Calcium carbonate precipitation influences processes across multiple trophic levels in the Little Colorado River.

Authors: Eric Moody, Jeffrey Muehlbauer, Phoebe Brown, Jessica Corman, Bridget Deemer, Theodore Kennedy, Charles Yackulic

Year: 2025

Abstract

Carbonate geology and karst aquifers are widespread, representing 15% of the global, ice-free, continental surface. Calcium carbonate precipitation in karst ecosystems reduces water clarity and light availability, streambed stability, and phosphorus (P) availability, all of which have the potential to limit primary and secondary production. We investigated these potential abiotic effects on ecosystem production and food web dynamics using a spatial gradient of calcium carbonate precipitation in the Little Colorado River, AZ, USA, which is one of the population strongholds of the threatened Humpback Chub (Gila cypha). We found that both light and phosphorus availability were higher in the upstream, low-precipitation reach of the river during karst spring-fed baseflow conditions, but this spatial gradient weakened when overland flows produced by rainfall events added to base flows. Nutrient assays demonstrated that primary production was limited by phosphorus availability throughout the river suggesting that the increased P availability we documented in upstream low-precipitation reaches is likely supporting higher rates of autochthonous production in these locations. Emergent aquatic insects, an important prey resource for Humpback Chub and other fishes, were also more abundant at upstream, low-precipitation sites. Previous work has demonstrated higher juvenile Humpback Chub growth rates in the upstream, low-precipitation sites, suggesting that calcium carbonate precipitation is regulating food web production and fish growth. As calcium carbonate precipitation is a common feature of karst-fed aquatic ecosystems worldwide, our work suggests that processes that regulate calcium carbonate precipitation rates are an underappreciated control on light and phosphorus availability with implications for freshwater food web and ecosystem processes.