Food web structure in the Alabama River: Stable isotope-based characterization of a large, dam-regulated southeastern U.S. coastal plain river system

Abstract

Riverine food webs are influenced by dams, which modify flow regimes, sediment transport, and nutrient cycling, reducing longitudinal connectivity and potentially altering trophic interactions. This study used carbon (δ13C) and nitrogen (δ15N) stable isotopes to characterize trophic relationships within the hydrologically altered Alabama River and evaluate how three lock-and-dam structures influence food web dynamics. Specific objectives were to (1) characterize isotopic patterns in pelagic and benthic resources across sites, (2) compare inter- and intra-specific variation in trophic position among five fish species, and (3) compare inter- and intra-specific variation in trophic (isotopic) niche among the same five species. A total of 2,795 samples, including five fish species, sediment, seston, and macroinvertebrates, were collected from six sites immediately upstream and downstream of the three lock-and-dam structures. Consumer δ13C values showed extensive overlap among taxa, suggesting shared reliance on similar basal carbon resources. In contrast, isotopic niche size was larger downstream than upstream for nearly every species-dam comparison, suggesting that intraspecific niche dynamics captured ecological responses to river regulation not apparent in overall trophic structure. Baseline δ15N values varied longitudinally, exhibiting an overall downstream decline across the river continuum. Despite this longitudinal shift in baseline δ15N, trophic position estimates showed limited variation among adjacent dam-paired sites. These results suggest that the Alabama River supports a broadly integrated food web despite hydrologic alteration and demonstrate the value of intraspecific isotopic niche variation for understanding ecological responses to river regulation.