Hydrobiologia (2015) 753:131–147 DOI 10.1007/s10750-015-2201-9 PRIMARY RESEARCH PAPER Stable isotope analysis reveals relative influences of seasonal hydrologic variation and impoundment on assimilation of primary production sources by fish in the Upper Yesilırmak River, Turkey Nehir Kaymak • Kirk O. Winemiller • Senol Akin • Zekeriya Altuner • Fatih Polat • Tarik Dal Received: 19 September 2014 / Revised: 27 January 2015 / Accepted: 28 January 2015 / Published online: 7 February 2015 Ó Springer International Publishing Switzerland 2015 Abstract This study analyzed variation in stable spatial variation. d13C of primary producers, d15NofS. isotope ratios of aquatic and terrestrial primary pro- cephalus, and both d13Candd15NofC. banarescui ducers and two common cyprinid fishes (Capoeta revealed significant seasonal variation. C. banarescui banarescui and Squalius cephalus) at seven sites in the biomass in the river channel was mostly derived from upper Yes¸ilırmak River Basin, Turkey, to estimate terrestrial herbaceous plants, and its biomass in the relative contributions of basal production sources to fish reservoir derived mostly from aquatic plants. Estimated biomass. We hypothesized that seasonal and spatial proportional contributions of herbaceous plants to both variation in fish assimilation of basal production species were greatest at the downstream site during sources would be affected by hydrological variation, spring, and declined during summer in the case C. with results for a reach downstream from a reservoir banarescui. Overall, the influence of the dam was small differing from those from upstream sites. Carbon and relative to effects from watershed characteristics and nitrogen isotopic ratios of primary producers and S. seasonal changes in temperature and hydrology. cephalus and d15NofC. banarescui revealed significant Keywords Capoeta banarescui Á Squalius cephalus Á Fluvial gradient Á Primary production Á Handling editor: M. Power River impoundment Á Stable isotope N. Kaymak (&) Á S. Akin F. Polat Department of Fisheries, Faculty of Agriculture, Department of Environmental Protection and Control, Gaziosmanpasa University, 60240 Tokat, Turkey Almus Vocational School, Gaziosmanpasa University, e-mail: [email protected] 60240 Tokat, Turkey e-mail: [email protected] S. Akin e-mail: [email protected] T. Dal Department of Aquaculture, Almus Vocational School, K. O. Winemiller Gaziosmanpasa University, 60240 Tokat, Turkey Program in Ecology and Evolutionary Biology and e-mail: [email protected] Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX 77843-2258, USA e-mail: [email protected] Z. Altuner Department of Biology, Faculty of Art and Science, Gaziosmanpasa University, 60240 Tokat, Turkey e-mail: [email protected] 123 132 Hydrobiologia (2015) 753:131–147 Introduction & Thorp, 2006; Herwig et al., 2007; Leigh et al., 2010), with only a few having examined highly regulated Major sources of primary production, food resources, rivers (Ward & Stanford, 1983; Hoeinghaus et al., and abundance and feeding habits of aquatic organ- 2007). Here, we investigate food web dynamics of the isms vary along fluvial gradients. Upstream processes upper Yesilirmak River in north-central Turkey, a in rivers influence downstream processes (river con- system impacted by two reservoirs (Almus and tinuum concept, RCC), which results in structural Atakoy). Our study area was centered on the Almus changes in aquatic food webs from headwater streams Reservoir at elevations between 640 and 1,040 m. The to lower reaches. Dams disturb natural processes climate is semi-arid and stream flows vary seasonally, associated with longitudinal fluvial gradients and alter with low flows between July and February and high biotic communities and food web dynamics (Romanuk flows between March and May. Using stable isotope et al., 2006). Availability of primary production ratios of two omnivorous fish species, Capoeta sources for aquatic consumers is influenced by banarescui and Squalius cephalus, we tested predic- hydrology (Roach et al., 2014), nutrient enrichment tions of alternative models for material and energy from agricultural and urban sources (Pingram et al., transfer in food webs and spatiotemporal variation of 2012), and dams that regulate flows in downstream contributions from different primary production reaches (Shannon et al., 2001; Hoeinghaus et al., 2007; sources. We hypothesized that the dam affects as- ¨ ¨ Marty et al., 2008;Vorosmarty et al., 2010). Several similation of carbon sources by omnivorous fishes, and studies concluded that flows released from reservoirs that this influence varies among locations within the shifted diets of fishes and invertebrates in downstream drainage network. Specifically, fish biomass at reaches (Chester & Norris, 2006; Smokorowski et al., relatively unimpacted upstream locations should 2011), whereas other studies concluded no major derive mostly from riparian terrestrial plants as effects (Chessmen et al., 2009; Rolls et al., 2012). predicted by the River Continuum Concept. In river In their description of the RCC, Vannote et al. reaches with greater channel width and relatively less (1980) emphasized the importance of different prima- canopy cover, importance of benthic algae and phyto- ry production sources supporting consumers in differ- plankton should be greater (Vannote et al., 1980; ent segments along the longitudinal fluvial continuum. Chang et al., 2012). Algae (phytoplankton) also should Two alternative models for food web ecology of rivers be important primary production source supporting fish are the riverine productivity model (RPM, Thorp & biomass in the reservoir (Hoeinghaus et al., 2007). Delong, 1994) and the flood pulse concept (FPC, Junk Downstream from the reservoir, drifting zooplankton et al., 1989). All three models have been supported by from the reservoir represent a high-quality food field studies. Herwig et al. (2004), Hadwen et al. resource for consumers and could thereby subsidize (2010), and Chang et al. (2012) obtained results the food web (Doi et al., 2008; Mercado-Silva et al., consistent with the RCC. Research on tropical and 2009; Helmus et al., 2013; Power et al., 2013; Wellard temperate floodplain rivers (Hoeinghaus et al., 2007; Kelly et al., 2013). We hypothesized that during spring, Zeug & Winemiller, 2008) revealed that flood pulses when flows are higher, fish within the main channel had a strong influence on the entrance of terrestrial assimilate more material derived from terrestrial primary production into aquatic food webs, a finding production sources (Junk et al., 1989; Huryn et al., consistent with the FPC. Phytoplankton, benthic algae, 2001; Herwig et al., 2004); during low-flow period in and aquatic vascular plants were found to be the the summer, when flow conditions are more suitable principal carbon sources assimilated by consumers in for algal growth, aquatic benthic sources (e.g., benthic temperate, tropical, and dry-land floodplain rivers algae, filamentous algae) and phytoplankton should be (Leigh et al., 2010; Hladyz et al., 2012; Jackson et al., more important because lower water velocity, greater 2013), but riparian vegetation also was important light penetration, and higher temperatures promote during periods of low as well as high flows, which is algal growth in the river (Huryn et al., 2001; Finlay consistent with the RPM. et al., 2002). To test these hypotheses, we analyzed and Most attempts to evaluate these models have compared carbon and nitrogen stable isotope ratios of investigated relatively large, lowland rivers over one the principal primary production sources and two or two seasons and limited ranges of discharge (Delong 123 Hydrobiologia (2015) 753:131–147 133 omnivorous cyprinid fishes (Capoeta banarescui and Reservoir, five locations in river reaches located Squalius cephalus) from locations above, within, and above the reservoir (812–1083 m elevation), and one below Almus Reservoir on the Yes¸ilırmak River location in a river reach downstream from the during three seasons. reservoir (Fig. 1). The reservoir shoreline has little vegetation cover and the region surrounding the reservoir is dominated by oak (Quercus spp.), poplar Methods (Populus spp.), black pine (Pinus nigra), and Scots pine (Pinus sylvestris). Upstream sites 1 and 3 were in Study area the main channel of the Yes¸ilırmak River, upstream site 2 was a tributary connected to the main channel. The Yes¸ilırmak River, the second longest river in Current velocities are faster at upstream sites 1 and 2. Turkey, originates within the Ko¨se Mountains in Channel width of upstream site 3 is wider compared to northeastern Turkey and flows into the Black Sea. the other upstream sites and downstream site. Tribu- Northeastern Turkey has a semi-arid climate with taries 1 and 2 are small and flow directly into the large seasonal variation in temperatures. Water reservoir. Oak, poplar, willow (Salix spp.), plane temperature ranges from -2to26°C annually. The (Platanus spp.) and mahaleb cherry (Prunus mahaleb) river catchment is 36.114 km2, and the river length is trees, and herbaceous plants dominate riparian vegeta- 519 km. Annual precipitation is 500 mm, and mean tion communities of the main channel and tributaries. annual discharge of the Yes¸ilırmak is 5.80 km3. The exceptions were site 2 bordered by pastures and Flows are low during winter and summer and high site 3 bordered only by herbaceous plants. A single during spring in response to snowmelt