2006 SOUTHEASTERN NATURALIST 5(1):165–174 Chironomid Emergence and Relative Emergent Biomass from Two Alabama Streams STEVEN K. REYNOLDS, JR.1,2,* AND ARTHUR C. BENKE1 Abstract - Chironomid pupal exuviae were sampled monthly using drift nets and hand sieves over an annual cycle from Hendrick Mill Branch (HMB; Blount County, AL) and Payne Creek (PC; Hale County, AL). Taxon richness, community composition, and emergence phenologies were similar despite marked differences in physical/chemical characteristics of the study streams. The highest emergence rates were observed in spring (PC) or both spring and fall (HMB). However, there was no significant relationship between emergence/emergent biomass and date. Estimates of daily emergence and emergent biomass were much higher in HMB than in PC. These patterns may have resulted from a more consistent flow regime, higher substrate stability, lower variation in temperature/chemical characteristics, greater channel surface area, and a more significant hyporheic zone in HMB than in PC. Introduction The emergence of adult chironomids from aquatic habitats represents a potential significant vector of the transport of energy (in the form of adult biomass) within and between aquatic habitats and to surrounding terrestrial areas (Jackson and Fisher 1986, Stagliano et al. 1998). For accurate estimations of energy flow (secondary production) into and out of aquatic habitats, it is necessary to quantify emergence (Benke et al. 1999, Jackson and Fisher 1986, Stagliano et al. 1998). Many studies have documented that larvae of the family Chironomidae are ubiquitous and abundant in streams, often contributing substantially to the energy budgets of various habitats (Benke 1998, Coffman and Ferrington 1996). However, few lotic studies have quantified the emergence of this family at taxonomic levels below family (i.e., Coffman 1973, Ferrington et al. 1995, Wartinbee 1979). Furthermore, comparatively few studies have quantified chironomid emergence within the context of overall energy flow simultaneously from different types of streams. In our study, pupal exuviae, the shed cuticles of the pupal stage remaining in the water after adult eclosion, were collected from two small perennial streams differing substantially in parent geology within two physiographic provinces of Alabama: the Valley and Ridge and the Coastal Plain. Pupal exuviae samples are representative of adult emergence from all microhabitats within a given stream segment (e.g., benthic, hyporheic, hygropetric, 1Department of Biological Sciences, Aquatic Biology Program, A-122 Bevill, Box 870206, University of Alabama, Tuscaloosa, AL 35487-0206. 2Current address - US EPA, National Risk Management Research Laboratory, Robert S. Kerr Environmental Research Center, PO Box 1198, Ada, OK 74821-1198. *Corresponding author - reynolds.steve@epa.gov. 166 Southeastern Naturalist Vol. 5, No. 1 etc.) because all adult midges emerging at the water surface can be directly sampled from this area (Ferrington et al. 1991, 1995). We compared genuslevel emergence phenologies and magnitudes of the potential export of adult biomass from pupal exuviae sampled from these two streams. Field-site Description Hendrick Mill Branch (HMB), in the Valley and Ridge physiographic province (Blount County, AL) is a small perennial stream within the Black Warrior River drainage, with a stable dolomite cobble/boulder/bedrock substrate and relatively deep hyporheic zone (≈ 40–50 cm, Reynolds 2002). HMB has a high alkalinity (> 100 mg CaCO3/L) and specific conductance (≈ 200 mS/cm) and a slightly basic pH (≈ 8). Water temperature variation is relatively low, ranging from 10 to 17 °C (mean = 15.5 °C) throughout the year (Huryn et al. 1995). Payne Creek (PC), in the Coastal Plain physiographic province (Hale County, AL) is also a small perennial stream within the Black Warrior River drainage; however it has a fine- to medium-grained sand substrate and limited hyporheic zone (≈ 5–20 cm, Reynolds 2002). PC has a low alkalinity (< 10 mg CaCO3/L) and slightly acidic pH (≈ 5–6). Water temperature variation in PC shows a much greater annual range (6–24 °C, mean = 18.5 °C) than HMB (G.M. Ward, University of Alabama, Tuscaloosa, AL, unpubl. data). Mean annual discharge and maximum current velocity are higher in HMB (0.065 m3/s and 20.9 cm/s, respectively) than in PC (0.015 m3/s and 9.9 cm/s, respectively). Mean annual current velocities across the active channel are similar in PC and HMB (6.4 and 5.2 cm/s, respectively—vector transect and water depth method using a Marsh- McBirney current meter; Reynolds 2002). Methods Emergence phenologies Daily emergence of chironomid taxa was determined by collecting pupal exuviae monthly (on consecutive dates every 28 to 32 d) from 5 sites in HMB and 4 sites in PC from March 1999 to February 2000. Exuviae were taken directly from the water surface using a 45-mm hand sieve, under littoral vegetation and around emergent macrophytes/large woody debris. Sampling effort was standardized between sites and dates by limiting the sample time to 10 min per site. Exuviae were washed from the sieve and preserved in the field with 80% ethanol. In the laboratory, exuviae were sorted, identified to genus using keys in Coffman and Ferrington (1996) and Wiederholm (1986), and counted. Emergence and emergent biomass Two drift nets (100-mm mesh, 0.097-m2 aperture) were used monthly in HMB and PC to quantify emergence phenologies and calculate emergent biomass. Sample dates were the same as those used for hand-sieve sampling. 2006 S.K. Reynolds, Jr. and A.C. Benke 167 Specific site selection was random among sample sites used and drift nets were placed upstream from the area sampled by hand sieve. Sampled material was collected over a 1-h period at dusk and preserved in 80% ethanol with Phloxine-B stain. Exuviae were sorted from the gross sample and identified to genus and counted in the laboratory, as above. Chironomid exuviae in each drift net were counted and converted to an estimate of hourly adult emergence per m2 of water surface (adults m-2 h-1) by dividing the number of exuviae in the sample by the aperture width of the drift net opening (0.312 m) and the current velocity (m/h). By extrapolation, an estimate of total daily emergence was calculated for each stream (mean hourly estimate [n = 12] x 24 h/d). Total emergent biomass was estimated in a similar fashion after substitution of the hourly emergence data used above with previously published estimates of adult biomass (from Stagliano et al. 1998). All biomass units were expressed as dry mass. Daily emergence data were considered estimates because the period of time and distance over which exuviae accumulated was unknown, and it is assumed that our measurement of hourly emergence was representative of emergence over a 24-h period. Statistical analyses Emergence and emergent biomass data from replicate drift nets (n = 2) in each stream were analyzed by one-way ANOVA by sample date, alpha-level = 0.05. Emergence phenology data from hand-sieve collections were not compared statistically because they were only semi-quantitative. However, samples were standardized by sampling effort, so relative abundances of chironomid taxa through time were comparable between the two streams. Results Emergence phenologies There was nearly continuous emergence throughout the year in both streams, with maximum emergence in spring and late fall for HMB and from February to June in PC (Fig. 1). A total of 32 and 30 genera were collected in monthly grab samples from HMB and PC, respectively (Tables 1 and 2). Corynoneura and Stempellinella were the most common genera, numerically dominating exuviae samples from both streams. Additionally, Rheosmittia, Polypedilum, and Tanytarsus were common in PC, whereas Parametriocnemus and Rheotanytarsus were common in HMB. Approximately 20% of the chironomid genera from each stream were present in all or nearly all the qualitative hand-sieve samples throughout the year (25 and 17% for HMB and PC, respectively). Although samples obtained by hand sieve were not strictly quantitative, the standardized sampling period at all sites provided an estimate of relative abundance across sample dates and between streams. Mean daily emergence from HMB (74 exuviae/date) was ≈ 2.5 times higher than from PC (29 exuviae/date). 168 Southeastern Naturalist Vol. 5, No. 1 Emergence and emergent biomass Emergence in HMB was highest in the spring and lowest in summer/fall (Fig. 2). However, because of high within-date variation, one-way ANOVA indicated no significant differences among dates (p > 0.05). Total daily emergence and biomass in HMB based on extrapolation of the hourly drift samples was 35.31 adults m-2 d-1 and 3.96 mg m-2 d-1, respectively (Table 3), with highest values in spring and lowest values in summer/fall (Fig. 2). Microtendipes, Polypedilum, Stempellinella, and Corynoneura were the numerically dominant midges in HMB (all with > 3 adults m-2 d-1 and > 0.37 mg m-2 d-1 emergent biomass; Table 3). Total daily chironomid emergence estimated from hourly drift samples from PC was 2.61 adults m-2 d-1. Total daily emergent biomass in PC was 0.22 mg m-2 d-1 (Table 3). However, on all but 3 sample dates (March, May, and June), no pupal exuviae were collected by drift despite presence of Figure 1. Mean total pupae abundance (no. exuviae/date) from Hendrick Mill Branch (A) and Payne Creek (B) based on 10-min hand-sieve samples. Error bars = ± 1SE. N = 5 for HMB, and n = 4 for PC. 2006 S.K. Reynolds, Jr. and A.C. Benke 169 exuviae in PC throughout the year. Based on these limited samples, 4 genera (Stempellinella, Tanytarsus, Corynoneura, and Parametriocnemus) were the numerically dominant midges emerging from PC (≥ 0.3 adults m-2 d-1 and > 0.02 mg m-2 d-1 emergent biomass; Table 3). No statistical analysis on daily emergence was performed because of the limited number of sample dates with pupal exuviae collections. Discussion In HMB and PC, monthly hand-sieve samples of chironomid exuviae indicated that adult emergence occurred continuously throughout the year, with emergence peaks in either spring and fall (HMB) or only spring (PC) Table 1. Emergence phenologies for Chironomidae and mean monthly relative abundances (%) of dominant (> 5% of mean relative abundance) taxa in Hendrick Mill Branch based on monthly 10-min hand-sieve samples. x = pupal exuviae sampled, but represent < 5% of mean relative abundance; - = was probable given presence of exuviae on other dates, however they were not sampled. Empty cells indicate that no exuviae were sampled nor would any be expected based on other sample dates. Genus M A M J J A S O N D J F Mean Ablabesmyia x Brillia x x x x - - Corynoneura 42 29 44 31 14 57 53 87 62 61 28 42 46 Cricotopus x Cryptochironomus x Dicrotendipes x x Einfeldia x Eukiefferiella x x x x - - x x x x x x Heterotrissocladius x Labrundinia x x - - x Larsia - x x x Micropsectra - x x x - x Microtendipes x x Nanocladius x x Orthocladius x x x x x x x - x Paracricotopus x Parakiefferiella x x x - x x x - x Paramerina x x - - x Parametriocnemus 4 3 12 17 32 14 18 4 11 12 4 3 11 Paratanytarsus x x x x x - x x x x x x Paratendipes x x Parorthocladius x Phaenopsectra x Polypedilum x x x x x - x - x x Psectrocladius x x x Rheocricotopus x x x x - x Rheotanytarsus 5 11 3 2 13 - 2 1 4 3 16 3 5 Stempellinella 9 16 4 37 10 4 9 1 1 5 24 17 11 Synorthocladius x x- x Tanytarsus x x x x x - x - x x x x Tvetenia x x x x x - - Xylotopus x 170 Southeastern Naturalist Vol. 5, No. 1 (Fig. 1). The lack of temporal emergence synchrony, recognized in this study as the continuous emergence by many taxa (Tables 1 and 2), is common in southern temperate climates where favorable thermal conditions allow for emergence throughout the year (Armitage 1995, Benke 1998, Corbet 1964). Comparable emergence patterns to those seen in our study for PC were documented in a wetland system < 1 km downstream of the current PC study sites (Stagliano et al. 1998). Daily emergent chironomid biomass from HMB was 3.96 mg m-2 d-1 based on drift net samples taken on single dates at monthly intervals. This value is only approximate because of assumptions made regarding discontinuous temporal sampling and the relationship between exuviae on a unit area of water versus a unit area of stream bottom. Nonetheless, emergence values for HMB were comparable to those from previously described temperate lotic systems, assuming that chironomids constitute a conservative Table 2. Emergence phenologies for Chironomidae and mean relative abundances (%) of dominant (> 5% of mean relative abundance) taxa in Payne Creek based on monthly 10-min hand sieve samples. x = pupal exuviae sampled, but represent < 5% of mean relative abundance; - = emergence was probable given presence of exuviae on other dates, however they were not sampled. Empty cells indicate that no exuviae were sampled nor would any be expected based on other sample dates. Genus M A M J J A S O N D J F Mean Ablabesmyia x Brillia x x x x Cladotanytarsus x x - x x Conchapelopia x Corynoneura 25 22 12 4 8 - 22 31 65 58 - 13 22 Cryptochironomus x - x Cryptotendipes x x x x Dicrotendipes x x x x x x Eukiefferiella x - - - x Glyptotendipes x Krenosmittia - x x - - Larsia x x x Meropelopia x Nanocladius x Orthocladius x - x x x Paracladopelma x Parakiefferiella x x - - x Paralauterborniella x x - x Paramerina x x x Parametriocnemus x x x x x - - x - x - x Paratanytarsus x Phaenopsectra x - x Polypedilum 2 2 12 8 8 - 19 - 6 - - 21 6 Psectrocladius x - - x x Rheocricotopus x - - x x x Rheosmittia - 38 - 1 86 - 10 Saetheria x - x Stempellinella 33 19 39 24 40 - 16 23 12 8 5 8 19 Tanytarsus 7 4 15 46 8 - 5 - 6 - 2 31 10 Xylotopus x x 2006 S.K. Reynolds, Jr. and A.C. Benke 171 (35%; Coffman 1995) amount of total macroinvertebrate emergent biomass (mean = 3.80 mg m-2 d-1, range = 1.96 to 6.79 mg m-2 d-1; data converted from annual values from Jackson and Fisher 1986). In contrast, daily emergent chironomid biomass in PC was only 0.22 mg m2 y-1. This value was extremely low compared to the lotic systems discussed above, and also much lower than previous estimates of emergent biomass (1.59 mg m-2 d-1) from a wetland in the same watershed (Stagliano et al. 1998). In our study, drift nets were placed in locations known a priori to have continuous annual flow. However, discharge of PC declined considerably during summer and fall, resulting in an increase in the number of isolated pools and backwaters (S. Reynolds, pers. observ.). Drift nets placed in areas of maximum current velocity would have undersampled exuviae present in pools and backwaters because exuviae from the latter would likely not have been captured by samplers in flowing water. Furthermore, peak emergence times may have been missed by the sampling protocol in PC. Drift was sampled in the early evening during the typically period of peak emergence (Coffman 1973, Vilchez-Quero and Lavandier 1986), although Figure 2. Mean hourly emergence and emergent biomass for Chironomidae from drift samples collected monthly from Hendrick Mill Branch (A) and Payne Creek (B). Error bars = ± 1SE. • = Density, o = Biomass. 172 Southeastern Naturalist Vol. 5, No. 1 other peak emergence times have been documented (i.e., early morning, throughout the night and/or day; Danks and Oliver 1972, Franquet and Pont 1996, Mundie 1971, Wartinbee 1979). Consequently, whereas hand-sieve samples would have captured exuviae in backwaters and pools during these periods of low flow (Tables 2 and 3), daily emergent chironomid biomass and emergence estimates for PC based on drift samples from the main channel likely are underestimated. Despite great differences in emergence numbers and biomass between HMB and PC, chironomid assemblages in HMB and PC were quite similar taxonomically. Thirty-two genera were found in HMB, 30 in PC, with 20 genera common to both streams. These patterns are comparable to previous studies in similar habitats. Stagliano et al. (1998) identified 31 genera in a wetland < 1 km downstream from our PC sites, and Coffman (1973) and Frommer and Sublette (1971) identified 53 and 46 genera, respectively, from streams similar in size, substrate and chemical characteristics to HMB. In both Coffman (1973) and Frommer and Sublette (1971), both larger substrate and longer stream reaches were studied, including more types of habitat, which may explain the higher taxon richness values relative to HMB. Table 3. Total daily abundance (adults m-2 d-1) and biomass (mg m-2 d-1) of chironomid emergence from Hendrick Mill Branch (HMB) and Payne Creek (PC) by monthly drift net samples. - = taxon not sampled. HMB PC Taxon Abundance Biomass Abundance Biomass Chironomini Cryptotendipes - - 0.07 0.01 Dicrotendipes - - 0.16 0.03 Microtendipes 3.09 1.17 - - Paratendipes 1.37 0.24 - - Polypedilum 4.26 0.44 0.16 0.02 Tanytarsini Micropsectra 1.20 0.15 - - Paratanytarsus 2.44 0.28 - - Rheotanytarsus 1.77 0.21 - - Stempellinella 3.12 0.37 0.42 0.05 Tanytarsus 0.17 0.02 0.28 0.03 Orthocladiinae Corynoneura 7.80 0.46 0.98 0.06 Eukiefferiella 2.06 0.12 - - Orthocladius 1.88 0.11 - - Parakiefferiella 0.91 0.05 - - Parametriocnemus 3.69 0.22 0.38 0.02 Rheosmittia - - 0.10 0.01 Tvetenia 0.39 0.02 - - Tanypodinae Labrundinia 0.40 0.06 - - Larsia 0.76 0.03 0.07 < 0.01 Total Chironomidae 35.31 3.96 2.61 0.22 2006 S.K. Reynolds, Jr. and A.C. Benke 173 Taxonomic and temporal similarity of emergence phenologies acquired by hand-sieve samples suggests that HMB and PC have similar chironomid assemblages despite their physical differences (Tables 1 and 2). Approximately 60% of genera were common to both streams and the genera Corynoneura and Stempellinella showed similar numerical dominance in both streams (Tables 1 and 2). However, mean emergence (no. exuviae/ date) based on hand-sieve samples, as well as, total emergence (no. m-2 d-1) and total emergent biomass (mg m-2 d-1) based on drift-net samples, were all higher (2.5–17.5 times) in HMB than in PC The true measure of the difference in emergence and emergent biomass between HMB and PC probably falls between the 13-fold (for abundance) and 18-fold (for biomass) differences suggested by the drift nets samples and the 2.5 to 3.5-fold differences measured previously by either alternative pupal exuviae techniques or in larval biomass (Reynolds 2002). 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