Zooplankton densities reduced by increases in resource N:P and hypereutrophic mesocosms

Authors: KELLY, PATRICK - Wisconsin Department Of Natural Resources; Taylor, Jason; ANDERSEN, ISABELLE - Baylor University; SCOTT, JEFFERSON - Baylor University

Submitted to: Hydrobiologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2023
Publication Date: 5/1/2024
Citation: Kelly, P.T., Taylor, J.M., Andersen, I.M., Scott, J.T. 2024. Zooplankton densities reduced by increases in resource N:P and hypereutrophic mesocosms. Hydrobiologia. https://doi.org/10.1007/s10750-024-05557-8.
DOI: https://doi.org/10.1007/s10750-024-05557-8

Interpretive Summary: Nutrient enrichment can impact lake organisms. Not only can excess Nitrogen and Phosphorus from agricultural runoff impact system by increasing algal blooms and hypoxia, but the relative ratios may also alter growth rates of aquatic organisms. ARS and University scientists conducted an experiment to see how different ratios of Nitrogen and Phosphorus influence zooplankton densities (important food resources for fish). Results indicate that increasing temperature and decreasing Phosphorus in relation to Nitrogen decrease zooplankton numbers during the summer growing season. This suggests that higher Nitrogen compared to Phosphorus inputs to lakes may decrease zooplankton populations. These animals are not only important as food resources to fish but also play a primary role in grazing of algae in lakes.

Technical Abstract: Crustacean zooplankton are often the primary grazers of pelagic phytoplankton in lakes, and as such are impacted by the availability and quality of algal resources. Nutrient loading of lake ecosystems may influence food resource quantity and quality for zooplankton by stimulating primary production and altering stoichiometric ratios of nutrients such as nitrogen (N) and phosphorus (P) in phytoplankton biomass. Ecological stoichiometry has been well-studied as a driver of zooplankton dynamics in lakes; yet, we still lack a thorough investigation of the impact of varying N loads specifically, and the resulting variation in resource N:P stoichiometry. Increased N loading relative to P is common especially in areas with high agricultural land use, high N deposition from industrial activity, and/or due to responses from P regulations. We used a mesocosm experiment in which we altered inputs of N and P to observe the impact of stoichiometric variation in nutrient concentrations on zooplankton density and community composition for a period of 10 weeks. Our experiment achieved a wide gradient of environmental N:P availability primarily by altering N loading, while keeping P inputs consistent across treatments. Despite highly variable N:P loading across treatments, particulate N:P was more constrained. We did not observe a treatment effect on zooplankton density. However, across all treatments, we observed declines in zooplankton density through time, and a reduction in cladocerans relative to copepods. These temporal patterns correlated to a loss of P in the water column and higher food resource N:P, as well as warming temperatures throughout the experiment. Zooplankton density did not relate to resource availability. Our experiment highlights the potential importance of seston quality over quantity for supporting higher zooplankton densities, as well as the impacts of high temperatures. These results suggest disproportionately high N loads relative to P as is common in agricultural landscapes could reduce zooplankton populations, despite stimulating greater primary production.