidae, and Heptageniidae. Farmland river sites were characterized by Ceratopogonidae, Corixidae, Oligochaeta, and Tricladida. The mixed land use group of river sites was faunistically more similar to sites in forested than in farmland areas. Forested sites were fast-flowing and had little benthic detritus; farmland sites were slow-flowing and had high levels of detritus. The biome dependency hypothesis, which predicts that similar assemblages of macroinvertebrates are likely to occur at river sites both within and among drainages if the basins occupy the same biome, was not largely supported because characteristic vegetation had been disturbed.

Effect of stress on a freshwater benthic detritivore: Scope for growth in Gammarus pulex

Scope for growth (SfG), a measure of energy balance (between food intake and metabolic output) within animals, has been used as an indicator of pollution stress in marine systems. However, it has not been used commonly in freshwater systems and here we investigate the sensitivity of SfG in Gammarus pulex, a benthic freshwater crustacean, under conditions often associated with pollution. The effects of four specific substances were investigated; a metal (zinc), an organic (3,4-dichloroaniline), and two dissolved gases (oxygen and ammonia). In all cases SfG was reduced by the stress, primarily due to a depression in energy intake. Only with ammonia was energy output (respiration) significantly affected.

The evolutionary importance of mechanoreception in three erpobdellid leech species.

The ability of an erpobdellid leech, Erpobdella montezuma, to discriminate between two congeneric amphipod prey (Hyalella montezuma and Hyalella azteca) through mechanoreception was examined. Both E. montezuma and H. montezuma, a pelagic filter feeder, are endemic to the near thermally constant environment of Montezuma Well, Arizona, and form a highly specialized predator-prey relationship. In contrast, H. azteca, a benthic detritivore, is widely distributed throughout North America. Erpobdella montezuma was highly responsive to the prey signals of the endemic H. montezuma and showed only a limited response to H. azteca. An inverse relationship occurred between the frequency of attack and size of the leech. The abilities of Erpobdella punctata and Nephelopsis obscura, neither found in Montezuma Well, to detect the signals of both amphipod prey were also examined. These species with non-specialized diets showed a low response to the signals of both prey species. This study demonstrates that very close predator-prey relationships can develop over evolutionary time in isolated aquatic systems through the use of mechanoreception.

Microbial and Animal Processing of Detritus in a Woodland Stream

The detritus standing crop, microbial respiration, and macroinvertebrate biomass were examined in monthly samples from the riffle sections of a first—order woodland stream. Total detritus was remarkably constant; the average (with 95% CL) ash—free dry mass standing crop was 426.4 ± 85.9 g/m2 over the 14 mo of the study. Throughout the year benthic detritus was dominated by fine particulate detritus (<1 mm), which made up 68.9% of the total ash—free dry mass. Woody debris made up 8%, whole leaves 3.5%, and leaf fragments and other coarse particulate detritus accounted for 19.7% of the total standing crop. Decreases in standing crop were attributable to microbial respiration, macroinvertebrate assimilation, and downstream export. Microbial respiration annually removed 150% of the average standing crop, with the major effect on the smallest particle size category. Macroinvertebrate assimilation, defined as the sum of respiration and growth, removed 11.6% of the detritus standing crop annually. Shredders accounted for 20% of total animal assimilation, with the remaining 80% attributable to collectors and grazers. Based on monthly changes, it appears that total detritus standing crop is the result of the past discharge regime, which determines the overall amount of detritus present, and the rate of biological (microbial and invertebrate) processes, which determine the size and quality of the detritus particles. This suggests that detritus in streams, while strongly affected by both biotic and abiotic factors, may be in equilibrium within physical and biological constraints such that an annual steady—state system exists, similar to that for soil systems.