The stable isotopes of nitrogen (8'5N) and carbon (8'3C) provide powerful tools for estimating the trophic positions of and carbon flow to consumers in food webs; however, the isotopic signature of a consumer alone is not generally sufficient to infer trophic position or carbon source without an appropriate isotopic baseline. In this paper, I develop and discuss methods for generating an isotopic baseline and evaluate the assump- tions required to estimate the trophic position of consumers using stable isotopes in multiple ecosystem studies. I test the ability of two primary consumers, surface-grazing snails and filter-feeding mussels, to capture the spatial and temporal variation at the base of aquatic food webs. I find that snails reflect the isotopic signature of the base of the littoral food web, mussels reflect the isotopic signature of the pelagic food web, and together they provide a good isotopic baseline for estimating trophic position of secondary or higher trophic level consumers in lake ecosystems. Then, using data from 25 north temperate lakes, I evaluate how 815N and 8'3C of the base of aquatic food webs varies both among lakes and between the littoral and pelagic food webs within lakes. Using data from the literature, I show that the mean trophic fractionation of b'5N is 3.4%o (1 SD = 1%M) and of 8'3C is 0.4%o (1 SD = 1.3%o), and that both, even though variable, are widely applicable. A sen- sitivity analysis reveals that estimates of trophic position are very sensitive to assumptions about the trophic fractionation of '5 N, moderately sensitive to different methods for gen- erating an isotopic baseline, and not sensitive to assumptions about the trophic fractionation of 8'3C when 8'3C is used to estimate the proportion of nitrogen in a consumer derived from two sources. Finally, I compare my recommendations for generating an isotopic baseline to an alternative model proposed by M. J. Vander Zanden and J. B. Rasmussen. With an appropriate isotopic baseline and an appreciation of the underlying assumptions and model sensitivity, stable isotopes can help answer some of the most difficult questions in food web ecology.

/pdf/using-stable-isotopes-to-estimate-trophic-position-models-16ajs0cxhj.pdf

Using stable isotopes to estimate trophic position: models, methods, and assumptions

Theory is developed to explain the carbon isotopic composition of plants. It is shown how diffusion of gaseous CO2 can significantly affect carbon isotopic discrimination. The effects on discrimination by diffusion and carboxylation are integrated, yielding a simple relationship between discrimination and the ratio of the intercellular and atmospheric partial pressures of CO2. The effects of dark respiration and photorespiration are also considered, and it is suggested that they have relatively little effect on discrimination other than via their effects on intercellular p(CO2). It is also suggested that various environmental factors such as light, temperature, salinity and drought will also have effects via changes in intercellular p(CO2). A simple method is suggested for assessing water use efficiencies in the field.

On the Relationship Between Carbon Isotope Discrimination and the Intercellular Carbon Dioxide Concentration in Leaves

Stable isotope ratios provide clues about the origins and transformations of organic matter. A few key reactions control the isotopic composition of most organic matter. Isotopic variations introduced by these reactions are often passed on with little change so that isotopic measurements can indicate natural pathways and flows “downstream” from these key reactions. When chemical and metabolic processes scramble the information content of molecules, isotopic compositions are often preserved. This realization has prompted increasing use of stable isotope analyses as a tool for understanding complex ecological processes.

δ13C Measurements as Indicators of Carbon Flow in Marine and Freshwater Ecosystems

The results of analyses of the distribution, structure and function of ericoid, ecto and vesicular-arbuscular mycorrhizas are used to challenge the conventional view, which was based largely upon studies of isolated plants and excised plant roots under controlled conditions, that the symbiosis is primarily involved in the capture of phosphate ions. In nature, each mycorrhizal type is associated with an ecosystem and soil environment with distinctive characteristics in which selection has favoured the development of a particular range of attributes. These attributes are evaluated and their importance for the individual plant and for the ecosystems in which they occur is assessed. It is concluded that knowledge of the full range of functions of each mycorrhizal type is essential for an understanding of the distribution and dynamics of the ecosystem in which it predominates.

Mycorrhizas in ecosystems

Summary Here, the coevolution of mycorrhizal fungi and roots is assessed in the light of evidence now available, from palaeobotanical and morphological studies and the analysis of DNA-based phylogenies. The first bryophyte-like land plants, in the early Devonian (400 million years ago), had endophytic associations resembling vesicular‐ arbuscular mycorrhizas (VAM) even before roots evolved. Mycorrhizal evolution would have progressed from endophytic hyphae towards balanced associations where partners were interdependent due to the exchange of limiting energy and nutrient resources. Most mycorrhizas are mutualistic, but in some cases the trend for increasing plant control of fungi culminates in the exploitative mycorrhizas of achlorophyllous, mycoheterotrophic plants. Ectomycorrhizal, ericoid and orchid mycorrhizas, as well as nonmycorrhizal roots, evolved during the period of rapid angiosperm radiation in the Cretaceous. It is hypothesised that roots gradually evolved from rhizomes to provide more suitable habitats for mycorrhizal fungi and provide plants with complex branching and leaves with water and nutrients. Selection pressures have caused the morphological divergence of roots with different types of mycorrizas. Root cortex thickness and exodermis suberization are greatest in obligately mycorrhizal plants, while nonmycorrhizal plants tend to have fine roots, with more roots hairs and relatively advanced chemical defences. Major coevolutionary trends and the relative success of plants with different root types are discussed.

Coevolution of roots and mycorrhizas of land plants

Summary
We review and reassess experimental evidence which indicates that the unstirred layers of solution bathing aquatic plant cells or organs represent a major factor limiting their rate of photosynthesis under natural conditions. Unstirred layers may also limit membrane transport of HCO3− where this occurs. Some authors have suggested that aquatic plants carry out C4photosynthesis and cite in support measurements of discrimination between 13C and 12C. We propose that this discrimination can also be a manifestation of the presence of unstirred layers or membrane transport or both.

https://www.jstor.org/stable/pdfplus/2432040.pdf

Photosynthesis by aquatic plants: effects of unstirred layers in relation to assimilation of co2 and hco3−and to carbon isotopic discrimination

Summary
We present a simple mathematical model of the infection of roots of Trifolium subterraneum by vesicular-arbuscular mycorrhizal fungi. The model permits separate calculation of the following.

(1) The frequency of infection in unit length of root in unit time from mycorrhizal propagules distributed uniformly and randomly in soil. A value of 40·8 m−1 day−1 was found for the soil used in the experiments.

(2) The rate of fungal growth along the root cortex from a single entry point, which was found to be 12·2 × 10−4 m day−1 for infections derived from propagules in the same soil.

An alternative method of estimating the frequency of infection close to the root tip (from measurements of the rate of root growth and distance between the apex and the most apical entry-point) is presented. Comparison of values obtained suggests that the root tip is nearly 10 times more infectible than the average for the whole root system.

Some of the assumptions implicit in the model have been investigated experimentally and the results are reported here.

The model should be useful in distinguishing between environmental effects on the phases of colonization occurring in the soil, and at the root surface and within the root.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1981.tb07485.x

A quantitative study of mycorrhizal infection in trifolium: separate determination of the rates of infection and of mycelial growth

SUMMARY
The effects of photon irradiance on the growth of young plants (up to three weeks old) of Trifolium subterraneum L. and on the development of mycorrhizal root systems were studied with plants grown in a soil/sand mixture inoculated with Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe. Total plant growth was lower when photon irradiance was 100 μmol m−2 s−1, compared with 450 μmol m−2 s−1. Fresh weight of shoots was unaffected, but the fresh weight/dry weight ratio was increased at the lower irradiance in shoots, but not in roots. The main effect of decreased irradiance on the growth of the root system was a reduction in the numbers of first- and second-order lateral roots initiated. The average rate of extension of axial and lateral roots was only slightly reduced. The fraction of the root length infected was lower at the lower irradiance, particularly in the first two weeks. Development of infection was analyzed in terms of the role of formation of mycorrhizal entry-points (A) and the average rate of growth of infection units (B). Reduction in the fraction of the root length infected was found to be due to a reduction in A. B was remarkably constant at the two irradiances and in the different root subsystems. Thus both roots and infection units respond by making fewer units of unaltered rate of growth.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1986.tb00623.x

Effects of photon irradiance on the growth of shoots and roots, on the rate of initiation of mycorrhizal infection and on the growth of infection units in Trifolium subterraneum L.

Summary
The processes of infection of roots of Trifolium subterr aneum by vesicular–arbuscular mycorrhizas are analysed using a modification of a model presented previously. The modified model uses as dependent variables the fraction of the root length infected and the linear density of infection units. It allows the evaluation of the rate of initiation of new infection units and of the rate of growth of hyphae within the root as functions of time.

The model is applied to data for T. subterraneum grown in a 20-foid range of densities of randomly-mixed propagules in soil; we show the rate of initiation of new infection units in both main and lateral roots was proportional to the propagule density over short periods (5 to 11 day). However, the rate of initiation of infection units in main roots during this period was approximately half that in lateral roots. The rate of growth of hyphae in the root did not depend on the propagule density.

As the main roots ceased to grow, both rates of infection and of mycelial growth fell: these rates did not fall for lateral roots during the 26 days of the experiment. The results raise the question of the dependence of mycelial growth-rate on the age of the infection unit, which is discussed.

The model predicts that the fraction of the root length infected approaches asymptotically a value less than one, as has been repeatedly observed. Other models require arbitrary modification to accommodate such findings.

The quantitative study of mycorrhizal infection

summary
This paper re-examines the question of the susceptibility of plant roots to infection by vesicular-arbuscular mycorrhizal fungi, particularly near the root apex. The cumulative distributions of the observed distance between the root apex and the most apical mycorrhizal entry points are compared with expected distributions based on (i) a random distribution of successful encounters between hypha and root and (ii) an uninfectible apical region (of length Zo). Data for both Allium porrum L. (leek) and Trifolium subterraneum L. (clover) are well fitted by the expected distributions and in the majority of cases (17/31) the best fit is obtained with zero Zo, with 8 additional cases being best fitted by values of Zo of 1 mm or less. Even where the best fits are obtained with values of Zo greater than zero, the distribution using Zo= 0 is still within the confidence limits for the data. These findings indicate that in neither species is there clear evidence for a subapical region that is immune to infection. The fact that there are short uninfected lengths immediately behind the apex is interpreted as representing a delay between an encounter and a detectable infection. Other consistent deviations of the data from the theoretical distributions are discussed.

Fitting the distributions yields values for the frequency of infection (A). These are compared for the two host plants at different propagule densities. The values for leek are much lower than for clover. In both species there are increases in A with increases in propagule density which are consistent with earlier findings.

https://www.jstor.org/stable/info/2557638

N. A. Walker

Papers

Photosynthesis by aquatic plants: effects of unstirred layers in relation to assimilation of co2 and hco3−and to carbon isotopic discrimination

A quantitative study of mycorrhizal infection in trifolium: separate determination of the rates of infection and of mycelial growth

Effects of photon irradiance on the growth of shoots and roots, on the rate of initiation of mycorrhizal infection and on the growth of infection units in Trifolium subterraneum L.

The quantitative study of mycorrhizal infection

Distribution of VA mycorrhizal entry points near the root apex: Is there an uninfectible zone at the root tip of leek or clover?