Morphological responses of the submerged macrophyte Vallisneria natans along an underwater light gradient: A mesocosm experiment reveals the importance of the Secchi depth to water depth ratio.

Bottom light availability (BLA), represented by the ratio of the Secchi disk depth to water depth (SD/WD), plays a fundamental role in the growth and reproduction of submerged macrophytes. However, studies thus far have mainly explored the interactions between macrophyte responses and BLA through field investigations; this means that knowledge of such responses to various underwater light conditions in mesocosm experiments is rudimentary at best. We hypothesized that the growth and clonal reproduction of submerged macrophytes decrease with decreasing BLA and collapse beyond a critical threshold. Here we performed a 42-day outdoor mesocosm experiment with a species of perennial submerged macrophyte, Vallisneria natans, along a decreasing SD/WD gradient. Over this gradient, the primary morphological traits (plant height, root length, plant biomass), relative growth rate, and shoot increment rate of V. natans exhibited a significant trend of initial increase followed by a decrease. The photoinhibition occurred at high and low-light stress, indicating that an intermediate SD/WD (0.55-0.65) provides optimal growth conditions. The number of ramets, ramet biomass, ramet/total biomass ratio, and root/shoot ratio all decreased with decreasing SD/WD ratio, suggesting that V. natans allocates more resources for clonal reproduction and population stability rather than increased shoot biomass at higher BLA conditions. The results of principal component analysis and threshold detection indicated that the growth traits of V. natans had a higher SD/WD tipping point value (0.55 vs. 0.50) than the reproductive capacity and stability, indicating that only values of SD/WD ≥ 0.55 ensured the growth and the vegetative reproduction of V. natans. Additionally, an inverted U-shaped relationship between growth traits and a linear relationship between reproduction and stability reflect the resource allocation strategies and resilience of V. natans to decreasing underwater light conditions.