ABSTRACT VARIATION AMONG FISH SPECIES IN THE STOICHIOMETRY OF NUTRIENT EXCRETION. by Lisette Esmeralda Torres

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1 ABSTRACT VARIATION AMONG FISH SPECIES IN THE STOICHIOMETRY OF NUTRIENT EXCRETION by Lisette Esmerld Torres This study investigtes how nutrient cycling rtes nd rtios vry mong fish species nd with body size, nd how they re relted to body nutrient contents ccording to predictions of ecologicl stoichiometry. Nutrient excretion rtes nd body nutrient contents of eight fish species in four fmilies were estimted in hypereutrophic reservoir. As predicted by llometry, per cpit nitrogen nd phosphorus excretion rtes incresed nd mss-specific excretion rtes decresed, with incresing mss. Body phosphorus content ws correlted with body mss, but this reltionship vried mong species. In ccordnce with stoichiometric predictions, species with low body phosphorus excreted more phosphorus thn those with high body phosphorus. However, this reltionship my be more of function of food nutrient content thn the nutrient content of the consumer. These results suggest tht stoichiometry cn provide frmework for vrition mong species in nutrient cycling nd for evluting the ecosystem consequences of biodiversity loss.

2 VARIATION AMONG FISH SPECIES IN THE STOICHIOMETRY OF NUTRIENT EXCRETION A Thesis Submitted to the Fculty of Mimi University in prtil fulfillment of the requirements for the degree of Msters of Science Deprtment of Zoology by Lisette Esmerld Torres Mimi University Oxford, Ohio 2005 Advisor Dr. Mike Vnni Reder Dr. Thoms O. Crist

3 TABLE OF CONTENTS Pge Title Pge...i Tble of Contents.ii List of Tbles iii List of Figures...iv Acknowledgements..vii Introduction..1 Methods...5 Nutrient Excretion Experiments...5 Body Nutrient Anlysis...6 Correltion of Body Nutrient Content, Nutrient Excretion, nd Gut Contents...6 Lke-wide Biomss nd Nutrient Excretion Rtes...7 Sttisticl Anlyses...8 Results...9 Nutrient Excretion Experiments...9 Body Nutrient Anlysis...9 Correltion of Body Nutrient Content, Nutrient Excretion, nd Gut Contents...11 Lke-wide Biomss nd Nutrient Excretion Rtes...12 Discussion...13 Nutrient Excretion Experiments...13 Body Nutrient Anlysis...14 Correltion of Body Nutrient Content, Nutrient Excretion, nd Gut Contents...16 Lke-wide Biomss nd Nutrient Excretion Rtes...18 Conclusion...18 Literture Cited...21 ii

4 LIST OF TABLES Pge Tble 1: Summry of the smple sizes nd txonomic composition of Acton Lke fish used in this study...29 Tble 2: Summry of regression prmeters (slope, intercept) nd Bonferroni-djusted sttisticl results for multiple comprisons of nutrient excretion mong gizzrd shd nd the other Acton Lke fish species. Asterisks indicte significnt differences. P-vlues for slopes test for whether shd re significntly different from other fishes. P-vlues for intercepts re not reported due to the presence of significnt interctions...30 Tble 3: Summry of regression prmeters (slope, intercept) nd Bonferroni- nd Dunnett-djusted sttisticl results for multiple comprisons of body nutrient content s function of wet mss mong gizzrd shd nd the other Acton Lke fish species. Asterisks indicte significnt differences. P-vlues for slopes test for whether shd re significntly different from other fishes. P-vlues for intercepts re not reported due to the presence of significnt interctions 31 Tble 4: Summry of regression prmeters (slope, intercept) nd Bonferroni- nd Dunnett-djusted sttisticl results for multiple comprisons of nutrient excretion s function of body nutrient content mong gizzrd shd nd the other Acton Lke fish species. Asterisks indicte significnt differences. P-vlues for slopes test for whether shd re significntly different from other fishes. P-vlues for intercepts re not reported due to the presence of significnt interctions...33 Tble 5: Averge summer lke-wide biomss nd nutrient excretion rtes for Acton Lke fish bsed on electroshocking dt djusted for the men mss of ech species. Vlues with sterisks were extrpolted using globl regression scertined from excretion rtes in this study...35 iii

5 LIST OF FIGURES Pge Figure 1: Sterner model (1990), modified by Elser nd Urbe (1990) nd sensu Sterner nd Elser (2002), depicting the N:P of nutrient relese by zooplnkton grzers s function of lgl (food) N:P nd consumer N:P. This model ssumes tht the grzers re homeosttic, hve equl mximum gross growth efficiencies (0.7), nd consume the sme food source...36 Figure 2: Per cpit excretion rtes for N (A) nd P (B) by fishes in Acton Lke...37 Figure 3: (A) Body crbon content s function of wet mss for freshwter fishes in Acton Lke. Solid trendline depicts incresing crbon content with incresing mss for gizzrd shd. (B) Body N content s function of wet mss. (C) Body P content s function of wet mss. Solid trendline shows slight but non-significnt increse in P content with mss for shd while dshed trendline shows significnt increse in P content with mss for bluegill sunfish...38 Figure 4: Men percent body nutrient content nd body nutrient rtios of fish species smpled. Letters indicte significnt slope differences, except for body C:N, in which significnt differences in intercept (i.e. mens) were used in the bsence of n interction; b.g. = bluegill sunfish, g.s. = gizzrd shd, gr.s. = green sunfish, l.p. = logperch drter, l.s. = longer sunfish, l.m. = lrgemouth bss, o.s. = orngespotted sunfish, nd shiner = golden shiner...39 Figure 5: Percent body N content versus percent body P content of smpled freshwter fishes. Dshed lines represent molr nutrient rtios. The dt re similr to Sterner nd George (2000) nd show tht the vrition in body P content tends to exceed vrition in body N content, except for shd which seems to hve similr vrince in both N nd P. Note tht the fmily Clupeide (gizzrd shd) hs lower body N nd P thn the fmilies Centrrchide (e.g. bluegill), Percide (logperch), nd Cyprinide (shiner)...40 iv

6 Pge Figure 6: (A) Body C:N rtios s function of wet mss for Acton fishes. Trendline shows body C:N incresing with mss for gizzrd shd. (B) Body C:P rtios s function of wet mss. Solid line shows slight, non-significnt increse in body C:P with mss for shd; dshed line shows body C:P decresing with incresing mss for bluegill. (C) Body N:P s function of incresing wet mss. Solid nd dshed lines depict decresing body N:P with incresing mss for shd nd bluegill, respectively...41 Figure 7: Per cpit (A) nd mss-normlized (B) per cpit N excretion versus body N content from ll smpled fish species...42 Figure 8: Per cpit (A) nd mss-normlized (B) per cpit P excretion versus body P content for Acton Lke fishes...43 Figure 9: Nutrient relese rtios of eight reservoir fish species s function of fish body N:P...44 Figure 10: The Sterner model (1990) pplied to the N:P of nutrient relese by gizzrd shd nd bluegill sunfish from Acton Lke. (A) It ws ssumed tht both fishes were homeosttic, hd equl mximum gross growth efficiencies for N nd P (GGE mx ), nd consumed the sme N:P food. The blck nd gry dt points represent the men recycled N:P of shd nd bluegill s determined experimentlly. (B) GGE mx ws djusted for ech species in order to obtin men recycled N:P vlues from the nutrient excretion experiment. Note tht differences in the recycled N:P of shd nd bluegill pper to be driven more by differences in food N:P thn solely by body N:P. For both A nd B, incresing recycled N:P with incresing food N:P re depicted reltive to line with slope of one (dshed line)...45 Figure 11: Percent biomss nd nutrient excretion ccounted for by gizzrd shd reltive to v

7 Pge the rest of the fish community in Acton Lke...46 vi

8 ACKNOWLEDGEMENTS I would like to express my sincere thnks to Dr. Mike Vnni for ll of his help nd dvice nd for giving me wonderful project to work on in such new nd exciting field of reserch. I would lso like to thnk Dr. Mrí González nd Dr. Tom Crist for their insightful comments nd suggestions on this thesis. I m gretly indebted to Jen Bobson nd Ter Rtliff; they re the best field ssistnts nyone could ever sk for. Mny thnks go to the following individuls for helping me in the field s well: Alberto Pilti, Todd Levine, Weston Nowlin, Leh Lurich, Mr. Bobson, Alici Gulke, nd Brrett Scurlock. I must thnk Ann Bowling, Peter Levine, nd Alberto Pilti for conducting ll of my nutrient nlyses; without them, this project would not hve come to fruition. Thnks go to Dr. John Biler nd Mike Hughes from the Deprtment of Mthemtics nd Sttistics t Mimi for hving the ptience nd the time to sit with me nd cquint me with SAS in order to conduct ll of my sttisticl nlyses. Specil thnks go to the entire Vnni nd González lbs for their comments, suggestions, nd friendship. Thnks to the Deprtment of Zoology t Mimi, the Ohio Division of Wildlife, nd NSF-LTREB for funding nd for the permission to electroshock on Acton Lke. Eternl grtitude is given to Dr. Crig Willimson, my mentor, friend, nd gurdin ngel who serves s constnt inspirtion for me. On similr note, I would like to express my undying thnks nd love to my fmily, especilly my sisters Cri nd Cookie, for ll of their unending support, encourgement, nd love; I do not know where I would be without you. Lst but not lest, I would like to thnk God for His eternl love, strength, nd guidnce. vii

9 INTRODUCTION Biodiversity hs decresed globlly due to humn impcts on the environment, such s deforesttion nd pollution. According to some ecologists, this decrese hs cused functionl shifts in ecosystem processes nd hs led to scientific debte s to the reltive importnce of the components of species diversity in ecosystem functioning (Neem et l. 1994; Tilmn 1997; Chpin et l. 2000; Loreu et l. 2001). Besides species richness, two chrcteristics of biodiversity tht re of concern re species identity nd species evenness (Chpin et l. 2000). Species identity refers to the prticulr species present within system. Different species my hve unique trits nd functions tht cn medite nd lter energy nd mteril fluxes (Chpin et l. 2000). Depending on the species present, individuls or popultions cn hve lrge impcts on n ecosystem in comprison to other species within the community (Hector et l. 1999; Chpin et l. 2000; Dukes 2002). Species evenness cn be defined s the evenness of the reltive bundnces of ll species in functionl group or community. Typiclly, the most bundnt species cn control the rtes nd directions within n ecosystem (Chpin et l. 2000). Therefore, both species identity nd bundnce hve importnt consequences to biodiversity nd its effect on ecosystem functioning. Though studies hve been conducted concerning the effect of biodiversity on ecosystem functioning, it is still uncler s to whether biodiversity effects re due to the system s dependence on few key species or the chrcteristics of severl different species (Tilmn 1997; Chpin et l. 2000; Loreu et l. 2001). Similrly, the reltive importnce of complementrity nd smpling effect (or dominnce) in systems hs not been ddressed. Complementrity is the concept tht locl interctions between species, such s fcilittion, cn increse the performnce of community bove tht expected from the performnce of individul species in monoculture (Loreu et l. 2001). In other words, species differ in their functionl roles nd together they cn hve prticulr impct on the functioning of system tht they my not hve hd if they were the only species present in the system. For exmple, Crdinle et l. (2002) found tht mixed ssemblge of cddisfly species cn increse the velocity of strem chnnels, incresing both individul nd totl consumption of resources. In contrst, smpling effect occurs when high richness (i.e. the number of species) increses the probbility of contining species with unique ecosystem function (Loreu et l. 2001). Thus, both complementrity nd smpling effect cn influence how biodiversity ffects ecosystem functioning. Yet, it is still 1

10 unknown to wht extent the impct of biodiversity on ecosystem functioning is due to species identity or species bundnce. Most reserch regrding the biodiversity-ecosystem functioning debte hs centered on plnts (e.g. Tilmn nd Downing 1994; Hector et l. 1999; Dukes 2002). More recently, studies hve exmined bcteri nd fungi (e.g. Hooper nd Vitousek 1997; vn der Heijden et l. 1998; Neem et l. 2000), lef-eting insects (e.g. Jonsson nd Mlmqvist 2000), gstropods (O Connor nd Crowe 2005), strem invertebrtes (e.g. Crdinle et l. 2002; Jonsson nd Mlmqvist 2003), nd zooplnkton (e.g. Steiner 2002). Some uthors hve concluded tht differentil impcts of species (i.e. species identity) on their environment nd the types of interctions tht tke plce my be the most importnt mechnisms by which biodiversity enhnces ecosystem functioning (e.g. Crdinle et l. 2002; Jonsson nd Mlmqvist 2000, 2003; Loreu et l. 2001; Bellwood et l. 2003; O Connor nd Crowe 2005). Unfortuntely, no study hs tried to ssess the vlidity of tht hypothesis in freshwter lkes. If the hypothesis is ccurte, however, it could led to vrious conservtion nd ecosystem mngement implictions, signling the need to cknowledge the importnce of species identity in decisions ffecting ecosystem functioning. One wy to test the hypothesis in qutic systems is by determining the effect of different species on the importnt ecosystem process of nutrient cycling. Since the rtes nd rtios of nutrient inputs hve direct nd indirect effects on utotrophs nd heterotrophic microbes, nutrient cycling by vrious species is essentil for lke ecosystem functioning. Phytoplnkton community structure nd primry productivity in lkes nd reservoirs re ffected by the rtes nd rtios by which limiting nutrients re provided to the wter column (Tilmn et l. 1982). Orgnisms cn distribute nutrients to phytoplnkton by excreting them into the wter column nd cn ultimtely hve lrge impct on species composition of phytoplnkton nd ecosystem functioning (i.e. nutrient cycling). Nutrient excretion cn be defined s the relese of nutrients, primrily nitrogen (N) nd phosphorus (P), in dissolved, biovilble forms, such s PO 3-4 nd NH + 4. Ecologicl stoichiometry theory (sensu Sterner nd Elser 2002) predicts tht nutrient relese by consumer is function of the imblnce between the consumer s body nutrient content nd its prey s nutrient content s well s the efficiency t which the consumer ssimiltes nutrients into biomss (Sterner nd George 2000; Vnni et l. 2002). Similrly, using stoichiometric 2

11 principles, model proposed by Sterner (1990) suggests tht the N:P rtio of nutrient relese by consumer such s zooplnkton is function of food (lgl) N:P nd consumer N:P, ssuming tht the consumers re homeosttic nd hve equl mximum gross growth efficiencies (GGE mx ) for ech nutrient (Figure 1). It predicts tht if two consumers differ in body N:P but ingest food items tht hve the sme food N:P, then the recycling rtios would be inversely relted to the body N:P of the consumer (i.e. the low N:P consumer should recycle nutrients t high N:P rtio wheres the high N:P consumer should recycle nutrients t low N:P rtio). Recently, it hs been discovered tht excretion of N nd P by fish cn lter the nutrient rtios found in the wter column nd phytoplnkton communities (Persson 1997; Vnni et l. 1997). Fish cn therefore lter qutic food webs vi excretion nd egestion by consuming prey of vrying nutrient qulity (Schindler nd Eby 1997; Schus nd Vnni 2000; Sterner nd George 2000; Sterner nd Elser 2002), s well s by direct nd indirect effects of consumption. However, it is not known whether nutrient relese by fish follows the predictions of ecologicl stoichiometry theory. Gizzrd shd (Dorosom cepedinum) re fculttive detritivores (Yko et l. 1996) tht re found in most reservoirs in the Midwest nd Southern United Sttes (Johnson et l. 1988; Pge nd Burr 1991; Vnni et l. 2005). This species cn hve strong impct on qutic food webs becuse its food selectivity chnges with growth (Shepherd nd Mills 1996; Yko et l. 1996). Lrve re zooplnktivorous, preferring to consume smll zooplnkton tx (Bremign nd Stein 1994, 1997; Mirnd nd Gu 1998). Depending on their density, they cn depress zooplnkton biomss, shift zooplnkton community structure, nd hinder the growth nd bundnce of other fish popultions vi direct nd indirect competition (DeVries nd Stein 1992; Welker et l. 1994; Stein et l. 1995; Grvey et l. 1998; Vnni et l. 2005). However, when gizzrd shd hve reched totl length of pproximtely mm, they re ntomiclly nd physiologiclly cpble of consuming detritus nd sediment (Heinrichs 1982; Mundhl 1991; Smoot nd Findly 2000; Schus et l. 2002), nd in reservoirs, the diets of non-lrvl shd re dominted by sediment detritus (Yko et l. 1996; Mundhl nd Wissing 1986; Sigler 2002). Recent studies hve reveled tht gizzrd shd bundnce increses with incresing productivity mong nturl lkes (Bchmnn et l. 1996) nd reservoirs (DiCenzo et l. 1996; Michletz 1997; Bremign nd Stein 1999, 2001; Vnni et l. 2005). In Ohio reservoirs, in prticulr, their bundnce is correlted with the percent of wtershed lnd comprised of 3

12 griculture (Sigler 2002; Vnni et l. 2005). Similrly, becuse gizzrd shd lter their food selectivity (from zooplnkton to detritus - Sigler 2002) nd forging res (from the pelgic zone to the benthos), it hs been hypothesized tht they my recycle nutrients into the wter column s lrve nd dults vi excretion, possibly stimulting or sustining primry productivity (Lzzro et l. 1992; Schus et l. 2002; Ady et l. 2003; Wtson et l. 2003; Shostell nd Bukvecks 2004). Schus et l. (1997) nd Schus nd Vnni (2000) demonstrted tht gizzrd shd dults ply vitl role in nutrient trnsloction, bringing nutrients up from the benthos nd into the pelgic zone during feeding nd excretion. Due to the low N:P rtios t which they excrete, these fish cn lter phytoplnkton communities, possibly fvoring the growth of cynobcteri (Schus et l. 1997). Yet, it is still not known whether the ecosystem-level importnce of gizzrd shd on nutrient cycling is due simply to their gret bundnce or becuse they hve higher per cpit excretion rtes thn other species. There re few studies tht ddress interspecific comprisons of nutrient excretion by different fish species (Mther et l. 1995; Gido 2002). Different species s well s lrger txonomic ggregtions my excrete t different rtes or N:P rtios, which my influence ecosystem-wide nutrient cycling (Vnni et l. 2002). For exmple, Brbrnd et l. (1990) looked t nutrient excretion of three fish species in different feeding guilds brem ( benthivore tht consumes invertebrtes), perch ( plnktivore), nd roch ( detritivore). They found tht roch relesed more P into the wter column per unit biomss thn the other two species, but they hd limited dt. In ddition, in times of yer where P-limittion could hinder lgl growth, the roch popultion ws responsible for supplying phytoplnkton with two times the mount of P tht ws being supplied by the wtershed. Similrly, Gido (2002) found tht nutrient excretion by fish differed mong gizzrd shd, river crp sucker, nd smllmouth bufflo, species tht occur within the sme feeding guild. Thus, nutrient excretion by fish cn be dependent on species identity nd on the type or qulity of prey being consumed. The effects of excretion on qutic systems lso depend on the bundnce of species. Hll et l. (2003) quntified the impct of n exotic freshwter snil (Potmopyrgus ntipodrum) on C nd N fluxes within highly productive strem. They found tht the snils excreted 7.8 mg N/m 2 /h, which ws pproximtely 65% of the totl N-demnd of the lgl nd microbil community. Yet, since their per-biomss consumption of lge ws low compred to other species within the strem, Hll et l. (2003) concluded tht the snil popultion dominted 4

13 the locl N cycle becuse of its high biomss nd bundnce. I investigte the effect of species identity nd bundnce on ecosystem functioning by mesuring the reltive importnce of different fish species nd sizes on nutrient regenertion vi excretion in hypereutrophic reservoir. The objectives were to quntify nd compre excretion rtes of severl species nd sizes, including the bundnt detritivore, gizzrd shd, which hs been found to ffect primry production vi nutrient trnsloction (Schus et l. 1997; Schus nd Vnni 2000). Body nutrient contents were quntified to correlte them with excretion rtes nd to test predictions of ecologicl stoichiometry theory. I hypothesized tht: (1) fish species other thn gizzrd shd within the sme reservoir would hve similr excretion rtes nd rtios (per individul or per biomss) s shd; (2) vrition mong species could be explined by ecologicl stoichiometry theory; nd, therefore, (3) the reltive importnce of gizzrd shd in nutrient cycling would be dependent on the species high bundnce rther thn its unique function s nutrient recycler nd trnsloctor of nutrients. METHODS Nutrient Excretion Experiments Nutrient excretion experiments were conducted July-September 2004 t Acton Lke, 253-h hypereutrophic reservoir locted in Hueston Woods Stte Prk (Butler nd Preble Counties) in southwestern Ohio. A totl of 177 fish of vrious body sizes were collected vi electroshocking in the upstrem portion of the reservoir. I studied 8 fish species, represented by 3 orders nd 4 fmilies (Tble 1). The fish ssemblge ws dominted by gizzrd shd nd bluegill sunfish (Lepomis mcrochirus). Smple size vried ccording to sesonl vilbility. Ech fish ws trnsferred to continer filled with Acton Lke wter tht ws pre-filtered in the lb using 142 mm Advntec high-volume filters. Wter ws kept t epilimnetic tempertures, which were monitored during the experiment nd rnged from C throughout the summer-erly fll. There were lso control continers filled with only filtered wter. Initil smples of filtered lke wter were tken from 50-L crboys before filling the continers with wter in order to quntify initil nutrient concentrtions prior to fish ddition. Fish nd controls were incubted for min. Then, the wter ws filtered through Gelmn A/E glss-fiber filter (1.0 µm pore size) nd tken to the lb to be nlyzed for NH 3 -N nd SRP using the phenol-hypochlorite technique (Solorzno 1969) nd molybdenum blue method (Stinton et l. 5

14 1977), respectively, using Lnchet uto-nlyzer. Excretion rtes were determined s the difference between initil nd finl nutrient concentrtions, corrected for controls (Schus et l. 1997). Body Nutrient Anlysis Upon completion of nutrient excretion experiments, fish were mesured for totl length nd weighed. They were euthnized nd frozen for quntifiction nd nlysis of tissue C, N, nd P content. The gstrointestinl trct ws dissected from mouth to nus nd removed for lter gut content nlysis. The fish were then oven-dried until they reched stble weight. Fish were ground into powder using Retsch Model ZM100 tissue grinder (for lrge individuls) nd mortr nd pestle (for smll individuls). These smples were nlyzed for C nd N using Perkin Elmer Series 2400 elementl nlyzer, nd body P content ws nlyzed using HCl digestion followed by SRP nlysis (Vnni et l. 2002). Correltion of Body Nutrient Content, Nutrient Excretion, nd Gut Contents Per cpit nutrient excretion rtes nd rtios were plotted ginst component body nutrient content nd body nutrient rtios to determine if nutrient excretion ws function of body nutrient content, s predicted by stoichiometry theory. Log-log plots were used in certin instnces to stbilize the vrince mong species. Log per cpit excretion rtes were expected to be correlted with log body mss, with n exponent less thn 1, ccording to llometry theory (Brown et l. 2002). Becuse excretion rtes re function of size, it is desirble to remove the size effect when compring species. Therefore, mss-normlized rtes were obtined using the following formul: Y n = _Y_ X b where Y is dependent vrible (in this cse non-normlized excretion rte), Y n is the normlized dependent vrible (i.e. normlized excretion rte), X is the independent vrible (e.g. mss), nd b is the scling exponent for prticulr species (Brown et l. 2002). According to stoichiometric predictions, diet nutrient content cn ffect excretion rtes s well s consumer body nutrient content. Therefore, lck of n effect of body nutrient content on excretion rtes or rtios would indicte tht the diets of the fish species were possibly driving 6

15 nutrient relese rther thn solely consumer body nutrient content. Thus, the gut contents of 6 lrge gizzrd shd nd 6 lrge bluegill sunfish were removed from the gizzrd nd the nterior portion of the stomch, respectively. Only lrge shd nd bluegill were used becuse (1) these fishes re the most dominnt species in Acton Lke, nd (2) using lrge individuls incresed the likelihood of finding fish with undigested food items. Smples were exmined under dissecting scope to identify nd quntify food items. Vlues of N:P for food items were estimted from the literture. Similrly, the Sterner model (1990) ws pplied to the results from gizzrd shd nd bluegill sunfish. I determined whether or not consumer body-nutrient content lone could explin vrition in nutrient excretion rtios mong fish species. For the purpose of this study, GGE mx for gizzrd shd nd bluegill were initilly kept t 0.7 s in the Sterner (1990) model nd the N:P of their respective food items were dded. The detritus N:P rtio for shd ws bsed on Sigler (2002) nd Higgins et. l. (in review) wheres the chironomid N:P rtio for bluegill ws bsed on n verge of vlues for dipterns from Cross et l. (2003) (n = 11 individuls from reference strem) nd Frost et. l. (2003) (n = 8 Cndin lkes, number of dipterns collected were not presented). The model ws then modified by djusting the GGE mx to obtin recycling rtios similr to the vlues from the nutrient excretion experiment. Lke-wide Biomss nd Nutrient Excretion Rtes To evlute whether the impct of gizzrd shd on nutrient recycling is influenced more by bundnce thn by high per cpit excretion rte, lke-wide biomss nd nutrient excretion rtes were clculted for ll resident fish species bsed on summer (June 29 th nd August 11 th, 2004) hydrocoustic nd electroshocking trnsect dt. To get lke-wide biomss, the number of ech fish species cught offshore vi electroshocking ws clculted by multiplying the totl number of fish offshore (individuls h -1, obtined from hydrocoustics) by the proportion of prticulr species present offshore (obtined from electroshocking). This eqution is shown here: # individuls offshore = (individuls h -1 ) x # species A totl # individuls The number of ech fish species inshore ws clculted by multiplying the number of fish of tht 7

16 species offshore by the ctch per unit effort (CPUE) inshore compred to the CPUE offshore: # individuls inshore = # individuls offshore x CPUE inshore CPUE offshore Length-weight reltionships from mesurements tken fter the nutrient excretion experiment nd from Schneider et l. (2000) were used to clculte the men biomss of ech fish species (kg individul -1 ), which were then multiplied by the number of fish offshore nd inshore. This eqution is shown here: offshore biomss (kg h -1 ) = (# individuls offshore) x (kg individul -1 ) inshore biomss (kg h -1 ) = (# individuls inshore) x (kg individul -1 ) Acton Lke is comprised of 80% open wter nd 20% littorl re; therefore, offshore biomss estimtes were weighted by 0.80 while inshore biomss estimtes were weighted by 0.20 nd summed in order to get totl lke-wide fish biomss (kg h -1 ). This eqution is shown here: totl lke-wide biomss (kg h -1 ) = (offshore biomss x 0.80) + (inshore biomss x 0.20) To estimte lke-wide nutrient excretion rtes for ech species, the men nutrient relese rte from the nutrient excretion experiments ws multiplied by the bundnce of tht species nd then converted to flux rte (mg m -2 dy). A globl regression bsed on the dt obtined from the nutrient experiment (excluding gizzrd shd) ws used to determine excretion rtes for fish species tht were cptured during electroshocking trnsects but for which nutrient relese rtes were not directly mesured. Lke-wide excretion rtes for June nd August were verged to get n overll summer excretion rte for ech species. Percent biomss, N excretion, nd P excretion ccounted for by ech fish species ws then clculted. Sttisticl Anlyses Dt were log-trnsformed when pproprite to equlize vrinces nd were nlyzed using n nlysis of covrince (ANCOVA) with Bonferroni (slopes) nd Dunnett (intercepts) djustments for multiple comprisons with gizzrd shd (proc glm SAS, Version 9.1). Fish body mss, body P, body N, nd body N:P were covrites nd, in ll cses, fish species ws the 8

17 ctegoricl vrible. The following were dependent vribles: per cpit N nd P excretion, mss-specific N nd P excretion, mss-normlized N nd P excretion, excretion N:P, body C, body N, body P, body C:N, body C:P, nd body N:P. Regression lines were lso fitted to the dt for ll vribles to obtin the slopes nd intercepts for ech species. RESULTS Nutrient Excretion Experiments Per cpit N excretion ws function of incresing mss for ll 177 fish nlyzed together (Fig. 2, p < , r 2 = 0.62). There ws lso significnt species effect (p < ) s well s mss x species interction (p < ). With the exception of lrgemouth bss nd ornge-spotted sunfish, ll slopes were found to be less thn 1, indicting decrese in mssspecific excretion with incresing mss. Logperch nd golden shiner were the only species with slightly negtive slopes, but the size rnges of these two species were quite nrrow (Fig. 2). Gizzrd shd hd higher N relese rtes thn shiner but lower rtes thn both lrgemouth bss nd green sunfish. Shd ws not sttisticlly different from other fish species (Tble 2, Fig. 2). Per cpit P excretion ws lso highly correlted with incresing mss for ll fishes (Fig. 2b, p = , r 2 = 0.68). There ws significnt species effect (p < ) s well s mss x species interction (p = ). Slopes were less thn 1 (indicting decrese in mss-specific excretion with incresing mss) for ll fish species except lrgemouth bss, which hd greter slope thn gizzrd shd. Once gin, logperch hd slightly negtive slope tht ws sttisticlly lower thn shd. Similrly, the slope for gizzrd shd ws sttisticlly similr to ll other fish species (Tble 2, Fig. 2b). Body Nutrient Anlysis There ws no significnt effect of mss on fish body C content (Fig. 3, p = , r 2 = 0.41). However, there ws species effect (p = ) s well s mss x species interction (p = ). Averge body C content for Acton Lke fishes rnged from 39-45%. Gizzrd shd tended to hve greter increse in body C with mss (i.e. slope) thn longer sunfish but ws sttisticlly similr to ll other fishes (Tble 3, Fig. 4). Shd (p < ) nd bluegill sunfish (p = ) were the only fishes in which body C content incresed slightly with mss (Fig. 3). In terms of body N content, there ws no significnt effect of mss (Fig. 3b, p = , 9

18 r 2 = 0.31) but there ws n effect of species (p = ) nd mss x species interction (p = ). Fishes tended to hve nrrow rnge of verge body N content of 10-12%. Bluegill sunfish ws the only species in which N content slightly incresed with mss (p < ). Gizzrd shd hd less body N thn bluegill but ws sttisticlly similr to ll other fish species (Tble 3, Fig. 4b). There ws no significnt overll effect of mss on body P content (Fig. 3c, p = , r 2 = 0.54). Yet, there ws species effect (p = ) s well s mss x species interction (p < ). Acton Lke fishes tended to hve rnge of verge body P content of 3-4% (Tble 3, Fig. 4c). Bluegill sunfish ws the only species tht hd incresing body P content with incresing mss (p < , Fig. 3c) though longer sunfish hd mrginlly significnt increse in body P with mss (p = ). Similrly, bluegill hd greter increse in body P content with mss (i.e. slope) thn gizzrd shd, which ppered to hve slight but nonsignificnt increse in body P with mss (Tble 3, Fig. 3c & 4c). Overll, reltive vrition in body P content ws greter thn for body N for the fishes in Acton Lke: body molr N:P rnged from bout 4 to bout 11. Fishes in the fmily Centrrchide (i.e. bluegill, green sunfish, longer sunfish, ornge-spotted sunfish, nd lrgemouth bss) hd prticulrly high P content wheres gizzrd shd (Clupeide) tended to hve lower P content nd lower N content reltive to other fishes. The fmilies Percide (i.e. logperch drter) nd Cyprinide (i.e. golden shiner) hd body N nd P content tht ws intermedite compred to the other two fmilies (Fig. 5). Body C:N rtios showed no mss x species interction (p = ) or mss effect (Fig. 6, p = , r 2 = 0.36), but there ws significnt effect of species (p < ). Men C:N rtios for Acton Lke fishes rnged from 4.0 to 5.5. When compring intercepts mong species, gizzrd shd hd significntly higher men body C:N rtio thn ll other fish species except ornge-spotted sunfish (Tble 3, Fig. 4d). It ws lso the only species tht hd n increse in body C:N with incresing mss (p = , Fig. 6). Body C:P did not increse with incresing mss for ll fishes (Fig. 6b, p = , r 2 = 0.50). There ws, however, significnt mss x species interction (p = ) nd n effect of species (p = ). Men body C:P rnged from pproximtely 25 to 40, nd gizzrd shd ws found to hve greter increse in body C:P with mss (i.e. slope) thn bluegill sunfish (Tble 3, Fig. 4e). Bluegill sunfish ws the only species tht hd decrese in body C:P with incresing 10

19 mss (p < ), presumbly due to the increse in body P content with mss. Gizzrd shd incresed slightly in body C:P with mss, but tht trend ws not significnt (p = , Fig. 6b). Body N:P incresed with mss for ll fish species (Fig. 6c, p = , r 2 = 0.62). Yet, there ws species effect (p < ) s well s mss x species interction (p = ). Men body N:P rnged from 6 to 8. Both gizzrd shd (p = ) nd bluegill sunfish (p < ) hd decresing body N:P with incresing mss, though the decrese ws steeper for bluegill thn it ws for shd (Tble 3, Fig. 6c). Gizzrd shd body N:P ws sttisticlly similr to ll other fish species (Tble 3, Fig. 4f). Lstly, there ws slight but non-significnt decrese in body N:P with mss for logperch (p = ). Correltion of Body Nutrient Content, Nutrient Excretion, nd Gut Contents Per cpit N excretion ws correlted with body N content (Fig. 7, p = , r 2 = 0.41). There ws lso significnt species effect (p < ) nd body N x species interction (p < ). N excretion incresed with incresing body N content for bluegill sunfish (p = ), green sunfish (p = ), nd lrgemouth bss (p < ). Lrgemouth bss nd green sunfish were lso found to hve greter N excretion rtes thn gizzrd shd (Tble 4). When per cpit N excretion rtes were normlized for mss, there ws still significnt body N x species interction (p = , r 2 = 0.79) nd species effect (p = ). However, there ws no longer min effect of body N (p = ). Gizzrd shd ws found to hve lower N excretion rte thn lrgemouth bss but ws sttisticlly similr to ll other fishes (Tble 4, Fig. 7b). Per cpit P excretion ws correlted with body P content (Fig. 8, p < , r 2 = 0.57). There ws lso significnt effect of species (p < ) but no body P x species interction ws present (p = ). Bluegill sunfish ws found to hve incresed P excretion with incresing body P (p < ). This trend ws slight but non-significnt for gizzrd shd (p = ), most likely driven by n outlier. In ddition, shd tended to excrete more P thn bluegill, longer sunfish, lrgemouth bss, nd golden shiner. Logperch drter excreted significntly more P thn shd while both green sunfish nd ornge-spotted sunfish were sttisticlly similr to shd. After normlizing the per cpit P excretion rtes for mss, there ws no body P x species interction (p = ) or effect of body P (p = ) but there ws still n effect of species (p < , r 2 = 0.68). Gizzrd shd ws found to excrete more P thn 11

20 ll other Acton fishes with the exception of logperch drter (Tble 4, Fig. 8b). When compring the N:P of nutrient relese with fish body N:P, there ws significnt effect of species (p < , r 2 = 0.39). There ws lso no effect of body N:P (p = ) nd no body N:P x species interction (p = ). Overll, gizzrd shd hd lower, less vrible excretion N:P thn ll other fish species except ornge-spotted sunfish (Tble 4, Fig. 9). Gut contents were exmined to determine the diets of the fish species with the lrgest smple sizes gizzrd shd nd bluegill sunfish nd to relte diets to literture N:P vlues for their preferred food types. Out of the 6 bluegills used for gut content nlysis, only one individul ws found to hve n empty stomch. The other 5 fish hd gut contents tht were comprised of invertebrtes, with >90% of the identifible food items being chironomids. As for gizzrd shd, 2 out of 6 individuls were found with empty gizzrds. The rest of the shd hd gut contents consisting of only detritus. Using the literture vlues, I estimted N:P rtios for chironomids to be 27 (Cross et l. 2003; Frost et l. 2003) nd for detritus to be 12 (Sigler 2002; Higgins et. l. in review). Using the observed body N:P in the originl Sterner model, nutrient relese (with GGE mx set t 0.70) showed tht the observed differences in body N:P re insufficient to ccount for observed differences in the N:P rtio excreted (Fig. 10). The model predicted N:P recycling rtios tht were slightly greter thn those obtined in the nutrient excretion experiment. Thus, the nlysis suggested tht vrition in food N:P my be driving observed differences in excretion N:P. Consequently, GGE mx ws djusted for ech species so tht the recycled N:P vlues in the model corresponded to those found in the nutrient excretion experiment. The GGE mx were 0.65 for bluegill nd for shd. The results indicted tht observed excretion N:P my be function of food N:P nd differences in GGE mx (Fig. 10b). Lke-wide Biomss nd Nutrient Excretion Rtes Totl verge fish biomss in Acton Lke for the summer of 2004 ws 1385 kg h -1, nd the verge N nd P excreted by the entire fish community ws pproximtely 130 nd 17 mg m -2 dy, respectively. Gizzrd shd represented 66% of fish biomss, followed by lrgemouth bss (8%), common crp (6%), nd quillbck (5.7%). Bluegill sunfish were only 2% of the verge biomss while ll other fish were 1.5% or lower (Tble 5). In terms of nutrient relese, gizzrd shd ws responsible for 81% nd 95% of ll N nd P excreted by fish (Tble 5, Fig. 11). 12

21 Golden shiner nd bluegill sunfish relesed bout 6% nd 4% of the totl N excreted in the wter column. All other fish contributed less thn 2% of lke-wide N nd P excretion (Tble 5). DISCUSSION Nutrient Excretion Experiments As llometry theory predicts, per cpit N nd P excretion incresed with incresing mss, i.e. lrge fish excreted more nutrients thn smll fish on per individul bsis while smll fish tended to excrete more nutrients thn lrge fish per unit biomss (e.g. Schus et l. 1997). Mss-specific P excretion rtes for gizzrd shd rnged from 0.13 to 0.80 µmol g -1 h -1, with 10 individuls (rnging from 2.5 to 4.9 g wet mss) excreting up to pproximtely 2 µmol g -1 h -1. These excretion rtes were similr to the rnges reported by for shd in Tylorsville Reservoir, Kentucky (0.22 to 0.64 µmol g -1 h -1 - Shostell nd Buckvecks 2004), nd those found in previous studies of shd in Acton Lke (0.10 to 0.44 µmol g -1 h -1 - Mther et l. 1995; 0.07 to 0.69 µmol g -1 h -1 Schus et l. 1997). However, the slope for P excretion is low compred to other studies in Acton Lke gizzrd shd (0.36 this study; 0.89 Schus et l. 1997; nd Higgins et l. in review), despite similrities in lke temperture. Gizzrd shd mssspecific N excretion rtes rnged from 1.8 to 27.7 µmol g -1 h -1, with 3 smll individuls excreting 34.3, 36.7, nd 56.1 µmol g -1 h -1. The rnge for these excretion rtes re considerbly wider thn the estimtes presented by Shostell nd Buckvecks (2004) (1.28 to 2.71 µmol g -1 h -1 ), Schus et l. (1997) (1.85 to 7.35 µmol g -1 h -1 ), nd Mther et l. (1995) (0.4 to 7.2 µmol g -1 h -1 ). Similrly, the rnges for mss-specific P nd N excretion for bluegill sunfish (0.01 to 0.99 µmol g -1 h -1 for P; 1.7 to 86.7 µmol g -1 h -1 for N) were considerbly wider thn lbortory vlues reported by Mther et l. (1995) (0.04 to 0.18 µmol g -1 h -1 for P; 2.5 to 3.4 µmol g -1 h -1 ). Unfortuntely, nutrient excretion rtes for the other fishes in Acton Lke could not be compred to literture vlues becuse no previous studies hve been conducted using those species. The originl hypothesis tht other fish species within Acton Lke would hve similr excretion rtes nd rtios (per individul or per biomss) s gizzrd shd ws not supported. Shd hd higher N relese rtes thn golden shiner but lower relese rtes thn both lrgemouth bss nd green sunfish. In ddition, gizzrd shd hd higher slope for P excretion rte thn logperch drter but ws sttisticlly similr thn the other fish species. However, when mss ws tken into ccount, shd tended to hve higher excretion rtes thn most fish species (Tble 13

22 4, Fig. 8). Thus, the effect of body size on nutrient recycling my be species dependent. Body Nutrient Anlysis Crbon content ws found to increse with incresing mss for only gizzrd shd nd bluegill sunfish. This my be due to the fct tht shd nd bluegill hve higher lipid contents s their mss increses, especilly in times of high cloric intke such s periods prior to reproduction nd over-wintering (Sterner nd Elser 2002). The other smpled fish species (e.g. logperch drter) lso hve smll size rnges. In this cse, ccumultion of C s lipids could be miniml nd chnges in C content with mss would be difficult to detect. Similrly, gizzrd shd ws found to hve higher men body C content thn some of the other Acton fishes. Once gin, this my be due to the fct tht shd tend to store more C s lipids thn the other fishes s evidenced by the drk nd oily texture of their skin (personl observtion). There ws no significnt effect of mss on fish body N content, contrry to Dvis nd Boyd (1978), who found tht N content decreses with incresing mss for bluegill sunfish nd lrgemouth bss. In fct, this study found slight increse in body N content with incresing mss for bluegill (slope = 0.6, Tble 3). This is possibly result of n increse in structurl protein such s collgen, which tends to contribute significntly to the biomss of mny vertebrtes. In ddition, bluegill hd higher body N content thn gizzrd shd, which once gin my be due to differentil lloction of N to proteins (Sterner nd Elser 2002). In terms of body P content, bluegill sunfish ws the only Acton Lke fish species tht hd incresing body P with incresing mss. A similr trend ws reported by Dvis nd Boyd (1978) nd ppers to be chrcteristic of the fmily Centrrchide (Sterner nd Elser 2002), though this reltionship ws not seen for the other members of this fmily in this study due to smll smple sizes. Bluegill lso hd higher men body P content thn gizzrd shd nd ws observed to hve greter number of nd stiffer scles thn mny of the fishes in this study (personl observtion). This suggests tht bluegill likely hve greter lloction of P to the formtion of bones nd scles (Sterner nd George 2000; Sterner nd Elser 2002). Overll, reltive vrition in body P content ws greter thn for body N for the fishes in Acton Lke, result similr to Sterner nd George (2000). The rnge of body P content ws lso similr to tht presented by Sterner nd George (rnge of %). However, they present rnge of 8-12% for body N content, which is nrrower thn wht ws found here (5.5-15%). 14

23 This is presumbly due to the presence of gizzrd shd, which tended to hve lower N content thn the other fishes in this study. Shd my hve lso influenced the rnge in body N:P (rnge of 4 to 11), which ws found to be slightly nrrower thn the N:P rnge given by Sterner nd George (rnge of 5 to 15). Fishes in the fmily Centrrchide (e.g. bluegill) hve prticulrly high P content wheres gizzrd shd hve lower P content nd lower N content reltive to the other Acton Lke fishes. The fmilies Percide (i.e. logperch drter) nd Cyprinide (i.e. golden shiner) hd body N nd P content tht ws intermedite to the other two fmilies. This finding is similr Sterner nd George (2000), who stted tht the closely relted fmilies Percide nd Centrrchide hd high body P content, suggesting tht orgnisms tht re txonomiclly similr my hve similr body nutrient contents (Vnni et l. 2002). It hs lso been proposed tht high nd low consumer body P (nd to lesser extent body N) my be mnifesttion of evolutionry dpttions in stoichiometric niches (e.g. development of reduced body demnds for prticulr element; Hessen et l. 2004). Thus, the body nutrient stoichiometry of freshwter fishes hs phylogenetic nd evolutionry component tht needs further explortion. The rnge of body C:N for Acton Lke fishes ws similr to the rnge presented for fishes in wetlnd of Lke Superior by Tnner et l. (2000). However, their rnges for C:P nd N:P body rtios (40.6 to 64.6 nd 8.4 to 12.8, respectively) were wider thn those shown in this study, possibly due to differences in fish ssemblges nd evolutionry history. Interestingly, gizzrd shd ws found to hve higher men body C:N thn the other fish species nd ws the only fish species in which body C:N rtio incresed with incresing mss. This ws due to shd s increse in body C with mss. Gizzrd shd lso hd higher men C:P thn bluegill sunfish, which ws most likely due to shd s higher lipid content. Similrly, bluegill ws the only fish species in which body C:P decresed with mss due to its increse in body P content with mss. Gizzrd shd hd higher men body N:P thn bluegill. This is consequence of (1) the low body P content reltive to body N for shd nd (2) the high body P content of sunfish (e.g. bluegill). Both gizzrd shd nd bluegill sunfish lso hd decresing body N:P with incresing mss, result tht supports Vnni's (1996) prediction tht there should be decresing llometric trend for body N:P with incresing fish size. On the other hnd, the other fish species in this study did not show the sme trend, indicting tht lrger smple size ws needed nd tht body 15

24 N:P my only decrese with size for fishes with lrge size rnges. It lso indictes tht decresing body N:P with incresing mss my be species-specific. Ultimtely, to evlute differences in body nutrient stoichiometry mong fish, one must consider constituent elements s well s their rtios nd their biochemicl form (Anderson et l. 2004). Correltion of Body Nutrient Content, Nutrient Excretion, nd Gut Contents Cn ecologicl stoichiometry theory explin the vrition mong Acton Lke fishes? Contrry to predictions of stoichiometry theory, N nd P excretion incresed with incresing body N nd body P, respectively. It is possible tht N excretion incresed with incresing body N for bluegill sunfish, green sunfish, nd lrgemouth bss s result of excess nitrogen in their diets reltive to their body nutrient requirements, diet shifts, or mss effect on excretion rtes. Similrly, bluegill sunfish hd incresing P excretion with incresing body P tht my be due to mss or diet shift effect on excretion rtes. On the other hnd, s predicted by stoichiometry theory, per cpit N nd P excretion were correlted with fish body N nd body P content, t lest mong species. Fishes with high body N (e.g. lrgemouth bss) or high body P content (e.g. bluegill sunfish) excreted less N or P thn fish species with low body N or low body P (e.g. gizzrd shd). Similrly, fishes with high body N:P (e.g. gizzrd shd) hd lower excretion N:P rtios thn fishes with low body N:P. However, when excretion rtes were corrected for mss, the effect of body nutrient content ws removed. There ws lso no effect of body N:P on recycling N:P, result similr to tht found by Elser nd Urbe (1999) using zooplnkton, though this ws t the individul level nd not t the species level. This suggests tht the predominnt fctor ffecting excretion N:P is food N:P. Consistent with other studies, exmintion of gut contents reveled tht gizzrd shd primrily ingest detritus (Yko et l. 1996; Mundhl nd Wissing 1986; Sigler 2002) wheres bluegills tend to consume chironomids (Olson et. l. 2003). Though bluegill diet preference hs not been ssessed in Acton Lke, it is resonble to ssume tht bluegills in the reservoir prefer chironomids being tht they re very bundnt in shllow, oxygented res in Acton Lke (Devine nd Vnni 2002). Admittedly, smple sizes for gut content nlyses were smll, nd it is possible tht bluegill my consume other prey items not represented in the smples. Similrly, the N:P of gut contents from bluegill sunfish used in the nutrient excretion experiments were not pproprite becuse of ltertion of food nutrient content s result of digestion. Thus, it ws 16

25 resonble to use literture vlues for detritus nd chironomid N:P rtios in the Sterner model. By including these N:P vlues nd djusting the GGE mx for ech species, differences in recycled N:P vlues for shd nd bluegill indicted tht nutrient relese is more function of food N:P thn consumer N:P. Elser nd Urbe (1990) rrived t the sme conclusion for zooplnkton by estimting the residuls of the regression between recycled N:P nd food N:P, plotting them ginst consumer N:P, nd finding significnt negtive reltionship between the residuls nd consumer N:P. Thus, diet, diet shifts, nd growth rtes my ply vitl roles in determining nutrient recycling within qutic systems. Aquculture literture hs consistently found tht body P nd P excretion increses s dietry P increses for few fish species (Atlntic slmon (Slmo slr) Vielm nd Lll 1998; rinbow trout (Oncorhynchus mykiss) - Bureu nd Cho 1999; juvenile hddock (Melnogrmmus eglefinus) - Roy nd Lll 2003). Brbrnd et l. (1990) lso suggested tht the P relese rtes of fish must be influenced by the food they consume becuse fish tht feed on sediment (e.g. roch) tend to hve higher P excretion thn benthivorous nd plnktivorous fishes (e.g. perch). Yet, it is possible tht even fish within the sme feeding guild my ffect nutrient recycling differently depending on slight differences in food (e.g. selection of different components of the sme food source) nd ssimiltion efficiencies (Yoss nd Arujo- Lim 1998). Lstly, shd growth rtes re strongly linked to diets; they grow fstest on diet consisting primrily of zooplnkton (Schus et l. 2002). Consequently, chnge in diet from one comprised of detritus to one dominted by zooplnkton could lter shd growth rtes nd, in turn, influence excretion rtes nd rtios vi higher requirements for P (Sterner nd Elser 2002; Elser et l. 2003), resulting in decresed P excretion. Results from this study suggest tht diet cn influence nutrient excretion rtes nd rtios. For exmple, gizzrd shd consume low N:P food, detritus, nd hve high body N:P, resulting in low recycling N:P rtio. Bluegill sunfish, on the other hnd, consume mcroinvertebrtes such s chironomids in the summer, especilly in lkes with low densities of bluegill like Acton Lke (Olson et l. 2003). By consuming this high N:P food nd by hving low body N:P, bluegill excrete t high N:P rtio. Therefore, it ppers tht the quntifiction of food nutrient content is necessry when studying the stoichiometric spects of consumer-driven nutrient recycling. 17