ABSTRACT We used dual-isotope mixing models (d 13 C/d 15 N and d 2 H/d 15 N) in a Bayesian framew... more ABSTRACT We used dual-isotope mixing models (d 13 C/d 15 N and d 2 H/d 15 N) in a Bayesian framework to partition allochthonous and autochthonous energy sources for salmonids in 2 headwater streams in the Kenai Lowlands, Alaska (USA). Our 1 st objective was to estimate the production base for juvenile coho salmon (Oncorhynchus kisutch) and Dolly Varden (Salvelinus malma). We hypothesized that consumers would be reliant on both autochthonous (filamentous algae and periphyton) and allochthonous sources, but that autochthonous sources would dominate because of the open canopy and lower-quality litter inputs provided by the riparian wetland vegetation, primarily bluejoint grass (Calamagrostis canadensis). Our 2 nd objective was to evaluate the utility of stable H isotopes for tracing energy pathways in a northern-latitude ecosystem. We hypothesized that d 2 H-based models would provide more precise estimates of source partitioning than d 13 C-based models because of greater source separation. Allochthonous source contributions consistently exceeded autochthonous sources for all fish species and size classes at both study sites. However, diet shifted during ontogeny, and larger Dolly Varden relied more on autochthonous sources than did smaller individuals of both species. Last, we found good correspondence and similar levels of precision between the d 13 C-and d 2 H-based models despite greater source separation by d 2 H. Our results highlight the importance of allochthonous sources in headwater streams, and we suggest that litter inputs from grasses may be an under-appreciated subsidy to salmon production. Stable H isotopes can be an effective foodweb tracer in northern-latitude streams, but source partitioning results were not sufficiently different from stable C isotope models for us to recommend unequivocally using them to replace or enhance d 13 C in similar studies. Ratios of stable H isotopes (2 H/ 1 H, d 2 H) have many applications in ecology, including the study of food webs (Smith and Epstein 1970), animal migrations (Hobson and Wassenaar 1997), hydrologic models (Gibson et al. 2002, Pham et al. 2009), and delineation of methanogenic pathways in lakes (Deines et al. 2009). The utility of d 2 H as a tracer of energy flow has been explored in the context of partitioning allochtho-nous and autochthonous resources in aquatic ecosys-tems (Doucett et al. 2007, Jardine et al. 2009, Finlay et al. 2010). Stable C isotopes (13 C/ 12 C, d 13 C) have been used successfully to quantify energy pathways in lotic ecosystems (Doucett et al. 1996b), but basal resource signatures are not always distinct, prohibit-ing source partitioning (France 1995, Doucett et al. 1996a, Finlay 2001). In contrast, separation between allochthonous and autochthonous resources may be more consistent for stable isotopes of H, thus providing
Executive Summary The main purpose of this report is to provide an up-to-date assessment and conc... more Executive Summary The main purpose of this report is to provide an up-to-date assessment and conceptual model of factors affecting Delta Smelt (Hypomesus transpacificus) throughout its primarily annual life cycle and to demonstrate how this conceptual model can be used for scientific and management purposes. The Delta Smelt is a small estuarine fish that only occurs in the San Francisco Estuary. Once abundant, it is now rare and has been protected under the federal and California Endangered Species Acts since 1993. The Delta Smelt listing was related to a step decline in the early 1980s; however, population abundance decreased even further with the onset of the “pelagic organism decline” (POD) around 2002. A substantial, albeit short-lived, increase in abundance of all life stages in 2011 showed that the Delta Smelt population can still rebound when conditions are favorable for spawning, growth, and survival. In this report, we update previous conceptual models for Delta Smelt to re...
ABSTRACT We used dual-isotope mixing models (d 13 C/d 15 N and d 2 H/d 15 N) in a Bayesian framew... more ABSTRACT We used dual-isotope mixing models (d 13 C/d 15 N and d 2 H/d 15 N) in a Bayesian framework to partition allochthonous and autochthonous energy sources for salmonids in 2 headwater streams in the Kenai Lowlands, Alaska (USA). Our 1 st objective was to estimate the production base for juvenile coho salmon (Oncorhynchus kisutch) and Dolly Varden (Salvelinus malma). We hypothesized that consumers would be reliant on both autochthonous (filamentous algae and periphyton) and allochthonous sources, but that autochthonous sources would dominate because of the open canopy and lower-quality litter inputs provided by the riparian wetland vegetation, primarily bluejoint grass (Calamagrostis canadensis). Our 2 nd objective was to evaluate the utility of stable H isotopes for tracing energy pathways in a northern-latitude ecosystem. We hypothesized that d 2 H-based models would provide more precise estimates of source partitioning than d 13 C-based models because of greater source separation. Allochthonous source contributions consistently exceeded autochthonous sources for all fish species and size classes at both study sites. However, diet shifted during ontogeny, and larger Dolly Varden relied more on autochthonous sources than did smaller individuals of both species. Last, we found good correspondence and similar levels of precision between the d 13 C-and d 2 H-based models despite greater source separation by d 2 H. Our results highlight the importance of allochthonous sources in headwater streams, and we suggest that litter inputs from grasses may be an under-appreciated subsidy to salmon production. Stable H isotopes can be an effective foodweb tracer in northern-latitude streams, but source partitioning results were not sufficiently different from stable C isotope models for us to recommend unequivocally using them to replace or enhance d 13 C in similar studies. Ratios of stable H isotopes (2 H/ 1 H, d 2 H) have many applications in ecology, including the study of food webs (Smith and Epstein 1970), animal migrations (Hobson and Wassenaar 1997), hydrologic models (Gibson et al. 2002, Pham et al. 2009), and delineation of methanogenic pathways in lakes (Deines et al. 2009). The utility of d 2 H as a tracer of energy flow has been explored in the context of partitioning allochtho-nous and autochthonous resources in aquatic ecosys-tems (Doucett et al. 2007, Jardine et al. 2009, Finlay et al. 2010). Stable C isotopes (13 C/ 12 C, d 13 C) have been used successfully to quantify energy pathways in lotic ecosystems (Doucett et al. 1996b), but basal resource signatures are not always distinct, prohibit-ing source partitioning (France 1995, Doucett et al. 1996a, Finlay 2001). In contrast, separation between allochthonous and autochthonous resources may be more consistent for stable isotopes of H, thus providing
Executive Summary The main purpose of this report is to provide an up-to-date assessment and conc... more Executive Summary The main purpose of this report is to provide an up-to-date assessment and conceptual model of factors affecting Delta Smelt (Hypomesus transpacificus) throughout its primarily annual life cycle and to demonstrate how this conceptual model can be used for scientific and management purposes. The Delta Smelt is a small estuarine fish that only occurs in the San Francisco Estuary. Once abundant, it is now rare and has been protected under the federal and California Endangered Species Acts since 1993. The Delta Smelt listing was related to a step decline in the early 1980s; however, population abundance decreased even further with the onset of the “pelagic organism decline” (POD) around 2002. A substantial, albeit short-lived, increase in abundance of all life stages in 2011 showed that the Delta Smelt population can still rebound when conditions are favorable for spawning, growth, and survival. In this report, we update previous conceptual models for Delta Smelt to re...
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