Bernard W. Sweeney

Size Variation and the Distribution of Hemimetabolous Aquatic Insects: Two Thermal Equilibrium Hypotheses

Adult body size and fecundity of a number of hemimetabolous aquatic insects depend largely on thermal conditions during the larval period. Small adults and reduced fecundity result when temperatures are either warmed or cooled with respect to more optimal thermal conditions. Temperature apparently affects adult size by altering the larval growth rate and the timing and rate of adult tissue development for each larva. The data suggest a new interpretation for the geographic distribution of aquatic insects.

Ephemerella Mayflies of White Clay Creek: Bioenergetic and Ecological Relationships Among Six Coexisting Species

Comparative data are presented on feeding, growth, respiration, mortality, adult emer- gence and fecundity, egg development, and calorimetry for Ephemerella subvaria, E. dorothea, E. verisimilis, E. funeralis, E. serrata, and E. deficiens in White Clay Creek (WCC), Pennsylvania, USA. All species are univoltine in WCC. The larvae of most species hatch from eggs in late summer- early fall and complete their growth by the following spring or early summer. The magnitude and rate of larval growth for each species appears to be affected largely by seasonal variation in both stream temperature and food quality. Net growth efficiencies (NGE) tend to be positively correlated with larval size for most species. NGE ranged from 3.5 to 81.6% for Ephemerella species. The timing of adult emergence and the size and fecundity of adults appears to be determined largely by the magnitude and pattern of temperatures that are experienced during the larval growth period. An hypothesis is presented to explain the interaction of temperature, insect developmental processes, and physiology in determining both the timing of adult emergence and the resultant size and fecundity of individuals. Empirical data are presented to support the hypothesis. Quantitative data concerning seasonal variation of larval densities are given for all species except E. funeralis. Results indicate that the remaining five species all coexist in riffle habitat in WCC (as opposed to pool habitat). There is also minimal overlap in the relative abundance of consubgeneric species. It is suggested that competition between pairs of consubgeneric species may be reduced somewhat by having the relative abundance and body size, as well as resource requirements (i.e., biomass production), of each pair segregated temporally.

Can DNA barcodes of stream macroinvertebrates improve descriptions of community structure and water quality

Abstract Four approaches to or levels of identifying macroinvertebrates (amateur/family, expert entomologist/genus, expert entomologist/genus–species, and DNA barcoding/species) were used to assess community structure and water quality in White Clay Creek, Pennsylvania, USA. Macroinvertebrates were collected in March 2008 from 2 riffle sites 3.9 km apart on the same stream. The downstream site was known to be degraded by land and water use. About 98% of the 1617 specimens used for analysis, including small, immature, and damaged specimens, were successfully barcoded (sequenced) for the mitochondrial cytochrome c oxidase subunit I gene. A criterion of 2 to 4% genetic divergence provided good separation of presumptive species. Barcodes increased the taxonomic inventory across the 2 sites by 475% (124 taxa) relative to the amateur level, and 125% (83 taxa) and 70% (62 taxa) relative to the expert genus and species levels, respectively. Barcoding revealed species not currently described in larval taxonomic keys, including multiple (2–11) coexisting congeneric species. That 150 species were revealed by barcoding samples collected on the same date and in the same habitat was unprecedented, as was the fact that 60 cm2 of stream bottom supported an average of 248 to 347 individuals representing 55 to 68 species. Most barcode species were rare, with 42% represented by ≤2 individuals. Across all species, 43 of 89 barcode species were unique to upstream site 11 and 60 of 107 were unique to downstream site 12. In terms of water-quality assessment, most of the 17 metrics studied changed significantly (α  =  0.05) when taxonomy changed from family to genus–species (79% and 93% for sites 11 and 12, respectively), and many also changed when taxonomy changed from genus to species (59 and 65% for sites 11 and 12, respectively). The proportion of metrics able to detect a difference (α  =  0.05) between sites 11 and 12 increased with improved taxonomic resolution (36, 47, 65, and 76% for family, genus, genus–species, and barcode, respectively). The results revealed a pollution-tolerance gap because barcoding pushed larval taxonomy beyond the available pollution-tolerance data. Regardless, the combined morphological and molecular approach provides a finer resolution for evaluating environmental change associated with both natural and anthropogenic processes. The ability to distinguish larvae at the species level through barcoding finally puts biodiversity assessments for aquatic communities in terms comparable to those used for terrestrial ecosystems where estimates of biodiversity for plants and animals are never quantified at the level of genus or family. We conclude that DNA barcodes of stream macroinvertebrates will improve descriptions of community structure and water quality for both ecological and bioassessment purposes.

Egg and Larval Development Times for 35 Species of Tropical Stream Insects from Costa Rica

We examined total development times for 5 mayfly species, 2 stoneflies, 10 caddisflies, and 18 chironomid midges collected from three streams that flow through tropical evergreen forest in northwestern Costa Rica. Most eggs, larvae, and pupae were reared in the laboratory in a photoperiod of 12:12 LD and at 20°C, which simulated field conditions. Algae, algal detritus, and leaves were provided as food for all species; predators were also given various animal prey. All study species had total development times that were rapid relative to the univoltine life histories observed or assumed for many temperate species. Egg development times ranged from a few days to approximately a week for chironomids and from 10 to 38 days for mayflies, stoneflies, and caddisflies. Most chironomids had short larval/pupal development times: development was completed in 19-29 d by seven species, in 30-40 d by nine species, and in >50 d by two species. Relatively short development times (including the pupal stage when present) were also observed for the mayfly Acerpenna sp. (28 d) and the caddisflies Wormaldia sp. (45 d) and Oecetis nr. prolongata (52 d). Larval/pupal development times were longer for the other four mayflies (76-159 d), two stoneflies (83-167 d), and eight caddisflies (72-209 d). No evidence of egg or larval diapause was observed. The combination of rapid development and absence of diapause suggests that all these species have multivoltine life histories. This finding has important implications for temporal changes in the structure and function of the aquatic insect assemblage in these streams.

Growth and Production of Stream Stonefly: Influences of Diet and Temperature

The natural influx of leaf litter from a hardwood forest to sections of two spring seeps and a spring brook was excluded for two years and replaced with equivalent amounts of leaves from speeific tree species. Larval growth, mortality, and productivity, as well as the timing of emergence and the size of adults, were assessed in the naturally occurring populations of Soyedina carolinesis, and the differences related to differences in available food and in temperature among study sites. During the first experimental year, larvae fed only on sugar maple or chestnut oak leaves exhibited the same rate and magnitude of growth as larvae fed the natural mix of leaves collected from the surrounding forest. Larval growth rates and adult size seemed lower on monospecific diets of hickory, American beech, and red oak leaves relative to the natural, mixed leaf diets. Adult emergence occurred on or about the same date for most diets. Larval production ranged from 1382 to 5500 mgm_2yr-1; there was no correlation between larval growth rate and productivity on a given diet. During the second experimental year, most sites that had been supplied with single-species leaf diets during the first experimental year were provided with sugar maple leaves only. This was an attempt to evaluate site-to-site differences in habitat quality (other than diet) that might affect the growth performance of larvae. The amount of variation in larval growth rate and adult size among sites having a common diet was equal to or greater than among-site variation during the previous year, when each site differed in leaf diet. In the spring brook experiment, both diet and location within the brook exerted significant effects on larval growth rate and adult size. The results of the two-year experiment suggest that diet exerts some influence on the seasonal pattern and magnitude of larval growth. However, site-to-site differences in the seasonal pattern and magnitude of temperatures (especially during the December-February period) exhibited the best cor? relation with observed differences in larval growth and adult size.

Climate Change and the Life Histories and Biogeography of Aquatic Insects in Eastern North America

One of the most important factors affecting the life history characteristics and biogeography of aquatic insects is temperature (Sweeney, 1984). Insects are poikilothermic (cold-blooded) animals whose metabolism, rate and magnitude of growth, development, and overall behavioral activities respond significantly to thermal change on a diel, seasonal, and annual basis (Ward and Stanford, 1982). Despite this sensitivity to temperature, most aquatic insect species can be found in aquatic habitats over a broad geographic area that includes a wide range of thermal regimes. Obviously, these aquatic insect species possess bioenergetic, developmental, and/or behavioral mechanisms that enable conspecific populations to survive and reproduce in very different environmental conditions.

Egg Development, Growth, and Metabolism of Sigara Alternata (Say) (Hemiptera: Corixidae) in Fluctuating Thermal Environments

Egg development and growth of Sigara alternata in southeastern Pennsylvania, USA were evaluated in six fluctuating temperature regimes with daily maxima ranging from 12.0° to 21. 5°C. Developmental rate was positively correlated with increased magnitude of the diel temperature pulse. Developmental acceleration was most apparent for environments pulsing beyond 15.0°C for both stages. Oviposition rates were studied at natural temperatures during the initial days of reproduction. Egg deposition was sporadic but highly correlated with maximum diel temperatures. A critical temperature region of 11.0—12.5°C was observed for initiation and maintenance of egg laying. Metabolic rates were measured using a different respirometer. The thermal response of metabolism varied with animal soze and the range of test temperatures. The energy demand per uniy weight of tissue was inversely related to body size at all temperatures. Adult population metabolism decreased exponentially during the peak reproductive period due to high mortality. Energy budgets were determined for nymphal development in all thermal regimes. Net growth efficiencies and production/respiration ratios for Sigara alternata ranged from 49.7—73.4% and 0.9—2.7 respectively. Assimilation rates increased with rinsing temperatures and varied between 1.05 and 3.52 J.mg—1.day—1 [=0.25 —0.84 cal.mg—1.day—1]. Net growth efficiencies tended to decrease with increased temperatures due to higher maintenance costs. The significance of fluctuating temperatures to the bioenergetics, developmental dynamics, and ecology are discussed .

Concentrations and Export of Solutes from Six Mountain Streams in Northwestern Costa Rica

Streamwater chemistry in six streams draining adjacent, undisturbed catchments of volcanic origin in northwestern Costa Rica was monitored between 1990 and 1993. The catchments ranged from 0.36 to 3.2 km2 in area, and from 600 to 1300 m in elevation. Rainfall averaged 2.4 m/yr at 600 m but was unmeasured and probably much greater at higher elevations. Runoff among the catchments ranged from 1.4 to 4.3 m/yr, with approximately 84% occurring during the May-December rainy season. Within individual streams, concentrations of NO3-, total dissolved phosphorus (TDP), and dissolved organic carbon (DOC) increased with streamflow while Ca2+, Mg2+, and Na+ decreased with streamflow. K+ showed little response to streamflow. Among streams, flow-weighted mean concentrations of NO3- (0.12-0.30 mg/L), DOC (0.63-1.79 mg/L), and TDP (0.008-0.029 mg/L) were intercorrelated and varied inversely with annual runoff. DOC concentrations were low compared with data from other tropical streams. Annual exports (kg ha-1yr-1) for these solutes were relatively uniform among catchments, suggesting that differences in runoff explain much of the stream-to-stream differences in solute concentrations. Cation exports were also relatively uniform among five of the streams, but were distinctly higher in the remaining stream, which is also enriched in SO42- and Cl-, and may receive water from a geothermal source.

Present Status and Future Directions of Tropical Stream Research

This paper presents the results of a survey of our understanding of the structure and function of streams and rivers based on the opinions of colleagues working in the tropics. The questionnaire examined the status of eight general topics: taxonomy, life history traits, standing crop biomass, production, trophic structure, transport/drift of material or organisms, nutrient dynamics, and biotic interactions. Two conclusions were reached: (1) significant progress has been made in our understanding of tropical streams and rivers; very few subjects have not been examined at one or more tropical locations. (2) Although most subjects have received at least some attention, they remain relatively unknown at most sites. Recommendations for future directions in tropical stream research are given.

A Model for Seasonal Synchrony in Stream Mayflies

Many mayfly species have synchronous univoltine life histories over broad geographic ranges, but the life-history adaptations underlying their seasonality remain unidentified. We investigated whether simple adaptations in the response of development rate to temperature might account for the observed phenology of adult emergence and the 1- and 2-year life histories of eight mayfly species studied in Piedmont streams of eastern North America between 34°N and 50°N latitude. We present a model consisting of two sequential life-history stages. Development rate in each stage is a linear function of streamwater temperature above a lower threshold but, in the first stage, a developmental quiescence occurs whenever a maximal temperature is exceeded. The five model parameters are assumed not to vary with latitude. Using daily field temperatures, the model simulated several successive generations beginning from an arbitrary day of the year. With parameters fitted from the field data, the model could reproduce both the univoltine life history and the latitudinal variation in the timing of adult emergence for six species. For two additional species, the model reproduced a northward transition to a semivoltine life history. The simulations suggest that nearly all development occurs in spring and autumn during periods of roughly equivalent thermal regime at all latitudes.