Stream Macroinvertebrates and Carbon Cycling in Tangled Food Webs

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.

Intercontinental analysis of temperate steppe stream food webs reveals consistent autochthonous support of fishes

“…Therefore, CSIA‐AA allows for partitioning of specific energetic pathways (i.e. autochthonous aquatic carbon, allochthonous terrestrial carbon and associated heterotrophic microbial carbon) that are usually entangled in freshwaters (Demars et al, 2021; Harada et al, 2022).…”

The application of the River Continuum Concept (RCC) on the headwaters of the Sampolawa River in Southeast Sulawesi, Indonesia

“…Macroinvertebrates also occupy the second and third trophic levels in the water chain. Demars et al (2021) stated that the highest C-Organic value was obtained in the rainy season, and soil organic matter levels usually increase with the mean annual rainfall; this is also supported by Muna et al (2020), who state that rainfall and temperature are climatic factors that can affect the amount of organic matter content. For example, an area that has low rainfall will have warm temperatures.…”

Water availability and seasonality shape elemental stoichiometry across space and time

“…The Coastal Plain region has experienced extensive human-induced alterations in water availability during the growing season due to climate warming, greater ET (Golladay et al, 2016), and groundwater withdrawals (Rugel et al, 2012) with summertime baseflows being approximately half of the historical flows (Site 5 flow record 1939 to present) based on long-term records (Appendix S1: Table S6; Golladay et al, 2007). Based on our results, reductions in flow due to drought (PDSI <0) and further compounded by groundwater withdrawals have reduced the C:N and C:P of dissolved solutes, which may result in changing productivity dynamics via the microbial loop (Demars et al, 2021;Meyer, 1994). It is exceedingly challenging to disentangle the relative influence of land use and land cover change versus water availability.…”