Phytoplankton biomass responses are amplified under nitrogen-limiting conditions

Research Team

Abstract

Research Question

Methods

Results

Discussion

Acknowledgements & Funding

Literature Cited

NACE Career Readiness Competencies

Research Team

Eden Waters-Carpenter

Biology and Environmental Science

Isabelle Andersen

Biology

Lesley Knoll

Biology

Abstract

The availability and relative abundance of nitrogen (N) and phosphorus (P) in lake ecosystems can have a high degree of seasonal variability, which can influence the nutrient limitation dynamics of phytoplankton communities. Along with nutrient inputs, temperature plays a key role in promoting phytoplankton growth, further intensifying eutrophication in water bodies. In this study, we combined classic N and P nutrient limitation bioassays with an elevated temperature treatment to investigate seasonal limitation patterns and the extent to which warming alters nutrient effects on the phytoplankton community in a hypereutrophic reservoir. From May to October, we quantified biomass responses of the total phytoplankton assemblage and the three dominant divisions present: chlorophytes, cyanobacteria, and diatoms. The total phytoplankton community was P-limited from spring through August, then shifted to co-limitation for the remainder of the study. Chlorophytes and diatoms showed similar late-summer co-limitation patterns, whereas cyanobacteria instead shifted to strict N limitation. Temperature did not directly influence nutrient limitation status, but during periods of N limitation, elevated temperatures significantly increased the phytoplankton biomass response of the +N and +NP treatments. Therefore, our results suggest that elevated temperatures may magnify cyanobacteria blooms under N-limiting conditions. These findings indicate that reductions in both N and P are necessary for effective eutrophication management, especially as global temperatures continue to rise.

Research Question

How does elevated temperature alter nutrient limitation of the phytoplankton assemblage in a hypereutrophic reservoir?

Methods

Investigated phytoplankton seasonal nutrient limitation in Acton Lake, a hyper-eutrophic reservoir in Ohio. It has a surface area 2.32 km2, max depth 8 m, and has a predominantly agricultural watershed.

Collected vertically integrated samples weekly from the euphotic zone, for 20 weeks (May - October).

Determined total nitrogen (TN) and total phosphorus (TP) from unfiltered water.

Incubated standard nutrient limitation bioassays for ~48 hours under consistent light conditions at average ambient euphotic zone temperatures or at +3°C.

Quantified the biomass of major phytoplankton groups in each flask with AlgaeLabAnalyser and calculated growth response ratio (ΔR).

ΔR(treatment) = avg chl a (treatment) / avg chl a (control)

PhycoTech processed samples to identify the cyanobacteria community to the genus-level and calculate biovolume.

Results

End-of-July declining TN:TP (Fig. 1a), coincided with increasing surface water temperatures (Fig. 1b) and Raphidiopsis dominance as the primary N-fixer (Fig. 2b), peaking at 43% of the cyanobacteria community.

Phytoplankton shifted from chlorophyte/diatom dominance to a midsummer cyanobacteria bloom that peaked as 75% of the assemblage. After early-September, divisions made up relatively equal proportions (Fig. 2a).

Nutrient limitation shifted seasonally from relative P limitation to relative N limitation (Fig. 3a, b).

Over the full study period, elevated temperature did not significantly alter relative N vs. P limitation of phytoplankton (p=0.29; Fig. 4a).

During periods of relative N limitation, warming significantly increased the strength of N limitation in cyanobacteria (p=0.03; Fig. 4b).

During periods of relative P limitation, warming did not significantly affect phytoplankton relative N vs. P limitation (p=0.91; Fig. 4b).

Figure 1. (a) Euphotic Zone TN:TP. Line graph of TN:TP ratios from May to October showing a steady decline from high values, indicating phosphorus limitation (>50) in early summer, to low values indicating nitrogen limitation (<20) by late summer, crossing the co-limitation range in between. (b) Lake Temperature Heatmap. Heatmap of lake temperature by depth (0–8 meters) and time (May - October). Warmer surface temperatures develop in summer while deeper waters remain cooler, showing thermal stratification that weakens toward fall.

Figure 2. (a) Phytoplankton Assemblage. Stacked area chart showing phytoplankton chlorophyll-a overtime (May – October). The community shifts from chlorophyte and diatom dominance in early summer to a midsummer cyanobacteria bloom, which becomes the dominant group, followed by a more even distribution among groups in early-September. (b) Cyanobacteria Community. Stacked area chart showing relative abundance of cyanobacterial genera overtime (May – October). Community composition changes through the season, with Raphidiopsis becoming dominant in mid to late summer and other genera contributing more evenly in early and late season.

Figure 3. Two line graphs showing seasonal changes in nitrogen (N) versus phosphorus (P) limitation from May to October for different phytoplankton groups under (a) ambient temperature and (b) elevated temperature (+3°C). Values above zero indicate stronger nitrogen limitation, while values below zero indicate stronger phosphorus limitation. Under ambient conditions, values fluctuate around zero early in the season and trend toward stronger nitrogen limitation by September and October. Under elevated temperature, patterns are more variable, with a sharper dip toward phosphorus limitation in August followed by stronger nitrogen limitation later in the season. Multiple colored lines represent different phytoplankton groups.

Figure 4. Two-panel chart comparing nitrogen (N) versus phosphorus (P) limitation under ambient and elevated temperatures from May to October. Left panel shows overall trends: warming slightly increases relative phosphorus limitation across total phytoplankton and most groups (cyanobacteria, chlorophytes, diatoms), though values are near zero with overlapping confidence intervals and no statistical significance. Right panel separates periods of P limitation (squares) and N limitation (circles) for each group. Most points cluster near zero, indicating weak or variable shifts, with one significant difference marked for cyanobacteria. Error bars represent 95% confidence intervals.

The following is an image of poster presented at the 2026 Undergraduate Research Forum.

Discussion

Elevated temperature may suppress cyanobacterial N fixation while increasing metabolic demand, thereby intensifying N limitation during periods dominated by N-fixing cyanobacteria.

Climate warming may reinforce or prolong N limitation in hypereutrophic lake ecosystems.

These findings highlight the importance of N and P management strategies for mitigating cyanobacterial blooms during the summer.

Acknowledgements & Funding

We would like to thank Amy Weber, Mike Vanni, Alexandra Bros, Madison Miller, Ellie Connett, Kate Halsey, Anna Maki, Ethan Krekeler, Tera Ratliff, the CAWS laboratory for field and laboratory support. We would also like to thank Lady Gaga for providing the soundtrack to our research at every stage. This work was funded by the National Science Foundation #2427185.

Literature Cited

Mulholland, P. J., & Hill, W. R. (1997). Seasonal patterns in streamwater nutrient and dissolved organic carbon concentrations: Separating catchment flow path and in-stream effects. Water Resources Research, 33(6), 1297-1306. 10.1029/97WR00490
Shostell, J., & Bukaveckas, P. A. (2004). Seasonal and interannual variation in nutrient fluxes from tributary inputs, consumer recycling and algal growth in a eutrophic river impoundment. Aquatic Ecology, 38, 359-373. 10.1023/B:AECO.0000035167.67399.63
Zhong, M., Xie, P., Feng, Y., Han, W., Du, E., Tian, D., Ma, R., Wang, Y., Jian, Z., Chen, J., Rao, Q., Sun, S., Wang, H., Jeppesen, E., & Su, H. (in press). Global Warming Amplifies Nitrogen Over Phosphorus Limitation in Aquatic Ecosystems: A Multi-Trophic Meta-Analysis. Global Change Biology.
Kraemer, B. M., Anneville, O., Chandra, S., Dix, M., Kuusisto, E., Livingstone, D. M., Rimmer, A., Schladow, S. G., Silow, E., Sitoki, L. M., Tamatamah, R., Vadeboncoeur, Y., & McIntyre, P. B. (2015). Morphometry and average temperature affect lake stratification responses to climate change. Geophysical Research Letters, 42(12), 4981-4988. 10.1002/2015GL064097
Toseland, A., Daines, S. J., Clark, J. R., Kirkham, A., Strauss, J., Uhlig, C., Lenton, T. M., Valentin, K., Pearson, G. A., Moulton, V., & Mock, T. (2013). The impact of temperature on marine phytoplankton resource allocation and metabolism. Nature Climate Change, 3(11), 979-984. 10.1038/nclimate1989
Kramer, B. J., & Gobler, C. J. (n.d.). Simulated heat waves promote the growth but suppress the N2 fixation rates of Dolichospermum spp. and cyanobacterial communities in temperature lakes. Ecological Indicators, 147, 109983. 10.1016/j.ecolind.2023.109983
Guildford, S. J., & Hecky, R. E. (2000). Total Nitrogen, Total Phosphorus, and Nutrient Limitation in Lakes and Oceans: Is There a Common Relationship? Limnology and Oceanography, 45(6), 1213-1223. 10.4319/lo.2000.45.6.1213

NACE Career Readiness Competencies

I have gained and developed several NACE Career Readiness competencies. I have furthered my career + self-development skills through identifying future career goals and maintaining professional relationships. I have developed skills in technology, using it to analyze and visualize data. I have gained skills in teamwork by collaborating with others to achieve common research goals.

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