Snag Habitat Research Articles

Environmental Determinates of Distribution for Dragonfly Nymphs (Odonata: Anisoptera) in Urban and Non-Urban East Texas Streams, USA

We collected environmental and habitat data for nymphs of 12 dragonfly species (Odonata: Anisoptera) from 91 stream sites throughout eastern Texas, including urban and non-urban locations. Understanding the relationship of dragonflies to habitat structure and other environmental variables is crucial for the purpose of conserving these insects and better using them as predictive tools for water quality assessments, and refining tolerance values. The objectives of this study were to determine the key environmental variables influencing the diversity and distribution of dragonflies in eastern Texas streams, and further determine if differences in those factors could be observed between urban and nonurban sites. We collected samples separately from benthic habitats and woody snag habitats. Significantly fewer sites were observed to have dragonfly species on snag habitat (mean = 1.25) compared to benthic samples (mean = 14.67) (t-test, p = 0.001). The number of dragonfly species collected among non-urban streams (mean = 9.83) was not significantly different than urban streams (mean = 6.08; t-test, p = 0.07). Detrended correspondence analysis of benthic and snag habitat data collected from non-urban and urban locations showed that most of the species are oriented most closely to benthic habitats in non-urban streams. Snag habitat was shown to be poorly ordinated for all of the species. A canonical correspondence analysis of 29 water quality and habitat variables as environmental determinants of dragonfly diversity and distribution showed that distributional relationships among species are complex and often described by multiple environmental factors.

Environmental Determinates of Stream Caddisfly (Trichoptera) Diversity in Eastern Texas, USA

We assessed Trichoptera diversity for 91 wadeable stream sites in eastern Texas, and caddisflies were found at 86 sites. Caddisfly larvae were collected separately from benthic samples and from snag habitat (woody debris). We collected 13 families and 28 genera from benthic samples and 9 families and 21 genera of Trichoptera from snag habitats, and there was broad overlap among taxa between the two habitat types. A significantly greater number of genera were collected from non-urban benthic samples (mean=4.29, range=1–11) than urban ones (mean=2.6, range 1–7) (t-testα=0.05, t=3.57, P=0.0006) (Fig. 3A). We also found significantly fewer genera on snag habitats (mean=2.12) in urban streams compared to non-urban streams (mean=3.48) (t-testα=0.05, t=4.14, p=0.0001). We used correspondence analysis to evaluate data from benthic and snag habitats in nonurban and urban locations. Several genera were more commonly associated with urban streams while other genera were more commonly associated with non-urban streams. Finally, we used Canonical Correspondence Analysis to evaluate more than two dozen water quality and habitat variables as environmental determinates of caddisfly diversity and distribution, and this analysis showed that the distributional relationships among genera are complex and often a function of multiple environmental factors.

Comparison of Macroinvertebrate‐Derived Stream Quality Metrics Between Snag and Riffle Habitats1

Abstract: We compared benthic macroinvertebrate assemblage structure at snag and riffle habitats in 43 Wisconsin streams across a range of watershed urbanization using a variety of stream quality metrics. Discriminant analysis indicated that dominant taxa at riffles and snags differed; Hydropsychid caddisflies (Hydropsyche betteni and Cheumatopsyche spp.) and elmid beetles (Optioservus spp. and Stenemlis spp.) typified riffles, whereas isopods (Asellus intermedius) and amphipods (Hyalella azteca and Gammarus pseudolimnaeus) predominated in snags. Analysis of covariance indicated that samples from snag and riffle habitats differed significantly in their response to the urbanization gradient for the Hilsenhoff biotic index (BI), Shannon’s diversity index, and percent of filterers, shredders, and pollution intolerant Ephemeroptera, Plecoptera, and Trichoptera (EPT) at each stream site (p ≤ 0.10). These differences suggest that although macroinvertebrate assemblages present in either habitat type are sensitive to detecting the effects of urbanization, metrics derived from different habitats should not be intermixed when assessing stream quality through biomonitoring. This can be a limitation to resource managers who wish to compare water quality among streams where the same habitat type is not available at all stream locations, or where a specific habitat type (i.e., a riffle) is required to determine a metric value (i.e., BI). To account for differences in stream quality at sites lacking riffle habitat, snag‐derived metric values can be adjusted based on those obtained from riffles that have been exposed to the same level of urbanization. Comparison of nonlinear regression equations that related stream quality metric values from the two habitat types to percent watershed urbanization indicated that snag habitats had on average 30.2 fewer percent EPT individuals, a lower diversity index value than riffles, and a BI value of 0.29 greater than riffles.

Temporal dynamics of snags and development of snag habitats in wet spruce–fir stands in east-central British Columbia

Patterns of tree mortality, rates and type of tree and snag fall, and relationships between snag characteristics and potential wildlife habitat value were estimated for hybrid spruce ( Picea glauca (Moench) Voss × engelmannii Parry ex Engelm.) and subalpine fir ( Abies lasiocarpa (Hook.) Nutt.) in east-central British Columbia in order to provide important parameters for deadwood modelling. We sampled 172 snags (52 spruce and 120 fir) for species, size, morphological, and habitat attributes, and used dendroecological techniques on a subsample of these ( n = 158) to estimate year of death. Input of snags appeared to occur at a nearly constant rate in these stands. Estimated annual tree fall, including live trees and snags, was 5.3% for hybrid spruce and 6.1% for subalpine fir and stem breakage was more frequent than uprooting. Long-term annual snag fall rates were 4.6% for hybrid spruce and 2.9% for subalpine fir. Discriminant analysis based on time since death correctly classified snags into three decay classes for 85% and 72% of spruce and fir, respectively. Snags that potentially could provide important functions for wildlife habitat were more prevalent in fresh and intermediate classes for hybrid spruce and in intermediate and old classes for subalpine fir. The results provide valuable parameters for further development of deadwood models, which are an important tool for development of best practices for deadwood management.

Influence of Riffle and Snag Habitat Specific Sampling on Stream Macroinvertebrate Assemblage Measures in Bioassessment

Stream macroinvertebrate communities vary naturally among types of habitats where they are sampled, which affects the results of environmental assessment. We analyzed macroinvertebrates collected from riffle and snag habitats to evaluate influences of habitat-specific sampling on taxon occurrence, assemblage measures, and biotic indices. We found considerably more macroinvertebrate taxa unique to snags (143 taxa) than to riffles (75 taxa), and the numbers of taxa found in both riffles and snags (149 taxa) were similar to that found in snags. About 64% of the 47 macroinvertebrate measures we tested differed significantly between riffles and snags. Eighty percent intercepts of regressions between biotic indices and urban or agricultural land uses differed significantly between riffles and snags. The Hilsenhoff biotic index calculated from snag samples explained 69% of the variance of riffle samples and classified 66% of the sites into the same stream health group as the riffle samples. However, four multimetric indices for snag samples explained less than 50% of the variance of riffle samples and classified less than 50% of the sites into the same health group as the riffle samples. We concluded that macroinvertebrate indices developed for riffle/run habitat should not be used for snag samples to assess stream impairment. We recommend developing an index of biotic integrity specifically for snags and using snags as an alternate sampling substrate for streams that naturally lack riffles.

INFLUENCES OF WATERSHED URBANIZATION AND INSTREAM HABITAT ON MACROINVERTEBRATES IN COLD WATER STREAMS1

ABSTRACT: We analyzed data from riffle and snag habitats for 39 small cold water streams with different levels of watershed urbanization in Wisconsin and Minnesota to evaluate the influences of urban land use and instream habitat on macroinvertebrate communities. Multivariate analysis indicated that stream temperature and amount of urban land use in the watersheds were the most influential factors determining macroinvertebrate assemblages. The amount of watershed urbanization was nonlinearly and negatively correlated with percentages of Ephemeroptera‐Plecoptera‐Trichoptera (EPT) abundance, EPT taxa, filterers, and scrapers and positively correlated with Hilsenhoff biotic index. High quality macroinvertebrate index values were possible if effective imperviousness was less than 7 percent of the watershed area. Beyond this level of imperviousness, index values tended to be consistently poor. Land uses in the riparian area were equal or more influential relative to land use elsewhere in the watershed, although riparian area consisted of only a small portion of the entire watershed area. Our study implies that it is extremely important to restrict watershed impervious land use and protect stream riparian areas for reducing human degradation on stream quality in low level urbanizing watersheds. Stream temperature may be one of the major factors through which human activities degrade cold‐water streams, and management efforts that can maintain a natural thermal regime will help preserve stream quality.

Macroinvertebrate Assemblages of Submerged Woody Debris in the Elm Fork of the Trinity River, Texas

ABSTRACT We studied the taxonomic composition and spatial and temporal variability of macroinvertebrates inhabiting coarse woody debris in an 8 km segment of the Elm Fork, a low gradient prairie river in Texas. Seventy-seven macroinvertebrate taxa were collected from submerged wood habitats compared with 38 taxa collected from soft benthic substrata. Assemblages were highly variable spatially and temporally, particularly among seasons. An ordination of species and environmental variables indicated some trends in the longitudinal distribution of flow-dependent species but overall did not find coarse woody debris-inhabiting taxa to have strong associations with many of the selected environmental variables. Significant seasonal differences occurred in the abundances of the Chironomidae (Diptera) with Orthocladiini dominating in the winter and Chironomini and Tanytarsini in the summer and autumn. Data gathered after an extended high flow period in the spring indicated a greater resilience of snag habitats than benthic substrata, with rapid re-colonization of coarse woody debris by macroinvertebrates following the high flow episodes.

Cavities in Snags along a Wildfire Chronosequence in Eastern Washington

We sampled occurrence of bird-excavated cavities in snags in a chronosequence of 26 wildfire burns (ages 1-81 years) on the east slope of the Washington Cascade Range, USA. Cavities occurred in 5.5% of the 1,867 recorded snags; most (69%) were in burns 250) of subalpine fir (Abies lasiocarpa; 0.2%) and lodgepole pine (Pinus contorta; 0.0%) snags. Cavities occurred in about 4% of the small samples (n 34 cm diameter at breast height [dbh]) snags >2 m tall and located in middle-age to older burns (>19 years old). Cross-validated accuracy of the classification tree model was 97% for cavity snags and 82% for snags without cavities. Cavity-bearing Douglas-fir snags were best characterized as large-diameter snags (>33 cm dbh), or as smaller soft snags (Class 4, 5) at elevations <1,200 m. Accuracy of the classification tree model was 88% and 73% for snags with and without cavities, respectively. In burns <20 years old, Douglas-fir snags with broken tops had higher cavity excavation rates (6%) than snags with whole tops (0.25%). Aspect, slope, slope position, and elevation had negligible or no value for classification. To ensure good post-fire snag habitat, manage pre-fire green stands for tree species, large size, defect (e.g., broken tops), and spatial pattern that will provide cavity snags in short and long terms. When salvaging burns, retain snags with defects incurred prior to fire, especially broken tops, and large-diameter snags of species known to be most suitable for cavity excavation in that area.

Habitat Distribution, Dietary Composition and Life History Characteristics of Odonate Nymphs in a Blackwater Coastal Plain Stream

The ecology and life histories of odonates were studied in a headwater, sand-bottomed coastal plain stream in Virginia. Quantitative sampling of odonates in sand and silt sediments, on submerged snags and in debris dams was conducted monthly for 13 mo. Six species of odonates were common in the stream. Calopteryx maculata (Calopterygidae) had a univoltine life history, whereas Boyeria vinosa (Aeshnidae) Cordulegaster maculata (Cordulegastridae), Gomphus cavillaris, Hagenius brevistylus and Progomphus obscurus (Gomphidae) were semivoltine. The odonates were most abundant in debris dams, less abundant in silt and sand sediments and least abundant on snags. Habitat-specific production of odonates was 1.3 g m−2 y−1 in debris dams and 0.1 − 0.3 g m−2 y−1 in the sand, silt and snag habitats. The production to biomass ratio (P/B) for Calopteryx was 5.9, whereas ratios for semivoltine species ranged from 2.0 − 4.0.Analysis of overlap in the use of habitat, food and time showed that greatest ecological separation of the species was in their different use of habitat. Boyeria vinosa and Calopteryx maculata primarily inhabited debris dams, Hagenius brevistylus and Gomphus cavillaris were most abundant in silt and Progomphus obscurus was found almost exclusively in sand. Cordulegaster maculata occurred throughout the stream except on snags. Narrow niche breadths for B. vinosa, C. maculata and P. obscurus based on their use of habitat suggest high fidelity of these species to one habitat, whereas Cordulegaster maculata, with the broadest habitat niche breadth, was a habitat generalist. There was little difference among the species in prey items. Trophic niche breadths of all species were broad, all species feeding on a wide variety of invertebrates, in particular Chironomidae, Ephemeroptera and Plecoptera. Little ecological separation of the species occurred based on their use of time, all species occurring in the stream throughout the year with little staggering of life history events or growth patterns. Seasonal patterns of changing resource availability and the dynamic nature of the stream environment likely are important in regulating the distribution, abundance and interactions of the odonate community in this stream.

Secondary production of riffle beetles on the snag habitat of a Coastal Plain river in the south-eastern USA

Secondary production of riffle beetles on the snag habitat of a Coastal Plain river in the south-eastern USA

Changes in fatty acid and hydrocarbon composition of leaves during decomposition in a southeastern blackwater stream

Fatty acid and hydrocarbon composition were determined in decomposing leaf packets of sweetgum (Liquidambar styraciflua) and water oak (Quercus nigra) from a snag habitat in a southeastern blackwater stream. The initial total fatty acid and hydrocarbon concentrations in sweetgum leaves were significantly greater than in the oak species. Higher concentrations of unsaturated fatty acids and dicyclic diterpenoid hydrocarbons accounted for most of this difference. Both of these biochemical subgroups are preferentially degraded relative to the bulk leaf material and other compounds within their respective lipid classes. No significant differences remained after 70 days of decomposition. Cuticular fatty acids are selectively preserved and thus, increased relative to noncuticular components during decomposition. The bacterially derived iso- and anteiso-branched-chain fatty acids increased markedly after 23 days. The results of this study suggest that qualitative differences in lipid composition may contribute to the observed difference in overall decomposition rate of leaves between these species.

Importance of flood regime to invertebrate habitat in an unregulated river–floodplain ecosystem

AbstractUnlike most large rivers of the northern hemisphere, several medium-sized rivers in the southeastern USA Coastal Plain remain unregulated. These smaller rivers possess 2 habitat types (snags and floodplain) that were historically important for invertebrate assemblages in many rivers and are strongly dependent on flood regime. I reviewed and compared 2 models of habitat inundation (snags and floodplain) that were developed for the Ogeechee River (Georgia, USA) to understand the ecological significance of these habitats. These models showed that snag habitat surfaces varied from only ∼20 to 50% of channel bottom surfaces, but floodplain inundation varied from 0 to 37 times the width of the channel at 100% inundation. Long-term analysis of inundation patterns from a 58-y record of discharge demonstrated that substantial flooding occurred almost annually for 1 to 2 mo/y. Habitat-specific invertebrate biomass was highest on snags (mostly aquatic insects), followed by the main channel (dominated by Corb...

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 &gt;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.

The Role of Snags in the Life of Woodpeckers and Snag Management in a Forest:A Review

We reviewed the relations between snags and the life of woodpeckers, and snag habitat management.A wide variety of wildlife uses snags for nesting, roosting, feeding and so on.Primary cavity nesting birds such as woodpeckers largely depend on snags and secondary cavity nesters such as titmice use the holes that the former excavate as well as natural cavities.Western countries noticed the importance of snags for wildlife earlier and have considered snag protection in forest management.While in Japan people have paid little attention to the role of snags for wildlife, and snags are not being accepted as part of forests.We discussed the necessity of snag management in Japanese forests.

Ontogenetic and Temporal Shifts in the Diet of the Amphipod Gammarus fasciatus, in the Ohio River

-A field study was conducted to determine if ontogenetic or temporal shifts occur in the diet of the amphipod Gammarus fasciatus Say in the Ohio River. Amphipods were collected monthly from cobble and snag habitats in the Ohio River for I yr. Gut contents of collected amphipods were analyzed microscopically for the presence of detritus, filamentous algae, diatoms and animal matter. Gammarusfasciatus consumed each food type in different amounts, depending on amphipod size and month collected. Detritus was the most common food item found in amphipod guts (100% of microscope fields in guts examined), followed in order by filamentous algae, diatoms and animal matter (the ranges of each food type in the guts of G. fasciatus were: 0.036-0.287, 0.061-0.281, 0.002-0.072, respectively). Food use shifted ontogenetically: small G. fasciatus were limited to a diet consisting mainly of detritus, whereas larger animals consumed significant amounts of filamentous algae, diatoms and animal matter. There were also monthly differences in foraging, presumably due to differences in seasonal abundances of food types and other environmental factors. We suggest that the abundance of, and ability to use filamentous algae, diatoms and animal matter allow populations of G. fasciatus to persist and maintain significant year-round populations in the Ohio River.

Habitat-specific distributions of some fishes in a tropical estuary

The species richness of fishes and the distributions of six fish species were compared among four habitat types, snaggy banks, clear banks, mud banks and mid channels, in an estuary in tropical Australia. Both day and night samples were collected in fish traps, on four occasions, three months apart. On each sampling trip the highest number of species was trapped from snaggy habitats, intermediate numbers from clear banks or mud banks, and the fewest species from mid channels. Although more species occurred in night than in day samples, the pattern of difference among habitats was consistent. The probability of capture of the six most abundant species, Acanthopagrus australis, Acanthopagrus berda, Arothron manilensis, Arius argyropleuron, Epinephelus coioides and Lutjanus russelli, was compared among habitat types and between day and night samples. Overall, snag habitats tended to be associated with high probabilities of capture of a number of species, particularly A. berda and L. russelli. At night, mid channels were characterized by high numbers of A. argyropleuron. Clear and mud banks, and day samples from mid channels were associated with low numbers of most species.

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.

The Relative Contribution of Benthic and Suspended Bacteria to System Biomass, Production, and Metabolism in a Low-Gradient Blackwater River

We compare the relative contribution of bacteria in four suspended and benthic habitats to total system bacterial standing stocks, production, and respiration in the Ogeechee River, a sixth order, low-gradient, blackwater river. The habitats considered are water column, sandy main channel sediment, backwater sediment, and snags. Total system bacterial biomass ranged widely with a mean of 3240 mg C m^-2. It was greatest during winter and early summer, and peaks were dominated by bacteria in sandy sediments where from 21 to 98% of total bacterial biomass was found. Bacterial biomass in both backwater and snag habitats remained consistently low. Dye studies revealed considerable water movement into sandy sediments, which were aerobic to the greatest depth sampled (60 cm). System bacterial production was less variable than biomass, ranging from 0.84 to 8.54 mg C m^-2 n^-1 >80% of which occurred in main channel sediments. System bacterial production could not account for system biomass; hence allochthonous sources of bacteria appear to be important. Bacterial respiration was calculated from rates of production and growth efficiencies measured previously in laboratory studies. The calculated contribution of bacterial respiration to measured system respiration had a modal value of 97%, but varied widely (from 9 to 161%). Benthic bacteria appear to be a major driver of system respiration, although uncertainties in these calculations point to the critical need for a better understanding of temporal variability in bacterial growth efficiency in nature.

EFFECTS OF SPATIAL ORIENTATION OF MULTIPLE PLATE ARTIFICIAL SUBSTRATES ON INVERTEBRATE COLONIZATION1

ABSTRACT: Jumbo multiple plate samplers were suspended in a river at 0.3 and 1 m depth in one of three orientations: interplate spaces closed to downwelling light and open to flow, open to light and flow, or open to light and closed to flow. Mean numbers of colonizing taxa and individuals were not significantly different among orientations after eight weeks of submergence. All variables tested decreased significantly with depth. Mean number of taxa, number of individuals (1 m only), and insect diversity decreased between samplings at four and eight weeks for samplers that were closed to light and open to flow. All orientations, depths, and durations had one‐half to two‐thirds of the total taxa. Colonization was affected by location in the river and position on sampler suspension equipment. The results indicate lack of orientation effects on colonization or high variability that obscured such effects. The sampler suspension equipment possibly increased among‐sampler variability by forming artificial snag habitats, and interplate light and flow conditions at different orientations may not have been sufficiently distinct to elicit different biological responses. Individual samplers provided diverse microhabitats regardless of orientation, but it would be prudent to include orientation among the variables considered in use of multiple plate samplers.

Invertebrate Drift Dynamics in a Subtropical Blackwater River

Invertebrate drift was studied for one year in a Coastal Plain blackwater river in southeastern Georgia, USA. Post-dusk samples were collected monthly at two sites, 160 km apart. Diel drift patterns were assessed seasonally at both sites, and a detailed analysis of diel drift was performed for two summer months at one site. Post-dusk drift densities (2-5 individuals/m³) and biomass (0.2-0.5 mg dry mass/m³) were relatively high throughout the year at both sites. Drift was composed primarily of larval black flies, caddisflies, beetles, midges, mayflies and copepods. Seventy-two to 82% of drift numbers and biomass consisted of animals normally found on submerged woody debris (the snag habitat). The remainder was from sandy and muddy benthic habitats. Drift had a relatively uniform distribution throughout the water column, suggesting that snag animals are swept far from their habitat. As a result, animals appear to drift for rather long distances (0.18-1.8 km), much further than found in most small streams. Drift periodicity was strongly night-active throughout the year, but the pattern was especially strong in summer. The summer diel analysis indicated that drift densities do not vary greatly in spite of wide fluctuations in discharge. However, a changing discharge pattern is reflected by a change in community composition. Black flies, in particular, increase rapidly in response to rising waters. Our results are consistent with the finding of concurrent analyses of production dynamics indicating that snags are an extremely important habitat for the invertebrate community in Coastal Plain rivers. We suggest that season and discharge together will determine temporal patterns of production and drift dynamics.