Highest primary production achieved at high nitrogen levels despite strong stoichiometric imbalances with phosphorus in hypereutrophic experimental systems

Authors: KELLY, PATRICK - Rhodes College; Taylor, Jason; ANDERSON, ISABELLE - Baylor University; SCOTT, JEFFERSON - Baylor University

Submitted to: Limnology and Oceanography Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2021
Publication Date: 11/18/2021
Citation: Kelly, P., Taylor, J.M., Anderson, I.M., Scott, J.T. 2021. Highest primary production achieved at high nitrogen levels despite strong stoichiometric imbalances with phosphorus in hypereutrophic experimental systems. Limnology and Oceanography Journal. Pages 4375-4390. https://doi.org/10.1002/lno.11968.
DOI: https://doi.org/10.1002/lno.11968

Interpretive Summary: Health of freshwater lakes is often impacted by excess nutrients (nitrogen and phosphorus) from agricultural and urban runoff, as well as human wastewater. Since the 1970s, the dominant scientific thinking regarding excess nutrients and water quality has been that phosphorus impacts water quality in freshwater and nitrogen impacts marine systems. However, debate is increasing on the utility of single nutrient reduction strategies to improve water quality in aquatic ecosystems, particularly in more southern lakes with high nutrient enrichment. We used experimental ponds to test if increasing nitrogen in ponds with high phosphorus impacted algal biomass and dissolved oxygen patterns in freshwater ecosystems. We found that the highest concentrations of algae and the highest levels of ecosystem production were achieved at the highest levels of nitrogen enrichment. These results challenge current views that water quality problems in enriched lakes can be solved by just managing phosphorus. Instead, our results support the need for reduction of nitrogen and phosphorus to improve ecosystem health in nutrient impacted freshwater lakes.

Technical Abstract: Lakes in human-dominated landscapes often face high loads of nutrients that may reduce ecosystem services and alter ecosystem function. In addition to high loading rates, nutrient loads may be extremely stoichiometrically imbalanced in nitrogen (N) relative to phosphorus (P). High N loads relative to P loads are especially common in agricultural watersheds, where nitrate (NO3) loads in particular are elevated due to fertilizer application and runoff. While past research has focused extensively on the impact of nutrient loads on phytoplankton biomass or reductions in water quality, we still lack assessment of the impact of high N loading and extreme stoichiometric imbalance on ecosystem process rates, specifically lake metabolism (i.e. measurements of gross primary production and ecosystem respiration). We used experimental mesocosms (limnocorrals) to determine the impact of increasing N loads relative to P on lake productivity, as measured by chlorophyll concentration and lake metabolism. We established four treatments of increasing N:P by increasing N loads only while leaving P loads consistent. We observed significant differences in chlorophyll and gross primary production (GPP), with higher biomass and rates in the highest N:P treatments. We did not observe any significant differences in ecosystem respiration (ER) across treatments, leading to high autotrophy in the high and medium-high N:P treatments. The peak in productivity observed here was far above commonly cited thresholds for likely P-limitation, as maximum productivity was achieved at a molar N:P of approximately 50. This suggests the potential for N deposition observed globally to contribute to increased primary production in lakes, even when the lakes are already stoichiometrically imbalanced. These results also indicate a dual nutrient management strategy may be more useful than focusing exclusively on P.