Tracking Trophic Interactions in Coldwater Reservoirs Using Naturally Occurring Stable Isotopes

Abstract

We measured signatures of naturally occurring stable isotopes of carbon ( 13 C) and nitrogen ( 15 N) in important invertebrate and fish taxa in two coldwater reservoirs in Colorado that had different food webs. One reservoir, Lake Granby, contained a large population of an opossum shrimp, Mysis relicta, and the other, Blue Mesa Reservoir, did not. We compared temporal dynamics of isotopic signatures of all taxa between lakes to quantify sources of variability in consumer signatures and to assess potential turnover rates of isotopic signatures in the top predator. Stable isotope signatures varied little across season or body size within most taxa, but large differences across reservoirs were observed. Nitrogen signatures of lake trout Salvelinus namaycush were enriched in larger individuals in both reservoirs, indicating increasing trophic position with body size. Carbon signatures in lake trout livers were consistently lower than those in muscle samples but did not change more rapidly than muscle signatures, suggesting that either lake trout diet was relatively invariant during the study or that liver signatures change at the same rate as muscle signatures in these fish. Inferences about diets from carbon and nitrogen signatures were corroborated by information from stomach analysis. Food web structure derived from stable isotope analysis indicated large differences in energy and materials pathways between the reservoirs with and without M. relicta. Rather than diversifying trophic structure by providing an alternative food source for planktivorous sport fish, as the introduction originally intended, M. relicta in Lake Granby short-circuited a major energy conduit, the zooplankton, from channeling pelagic primary production into sport fish biomass.

Turnover rates of nitrogen stable isotopes in the salt marsh mummichog, Fundulus heteroclitus, following a laboratory diet switch

“…Previously observed similarity between liver and muscle isotopic turnover rates in broad whitefish, Coregonus nasus, (Hesslein et al 1993) and in lake trout, Salvelinus namaycush, (Johnson et al 2002) could have been related to cold water conditions.…”

“…Juvenile broad whitefish were maintained at 10ºC (Hesslein et al 1993) while lake trout were collected from cold water reservoirs (Johnson et al 2002), and typically occupy colder waters with ideal temperatures near 10ºC (Scarola 1987).…”

“…While isotopic turnover rates in ectotherms should theoretically also vary according to relative tissue metabolic activity, experimental results have not been able to show significant variation amongst tissues in ectotherms (Hesslein et al 1993;Johnson et al 2002).…”

Predation on Fishes in the Sacramento-San Joaquin Delta: Current Knowledge and Future Directions

“…It is likely that Delta predators prefer fish prey that are energy-rich (Hartman and Brandt 1995), easily handled and consumed (i.e., small, softrayed and fusiform) and naïve to invasive predators (Kuehne and Olden 2012;Carthey and Banks 2014). Several authors document the importance of behavioral naiveté to predation mortality by demonstrating that invasive Lake Trout and Northern Pike feed disproportionately on salmonids despite the apparently higher abundance of native catostomid prey (Johnson and Martinez 2000;Johnson et al 2002;Lepak et al 2012). Given that the majority of fish predators in the Delta are invasives (Table 1), native fishes in the Delta may suffer increased predation because a lack of shared evolutionary history could produce predator naiveté in prey species (reviewed by Carthy and Banks 2014).…”

Importance of benthic production to fish populations in Lake Mead prior to the establishment of quagga mussels

“…While there is limited information available on the reliance of benthic fishes from manmade reservoir ecosystems, Johnson et al (2002) utilized isotoptic collections to determine the food web structure of 2 reservoirs in Colorado. The game fish in one reservoir were largely supported from resources from the bottom of the lake; however, the addition of one nonnative invertebrate species greatly altered the structure of a similar reservoir whose top predators fed mostly in the pelagic zone.…”