Nitrogen and Phosphorus Controls on Phytoplankton Growth and Bacterioplankton Communities in Wetland Impoundments

Nutrient enrichment in shallow lakes and ponds can shift systems from clear water, macrophyte dominance to turbid, phytoplankton dominance. Because the identity and threshold of the limiting nutrients vary among sites, system‐specific evidence is needed to guide water quality management.

We conducted 3‐day in situ nutrient‐enrichment bioassays in two wetland impoundments adjacent to Great Salt Lake (Utah, USA) to identify the limiting nutrient for phytoplankton growth, quantify growth responses to nitrogen (N) and phosphorus (P), estimate an operational nutrient threshold using Monod kinetics and evaluate bacterioplankton community and functional responses across the same nutrient gradient. Phytoplankton biomass (chlorophyll a ) was used to calculate growth rates, and bacterioplankton communities were profiled by 16S rRNA gene sequencing with inferred functional annotation.

Across both wetlands, phytoplankton growth increased with N additions but not with P additions, indicating N limitation under high ambient P. Monod models yielded a half‐saturation constant of approximately 0.7 mg N L ⁻¹ (as NO ₃ –N), which we interpret as an N threshold for phytoplankton growth in these impoundments. Bacterioplankton composition differed by site, with Cyanobiaceae dominating at both locations, and community evenness declining above the N threshold. Inferred metagenomes indicated a higher relative abundance of taxa carrying the microcystin synthetase gene mcyE above the N threshold, suggesting increased cyanotoxin potential.

Together, these results support management actions that control N in addition to P and demonstrate the value of bioassay‐derived thresholds for informing numeric criteria in managed wetlands.