This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Behaviors Responses of Drunelh coloradensis Ephemeroptera Nymphs to Short-Term pH Reductions Christopher M. Pennuto and Frank deNoyelles, Jr. Department of Systematics and Ecology, University of Kansas, Lawrence! KS 6604.5-2 7 06, USA Pennuto, C.M., and F. deNoyebles, Jr. 1993, Behavioral responses of Drupsella coloradensis (Epherneroptera) nymphs to short-term pH reductions. Can. 1. Fish. Aquat. Sci. 50: 2692-2697. Behavioral responses of Drunelba cs/oradensis nymphs were examined in outdoor experimental stream channels aker pH reductions of 7 and 2 pH units below ambient. The severity of pH decline below the ambient of 7.8 influenced the behavior patterns displayed by nymphs. At pH 7.01 (an intermediate pH decline) nymphs sat less frequently and burrowed more than controls. Burrowing behavior frequency returned to control levels and drifting and crawling behaviors increased relative to controls at pH 6.02, Ventilatory behaviors increased with pH decline, but were independent of the severity of acidity increases. These results suggest that individual behaviors may offer a more sensitive indicator of sub-acute stress in aquatic insect communities than population or community monitoring. Behaviors leading to increased activity levels in stream insects may have community-level effects via changes in predator-prey encounter rates or increased susceptibility to passive drift. These potential changes are discussed in reference to monitoring for acidification effects. On a 6tudi6 les comportements de nymphes de Druneh cobradensis dans des tronCons de cours d'eau exp6rirnentaux aprPs une r6duction de I et 2 unites de pH par rapport au pH ambiant. La gravit6 de la baisse de pH par rapport au pH arnbiant de 73, a influenc6 les comportements manifest& par les nymphes. A pH 7,01 (soit une baisse intermediaire du pH), les nymphes se posaient rnoins fr6quemrnent et s'enfouissaient davantage que ies temoins. La fr6quence du comportement d'enfouissernent retrouvait celle des groupes-tkrnoins et les comportements de dkrive et de rampement se faisaient plus fr6quents par rapport aux temoins, 21 pH &,02. Les comportements ventilatoires se sont accrus ii mesure que le pH baissait, mais ils 6taient independants de la gravite de ['acidification. Ces resultats donnent 5 penser que les comportements individuels peuvent csnstituer un indicateur plus sensible du stress subaigii dans des communaut6s d'insectes aquatiques que ne l'est la surveillance des populations ou des communaut6s. Des comportements qui conduisent ii une activit6 accrue chez les insectes lotiques peuvent avoir des effets au niveau de la cornmunaute dans la mesure oG sont modifi6s les taux de rencontre entre predateurs et proies ou encore est accrue la susceptibilit6 2 une derive passive. (I est question de ces changements en termes de surveillance des effets de I'acidiiication. Received April 7, 1993 Rep 1e 7 a vril 1993 Accepted luly 9, 1993 Accept4 le 9 juilBet d 993 (JB878) .ic ecologists have demonstrated that reduced pH and environmental) might also act to increase or decrease the A:; and have negative effects on individuals, populations, likelihood of drift and no research demonstrates whether other communities of aquatic insects. To assess the behaviors in insects are altered under acid stress (but McCahon potential impacts of acidification in lotic systems, researchers et al. (1989) have addressed changes in feeding rates of have monitored colmgmity level effects on benthic density, amphipods and snails in relation to increased acidity). The diversity, or functional group abundance (Fiberg et al. 1980; importance of insect behaviors in response to acidification and Hal1 et al. 1980; Hildrew et al. 1984; Pallard and Moreau 1987; the implications of behavioral alterations in determining the Hopkins et al. 1989; Hall 1990). Studies concerning individual outcome of higher level interactions has not yet been addressed. responses to acidification have focused on physiology, body Aquatic insects might use behaviors such as drifting or mass changes, and survival or emergence (Bell 1971; Fiance burrowing to evade exposure to short-tern1 increases in acidity 1978; Mavas and Hutchinson 1983; Allad and Msreau 1986; One might expect some behaviors to predominate depending on Cornea et al. 1986; Hemann and Andersson 1986; Wowe et al. the severity of the disturbance (Resh et al. 1988) and the 1988, 1989; Mackie 1989; Chmielewski and Hdl 1992). Drift energetic trade-offs between behavior and physiology (Dill rates are probably the most widely examined response to 1987). In particuIar, regulatory behaviors (e.g., Wiley and Kohler acidification (Hall et al. 1980; Pratt and Hdl 1981; Bmerod 1984) often involve conflicts with other behaviors md we likely et al. 1987; Hopkns et al. 1989; Bernard et al. 1990): generally to be the most sensitive indicators sf stress. For example, under drift increases in most taxa. Grazing mayflies (e.g., baetids and low oxygen conditions metabolic carbon dioxide must be siphlonurids), and small chironomid midges (Orthcladiinae) counteracted before tissue acidosis occurs (Nation 1985). appear to be the most sensitive insects to reduced pH. Although Mayflies might increase gill ventilation rates to overcome a increased drift rates have been documented with experimental minimal oxygen debt or drift if the oxygen stress is severe acid additions, it is unknown whether this drift response is active, (Eriksen 1963; Wiley and Kohler 1988). Drifting, howeven; may passive, or a combination of these. Other factors (both genetic pose added predation risks from visually feeding fish (Waters 2692 Can. 9. Fish. Aguat. Sci., \Gl. 50, 1993 1972; Allan 1978, 1984; Stewart and Szcytko 1983; Skinner TABLE1. Water chemistry for the Mullem Creek (MC) shady site and 1985). Thus, a conflict would arise if predation risk was high and Little Brooklyn Lake inlet (EB) nymph collection site on 8 August 1991. oxygen stress was greater than could be overcome by increases All measurements are in ~eqLunless stated otherwise. in ventilation rate. site As with a low oxygen stress, increased acidity may lead to a physiological disruption that requires rectification. In mayflies, metabolic ion concentrations are disrupted with increasing acidity (Rowe et a%.1988, 1989; but see Bemill et al. 1991), and pH depending on the costs, certain behavioral responses might be Ternperatua-e ("C) Conductivity (y~/crn~ expected as counter measures. For example, behaviors which Alkalinity ensure adequate evasion of the stress, yet do not increase ca2+ predation risks, might be favored. If behavioral responses to kIg2+ stress maximize regulatory efficiency based on energetic N$ trade-offs, drift (a physiologically inexpensive, direct escape) should increase under stressful conditions (i.e., reduced pH) if there are no other associated costs. If there are costs to drifting, behaviors other than drifting may be even more sensitive indicators of acid stress in aquatic insects. At low-level pH stress, trade-offs may exist between risks associated with physiological intolerance and predation such that drifting is avoided until physiologically necessary. than 1780 rn (Jensen 1966; Wapd 1985). Like most Here we report the results of a replicated experimental stream emphemerellids, D. coloradensis is a poor swimmer, generally study in which we reduced ambient pH and observed Brunella slow-moving, and exhibits a clinginglsprawling life style coloradensis nymphs for behavioral changes. We predicted that: (Edmunds 1984). Primary food items include diatoms at early (1) nymphal behavior would be influenced by low-level, short- instars and animal matter during later instaps. In Oregon this tern pH reductions and (%) drifting (a potentially expensive species exhibits a univoltine life history and shows very behavior in terns of predation, but energetically inexpensive) synchronous nymphal growth (Hawkins 1990). Adults emerge would occur more often at the lowest pH levels examined com- in late sumner and early autumn (Jensen 1966; personal pared to lesser pH declines. Our results suggest that nymphal observation). This species occurs in both LB and MC, but in low behavior is influenced by acidity increases, but to different numbers in MC. degrees depending on the severity of pH reductions. We discuss these results in reference to monitoring for acidification effects and Experimental Procedures the implications of behavioral alterations ira response to stress. Drunella ccslomdensis nymphs were collected from LB on Study Site and Methods 3 1 July and 6 August 1991. Nymphs were picked from stones using a fine bmsh or forceps, placed in a cooler with LB stream water, and transported to the study reach on MC, nymphs were Site Description placed in in-stream enclosures for at least 48 h prior to use in Two montane streams in the Rocky Mountains of southeast experiments. In-stream enclosures were weighted sieving Wyoming were used; Little Brooklyn Lake irdet (LB) and buckets and contained MC substrate with attached periphyton. Mullen Creek (MC). LB is a second order stream at 3158 rn Only insects that responded to stimulation with a bmsh were elevation in a remote location of the Medicine Bow National used for experiments. Nyn~phswere identified using keys of Forest in southeast Wyoming (4 1'22'8%: 106" 15'N). MC is a first Edmunds (1984) and Ward (1985). order stream at 22572 m elevation located adjacent to the United Experimental stream chmbers were constructed of clear States Forest Service (USFS) research station 1 h west of Plexiglas. The chambers measured 57.0 x 33.5 x 20 cm with a Centennial, WY (41 " 18W, 1106OO9'N). Substrate, flow and water 10-cm collecting basin at each end (Fig. 1). Five panels divided chemistry?especially pH and alkalinity, are similar between the each chamber into six, 5 x 34.5-cm stream channels. A l-m LB and MC sites, which are approximately 10 h apart mesh plastic screen sewed as a barrier on the upper and lower (Table 1).