Rethinking Food Web Tracing: A Conceptual Framework Utilizing a Multi Biomarker Approach to Better Understand Trophic Relationships

Abstract

In this dissertation, a conceptual framework for tracing trophic interactions and providing more clearly defined food web reconstructions is introduced, the Tracer Trichotomy. This framework involves the use of three separate trophic tracing methods to inform trophic interactions: stable isotopes (carbon and nitrogen), polyunsaturated fatty acid biomarkers, and bioaccumulative contaminants (replaced with stress/ health biomarker in conservation scenarios). The use of the Tracer Trichotomy provides a more complete and accurate understanding of trophic pathways and contaminant transport than the use of any individual method. In this dissertation, four separate projects are described, each assessing environmental or individual life cycle variables that could impact the values of these food web tracers. The environmental effects associated with time of year and annual time changes are assessed in a sentinel spider species in chapters I and II. In chapter I, it was found that spider mass and length steadily increased from April to September to a maximum average value of 0.078  0.03g, then decreased in October. Seasonal trends were observed for carbon and nitrogen stable isotopes, with significantly decreased signatures occurring late in the active season. Overall, methyl mercury concentrations (range: 12.1-134.4 ng/g) and the methyl:total mercury ratio (range: 49-98% methyl mercury) increased throughout the active season, with higher variability observed at the end of the active season. These results indicated that seasonality impacted several important endpoints and that spiders collected during the end of the active season may not be representative of spiders during the entire active season. In chapter II, it was found that carbon and nitrogen stable isotopes, ⍵3:⍵6 ratios, and mercury concentrations differed significantly in tetragnathid spiders between consecutive years at the same site location. These results indicated that enough variation occurs annually that the data of one year cannot be applied to following years at the same sampling site using tetragnathid spiders. In chapters III and IV, the individual life cycle variable of metamorphosis and altered metamorphosis processes are assessed in laboratory-reared and field-caught mayflies and an endangered species of freshwater mussel. In chapter III, the results showed that the δ15N and %N increased significantly with metamorphosis in both laboratory-reared N. triangulifer and field-collected Heptageniidae mayflies. There were no significant differences in polyunsaturated fatty acid profiles between larval and adult stages of field-collected mayflies; however, there was a significant increase in % arachidonic acid in laboratory-reared individuals. The results of this study indicate that the metamorphosis has a significant impact on food web tracers in laboratory and field mayflies, which should be considered when using mayflies or potentially other emergent aquatic insects in calculations connected to ecological risk assessments. In Chapter IV, for the endangered mussel Toxolasma cylindrellus, in vitro propagated individuals were significantly larger and had lower δ13C values than fish propagated mussels. Otherwise, there were no differences found between in vitro propagated, and fish propagated mussels for nitrogen stable isotopes, total carbon, and nitrogen, lipids, polyunsaturated fatty acids, or glycogen content. The results of this study indicate that in vitro propagation is a viable method for T. cylindrellus conservation and displays minimal differences in juvenile health and nutrient uptake between propagation methods.