Stream Macroinvertebrates and Carbon Cycling in Tangled Food Webs

Benoît O. L. Demars,Barry Thornton,Baptiste Marteau,Nikolai Friberg,Joanna L. Kemp

doi:10.1007/s10021-021-00626-8

Abstract

The annual global loss of organic carbon from terrestrial ecosystems into rivers is similar to the organic carbon stored in soils each year. Dissolved organic matter (DOM) flows through the food web to macroinvertebrates, but little is known about the effect of DOM increase on stream food webs and how much macroinvertebrates may contribute to the regulation of carbon fluxes in rivers. Using a before and after control impact (BACI) experimental design, we increased by 12% (+ 0.52 mg C L−1) the concentration of DOM in a stream for three weeks by adding sucrose, with a distinctive δ13C signature, to simulate a pulse of natural DOM supply from soils. We partitioned the diet of macroinvertebrates from carbon sources according to the green pathway (autotrophs) and detrital pathways (bacteria and terrestrial organic matter). Our flow food web approach based on C fluxes, with bacteria as a key node, showed the dominant contribution of the detrital pathways for macroinvertebrates in the reference stream. DOM addition induced changes in the diets of individual taxa, but did not have any strong effects on the relative overall contribution of the detrital pathways versus the green pathway. Autotrophic uptake of CO2 respired by bacteria was much larger than bacterial C flux to invertebrates (that is, the classic microbial loop) and allowed a significant fraction of natural allochthonous organic carbon to make its way to macroinvertebrates via autotrophs fixing CO2 respired by bacteria. Overall macroinvertebrates did not regulate directly to any great extent the flux of stream DOM towards downstream ecosystems.

Highlights

The annual global loss of organic carbon from terrestrial ecosystems into rivers is similar to the organic carbon stored in soils each year (Dawson 2013)
Autotrophic use of CO2 respired by bacteria was 13 ± 11 times larger in the reference and 24 ± 20 times larger in the treatment than bacterial C uptake by invertebrates. This autotrophic use of CO2 respired by bacteria allowed a significant fraction of natural organic carbon to make its way to macroinvertebrates via autotrophs fixing CO2 respired by bacteria: 6.6 ± 4.2% in the reference and 2.2 ± 1.8% in the treatment (10 ± 6% including sucrose)
Autotrophic use of CO2 respired by bacteria allowed a significant fraction of natural organic carbon (7% in the reference) to make its way to macroinvertebrates via autotrophs fixing CO2 respired by bacteria

Summary

Introduction

The annual global loss of organic carbon from terrestrial ecosystems into rivers is similar to the organic carbon stored in soils each year (Dawson 2013) The delivery of this organic matter to rivers is currently increasing in many parts of the world due to changes in climate, atmospheric pollution, land use and urbanisation (Freeman and others 2004; Monteith and others 2007; Noacco and others 2017). The fate of this organic matter in rivers remains difficult to characterise, because of the different forms (for example, leaf litter and dissolved organic matter) and varying quality of organic matter, as well as spatial heterogeneity and temporal dynamics (Raymond and others 2016; Bernhardt and others 2018; Drake and others 2018). The productivity of closed canopy temperate streams can increase the productivity of the connected riparian food webs (Nakano and Murakami 2001; Baxter and others 2005)

Methods

Results

Discussion

Conclusion