Joseph B. Rasmussen

PRIMARY CONSUMER δ13C AND δ15N AND THE TROPHIC POSITION OF AQUATIC CONSUMERS

Stable nitrogen isotope signatures (δ15N) are increasingly used to infer the trophic position of consumers in food web studies. Interpreting the δ15N of consumers relative to the δ15N characterizing the base of the food web provides a time-integrated measure of trophic position. We use primary consumers (trophic level 2) as baseline indicator organisms and investigate the variation in baseline δ15N values in 14 lakes in Ontario and Quebec. Values of δ15N ranged from −2 to +9‰ and varied significantly as a function of lake habitat (mean littoral = 1.6‰, pelagic = 3.1‰, profundal = 5.2 ‰). Stable carbon isotopic signatures (δ13C) of primary consumers decreased along this same habitat gradient (mean littoral = −23.8‰, pelagic = −28.4‰, profundal = −30.5‰). Primary consumer δ13C and a categorical lake variable explained 72% of the variability in primary consumer δ15N. This relationship was corroborated by primary consumer δ15N and δ13C data from the literature, indicating that habitat-specific variation in ba...

Stable isotope evidence for the food web consequences of species invasions in lakes

Species invasions pose a serious threat to biodiversity and native ecosystems; however, predicting and quantifying the impacts of invasive species has proven problematic. Here we use stable isotope ratios to document the food-web consequences of the invasion of two non-native predators, smallmouth bass and rock bass, into Canadian lakes. Invaded lakes had lower littoral prey-fish diversity and abundance than uninvaded reference lakes. Consistent with this difference, lake trout from invaded lakes had more negative δ13C values (-29.2‰ versus -27.4‰) and reduced trophic positions (3.3 versus 3.9) than those from reference lakes, indicating differences in food-web structure. Furthermore, a comparison of the pre- and post-invasion food webs of two recently invaded lakes showed that invasion was followed by substantial declines in littoral prey-fish abundance and the trophic position of lake trout, reflecting a shift in the diet of lake trout towards zooplankton and reduced dependence on littoral fish. This study demonstrates the use of stable isotope techniques to detect changes in food-web structure following perturbations; in this instance, bass-induced food-web shifts may have severe consequences for native species and ecosystems.

The effect of temperature and algal biomass on bacterial production and specific growth rate in freshwater and marine habitats.

We analyzed heterotrophic, pelagic bacterial production and specific growth rate data from 57 studies conducted in fresh, marine and estuarine/coastal waters. Strong positive relationships were identified between 1) bacterial production and bacterial abundance and 2) bacterial production and algal biomass. The relationship between bacterial production and bacterial abundance was improved by also considering water temperature. The analysis of covariance model revealed consistent differences between fresh, marine and estuarine/coastal waters, with production consistently high in estuarine/coastal environments. The log-linear regression coefficient of abundance was not significantly different from 1.00, and this linear relationship permitted the use of specific growth rate (SGR in day−1) as a dependent variable. A strong relationship was identified between specific growth rate and temperature. This relationship differed slightly across the three habitats. A substantial portion of the residual variation from this relationship was accounted for by algal biomass, including the difference between marine and estuarine/coastal habitats. A small but significant difference between the fresh- and saltwater habitats remained. No significant difference between the chlorophyll effect in different habitats was identified. The model of SGR against temperature and chlorophyll was much weaker for freshwater than for marine environments. For a small subset of the data set, mean cell volume accounted for some of the residual variation in SGR. Pronounced seasonality, fluctuations in nutrient quality, and variation of the grazing environment may contribute to the unexplained variation in specific growth.

The dynamics of spatially coupled food webs

The dynamics of ecological systems include a bewildering number of biotic interactions that unfold over a vast range of spatial scales. Here, employing simple and general empirical arguments concerning the nature of movement, trophic position and behaviour we outline a general theory concerning the role of space and food web structure on food web stability. We argue that consumers link food webs in space and that this spatial structure combined with relatively rapid behavioural responses by consumers can strongly influence the dynamics of food webs. Employing simple spatially implicit food web models, we show that large mobile consumers are inordinately important in determining the stability, or lack of it, in ecosystems. More specifically, this theory suggests that mobile higher order organisms are potent stabilizers when embedded in a variable, and expansive spatial structure. However, when space is compressed and higher order consumers strongly couple local habitats then mobile consumers can have an inordinate destabilizing effect. Preliminary empirical arguments show consistency with this general theory.

A Trophic Position Model of Pelagic Food Webs: Impact on Contaminant Bioaccumulation in Lake Trout

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Patterns of Food Chain Length in Lakes: A Stable Isotope Study

Food web structure is paramount in regulating a variety of ecologic patterns and processes, although food web studies are limited by poor empirical descriptions of inherently complex systems. In this study, stable isotope ratios (δ15N and δ13C) were used to quantify trophic relationships and food chain length (measured as a continuous variable) in 14 Ontario and Quebec lakes. All lakes contained lake trout as the top predator, although lakes differed in the presumed number of trophic levels leading to this species. The presumed number of trophic levels was correlated with food chain length and explained 40% of the among‐lake variation. Food chain length was most closely related to fish species richness ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Application of Stable Isotope Techniques to Trophic Studies of Age-0 Smallmouth Bass

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QUANTIFYING ASSIMILATION OF SEWAGE‐DERIVED ORGANIC MATTER BY RIVERINE BENTHOS

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