Stable Carbon and Nitrogen Isotope Tracers of ...

tope Tracers of Trophic Dynamics e Carbon and N in Natura ns and Fisheries of the Lahontan Lake System, Nevada Marilyn L. F. Estep Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by STANFORD UNIV. on 08/05/13 For personal use only.

Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20008, USA

and Steven ~ i g g ' Biological Sciences Center, Desert Research Institute, P.O. Box 60220, Reno, NV 89506, USA

Estep, I%%. L. F., and S. Vigg. 1985.Stable carbon and nitrogen isotope tracers of trophic dynamics i n natural populations and .fisheries of the bahontan bake system, Nevada. Can. 8 . Fish. Aquat, Sci, 42: 1712-1719. The influence of combined inorganic N o n the growth of N2-fixing blue-green algae (Aphanizomenon flos-aquae) in Lahontan Reservoir was detected with N isotopic compositions (6I5N = +I ,2 to +7.0) that I n Pyramid Lake, however, the 6 " ~of the entire food web was influenced by a were enriched i n contribution s f isotopically light N released from N2-fixing blue-green algae (NoduBaria spumigena). Carbon isotope ratios (bflc) of these unpalatable, blue-green algae from both lakes were different enough from the zooplankton and higher animals to preclude a direct trophic link. An enrichment i n l3cwith trophic level i n the food chain was measured i n both lakes. Carbon isotope ratio measurements clearly illustrated the isotopic similarity between hatchet-y-raised Lahontan cutthroat trout (Salms c l a ~ kheni shawi) and cui-ui (Chasmistes cujads) and the artificial diet, but differed from the 6'" of their wild counterparts consuming natural foods. There is a consistent isotope fractionation between the 6I3C of scales and the 6% of muscle from fish that is species specific. This finding demonstrates that fish scales, as well as muscle, can b e used t o determine diet. h'influence de l'azote inorganique combine sur la croissance d'une algue bleu-vert (Aphawizomenon flos-aquae) fixatrice de I ' a o t e dans le r6servoir bahontan a ete dktectee avec des compositions isotspiques d'azote (8% = +1,2 3 +7,0)enrichies de I5P4. mans le lac Pyramid, le 6I5bJ de toute la chaine alimentaire etait toutefois ssus I'influence d'un apport d'ura isotope Ieger de N emis par une algue bleu-vert (NoduBaria spumigena) fixatrice de I'azote. Les rapports des isotopes de C (613c) dans ces akues bleu-vert desagr6ables au goist provenant de ces deux lacs ktaient suffisamment differents de ceux d u nooplancton et des animaux superieurs pour exclure u n lien trophique direct. O n a quantifie Efenrichissement en I 3 Cen fsnction d u niveau trophique de la chaine alimentaire dans les deux bassiws. hes quantifications d u rapport des isotopes de C ont clairement illustre la ressemblarace isotopique entre, d'une part, la truite fard6e de hahontan (SaBms cBarki henshawi) et le caei-ui (Chamistes cujus) produits en eclsserie et, d'autre part, les aliments artificiels; ces quantifications etaient toutefois differentes de 6I3c garbsent chez leurs cousins sauvages se nourrissant d'aliments naturels. II existe u n fractionnernent isotopiqaee uniforme particulier A chaque espece entre le 6I3c des 6cailles et Be des muscles de poisson. Cette d6couverte revele que les 6cailIes de poisson ainsi que les muscles peuvent &re utilises pour gtablir le regime alimentaire. Received February 6/ 198.5 Accepted ]uBy 22, 198.5 (J8l08)

nergy flow though trophic levels of biological systems can sometimes be traced by measuring variation in the natural abundance of stable isotopes (e.g . C, N, H, and S ) . Stable isotopes can be useful tracers because inorganic nutrients as well as organic dietary sources in the food web may have different isotopic compositions (see Fry and Shem 1984for a recent comprehensive review). In plants the isotopes in inorganic C and N sources are fractionated as the elements a e "resent address: U.S. Fish and Wildlife Service, Willard Substation, Seattle National Fishery Resemh Center, Cook, WA 98605, USA.

metabolized by specific biochemical pathways. For example, @ isotope ratios can distinguish between vascular plants that fix C from C 0 2into a 3- versus 4-carbon molecule (Smith and Epstein 1971). Stable N isotopic compositions may distinguish plants that fix atmospheric N2 from those that metabolize soluble nitrogenous compounds (Virginia and Delwiche 1982). En recent yeas, stable isotope techniques have been successfully applied to the study of bioenergetics in estuarine and marine ecosystems. Marine algae and phytoplankton cornonly have C and I\% isotopic compositions different from those of terrestrial higher plants and seagrasses. The relative importance of each type of plant to the food web can thus be assessed. Cun. J . Fish. Aquat. Sci., VoB. 42, 6985

Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by STANFORD UNIV. on 08/05/13 For personal use only.

In contrast, stable isotopes have had only minimal application to freshwater ecology. Differences in the isotopic compositions of the primary producers, algae and phytoplankton, can be more variable in freshwater systems. For example, in several New England lakes, Bana and Deevey (1960) measured a HO%o variation in the C isotope ratios of plankton, which was due to differences in the 6 ' " ~of the inorganic C. The influence of seasonal variations in the biogeochemical cycle can also change ~ and Nissenbaum the 813c of phytoplankton by 1 5 % (Stiller 1980). En a food web study, Rau (1980) used C isotope ratios to determine the contribution of allochthonous organic matter to aquatic insects in a subalpine lake. Examples of N isotope studies in freshwater systems are fewer. Pang and Nriagu (19'97) measured 8 " of ~ phytoplankton and zooplankton samples from Lake Superior in addition to their extensive study of N fractions from sediments. In comparison with marine plankton, the lacustrine samples had more negative 6 1 5 values. ~ The behavior of multicomponent stable isotope traces of organic substances in lakes and rivers is not well defined or understood. In this paper several potential applications of the stable isotope technique as a tracer of biological processes in limnology and freshwater fisheries have been explored in two contrasting lakes in northern Nevada: Pyramid Lake and Lahontan Reservoir. Firstly, the effects of pollution on the N cycle were measured on inorganic m d organic N in Lahontan Reservoir. Allwhthonous input of N into a lake limited with respect to this nutrient can be calculated by measksing flow rates of rivers emptying into the lake and the concurrent concentrations of inorganic N species on a temporal basis. The fate of this N and its residence time in the lake are not straightforward measurements, but are crucial to the ecology of the organisms and the biogeschemical cycling. Stable isotopes could be useful tracers of particular sources of N because the isotopic conpositions of N in either inorganic species or organic matter may be source and pathway dependent. Secondly, the 8 1 ' ~of blue-green algae from Lahontan Reservoir were related to the number of heterocysts and their biochemical activity. Mzny filamentous blue-green algae that me capable of fixing N2 from the atmosphere have specialized cells called heterocysts that contain the enzymes necessary for Nz fixation. These blue-green algae are stimulated by low, combined-N levels to form heterocysts because the presence or uptake of ammonia often represses the induction sf heterocyst formation (Bottomley et al. 19'99). Other strains of blue-greens, however, continue to have active nitrogenase, the enzyme responsible for fixing N2 (Bottomley et al. 1979), even when ;ammonia is still available. The SI51\1 of N2-fixing blue-green dgae that have been measured is approximately 0.0%0(Wada a d Hattori 1976; Macko et al. 1982; Estep and Macko 1985) because there is little isotope fractionation during N2 fixation (Hoering and Ford 1960; Macko et al. 1982). Thirdly, one of the questions concerning N-limited lakes is the relative importance of N2-fixing blue-green algae to the nutrient cycle. Presumably, these algae are unpalatable to most zooplamkters, the primary diet of small fish (Cooper and Vigg 1985). Both lakes host a considerable population of filamentous blue-green algae during the summer months. The importance of these organisms to the N cycle and the food chain is assessed. Fourthly, stable isotopes have been used as tracers of multicomponent diets through the food webs sf the two lakes, which have distinctive fish populations. Pyramid Lake has a native fish fauna endemic to the Lahontan Basin (Vigg 1981). Notably, cui-ui (Chasmistes cujus), an endangered species found only in Can. 9. Fish. A q w t . Sci., Vol. 42, 1985

Pyramid Lake, and Lahontan cutthroat trout (Saimo clarki henshawi) are both threatened by lack of natural reproduction. Lahontan c u t t h a t trout achieves its potential as a lacustrine piscivore only in the uniquely suitable environment of Pyramid Lake. Lahontan Reservoir, in contrast, has an exotic and diverse fish fauna with little resemblance to the short and efficient food chain that evolved in Pyramid Lake. Introduced species, including c q (C'glarinus carpio) , catfish (Ictalurus spp .), Sacramento blackfish (Orthodon microlepidotus), and white bass (Morone chqyssps), dominate the fish community. Lastly, fish hatcheries and rearing facilities me managed by the Pyramid Lake Paiute tribe, who stock both cui-ui and Lahontan cutthroat trout into the lake in large numbers. The studies presented here are attempts at defining the isotopic relationship of a hatchery fish with its food and comparing hatchery-reared animals with wild ones. In addition, the isotopic composition of fish scales is compared with that of muscle to evaluate the use of tracing diet and potentially tracing agespecific scale regions to trophic level. Obviously, owing to depleted population levels and their legal status as an endangered species, cui-ui cannot be sacrificed for food-habit studies. Thus, sampling of scales is a nondestructive method for accomplishing this objective.

Study Sites The Tmckee River, Pyramid Lake, and Lahontan Reservoir are remnants of late-Pleistocene Lake Lahontan that inundated some 22 300 km2 during the last pluvial period, about 12 000 yr ago. The Tmckee River emanates from oligotrophic Lake Tahoe in the Sierra Nevada; it flows about 192 km and descends 7 15 m before entering the southwest end of Pyramid Lake (Fig. 1). Snowpack is the primary source of the river; maximum Wows occur during the spring snowmelt, and the historical mean annud discharge is about 5.9 x 10'm3. Upstream, the river water has a nearly neutral pH and a low ionic concentration (8), high temperature (>20°C), and large periphyton biomass occur. Pyramid is a teminal lake, i.e. it has no outflow. This lake is saline (-5 g TDS/L), is N limited, has seasonal blooms of the blue-green alga Nsdularia spurnigena, and is classified as mesotrophic (Gdat et al. 1981). Derby Dam, constructed in 1905, is situated on the Tmckee River, 56 Eurm upstream from Pyramid Lake. It diverts water via the Tmckee Canal to Lahontan D m and Reservoir, which were completed in 1915 on the lower Carson River. In addition to receiving the entire flow of the Carson, Lahontan Reservoir has historically received about half sf the total flow of the Tmckee River. The reservoir water is fresh (e280mg TDS/L), is N limited, has seasonal blooms of the blue-green alga Aphanizomenonfis-aquae, and may be considered eutrophic (Cooper et al. 1983).

Methods Water, sediment, and biological samples were collected from Pyramid Lake, Lahontan Reservoir, the Tmckee River, the R-SJWPCP, and the Tflackee Canal during 1982-84. Biological samples were kept frozen until they were dried at 60°C.Before

1

i

NEVADA

a*=

[RW

sample standard


where X = 13cor 15N and R = "c/"c or ' % / ' 4 ~ . The standard for C is the Peedee Belemnite (limestone) that has been assigned a 6I3c value of 0.0. The standard for N is air N2: 6 1 5= ~ 0.0-Samples with positive isotope ratios are enriched in the heavy isotope relative to the standard, whereas those with negative isotope ratios are depleted with respect to the standard. The reproducibility of the measurement is 263.1 for g3cand k0.2 for S1"N.

Results and Discussism Tracing the Inorganic Source of N with Stable Isotopes

. 1 . T m c k e e River hainage basin including Pyrmid Lake and Lahontan Resewsir, Nevada. DD = Derby Dam; STP = sewage treatment plant. I

dissection, the thawed fish were washed with tap water. The muscle of fish was used routinely as the representative sample of the whole fish. Fish scales and sediment were acidified with 2 mol HCL/L, which was then decanted to remove carbonate and surface bacteria prior to analysis. The N isotopic cornposition of dissolved inorganic N species was determined on H20 samples after centrifugation followed by filtration through a 8 . 8 - ~ mMillipore filter to remove fine organic materials. The organic residue was dried in an oven at 60BCand the supernatant and filtrate were evaporated. These samples were subsequently preserved in H2S04. The organic residue was subjected b Kjeldahl digestion, and the ammonia from either the water or orgmic samples was distilled quantitatively into a solution of MCl and subsequently dried before analysis. A known sample of ammonium chloride was subjected to the distillation process as a control. The 6 l ' ~of the initial NM4Cl control was +0.3%~? and after processing it was +8.2%0. For the analysis of the stable C and N isotopic composition, samples were converted to C 0 2 and N2 by a modification of the Dumas dry-combustion method (Stump and Frazer 1973). The dried powder was mixed with high-purity copper and precombusted copper oxide in a quartz tube, which was then evacuated, sealed, and combusfed at 900°C for I h. C 0 2 and N2 were isolated cryogenically and analyzed with isotope-ratio mass spectrometers (Nuclide Corp., model RMS-6-68, for C02, and a similar type sf instrument constructed at the Geophysical Laboratory for NZ). The stable isotopic ratio is calculated in terns of 6 X as follows:

Lahontan Reservoir usually receives about 50% of its total N from the Tmckee Canal and the remainder from the Carson River, according to a simplified model which assumes that N2 fixation is negligible (Cooper et al. 1983). In B 983, however, input from the Carson River was much greater due to high river flows. The total inorganic N in the Tmckee River above the sewage-treatment plant has a 6I5N value (- 1 to + 1) typical of input h m fertilizer and the natural biogeochemical cycle (beitler 1975; Table 1). B e f ~ r ethe water enters the Tmekee Cmal, a substantial amount of inorganic N enters from the effluent of the R-SJWPCP that has an isotopic composition almost 9760 heavier than that of the incoming, ambient N. Because the concentrations of %a are at least 100 times background, the periphyton in the Lower River and Truckee Canal are receiving an elevated amount of N compared with the Upper Tmckee River m d Lahontan Reservoir. Algae growing in nonlimiting conditions will incorporate isotopically light N into their cellular proteins (Wada and Hattori 1978; Macko et al. 1982). The residual total inorganic N is thus enriched in "N. Within Lahontan Reservoir, where the summer, growingseason levels of dissolved N are essentially zero, eke 615N of inorganic N is only + 1.1, yet the 615N of algae (blue-green) is +8.4. Thus, the isotopic cornposition of N2-fixing blue-green algae in Lake Lahontan is anomalous and indicates an additional N source, perhaps the isotopically heavy N from the polluted Tmckee Canal or from unmeasured sources along the Carson River. ion of Rebationship ofN2fiat ion to the isotopic co~nposit blue-green algae The number of heterocysts and their biochemical activity are related to the 615N of A . jos-aquae in the same samples (Table 2). Hn Lakontan Reservoir, samples with an increased number of heterocysts (>20 per 1000 cells) have 6% nearer to the expected value of O.63%0. Samples with

@d

A3 A3S

Cui-ui muscle scales

-18.1 -15.0

+10.6 +10.1

A4 A43


-17.4 -15.0

+10.4 +10.2

A42 A42S


-18.2 -16.1

+12.5 +lo.$

A43 A43S


-18.0 -15.3

+11.6 +14.0

A44 A44S

Gui-ui muscle scales

-18.0 -15.5

+12.2 4-11.7

A45 645s

Gui-ui muscle scales

-18.2 -15.4

+11.% +lo.%

A5 65s

Tmuta muscle scales

- 19.8

+%.3 4-10.2

A18 A18S

TAoe sucker muscle scales

- 22.8

-23.2

4-8.5 +7.2

A19 A19S

T&oe sucker muscle scales

- 2 1 .9 -23.0

+8.2 +4.1

-14.8

composition of trout from Naumauna Hatchery is anomalous md may reflect a different, moist commercial feed or medicated

feed that was used there ((A. Ruger, Pyramid Lake Fisheries, Sutcliffe, NV 89%18, pers. c o r n . ) . Eout reared in Pyramid raceways were intermediate in isotopic composition, although closer to that of the wild fish. The intermediate isotope ratios are logical because the diet of the trout includes both hatchery f e d and zooplankton (8'" = Zil5P4 = +10) present in the continuously circulating Pyramid Lake water supply.

-w

Isotopic composition of$sh scales A comp&son between the 8°C and 6 1 5 of ~ fish scales and muscle from three species of fish is reported in Table 7. Each species of fish has a characteristic isotope fractionation between scdes and muscle tissue. The results of the c o q ~ s o among n cui-ui indicate that there is a consistent 2-3% difference in 813C among samples of six individual fish. The Tahoe sucker samples show a reverse fractionation, indicating a difference in initial diet or subsequent metabolism. The N isotope patterns are more variable and would not be as valuable a tracer as C isotopes.

Conclusions Nitrogen isotope ratios have been used to trace very subtle additions and pathways of N in two lakes that are similar in that both are N limited and host large blooms of blue-green algae. The lakes are dissimilar in that Lahontan Reservoir (pH 7.0-9.0) has a proportionally large addition of N from the polluted Tmckee Canal and consists of freshwater, whereas Pyramid Lake (pH 9.2) is relatively unpolluted and considerably more saline and alkaline. These differences in chemistry we expressed in the isotopic composition of the organisms living in the lake. The uptake of other available combined forms sf N into blue-

green algae that are also fixing N2 is probably occurring in Lahontan Reservoir. Previously, all other 8"N measurements of N2-fixingblue-green algae were approximately 0 . m ~ ;hence, analysis of the results presented in this paper would urge caution in either dismissing or overestimating the importance of N2 fixation based on 6 1 5 or ~ biological measurements alone. In Pyramid Lake, by examining the 8 1 5 and ~ 8°C of the many aspects of the food web, one can see that even though bluegreens are unpalatable, their fixed N eventually enters the food chain after mineralization and recycling. The C isotope ratios are a further example of how the chemistry s f the lake and the physiology of the organisms caw be examined by isotopes. The 6 " ~of blue-green algae from Lahontm Reservoir was very negative ( -26 to -3@%0)(Table 2) compared with that of the sample from Pyramid Lake (8I3c = - 17) (Table 3). Increased salinity slows the diffusion of C 0 2 into plants, creating C 0 2 limitation, which results in less isotope fractionation (Guy et al. 1980). Conversely, the 813c of the total C 0 2 in each of the lakes may be different. A sample of blue-green algae (Nodularia) from Walker Lake, also a remnant of luvial Lake Lahontan, had a 6 'C of -24. The divergence of 6l C in such similar organisms could be useful in studying the physicd and chemical properties of freshwater lakes (Estep 1984). Because the relationship between the isotopic composition of an animal and its diet have been investigated in various studies, deviations in these intercomparisons can be useful in detemining other sources of nutrients in the animal's diet that cannot be seen from direct observation. These deviations can be expressed in the following foms. Firstly, temporal variations within a species, e.g. the 6I5N of inshore tui chubs (Table 41, reflect the different pools sf inorganic nutrients and the differences in dietary species compositions, which occur during the yearly biogeochemical cycle. Secondly, variability within a population at one time, e.8. inshore tui chubs, is indicative of a generalized rather than a specific diet. Thirdly, excessive or nonexistent enrichment with trophic level, e.g. the 8I5PJ of Lahontan cutthroat trout (Fig. 2), suggests the addition of an unsuspected component in an animal's diet. In this case, the component in the diet was most likely tui chubs having a different isotopic composition owing to seasonal variations in the N source. The application of stable isotopes as tracers in aquatic systems thus becomes more powerful as a tool for investigating f d webs. A find application of stable isotopes in determining recmitment of fish from natural reproduction compared with artificial propagation has been evaluated. Stable C isotopes of muscle tissue distinguish a hatchery-reared fish from one grown in the , consistent difference between the 813c wild. F u r g h e m o ~the of muscle tissue and that of scales from the same fish could become a very useful, nondestructive tool for tracing the fate and growth of hatchery-reared fish after they have been released. Different hatcheries could simply use fish food with a different isotopic composition as a biochemical tag so that techniques in hatchery practices could also be evaluated. A very slight artificial enrichment with 13C-labeled compounds would be feasible. Carbon isotope ratio measurements can be determined commercially at a reasonable cost from a number of laboratories, thus facilitating the use of stable isotopes as tracers for fishery management agencies. In summary, trophic dynamics in a lacustrine system in Nevada have been studied with stable isotope tracers. At the primary trophic level, isotopically heavy N entering Lahontan

P

Can. I . Fish. A q u a . Sci., h l . 42, 1985


Reservoir, possibly from the Reno-Sparks sewage plant, is an impsrtamt nutrient for the growth of blue-green algal blooms. The N in blue-green algae apparently enters the food web after they have decayed, and the N fixed by these organisms enters the water column as ammonia, which is then incorporated by palatable phytoplankton. At higher trophic levels, analyses or observation of gut content can determine the diet in an animal only at a specific time and cannot determine digestive efficiencies or metabolism. Stable isotope ratios of animals, however, ape integrative values reflecting the complexities in feeding habits. Variations in the natural abundance level of C and N isotopes within either wild or hatchery-reaped organisms yield information on seasonal variations in diets, generalized versus specific feeding, and species-specific assimilation and metabolic differences.

Acknowledgments The P y r m i d Lake Pauite tribe cooperated in the collection of s m p l e s ; Alan Wuger, Director of Pyramid Lake Fisheries, provided technical support. The U.S. Fish and Wildlife Service, Reno Office, provided cui-ui, T&oe sucker, and Lahontan cutthoat trout samples. The authors thank M q Miller, Desert Research Institute (BWI), for processing the samples of inorganic N from the %&ontan water system and 9. 9. Cooper (DWI) and D. %. Galat ( D M ) for assistance in collecting some of the samples, 5 . 9. Cooper measured the nitrogen fixation in algd samples from kahontaan Reservoir; Mary Peacock counted the algal cells from the same samples. T. C. Hoering is thanked for help with the stable isotope analysis. D. E. Galat, A. Gize, and T. 6 . Hoering critically reviewed the manuscript. This study was funded in part by a grant to Estep from the Charles E. Culpeper Foundation.

References BOTTOMLEY, B. J., 3. F. GWILLO, C. VANBAALEN, AND H. R. TABITA. 1979. Synthesis of nitrogenase and heterwysts by Anabaena sp. CA in the presence sf high levels of ammonia. J. Bacterial. 14Q:938-943. COOPER,J. J., AND S. VIGG. 1985. Species composition and seasonal succession of the zooplankton community of eutrophic Lahontan Reservoir, Nevada. Southwest. Nat. 30: 239-252. COOPER, 9. J.. S. VIGG,R. W. BRYCE, ANDR.L. JACOBSON. 1983. Limology of Lahontan Reservoir, Nevada. Biological Sciences Center h b l . No. 5021, Desert Research Institute, Reno, NV. 186p. ESTEP,M. L. F. 1984. Carbon and hydrogen isotopic composition of algae and bacteria from hydrothermal environments, Yellowstone National Pa&. Geochim. Cosmochim. Acta 48: 591-599. ESTEP,M. L. F., AND S. A. MACKO.1985. Nitrogen isotope biogeechemistry of thermal springs. 8rg. Geochem. 6: 779-785. Kku, B., A N D C. ARNOLD.1982. Rapid "C1"C turnover during growth of brown skimp (Penaeus aztecus). Oecologia 54: 200-204. FRY,B., A. JOERN,AND P. L. PARKER. 1978. Grasshopper food web analysis: use sf carbon isotope ratio to examine feeding relationships m o n g terrestrial herbivores. Ecology 59: 498-504. FRY,B., AND E. B. SHEWR.1984. 8l3C measurements as indicators of carbon Wow in marine and freshwater ecosystems. Contkb. Mar.Sci. 27: 13-47. GALAT,D.L., E. L. LIDER,S. VIGG,AND S. R. ROBERTSON. 1981. Limnology of a large, deep, North American terminal Bake, Pyramid Lake, Nevada, U.S.A. Hydrobiologia 82: 281-317.

GEARING,3. N., P. 3. GEARING, D. T. RUDNICK, A. G. REQUEBO, AND M. J. HUTCHINS. 1984. Isotopic variability of organic carbon in a phytopkmktonbased, temperate estuary. Geochim. Cosrnwhim. Acta 48: 1089-1098. GUY,It. D., D. M. WID, AND H. R. KROUSE.1980. Shifts in carbon isotope ratios of two C3 halophytes under natural and artificial conditions. Becologia 44: 24 1-247. HOERING, T. C., AND H. T. FORD.1960. The isotope effect in the fixation of nitrogen by Azotobacter. 3. Am. Chem. Soc. 82: 376-378. KOCM,D, L. 1972. Life history information on the cui-ui lakesucker (Chasmistes CU~US, Cope 1883)endemic to Pyramid L&e, Washoe County, Nevada. Ph.D. thesis, University of Nevada, Reno, NV. KREITLER, C. W. 1975. Determining the source of nitrate in ground water by nitrogen isotope studies. Rep. Invest. Bm.Econ. Geol. Univ. Texas No. 83. LANGDBN, R. W. 1978. Food habits of the tui chub (Gila bicolsr) in Pyramid M e . M.S. thesis, Hurnboldt State University, Arcata, CA. LA RIVERS,1. 1962. Fishes and fisheries of Nevada. Nevada Fish and Game Commission, Reno, NV. 782 p. MACKO,S. A., AND M. L. F. ESTEP.1985. Microbial alteration of stable nitrogen and carbon isotopic compositions of organic matter. Brg. Geochern. 6: 787-790. MACKO,S. A., M. L. F. ESTEP,AND T. C. HOERING.1982. Nitrogen isotope fractionation by blue-green algae cultured on molecular nitrogen and nitrate. Cmegie Inst. Wash. Year Book 8 1: 413-417. MINAGAWA, M., AND E. WADA.1984. Stepwise enrichment of '%I dong food chains: further evidence and the relation between 6I5N and animal age. Geochirn. Cosmochim. Acta 48: 1135-1 146). MIYAKE, Y., AND E. WABA.1967. The abundance ratio of I5N/I4N in marine environments. Rec. Bceanogr. Works Jpn. 9: 32-53. BANA,S., AND E. S. BEBVEY.1960. Carban- 13 in lake waters and its possible bearing on pdeolirnnology. Am. 3. Sci. 258-A: 253-272. PANG,B. C., AND J. 0 . NRIAGU.1977. Isotopic variations of the nitrogen in Lake Superior. Geochim. Cosmochim. Acta 41: 8 11-8 14. RAU,6. H. 1980. Carbon-13lcarbon-12 variation in subalpine lake aquatic insects: food source implications. Can. J. Fish. Aquat. Sci. 37: 742-745. SIGLEW, W. F., W. T. HELM,P. A. KUCBWA, S. VIGG,AND G.WORKMAN. 1983. Life history of Lahontm cutthroat trout, Salmo chrki henshawi, in Pyramid Lake, Nevada. Great Basin Nat. 43: 1-29. SMITH,B. N.. AND S. EPSTEIN.1971. Two categories of I3C/'*C ratios for higher plants. Plant Physiol. 47: 380-384. STILLER,M., AND A. NISSENBAUM. 1980. Va.riations of stable hydrogen isotopes in plankton from a freshwater lake. Geochim. Cosmoshim. Acta 44: 1099- 1101. STUMP,R. K., AND J. W. FRAEBR.1973. Simultaneous determination of carbon, hydrogen, and nitrogen in organic compunds. Rep. 1973, UCID- 16198. University of California, Livemore, CA. 7 p. TIESZEN,L. L., T.W. BOUTTON, K. 6. TESDAHL, AND N. A. SLADE.1983. Fractionation and turnover of stable carbon isotopes in animal tissues: implications for 6'" ma1ysis sf diet. Becologia 57: 32-37. VIGG,S. 1981. Species composition and relative abundance of adult fish in Pyramid Lake, Nevada. Great Basin Nae. 4 1: 395-488. 1983. Gill raker differentiation of tui chubs in remnant Lahontan waters. Presented at the Desert Fishes Council, 15th Annual Symposium, Nov. 17- 19, 1983, Furnace Creek, Death Valley, CA. VIRGINIA,R. A., AND C. C. DELVVTCHE. 1982. Natural I5N abundance of presumed N2-fixing and non-&-fixing plants from selected ecosystems. Becologia 54: 317-325. WABA,E., AND A. HATTOM.1976, Natural abundance of "N in particulate organic matter in North Pacific Ocean. Geochim. Cosmochim. Acta 12: 97- 102. 1978. Nitrogen isotope effects in the assimilation of inorganic nitrogenous compounds by marine diatoms. Geomicrobiot. J. 1: 85- 101.

Can. J. Fish. Aquar. Sci., Vol. 42, 1985

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