Robert A. Short

Production of Corydalus cornutus (Megaloptera) in Four Streams Differing in Size, Flow, and Temperature

Estimates of annual production for the predaceous aquatic insect Corydalus cornutus (Megaloptera) revealed considerable variation among four Texas streams with similar water quality characteristics but differing in size, flow, and temperature. Values of production varied from 1.62 g dry mass m-2 yr-1 to 13.19 g dry mass m-2 yr-1. This variation was largely due to differences in growth and development which occurred as a result of differences in thermal regime. At three of the streams, C. cornutus appeared to be univoltine with a cohort production interval (CPI) of 333 days. At a fourth site with higher summer temperatures, development from egg to mature larva occurred in 6-7 months or less, leading to a CPI of 210 days and suggesting bivoltinism (not previously reported for this group). Of the factors thought to control secondary production, including habitat, temperature, food quality and quantity, and biological interactions, temperature seems to be the most important. In addition, a direct relationship was found between production and stream width. This effect could result from limited primary production in canopied, narrow streams which might reduce prey species production and thus lower prey availability to C. cornutus. Permanency of flow appeared not to strongly influence production because the highest production was found in an intermittent stream. Our estimates of production for C. cornutus are among the highest yet reported for a predaceous species and approach the values obtained for primary consumer species in desert streams.

Variation in nutrient limitation of lotic and lentic algal communities in a Texas (USA) river

Nutrient limitation of periphyton and phytoplankton was assessed in the Upper Guadalupe River, Texas USA. Nutrient-diffusing substrates with added nitrogen (N) and phosphorus (P) were used to identify the limiting nutrient for lotic algae at three river sites in summer, fall, and winter. Pots enriched with P had significantly higher chlorophyll a concentrations for 7 of 9 trials. Added N alone did not significantly increase algal standing crops, although it was found to be secondarily limiting on one (and possibly two) occasions. Flow-through enrichment experiments were conducted in order to quantify the concentration of P needed to significantly increase algal standing crops. Response to enrichment was rapid when ambient P concentration was low (< 0.010 mg L−1), but more moderate when ambient P levels were higher (0.015–0.025 mg L−1). Nutrient limitation of phytoplankton in small surface-release reservoirs varied throughout the study, but N was either primarily or secondarily limiting in 6 of 8 trials; shifts in the limiting nutrient were correlated with fluctuations in flow into the reservoirs. Our enrichment studies show that algal response to nutrient addition was unpredictable as phytoplankton tended to be N-limited while periphyton was mainly P-limited. Further, while discharge apparently dictated the nutrient-biomass relationship for phytoplankton in reservoirs, ambient nutrient level is an important determinant of lotic periphyton response to enrichment.

Trophic Ecology of Three Winter Stoneflies (Plecoptera)

Winter stoneflies (Zapada oregonensis, Z. cinctipes and Capnia confusa) used detritus as a nymphal food almost exclusively and dominated the shredder biomass in a third-order mountain stream in Colorado. Laboratory feeding trials showed that ingestion rates and fecal production were temperature-dependent, although not greatly different between aspen or alder leaf discs. Assimilation efficiency (AE) was not temperature-dependent. Higher AE values for alder indicate that it is superior to aspen as a food source. High consumption rates (ca. 30% of body weight/day) by the shredders, even at low temperatures, would result in the conversion of considerable quantities of leaf litter to finer particles. However, size fraction analysis of egested material indicated that particle size reduction by shredder processing only partially explains detrital composition in a stream dominated by fine-particle feeding detritivores.

Seasonal Comparison of Leaf Processing in a Texas Stream

-A seasonal comparison of the processing of hackberry (Celtis laevigata) leaves in a Texas stream showed more rapid weight loss during the summer than during the winter. Processing coefficients (-k) of 0.124 (summer) and 0.074 (winter) were significantly different. The taxonomic and functional group compositions of the macroinvertebrates colonizing the packs were very similar with very few shredders present. When compared on a degree-day basis, there was no significant difference in processing rates between the two seasons. The lack of a change in the macroinvertebrate community and the results of the degree-day comparison indicate that temperature is the primary factor influencing processing rate in this stream.

Annual Production of Corydalus cornutus (Megaloptera) in the Guadalupe River, Texas

Annual production of the predaceous stream insect Corydalus cornutus (Megaloptera) was determined at five sites along the longitudinal profile of the Guadalupe River in S-central Texas using the size-frequency method. Production values progressively decreased from 22.9 g ash-free dry mass/m2/year to 4.6 g ash-free dry mass/m2/year from the upstream to the downstream site. These values are among the highest reported for a single species of aquatic insect. INTRODUCTION During the past 10 years there has been a resurgence of interest in the determination of secondary production in stream ecosystems, largely due to improved methods for estimating secondary production (e.g, Waters and Crawford, 1973; Benke, 1984). These methods have promoted studies on the trophic dynamics of individual species and their role in the ecosystem (Benke and Wallace, 1980; Benke et al., 1984). The production of carnivorous species is of particular interest since its magnitude is often a reflection of prey populations and the factors which influence prey populations. This study deals with the production of the predator Corydalus cornutus L. (Megaloptera: Corydalidae) at five sites along the longitudinal profile of a Texas river. Corydalus cornutus is widely distributed throughout central and eastern North America (Merritt and Cummins, 1978) where it normally inhabits riffle regions of welloxygenated streams. The larvae, which are often referred to as hellgrammites, have been reported as being predators feeding on a variety of aquatic insects (Stewart et al., 1973). Pupae are terrestrial and the aerial adults deposit egg masses on objects which overhang the stream. After hatching, the larvae drop down into the water to begin the aquatic phase of their existence. STUDY AREA The Guadalupe River arises in the Edwards Plateau region of S-central Texas and flows in a southeasterly direction for 712 km before entering the Gulf of Mexico at San Antonio Bay. Throughout its length the river is highly buffered and exhibits high conductivity (Table 1). The water temperatures normally range from near 10 C during winter to over 30 C at times during the summer (Table 1). Five sites located along the longitudinal profile of the river from near the headwaters to within 112 km of the coast, all riffle areas with rubble and gravel substrata, were sampled. The major difference TABLE 1.-Physical and physicochemical characteristics of the Guadalupe River at each sampling site. Physicochemical characteristics are shown as annual means and ranges

Temperature effects on warmwater stream insects: a test of the Thermal Equilibrium Hypothesis

Stanley, E. H. and Short, R. A. 1988. Temperature effects on warmwater stream insects: a test of the Thermal Equilibrium Hypothesis. - Oikos 52: 313-320. Population density and biomass, individual size, and life cycles of mayflies were determined in two Texas (USA) streams to test predictions of Vannote and Sweeneys Thermal Equilibrium Hypothesis with warmwater stream insects. Population density and biomass patterns corresponded well with each other for most but not all species. Austral mayflies at one site (Blanco River) had high summer but low winter densities while fecundity of mature female larvae was lowest in the summer and greatest during the winter. Female size and population density were less predictable at the second site (Honey Creek). At both sites, all species had highly asynchronous, overlapping cohorts, indicating that many generations are completed within one year. Although some results were in agreement with predictions of the hypothesis, other findings were inconsistent. Dominating abiotic conditions such as flooding or intermittence may favor rapid growth and development at the expense of egg production for warmwater mayflies. Due to the prevalence of multivoltinism, thermally optimal conditions are a function of both location and time. Additionally, population level characteristics are more appropriate indicators of thermal optimality than is individual body size of mature larvae or adults.