Temperature and nutrients are key regulators of phytoplankton growth. Under global warming and intensified eutrophication, understanding their interactive effects on phytoplankton is critical. While previous lab studies on single species show a left-skewed unimodal growth response to temperature and a saturating response to nutrients, whether these patterns hold for natural communities under coupled drivers remains unclear. This study conducted monthly in-situ incubation experiments over 15 months in a semi-enclosed subtropical coastal bay (Dongshan Bay). Seawater was collected from two stations (inner-bay DS07 and outer-bay DS05) with distinct environmental conditions. A 6×6 orthogonal factorial design (6 temperature × 6 nutrient gradients) was used to examine the coupled effects on phytoplankton growth, comparing whole-community vs. group-specific responses. Key findings:
(1) Phytoplankton community growth rate showed a clear unimodal response to warming, regardless of nutrient levels. However, high vs. low nutrients significantly modulated the activation energy (E_a), which followed a saturating function with nutrient concentration. Optimal growth temperature (T_opt) was co-regulated by long-term acclimation (in situ temperature) and short-term resource supply (nutrients). Community growth response to nutrients followed the Monod equation. Both maximum growth rate (μ_max) and half-saturation constant (K_s) increased linearly with temperature.
(2) For pico-phytoplankton groups under nutrient-replete conditions, pico-eukaryotes (PEUK), Synechococcus (SYN), and Prochlorococcus (PRO) all exhibited unimodal growth rate responses to warming. Under constant in situ temperature, PEUK’s T_opt increased with nutrients, while SYN and PRO’s T_opt decreased. Under constant nutrients, PEUK’s T_opt increased then saturated with rising ambient temperature; SYN’s T_opt continuously increased; PRO’s T_opt first decreased then increased. PEUK’s E_a increased with nutrients, whereas SYN and PRO’s E_a decreased in some treatments. Thus, PEUK is more competitive under high nutrients, while SYN and PRO are better adapted to low nutrients, indicating niche differentiation.
This study shows complex temperature–nutrient regulation of phytoplankton growth, offering in situ data for mechanistic production models. Biological responses arise from physiological trade-offs, competition, adaptation, and niche differentiation among phytoplankton groups. Under warming and eutrophication, short-term single‑stressor or single‑species lab studies cannot capture the nonlinear, lagged effects of multi‑factor interactions and community feedbacks. Upscaling to community and ecosystem levels—incorporating multi‑factor coupling, long‑term adaptation, and interspecific interactions—is essential to accurately predict climate impacts on community structure, primary productivity, and biogeochemical cycles, thereby informing global change responses.
 

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