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

Abstract

Lakes in human‐dominated landscapes often face high loads of nutrients that may alter ecosystem function. High N loads relative to P are especially common in agricultural watersheds, where nitrate (NO3) in particular is elevated due to fertilizer application and runoff. While past research has focused extensively on the impact of nutrient loads on reductions in water quality, we still lack assessment of the impact of high N loading and extreme stoichiometric imbalance on ecosystem process rates, specifically measurements of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem productivity (NEP). We used open‐bottomed experimental mesocosms (limnocorrals) to establish four treatments of increasing N : P by increasing N loads while leaving P loads consistent, and measured chlorophyll concentration and metabolic rates. We observed significant differences in productivity among treatments, with higher biomass and GPP in the highest two N : P treatments and a unimodal pattern between GPP and N concentrations. Declines in GPP at high N may have been due to limitation by P or light. In contrast to other studies, we did not observe any significant differences in ER with fertilization, potentially from a lack of dissolved organic carbon loading often associated with higher nutrient concentrations in those lakes. Maximum GPP and NEP observed here was far above commonly cited thresholds for likely P limitation at a molar N : P of approximately 330. Our results highlight the potential for high productivity despite stoichiometric imbalance that is common in agricultural systems, suggesting N management may be important in reducing primary production in hypereutrophic lakes.

Response of lake metabolism to catchment inputs inferred using high‐frequency lake and stream data from across the northern hemisphere

“…This result coupled with the strong relationships between N and P suggest that both nutrients are important for regulating lake metabolism. Indeed, other lakes with high N : P have shown increases in GPP with high N loading (e.g., Bogard et al 2017; Kelly et al 2021).…”

A Broad-Scale Look at Nutrient Limitation and a Shift toward Co-limitation in United States Lakes

“…Enrichments of nitrogen (N) and phosphorus (P) are often responsible for accelerating eutrophication, a major threat to freshwater ecosystem health, biodiversity, and ecosystem services across the globe. − Eutrophication often occurs due to an increase in the limiting nutrient or the nutrient in the lowest supply relative to the nutrient demands by organisms . There is an ongoing debate about whether nutrient reductions for eutrophication management should solely focus on P or whether an approach reducing and balancing both N and P is more useful. − The paradigm of P being the dominant limiting nutrient in lakes has been based on experimental evidence, lower cost of reducing P, and the assumption that biological N-fixation prolongs P-limitation. − Furthermore, the use of the Redfield Ratio (16N:1P molar) may lead to the assumption of primary P-limitation as the ratio is typically too low for freshwaters, which are often closer to 20N:1P . However, management-driven P reductions have resulted in accumulation of N in large lakes worldwide, leading to an additional nutrient problem …”