20 Nicholas School of the Environment 21 Duke University 22 Durham, NC 27708 23 USA 24 25 Author Contribution Statement 26 KRS conducted modeling and analysis efforts. RMC provided expertise on model application 27 and validation. SLS and JJV provided guidance on the research questions. KRS, RMC, JJV, and 28 SLS analyzed model fit, guided scenario analysis, and provided interpretations of results. SNH 29 and MJP provided guidance on historical data analyses, model input data, and postprocessing 30 analyses. KRS wrote the manuscript, and all coauthors contributed edits to the manuscript. 31 32 Significance Statement 33 Harmful algal blooms and eutrophication are key water quality issues worldwide. 34 Managing algal blooms is often difficult because multiple drivers, such as climate change and 35 nutrient loading, act concurrently and potentially synergistically. Long-term datasets and 36 simulation models allow us to parse the effects of interacting drivers of blooms. The 37 performance of our model depended on the ratio of nitrogen to phosphorus inputs, suggesting 38 that complex biological dynamics control blooms under variable nutrient loads. We found that 39 blooms were dampened under a "no climate change" scenario, suggesting that the interaction of 40 nutrient loading and increased temperature intensifies blooms. Our results highlight successes 41 and gaps in our ability to model blooms, helping to establish future management 42 recommendations. 43 44 Data Availability Statement 45 Data and metadata will be made available in a GitHub repository 46 (https://github.comAbstract 53 Phytoplankton blooms respond to multiple drivers, including climate change and nutrient 54loading. Here we examine a long-term dataset from Lake 227, a site exposed to a fertilization 55 experiment (1969-present). Changes in nitrogen:phosphorus loading ratios (high N:P, low N:P, 56 P-only) did not impact mean annual biomass, but blooms exhibited substantial inter-and intra-57 annual variability. We used a process-oriented lake model, MyLake, to successfully reproduce 58 lake physics over 48 years and test if a P-limited model structure predicted blooms. The timing 59 and magnitude of blooms was reproduced during the P-only period but not for the high and low 60 N:P periods, perhaps due to N acquisition pathways not currently included in the model. A 61 model scenario with no experimental fertilization confirmed P loading is the major driver of 62 blooms, while a scenario that removed climate-driven temperature trends showed that increased 63 spring temperatures have exacerbated blooms beyond the effects of fertilization alone. 64 65