Estimating terrestrial contribution to stream invertebrates and periphyton using a gradient-based mixing model for...

Area of Science:

• Ecology
• Biogeochemistry
• Stable Isotope Ecology

Background:

• Stream food webs often rely on both internal (autochthonous) and external (allochthonous) carbon sources.
• Distinguishing these sources using stable isotopes like carbon-13 (δ¹³C) is challenging due to overlapping signatures.
• Spatial gradients in δ¹³C signatures can provide a novel approach for source partitioning.

Purpose of the Study:

• To develop and apply a gradient-based mixing model for partitioning autochthonous and allochthonous carbon contributions to stream invertebrates.
• To assess the proportion of terrestrial carbon consumed by different invertebrate functional groups.
• To quantify the allochthonous content of periphyton and understand selective feeding by herbivores.

Main Methods:

• Developed a gradient-based mixing model utilizing spatial variations in δ¹³C signatures.
• Applied the model to stream invertebrate δ¹³C data to estimate terrestrial consumption (p(T)).
• Analyzed δ¹³C gradients in periphyton and compared consumer assimilation relative to periphyton and algal signatures.

Main Results:

• Terrestrial detritus lacks a significant downstream δ¹³C gradient, unlike autochthonous production.
• Estimated terrestrial consumption (p(T)) varied significantly among invertebrate groups: herbivores/grazers (0.15) < filterers (0.38) < collector/gatherers (0.43) < shredders (0.85).
• Periphyton showed a weaker downstream δ¹³C slope than pure algae, indicating ~30% allochthonous content; herbivores selectively consumed the autochthonous portion of biofilms.

Conclusions:

• Gradient-based modeling effectively partitions carbon sources even with overlapping signatures.
• Terrestrial carbon input varies substantially across stream invertebrate functional groups.
• Herbivores demonstrate selective feeding, preferentially assimilating autochthonous resources from mixed biofilms.