Light Requirement Dynamics in Three Common Submerged Macrophytes: From Establishment to Peak Biomass

The deterioration of the underwater light environment is a key driver of submerged vegetation decline in shallow lakes. However, previous studies have largely focused on the short-term light needs of plants, failing to capture their dynamic requirements across the entire growth period. To quantify these stage-specific dynamics, we investigated three common submerged macrophytes: the rosette-type Vallisneria natans, erect-type Hydrilla verticillata, and canopy-forming Myriophyllum spicatum. Using mesocosm enclosures, we established eight bottom light gradients (0–20% of ambient light intensity) during both seedling and rapid growth stages to assess growth responses. Key findings are as follows: (1) Light requirements varied by species: V. natans < H. verticillata < M. spicatum. (2) Growth traits exhibited distinct responses: shoot density and biomass increased progressively with light, while plant height showed a unimodal response (increasing then declining), reflecting a shift in energy allocation. (3) Light requirements increased with developmental stage: the light compensation point for V. natans, H. verticillata, and M. spicatum increased from 2.1%, 4.4%, and 4.7% (seedling stage) to 3.3%, 10.5%, and 24.1% (rapid growth stage), respectively. (4) An integrated light–biomass model showed that achieving specific biomass targets required 2.4 to 4.7 times more light during rapid growth than during the seedling stage. This study quantifies stage-specific light requirements for submerged macrophytes, providing a theoretical basis for vegetation restoration and light management in shallow lakes.