Portal de Periódicos da CAPES

Terrestrial support of detritivorous fish populations decreases with watershed size

2011; Wiley; Volume: 2; Issue: 7 Linguagem: Inglês

10.1890/es11-00043.1

2150-8925

Allison L. Babler, Alberto Pilati, Michael J. Vanni,

Mercury impact and mitigation studies

EcosphereVolume 2, Issue 7 art76 p. 1-23 ArticleOpen Access Terrestrial support of detritivorous fish populations decreases with watershed size Allison L. Babler, Allison L. Babler Department of Zoology, Miami University, Oxford, Ohio 45056 USA Present address: Graduate School of Education, St. Mary's University, Winona, Minnesota 55997 USA.Search for more papers by this authorAlberto Pilati, Alberto Pilati Department of Zoology, Miami University, Oxford, Ohio 45056 USA Present address: Departamento de Ciencias Naturales, Universidad Nacional de La Pampa, Uruguay 151, 6300, Santa Rosa, Argentina.Search for more papers by this authorMichael J. Vanni, Corresponding Author Michael J. Vanni Department of Zoology, Miami University, Oxford, Ohio 45056 USA Graduate Program in Ecology, Evolution and Environmental Biology, Miami University, Oxford, Ohio 45056 USAE-mail:[email protected]Search for more papers by this author Allison L. Babler, Allison L. Babler Department of Zoology, Miami University, Oxford, Ohio 45056 USA Present address: Graduate School of Education, St. Mary's University, Winona, Minnesota 55997 USA.Search for more papers by this authorAlberto Pilati, Alberto Pilati Department of Zoology, Miami University, Oxford, Ohio 45056 USA Present address: Departamento de Ciencias Naturales, Universidad Nacional de La Pampa, Uruguay 151, 6300, Santa Rosa, Argentina.Search for more papers by this authorMichael J. Vanni, Corresponding Author Michael J. Vanni Department of Zoology, Miami University, Oxford, Ohio 45056 USA Graduate Program in Ecology, Evolution and Environmental Biology, Miami University, Oxford, Ohio 45056 USAE-mail:[email protected]Search for more papers by this author First published: 12 July 2011 https://doi.org/10.1890/ES11-00043.1Citations: 33 Corresponding Editor: E. García-Berthou. AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Abstract Consumers in aquatic food webs can be supported by terrestrial and aquatic primary production. However, we know little about how allochthony, i.e., the proportion of consumer biomass derived from terrestrial organic matter, varies along environmental gradients. Using hydrogen isotopes (deuterium), we quantified allochthony of an ecologically dominant detritivorous fish species (gizzard shad, Dorosoma cepedianum) in reservoir ecosystems, along a gradient of watershed land use (agriculture vs. forest). We predicted that allochthony would decline with an increase in the proportion of watershed land composed of agriculture (% agriculture). This is based on the supposition that as % agriculture increases, so does the export of dissolved inorganic nutrients to lakes, stimulating algal production and reducing the importance of terrestrial organic carbon subsidies. Allochthony accounted for ∼34% of gizzard shad production (mean of 11 lakes), although this fraction varied greatly (0–68%) among lakes and isotope mixing model assumptions. Contrary to our hypothesis, we found no relationship between allochthony and % agriculture. However, allochthony was inversely related to total watershed area, as well as the absolute area of the watershed (rather than the percentage) composed of agriculture. We speculate that watershed area and allochthony are negatively correlated because watershed area exerts a strong control on the relative subsidies of dissolved inorganic nutrients vs. particulate organic carbon. Gizzard shad biomass was positively related to phytoplankton primary production but negatively related to allochthony, suggesting that phytodetritus is a higher quality resource than terrestrial detritus. Overall, our results show that both autochthonous and allochthonous carbon fuel the production of this ecologically important detritivore, the relative importance of allochthony decreases with increasing watershed size, and variation in gizzard shad production is closely tied to variation in autochthonous primary production. Introduction Historically, it was assumed that consumers in lakes are supported by in-lake (autochthonous) carbon fixation, usually by algae. However, organic carbon subsidies from terrestrial ecosystems can be an important energy source for aquatic food webs (Wetzel 1995, Polis et al. 2004, Reynolds 2008). Several studies show that terrestrial (allochthonous) organic carbon subsidies directly or indirectly fuel the production of a variety of lentic consumers, including bacteria (Kritzberg et al. 2004, Karlsson 2007, Berggren et al. 2010), protists (Berggren et al. 2010), metazoan zooplankton (Grey et al. 2001, Matthews and Mazumder 2006, Karlsson et al. 2003, Pace et al. 2007, Cole et al. 2011), benthic invertebrates (Solomon et al. 2008, 2011) and fish (Carpenter et al. 2005, Weidel et al. 2008, Solomon et al. 2011). In contrast, other studies suggest that in general terrestrial organic matter contributes little to the nutrition of aquatic consumers (in particular crustacean zooplankton; Brett et al. 2009, Mohamed and Taylor 2009). Thus, a major challenge is to ascertain the factors regulating "allochthony" (Cole et al. 2006), i.e., the proportion of consumer biomass derived from terrestrial organic matter. Detritivores are often an important component of aquatic food webs, but there is uncertainty surrounding their carbon sources (e.g., Hall and Meyer 1998, Moore et al. 2004). Sediment-feeding "detritivores" often consume a relatively amorphous mix of algal detritus, terrestrial detritus, and living heterotrophic and autotrophic microbes (e.g., Smoot and Findlay 2010a, b). Because it is difficult to distinguish and quantify these energy resources, little is known about the prevalence of allochthony in aquatic detritivores. Allochthony may vary along environmental gradients. For example, allochthony may decrease with lake size and in-lake primary production, because the ratio of algal production to allochthonous organic C inputs increases along these gradients (Jones 1992, Pace et al. 2007). Reservoirs have relatively large watersheds, and may therefore receive large subsidies of both terrestrial organic carbon and inorganic nutrients (Thornton 1990, Wetzel 1990). Reservoirs in agricultural watersheds receive particularly large nutrient subsidies and generally have high phytoplankton production (Carpenter et al. 1998, Knoll et al. 2003, Vanni et al. 2011). Elevated subsidies of particultate organic carbon may increase allochthony by providing a food resource for detritivores, while dissolved inorganic nutrient subsidies stimulate phytoplankton production and may thus favor autochthony. Thus, allochthony may depend on watershed size, watershed land use, lake size, and algal productivity. In reservoirs of the midwestern and southern USA (hereafter, "midwestern reservoirs"), fish biomass is often dominated by gizzard shad (Dorosoma cepedianum), an omnivore that exerts strong effects on food webs and nutrient cycling (Stein et al. 1995, Vanni et al. 2005, 2006b). As larvae, gizzard shad are zooplanktivores, but when they reach ∼30 mm in length (∼2–3 months of age) they develop morphological adaptations for sediment feeding. Individuals above this size feed primarily on sediment detritus (Mundahl and Wissing 1987, Gido 2002, Schaus et al. 2002, Higgins et al. 2006, Zeug and Winemiller 2008). Evidence based on energetics and stable isotopes shows that gizzard shad can assimilate both autochthonous and allochthonous carbon (Pilati 2007, Zeug and Winemiller 2008, Smoot and Findlay 2010a, b). However, little is known about the relative contributions of autochthonous vs. allochthonous detritus to gizzard shad production, and how these contributions vary along environmental gradients. Information on resource utilization by gizzard shad populations is critical, so that ecologists and fisheries managers can better quantify the factors limiting this ecologically dominant species. In reservoirs, gizzard shad biomass and relative abundance increase disproportionately as watershed agriculture and reservoir productivity increase, and the ecosystem effects of gizzard shad are particularly strong in productive systems (Vanni et al. 2005, 2006b, Hale et al. 2008, Schaus et al. 2010). As mentioned, agriculture increases subsidies of both inorganic nutrients and allochthonous detritus to reservoirs. Thus, it is not clear how gizzard shad allochthony varies along gradients of watershed land use and algal productivity, or how important allochthony may be in driving the high gizzard shad biomass observed in agriculturally-impacted, high productivity systems. More broadly, we know of no studies that have examined the importance of allochthonous carbon subsidies for a lake detritivore along environmental gradients such as productivity or land use. We quantified gizzard shad allochthony in 11 midwestern reservoirs, spanning a wide gradient in watershed land use and phytoplankton primary productivity. We hypothesized that, due to increases in algal primary production caused by agricultural nutrient subsidies, allochthony by gizzard shad decreases with an increase in the percentage of watershed area composed of agriculture. An alternative hypothesis is that allochthony is unrelated to watershed land use but rather is driven more by in-lake variable such as lake size or shape; this could occur if increased subsidies of sediment detritus increase with agriculture in proportion to nutrient subsidies and primary productivity. We tested these hypotheses using naturally occurring hydrogen isotopes to quantify allochthony. Methods General approach and study sites We used naturally occurring stable hydrogen isotopes to quantify allochthony of gizzard shad. δD, the ratio of deuterium (2H) to hydrogen (1H) expressed relative to a known standard, has emerged recently as an effective means of separating organic matter produced via aquatic vs. terrestrial photosynthesis (Doucett et al. 2007, Solomon et al. 2009, 2011, Caraco et al. 2010, Finlay et al. 2010, Cole et al. 2011). These studies show that δD is more effective than other naturally occurring stable isotopes, such as δ13C, in distinguishing terrestrial and autochthonous energy sources. We sampled 11 reservoirs in Ohio, USA, covering a gradient of watershed land use (forest vs. agriculture) and algal productivity (Table 1; Knoll et al. 2003, Vanni et al. 2006b). These ecosystems are representative of midwestern reservoirs in that they have high watershed area:lake surface area ratios and are warm and shallow, compared to natural north-temperate lakes. Gizzard shad biomass is often quite high, especially in productive reservoirs with agricultural watersheds (Table 1; Vanni et al. 2006b, Hale et al. 2008). To estimate gizzard shad allochthony, we quantified δD of gizzard shad tissue, autochthonous and allochthonous food sources (phytoplankton and terrestrial vegetation, respectively), and lake water (necessary to correct for direct uptake of water, and hence H, by fish). Because these lakes are turbid with high phytoplankton biomass, macrophytes and benthic algae contribute very little to autochthonous production; therefore, we did not sample these groups. Table 1. Characteristics of study lakes and watersheds. Values are means from samples taken at least monthly from May-August 2008, except for Grand Lake St. Mary's (GLSM), for which only a single measurement was available for 2008. Gizzard shad biomass is the mean of 3 years (2006–2008) based on hydroacoustic surveys in August; 2006 data were taken from Hale et al. (2008) and subsequent years provided by J. Denlinger, Ohio Department of Natural Resources-Division of Wildlife (no data are available for GLSM). Fish Gizzard shad were collected via electrofishing in shallow (≤3m), well oxygenated, "uplake" areas where they tend to thrive. We sampled age-1 and older gizzard shad (mean length = 206 mm, range 133–365), because these individuals have been detritivorous the majority of their lives. Except for Acton, each reservoir was sampled for gizzard shad on one date between 4 May and 12 June of 2008. Acton gizzard shad were sampled on 6 dates, in June 2007, July 2007, May 2008 (2 dates), June 2008 and October 2008. All fish were immediately frozen, returned to the lab and kept at −80°C until they were processed for isotope analyses, at which time they were thawed in a water bath for ∼40 minutes. A small sample of white muscle (∼2 g wet mass) was then collected from the dorsal region between the operculum and the front of the dorsal fin, and rinsed in deionized water prior to isotope analysis, described below. Phytoplankton and water We collected a 20 L water sample from the euphotic zone of each lake to characterize δD of water and seston (phytoplankton and similarly-sized particles that cannot be easily separated). For all reservoirs except Deer Creek, Pleasant Hill, and Tappan, seston and water were sampled on the same day as fish; for these 3 lakes water and seston were sampled within 2 weeks of fish sampling. To estimate δD of water, two 4 mL aliquots were filtered through 0.2 μm membrane filters and stored in brown HDPE Nalgene bottles with no air space at 4°C until analysis. Water samples from Berlin were lost in processing, so we used the mean water δD value of nearby Pleasant Hill. Seston was collected to estimate the isotopic composition of autochthonous organic material, i.e., organic matter derived from phytoplankton photosynthesis. To obtain seston samples, we filtered water (≤10 L) through a 142 mm diameter GF/F filter (nominal pore size ∼0.7 μm) in a high volume filtration apparatus until the filter became clogged. The seston was then gently scraped into a metal weighing dish for drying. A subset of samples examined under 40× magnification verified that filter particles did not contaminate seston samples. Two replicate samples were collected from each 20 L container. Seston represents a mix of phytoplankton, phytodetritus, allochthonous detritus, and bacteria nourished by autochthonous and allocthonous sources. It is difficult or impossible to physically separate phytoplankton from other sestonic particles, which presents a challenge when trying to estimate the isotopic signature of phytoplankton (Caraco et al. 2010, Cole et al. 2011, Solomon et al. 2011). Because reservoirs have large watersheds, "contamination" with terrestrial particles may be significant, and is likely most pronounced soon after storm events that deliver large quantities of allochthonous sediments. We corrected for allochthonous contamination of seston using concentrations of non-volatile suspended solids (NVSS) in lake water. During storms that cause runoff of allochthonous sediments, NVSS concentrations and loads increase greatly in inflow streams (Vanni et al. 2001), and this leads to a large increase in reservoir NVSS concentration just after storm events (e.g., Vanni et al. 2006a). Although NVSS is composed of inorganic particles, we hypothesized that NVSS concentrations and loads are correlated with those of allochthonous particulate organic carbon (POC). To test this hypothesis, we measured concentrations of both NVSS and POC, on the same stream water sample, on 56 samples collected from Acton Lake's two largest inflow streams during 2006–08 (NVSS methods are given below; POC was measured with a Perkin Elmer CHN analyzer). Stream NVSS and POC concentrations were highly correlated (r2 = 0.818, P < 0.0001, n = 56 samples pooled from the two streams; Appendix: Fig. A1). NVSS and POC loads (stream discharge × concentration) were even more strongly correlated (r2 = 0.921; Appendix: Fig. A1). Although these data are from only one lake/watershed (Acton), they clearly show that watershed POC inputs are strongly associated with NVSS inputs. Although we do not have such data for all watersheds, we have observed similar increases in NVSS (in inflow streams as well as recipient lakes) in Burr Oak and Pleasant Hill, where we also have monitored watershed inputs (Vanni et al. 2011). Therefore, the relative contribution of NVSS to total suspended solids (%NVSS) in the lakes can serve as a valid measure of terrestrial detritus "contamination" of seston samples. Concentrations of suspended solids (SS) and non-volatile suspended solids (NVSS) were measured (in addition to seston δD) in 7 reservoirs on multiple dates (except that Berlin was sampled only once) in 2007, 2008, and/or 2009 (n = 30 lake-dates). We used %NVSS, i.e., 100 × (NVSS/SS), as an index of terrestrial influence. We then created regressions of %NVSS vs. seston δD, and used the y-intercepts of these regressions as the estimated δD values of phytoplankton. This approach assumes that when %NVSS is zero, phytoplankton dominate seston, and therefore seston δD = phytoplankton δD. Using this general approach, we explored several scenarios, as described in the Allochthony scenarios section below. To measure SS and NVSS, water samples were filtered onto pre-weighed glass fiber filters previously combusted at 475°C. The filters were then dried at 60°C for ≥24 hours and weighed to obtain SS. Then, NVSS was determined by combusting the organic material on the filter in a muffle furnace at 550°C for 4 hours and re-weighing the filter (Vanni et al. 2006a). Terrestrial vegetation Terrestrial vegetation was sampled from the Acton, Pleasant Hill, and Burr Oak watersheds, which span our land use gradient and are geographically distributed across our study area (Vanni et al. 2005, 2011). Thus, they should represent the range of hydrologic and geologic conditions in the area. We sampled the two plant species that account for the vast majority of cropland, corn (maize, Zea mays) and soybean (Glycine max), and the most abundant tree species, maple (Acer spp.) and oak (Quercus spp.) Corn and maple were sampled in all 3 watersheds, while maple was not sampled in Pleasant Hill and soy was not sampled in Burr Oak. Tree leaves were senescent and were often collected from the ground. Leaves of corn and soy were collected from crop waste remaining on the field after autumn harvest. Within watersheds, sampling sites were selected by stratifying a watershed into three sections of equal length along its longest dimension and then randomly selecting 3 accessible sites within each stratum. If a species was present at more than two sites within a watershed, the samples from the two most geographically distant sites were used. Acton watershed vegetation was also sampled near the lake, and means for Acton include these data. A mean terrestrial value (δDalloc) was calculated for all 11 reservoirs using species-specific δD values from the nearest sampled watershed, weighted by land use proportions. To obtain δD for forested land, we used a simple mean of maple and oak δD; this approach is similar to other studies that have characterized δD of terrestrial vegetation for aquatic food web studies (e.g., Doucett et al. 2007, Cole et al. 2011). An overall δD for agricultural land was obtained by weighting by the proportion of cropland in corn, estimated from the USDA 2007 census of agriculture (USDA 2009); soy was assumed to constitute the remaining cropland area. Finally, an overall watershed-scale terrestrial δD was obtained using the proportion of land composed of agriculture vs. forest (Table 1). To assess whether incoming allochthonous sediments exhibited δD similar to terrestrial vegetation, we sampled sediments delivered to Acton Lake via its inflow streams. Seven samples were collected using automated samplers at Four Mile and Little Four Mile Creeks (Vanni et al. 2001, Renwick et al. 2008) during storm events in May 2008. Together these two streams drain 81% of the Acton watershed. We assume that most of this material is terrestrial in origin. Isotope analysis All samples for δD were dried to constant mass at 60°C and ground to a fine powder using a Retsch centrifugal mill (terrestrial vegetation) or a mortar and pestle (all other samples). Approximately 350 μg of the powdered material was packaged in silver capsules, and analyzed at the Colorado Plateau Stable Isotope Laboratory (Doucett et al. 2007, Cole et al. 2011). Samples were pyrolized at 1400°C and isotope concentrations were measured using gas isotope-ratio spectrometry. Water was measured using headspace equilibration with H2 gas and a Pt catalyst. Samples were normalized for exchangeable hydrogen using 3 standards as described in Doucett et al. (2007) and Solomon et al. (2009). All data are presented as delta units (δD) in per mil (‰) notation. Allochthony calculations We estimated allochthony of gizzard shad in each lake using a two-source mixing model, with phytoplankton and terrestrial vegetation as possible sources. However, because fish can obtain H from water they drink ("dietary water") as well as their food, we first corrected for dietary water using equations adapted from Solomon et al. (2009): where δD′fish and δDfish represent δD of fish tissue before and after correction for dietary water, respectively; ω is the proportion of tissue H obtained from dietary water; and δDwater is δD of lake water. This assumes no fractionation of H by gizzard shad, which is reasonable given that Solomon et al. (2009) found negligible H fractionation by aquatic consumers. We attempted to measure the contribution of dietary water (ω) for gizzard shad using controlled lab experiments, following methods of Solomon et al. (2009). However, we were unsuccessful because of high mortality, which unfortunately is typical for gizzard shad in lab settings. Thus, we explored several scenarios in which we varied ω, as described in the Allochthony scenarios section below. Allochthony, defined as the proportion of gizzard shad biomass derived from terrestrial organic matter (palloc), was estimated as: where δDautoc and δDalloc are values for phytoplankton and terrestrial vegetation, respectively. Allochthony scenarios Because we could not directly measure the contribution of dietary water (ω) or the isotopic signal of phytoplankton (δDautoc), we had to estimate these parameters through other means. We recognize that this increases the uncertainty of our allochthony estimates. To address this uncertainty, we explored several scenarios in which we varied ω and δDautoc and examined the sensitivity of palloc to this variation. Our approach to estimating uncertainty in palloc is thus similar to that of other recent papers using deuterium to study aquatic food webs (Caraco et al. 2010, Cole et al. 2011, Solomon et al. 2011). We explored 24 scenarios for each lake, in which ω and δDautoc were varied over reasonable ranges (Fig. 1). Each scenario yielded an allochthony (palloc) estimate for each lake. We then evaluated the mean and distribution of palloc produced by these scenarios. Our scenarios (Fig. 1) represent a fully factorial design in which we crossed 3 values of ω, δDautoc values generated by 4 different NVSS-seston δD regressions, and 2 corrections to δDautoc based on partial algal decomposition (no correction or correction). Figure 1Open in figure viewerPowerPoint Schematic representation of the allochthony scenarios explored. 24 scenarios were explored, consisting of a 3-way factorial design crossing 3 levels of dietary water, 4 regression methods and 2 levels of algal decomposition. For scenarios with algal decomposition ("Yes"), the estimated phytoplankton δD (δDautoc) obtained with the regression method was increased by 20‰, determined by experimentally decomposing phytoplankton. The filled cell represents the "baseline" scenario as explained in the text. Our moderate value of ω (which we consider most realistic) was 0.17 (Fig. 1). This is the mean ω estimated by Solomon et al. (2009) from a variety of aquatic consumers, including invertebrates and fish. We also explored a high ω > estimate (0.23), which is the maximum ω observed from several fish species (all salmonids) as reported in Solomon et al. (2009), and a low estimate (0.08). The latter value is based on data from six gizzard shad individuals collected from Acton and Burr Oak Lakes, from which we estimated δD of both muscle tissue and gut contents. For these six fish, we calculated ω by rearranging equation 1, i.e., by using a two-source mixing model in which fish derive H from their diet (gut contents) and water (Solomon et al. 2009). As described above, we regressed %NVSS vs. seston δD to correct for terrestrial contamination and obtain δDautoc; however, within this general scheme we used four different regression approaches (Fig. 1). For our simplest approach, we obtained a single regression of %NVSS vs seston δD using data pooled from all lakes (n = 30 lake-dates). Then we used the intercept of this regression, i.e., the δDautoc value corresponding to %NVSS = 0. A potential advantage of this approach, which we refer to as the "Common Regression" approach, is that it utilizes all the data. However, it produces a single δDautoc value for all lakes, and therefore does not allow for any variation among lakes. In addition, most of the points in this regression were derived from Acton Lake, where we have an intensive long-term sampling program, and the regression could be biased by this one lake. Therefore, our three other regression approaches addressed these issues. In the "Separate Regressions" scenarios, we used two separate regressions of %NVSS vs. seston δD, one for Acton and another for all other lakes pooled. We then used the respective intercepts as estimates of δDautoc, yielding one δDautoc value for Acton, and another for all other lakes. This removes potential bias due to frequent sampling of Acton, but does not allow for variation among the other lakes. We accounted for variation among lakes using two more regression approaches, each using regression residuals to derive lake-specific δDautoc values (Fig. 1). In the "Common Regression w/ Residuals" approach, we used the residuals of the all-lakes regression of %NVSS vs. seston δD. Thus, for each of the 7 lakes for which he had %NVSS data, we obtained a mean residual (from residuals on individual sampling dates) and added the residual to the intercept to get δDautoc for that lake. For the 4 lakes for which we lacked NVSS data, we used the intercept to estimate δDautoc, as in the Common Regression scenario. Finally, in the "Separate Regressions w/Residuals" scenarios, we employed the same approach except that for lakes other than Acton we used residuals of the regression specific to this set of lakes. In this scenario, we could not obtain a residual for Acton because it contributed all the points to its own regression; therefore we used the Acton-specific intercept in this scenario. We also wished to account for any change in δDautoc that may occur if phytoplankton partially decompose before being consumed by gizzard shad. Because these fish feed on surface sediments, it is possible that algae decompose to some extent before being consumed, and this could conceivably alter δDautoc. Thus, all of the 12 scenarios described above were run both with no adjustment to δDautoc (i.e., using intercepts and residuals as described in the preceding paragraph), or with an adjustment that accounted for changes in δDautoc during partial decomposition of phytoplankton (Fig. 1). To estimate the change in δDautoc due to partial decomposition, we conducted an experiment in which we incubated two samples of Acton Lake seston in the lab in the dark to simulate conditions at the sediment-water interface, measured the change in δD, and used this change to adjust δDautoc, relative to the scenarios without decomposition (Fig. 1). We sampled the decomposing seston after 3, 6 and 9 days incubation. However, we could not use data from any of the samples that had incubated for >3 days because they contained insufficient organic matter to obtain reliable deuterium values; most algae had already decomposed. Therefore, we used data only from the day 3 sampling. Seston δD increased by 20‰ after 3 days. Thus, in the decomposition scenarios, we increased δDautoc by 20‰ relative to the same scenario without decomposition. In total, we thus explored 24 scenarios: 3 ω values × 4 regression approaches × 2 algal decomposition states. We consider the most feasible scenario to be that in which we set ω = 0.17, used the Separate Regressions w/ Residuals method, and assumed no algal decomposition (Fig. 1). Thus we consider this our "baseline" scenario. We consider 0.17 to be the most realistic estimate of ω, even though it is derived from literature values and not our own comparison of gizzard shad tissue and their gut contents, for two reasons. First, our sample size is only six fish, from only two lakes. Second, although estimating ω by comparing consumer tissue vs. diet δD is valid when one cannot conduct controlled lab experiments, this method seems to yield lower ω values than controlled experiments (Solomon et al. 2009). We feel that the Separate Regressions w/ Residuals scenarios offer the most realistic means of estimating δDautoc because it provides lake-specific values and avoids possible bias in the regression due to disproportionate sampling of Acton. Finally, we contend that scenarios without algal decomposition are most parsimonious given our current knowledge, for a couple of reasons. First, our decomposition experiment was based on only two replicates and one sampling period; thus we are not as confident as we would like to be in our e