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Stable isotope mixing models are often used to quantify source contributions to a mixture. Examples include pollution source identification; trophic web studies; analysis of water sources for soils, plants; or water bodies, and many others. A common problem is having too many sources to allow a unique solution. We discuss two alternative procedures for addressing this problem. One option is a priori to combine sources with similar signatures so the number of sources is small enough to provide a unique solution. Aggregation should be considered only when isotopic signatures of clustered sources are not significantly different, and sources are related so the combined source group has some functional significance. For example, in a food web analysis, lumping several species within a trophic guild allows more interpretable results than lumping disparate food sources, even if they have similar isotopic signatures. One result of combining mixing model sources is increased uncertainty of the combined end-member isotopic signatures and consequently the source contribution estimates; this effect can be quantified using the IsoError model (http://www.epa.gov/wed/pages/models/isotopes/isoerror1_04.htm). As an alternative to lumping sources before a mixing analysis, the IsoSource mixing model (http://www.epa.gov/wed/pages/models/isosource/isosource.htm) can be used to find all feasible solutions of source contributions consistent with isotopic mass balance. While ranges of feasible contributions for each individual source can often be quite broad, contributions from functionally related groups of sources can be summed a posteriori, producing a range of solutions for the aggregate source that may be considerably narrower. A paleohuman dietary analysis example illustrates this method, which involves a terrestrial meat food source, a combination of three terrestrial plant foods, and a combination of three marine foods. In this case, a posteriori aggregation of sources allowed strong conclusions about temporal shifts in marine versus terrestrial diets that would not have otherwise been discerned.

Previously we found that cloned cottonwood saplings (Populus deltoides) grew twice as large in New York, New York, USA, compared to surrounding rural environments and that soils, temperature, CO₂, nutrient deposition, and microclimatic variables could not account for the greater urban plant biomass. Correlations between final season biomass and cumulative O₃ exposures, combined with twofold growth reductions in an open-top chamber experiment provided strong evidence that higher cumulative O₃ exposures in rural sites reduced growth in the country. Here, we assess the field gas exchange, growth and development, and allocation responses underlying the observed growth differences and compare them with isolated O₃ responses documented in the open-top chamber experiment. Cottonwoods showed no visible foliar injury, reduced photosynthesis of recently expanded foliage, early leaf senescence, protective reduction in stomatal conductance, or compensatory allocation to shoot relative to root biomass for either the chamber or field experiment. Instead, O₃-impacted chamber plants had significantly higher conductance and reduced photosynthesis of older foliage that led to reduced leaf area production and a twofold biomass reduction in the absence of visible injury. Rural-grown field plants showed the same pattern of significantly higher conductance in the absence of concomitant increases in photosynthesis that was indicative of a loss of stomatal control. Incremental changes in foliar production were also significantly inversely related to fluctuations in ambient O₃ exposures. The similarity in biomass, gas exchange, phenological, and allocation responses between chamber and field experiments indicate that mechanisms accounting for reduced growth at rural sites were consistent with those in the open-top chamber O₃ experiment. This study shows the limitation of visible symptoms as a sole diagnostic factor for documenting detrimental O₃ impacts and points toward a new approach to show O₃ impacts when visible injury is not present. Namely, O₃-impacted vegetation showed an unusual inverse relationship of increased conductance with lower photosynthesis of older foliage that was indicative of a loss of stomatal control. This increased stomatal conductance of O₃-impacted vegetation accentuates pollutant flux into affected foliage and has important implications for system water balance during warm, dry portions of the growing season when O₃ concentrations are highest.

Where is your church located? While 11,328 Presbyterian congregations can point to a visible location, approximately 400 congregations can also claim a virtual location--on the Internet.

Modeling and monitoring plankton functional types (PFTs) is challenged by the insufficient amount of field measurements of ground truths in both plankton models and bio-optical algorithms. In this study, we combine remote sensing data and a dynamic plankton model to simulate an ecologically sound spatial and temporal distribution of phyto-PFTs. We apply an innovative ecological indicator approach to modeling PFTs and focus on resolving the question of diatom–coccolithophore coexistence in the subpolar high-nitrate and low-chlorophyll regions. We choose an artificial neural network as our modeling framework because it has the potential to interpret complex nonlinear interactions governing complex adaptive systems, of which marine ecosystems are a prime example. Using ecological indicators that fulfill the criteria of measurability, sensitivity and specificity, we demonstrate that our diagnostic model correctly interprets some basic ecological rules similar to ones emerging from dynamic models. Our time series highlight a dynamic phyto-PFT community composition in all high-latitude areas and indicate seasonal coexistence of diatoms and coccolithophores. This observation, though consistent with in situ and remote sensing measurements, has so far not been captured by state-of-the-art dynamic models, which struggle to resolve this \"paradox of the plankton\". We conclude that an ecological indicator approach is useful for ecological modeling of phytoplankton and potentially higher trophic levels. Finally, we speculate that it could serve as a powerful tool in advancing ecosystem-based management of marine resources.