Hydrobiologia (2020) 847:3091–3101 https://doi.org/10.1007/s10750-020-04317-8 (0123456789().,-volV)( 0123456789().,-volV) PRIMARY RESEARCH PAPER Trophic basis of production of stream detritivores shifts with reduced forest inputs Susan L. Eggert . J. Bruce Wallace . Judy L. Meyer . Jackson R. Webster Received: 25 September 2019 / Revised: 23 May 2020 / Accepted: 27 May 2020 / Published online: 6 June 2020 Ó This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2020 Abstract Estimating changes in organic matter flow declined after 3 years of litter exclusion. After wood from resource to consumer using trophic basis of removal, production of detritivores declined again. production (TBP) is a way to examine resource Steepest declines in Pycnopsyche gentilis production limitation effects on ecosystem function. We exam- occurred within year 1. Tipula spp. and Tallaperla spp. ined diet shifts and production of insect detritivores to production declined after wood removal. Diets shifted assess changes with reduced detrital inputs to a from leaves to wood to fine particulate organic matter forested headwater stream. Organic matter was (FPOM) for Tipula spp. and Tallaperla spp., but not excluded for 7 years using a canopy net. Small and for P. gentilis. Resource flows to detritivores shifted in large wood were removed from the stream after the 3rd the exclusion stream from leaves to wood to FPOM and 5th year, respectively. Detritivore production after leaf standing crops declined and wood removal. Small wood was an important food resource. TBP results showed shifts in food resource use by two Handling editor: Marcelo S. Moretti detritivores with terrestrial input reduction. These Electronic supplementary material The online version of findings suggest that maintaining diverse riparian this article (https://doi.org/10.1007/s10750-020-04317-8) con- inputs of organic matter is important for detritivore tains supplementary material, which is available to authorized productivity in forested headwater watersheds. users. S. L. Eggert (&) Keywords Organic matter Á Detritus Á Gut contents Á USDA Forest Service, Northern Research Station, Freshwater invertebrate Á Riparian Á Flow food web Grand Rapids, MN 55744, USA e-mail: [email protected] S. L. Eggert Á J. B. Wallace Department of Entomology, University of Georgia, Introduction Athens, GA 30602, USA In forested headwater streams, the majority of energy J. B. Wallace Á J. L. Meyer Odum School of Ecology, University of Georgia, Athens, inputs are detrital (e.g., Fisher & Likens, 1973; GA 30602, USA Webster & Meyer, 1997). Positive relationships between terrestrially derived organic matter and J. R. Webster stream invertebrate productivity have been demon- Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, strated in a long-term exclusion of allochthonous VA 24061, USA 123 3092 Hydrobiologia (2020) 847:3091–3101 inputs to a headwater stream (e.g., Wallace et al., effects of riparian disturbance on functioning of the 1997, 1999, 2015) and short-term leaf litter additions lowest trophic levels. We anticipated shifts in organic (e.g., Richardson, 1991). Leaf litter is the preferred matter flows from leaves to wood during initial stages food resource for many stream invertebrates, but may of the litter exclusion and another shift toward become scarce during certain times of the year (e.g., amorphous detritus flows (the next largest pool of Grubbs & Cummins, 1996). Wood without biofilm is a organic matter remaining) following the removal of less-palatable source of carbon, but with its epixylic wood resources. Additionally, our intent was to biofilm of amorphous detritus, fungi, and bacteria, can differentiate the importance of different detrital also be an important food resource for some inverte- resources (i.e., more palatable leaf detritus vs. more brates (e.g., Tank & Winterbourn, 1996; Eggert & refractory wood without biofilm detritus), and to better Wallace, 2007). The physical structure of wood makes understand mechanisms of resource partitioning it a long-lasting resource for microbes and inverte- among a guild of stream detritivores. brates alike (e.g., Benke, 1985; Hax & Golladay, 1993; Tank et al., 1993). Fine particulate organic matter (FPOM) stored in the stream may serve as an Methods additional detrital resource (e.g., Tank & Winter- bourn, 1995). Study sites The trophic basis of production calculation traces organic matter flow between resources and consumers The reference and litter exclusion streams draining in a food web, making it a useful tool to quantify the Catchments 53 and 55, respectively, are located within strength of resource-consumer linkages in ecosystems Coweeta Hydrologic Laboratory in the Blue Ridge (e.g., Benke & Wallace, 1980, 1997; Benke et al., province of the southern Appalachian Mountains in 2001; Benke, 2018), describe the energy base of western North Carolina. Leaf litter and wood were streams (e.g., Smock & Roeding, 1986), and demon- excluded from 170 m of the treatment stream from strate alterations of organic matter flow with changes August 1993 to August 2000 using a 2.5-cm mesh gill in food resources (e.g., Hall et al., 2000; Rosi-Marshall net canopy over the entire stream and 1-cm mesh bird & Wallace, 2002). The method relies on estimates of netting along each bank. The sides of the canopy were secondary production, resources consumed, and open to allow for aerial insect recolonization. The assimilation of resources by consumers (Benke & canopy started at the spring-seep, preventing organic Wallace, 1980). Our objective here was to use the matter inputs from upstream. In August 1996, small trophic basis of production method to assess changes wood (\ 10 cm diameter) was removed by hand from in resource use among detritivores in response to a the exclusion stream (Wallace et al., 2000). Large long-term, whole-stream manipulation of detrital wood was removed by hand from the exclusion stream resources. in August 1998 (Wallace et al., 2001). The reference The experimental exclusion of litter inputs to an and litter exclusion streams are similar with respect to Appalachian headwater stream and the subsequent discharge, thermal regime, elevation, aspect, size, and removal of wood altered the resource base of the canopy cover (Electronic Supplementary Material: stream (Wallace et al., 1997, 1999, 2015). Leaf litter Table S1). Both watersheds are heavily forested by standing crops declined quickly in the exclusion Quercus spp. (oak), Acer rubrum L. (red maple), stream, but alternate food resources such as wood Liriodendron tulipifera L. (tulip popular), and dog- and fine particulate organic matter (FPOM) remained wood Cornus florida L. (flowering dogwood), with a abundant after 3 years of litter exclusion (Eggert et al., dense riparian understory of Rhododendron maxima L. 2012). We used the trophic basis of production method (rhododendron), which provides year-round shading. to quantify changes in organic matter flow associated with a step-wise reduction in detrital resources avail- Study organisms able to a detritivore assemblage in a headwater stream over an 8-year period. Gut content analyses, inverte- We estimated organic matter flows (hereafter referred brate production estimates, and assimilation efficien- to as flows) from food resources to insect detritivores cies for various food resources were used to measure using the trophic basis of production calculation 123 Hydrobiologia (2020) 847:3091–3101 3093 (Benke & Wallace, 1980). We focused on the dom- Gut analyses inant (54–77% of shredder production) shredders Pycnopsyche gentilis (McLachlan, 1871) (caddisfly), Pycnopsyche gentilis, Tipula spp., and Tallaperla spp. Tipula spp. (fly), and Tallaperla spp. (stonefly). individuals were collected seasonally (au- Additionally, we estimated flows to uncommon tumn = September, October, November; win- shredder taxa Leuctra spp. (stonefly), Lepidostoma ter = December, January, February; spring = March, spp. (caddisfly), Molophilus spp. (fly), Fattigia pele April, May; summer = June, July, August) from each (Ross, 1938) (caddisfly), Psilotreta spp. (caddisfly), site from spring 1993 to summer 2000. Three to five Limonia spp. (fly), Anchytarsus spp. (beetle) and similar-sized individuals of each taxon were examined Lipsothrix spp. (fly); hereafter referred to as ‘other seasonally for each stream. A minimum sample size of shredders.’ We measured secondary production for three was determined by calculating a coefficient of each of the rare shredders and estimated diets based on variation of percent food type consumed for various the mean diets of the three dominant shredders. Mean sample sizes of individuals of a taxon. Individuals of annual organic matter flows from each food resource each taxon were not found during every seasonal to the study organisms were calculated for each stream collection. Diets of organisms collected seasonally for the pre-treatment period (1992–1993), the litter were examined by measuring the relative area of exclusion only period (1993–1994, 1994–1995, and particles of each food type in the guts using the 1995–1996), the period following small wood removal methods of Cummins (1973). The proportional area of (1996–1997 and 1997–1998), and the period after each food type (leaf detritus, wood, amorphous large wood removal (1998–1999 and 1999–2000). detritus, animal, diatoms, fungi, and miscellaneous which included moss) was calculated for each indi-