Carbon storage reservoirs in watersheds support stream food webs via periphyton production

We measured the natural abundances of radiocarbon (C-14) in macroinvertebrates, fishes, and their potential food sources, collected from the upper and lower reaches of six temperate streams in Lake Biwa basin (central Japan), three of which flow on limestone bedrock. Several carbon storage reservoirs in the watersheds show distinctive C-14 signatures (e.g., ancient carbonate rocks, -1000 parts per thousand; modern atmospheric CO2, +50 parts per thousand). Our analyses showed that the C-14 values for periphytic algae range from -361 parts per thousand to +21 parts per thousand, reflecting C-14-depleted signals from watershed storage reservoirs (carbonate rocks and/or soils). In contrast, the C-14 values for coarse particulate organic matter (CPOM) range from -6 parts per thousand to +62 parts per thousand, reflecting C-14-enriched signals from modern atmospheric CO2. The periphyton from streams on limestone bedrock was more C-14-depleted than that from streams in non-limestone areas, although the C-14 values for periphyton from the latter were less than modern atmospheric (CO2)-C-14 concentration. The C-14 values for most of the consumers were between those for periphyton and CPOM. Based on a C-14 two-source mixing models, the results suggested that the grazers rely on periphyton, while the carbon source for collectors and predators shifts from CPOM in the upper reaches of streams to periphyton in the lower reaches. The C-14 signature can trace carbon from watershed storage reservoirs to benthic production, which suggests that stream food webs are composed of mixtures of carbon originating from various sources of different ages.