Catastrophic ecological regime shifts may be announced in advance by statistical early-warning signals such as slowing return rates from perturbation and rising variance. The theoretical background for these indicators is rich but real-world tests are rare, especially for whole ecosystems. We tested the hypothesis that these statistics would be early-warning signals for an experimentallyinduced regime shift in an aquatic food web. We gradually added top predators to a lake over three years to destabilize its food web. An adjacent lake was monitored simultaneously as a reference ecosystem. Warning signals of a regime shift were evident in the manipulated lake during reorganization of the food web more than a year before the food web transition was complete, corroborating theory for leading indicators of ecological regime shifts.

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Stable isotopes of carbon, nitrogen, and sulfur are used as ecological tracers for a variety of applications, such as studies of animal migrations, energy sources, and food web pathways. Yet uncertainty relating to the time period integrated by isotopic measurement of animal tissues can confound the interpretation of isotopic data. There have been a large number of experimental isotopic diet shift studies aimed at quantifying animal tissue isotopic turnover rate λ (%·day-1, often expressed as isotopic half-life, ln(2)/λ, days). Yet no studies have evaluated or summarized the many individual half-life estimates in an effort to both seek broad-scale patterns and characterize the degree of variability. Here, we collect previously published half-life estimates, examine how half-life is related to body size, and test for tissue- and taxa-varying allometric relationships. Half-life generally increases with animal body mass, and is longer in muscle and blood compared to plasma and internal organs. Half-life was longest in ecotherms, followed by mammals, and finally birds. For ectotherms, different taxa-tissue combinations had similar allometric slopes that generally matched predictions of metabolic theory. Half-life for ectotherms can be approximated as: ln (half-life) = 0.22*ln (body mass) + group-specific intercept; n = 261, p<0.0001, r2 = 0.63. For endothermic groups, relationships with body mass were weak and model slopes and intercepts were heterogeneous. While isotopic half-life can be approximated using simple allometric relationships for some taxa and tissue types, there is also a high degree of unexplained variation in our models. Our study highlights several strong and general patterns, though accurate prediction of isotopic half-life from readily available variables such as animal body mass remains elusive.

Cross-ecosystem subsidies to food webs can alter metabolic balances in the receiving (subsidized) system and free the food web, or particular consumers, from the energetic constraints of local primary production. Although cross-ecosystem subsidies between terrestrial and aquatic systems have been well recognized for benthic organisms in streams, rivers, and the littoral zones of lakes, terrestrial subsidies to pelagic consumers are more difficult to demonstrate and remain controversial. Here, we adopt a unique approach by using stable isotopes of H, C, and N to estimate terrestrial support to zooplankton in two contrasting lakes. Zooplankton (Holopedium, Daphnia, and Leptodiaptomus) are comprised of ≈20-40% of organic material of terrestrial origin. These estimates are as high as, or higher than, prior measures obtained by experimentally manipulating the inorganic 13 C content of these lakes to augment the small, natural contrast in 13 C between terrestrial and algal photosynthesis. Our study gives credence to a growing literature, which we review here, suggesting that significant terrestrial support of pelagic crustaceans (zooplankton) is widespread.allochthony | terrestrial subsidy

Aquatic food webs are subsidized by allochthonous resources but the utilization of these resources by consumers can be difficult to quantify. Stable isotope ratios of hydrogen (deuterium:hydrogen; deltaD) potentially distinguish allochthonous inputs because deltaD differs between terrestrial and aquatic primary producers. However, application of this tracer is limited by uncertainties regarding the trophic fractionation of deltaD and the contributions of H from environmental water (often called "dietary water") to consumer tissue H. We addressed these uncertainties using laboratory experiments, field observations, modeling, and a literature synthesis. Laboratory experiments that manipulated the deltaD of water and food for insects, cladoceran zooplankton, and fishes provided strong evidence that trophic fractionation of deltaD was negligible. The proportion of tissue H derived from environmental water was substantial yet variable among studies; estimates of this proportion, inclusive of lab, field, and literature data, ranged from 0 to 0.39 (mean 0.17 +/- 0.12 SD). There is a clear need for additional studies of environmental water. Accounting for environmental water in mixing models changes estimates of resource use, although simulations suggest that uncertainty about the environmental water contribution does not substantially increase the uncertainty in estimates of resource use. As long as this uncertainty is accounted for, deltaD may be a powerful tool for estimating resource use in food webs.

Fluxes of organic matter across habitat boundaries are common in food webs. These fluxes may strongly influence community dynamics, depending on the extent to which they are used by consumers. Yet understanding of basal resource use by consumers is limited, because describing trophic pathways in complex food webs is difficult. We quantified resource use for zooplankton, zoobenthos, and fishes in four low-productivity lakes, using a Bayesian mixing model and measurements of hydrogen, carbon, and nitrogen stable isotope ratios. Multiple sources of uncertainty were explicitly incorporated into the model. As a result, posterior estimates of resource use were often broad distributions; nevertheless, clear patterns were evident. Zooplankton relied on terrestrial and pelagic primary production, while zoobenthos and fishes relied on terrestrial and benthic primary production. Across all consumer groups terrestrial reliance tended to be higher, and benthic reliance lower, in lakes where light penetration was low due to inputs of terrestrial dissolved organic carbon. These results support and refine an emerging consensus that terrestrial and benthic support of lake food webs can be substantial, and they imply that changes in the relative availability of basal resources drive the strength of cross-habitat trophic connections.

Allochthonous organic carbon can subsidize consumers in aquatic systems, but this subsidy may only be significant in relatively small systems with high organic matter loading. We tested the importance of allochthonous carbon to consumers in a relatively large (258,000 m 2 ) clear-water lake by adding H 13 CO 3 daily for 56 d. Dissolved inorganic carbon (DIC) was substantially enriched in 13 C by the addition, but it was also variable over diel cycles because of exchange with the atmosphere and photosynthesis. By measuring the d 13 C value of a physically separated phytoplankton concentrate as well as the d 13 C of phospholipid fatty acids, we were able to follow 13 C-labeling dynamics of specific groups of phytoplankton and bacteria. The d 13 C values of particulate organic carbon (POC), dissolved organic carbon (DOC), phytoplankton, bacteria, zooplankton, and the invertebrate predator, Chaoborus spp. all increased to a maximum during the addition and declined once the addition ceased. Autochthony (% C derived from internal primary production) of carbon pools (POC, DOC) and consumers was assessed by fitting dynamic models to time series of d 13 C. Autochthonous carbon was the dominant source (88-100%) for POC, grampositive bacteria, a copepod, zooplankton biomass, and Chaoborus spp. Autochthonous carbon provided a lower fraction (,70%) of carbon to DOC, gram-negative bacteria, and cladoceran zooplankton. In comparison to smaller and more humic lakes, terrestrially derived allochthonous C was less significant to the pelagic food web in this larger, clear-water lake. Among lakes, the relative importance of autochthonous versus allochthonous carbon to planktonic consumers is positively correlated to the ratio of color (absorbance of light at 440 nm, an indicator of terrestrially derived organic carbon) to chlorophyll.

Inputs of terrestrial organic carbon (t-OC) into lakes are often considered a resource subsidy for aquatic consumer production. Although there is evidence that terrestrial carbon can be incorporated into the tissues of aquatic consumers, its ability to enhance consumer production has been debated. Our research aims to evaluate the net effect of t-OC input on zooplankton. We used a survey of zooplankton production and resource use in ten lakes along a naturally occurring gradient of t-OC concentration to address these questions. Total and group-specific zooplankton production was negatively related to t-OC. Residual variation in zooplankton production that was not explained by t-OC was negatively related to terrestrial resource use (allochthony) by zooplankton. These results challenge the designation of terrestrial carbon as a resource subsidy; rather, the negative effect of reduced light penetration on the amount of suitable habitat and the low resource quality of t-OC appear to diminish zooplankton production. Our findings suggest that ongoing continental-scale increases in t-OC concentrations of lakes will likely have negative impacts on the productivity of aquatic food webs.