Longitudinal trends in a community trophic niche in temperate tributaries across forested and agricultural watersheds pre- and post-monsoon

Monsoon rains trigger pulsed flows from tributaries, which in turn impact the structure of riverine food webs. However, the mechanisms driving food web dynamics in tributaries in response to these pulsed flows are not yet fully understood. We employed carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes of macroinvertebrates and fish to quantify changes in the trophic base and diversity, food chain length, and food web trophic niches before and after the monsoon in two tributaries of the northeast Asian monsoon region. The δ¹³C and δ¹⁵N values of primary basal resources (leaf litter and biofilms) were consistent before and after the monsoon, with a notable increase in δ¹⁵N values from forest streams to agricultural channels. Consumer δ¹³C and δ¹⁵N values remained stable over time but exhibited a longitudinal increase due to greater nutritional contributions from local resources. Community isotopic niche metrics were consistent across locations and seasons, while trophic niches diverged between watersheds and closely overlapped seasonally in isotopic space. These results highlight the significant impact of agricultural inputs on downstream channel food webs and demonstrate the limited effect of monsoonal rains on altering the longitudinal trajectory of trophic niches across tributaries.