水深对沉水植物苦草(Vallisneria natans)和穗花狐尾藻(Myriophyllum spicatum)生长的影响

水深是影响浅水湖泊沉水植物生长的主要因素之一.莲座型苦草（Vallisneria natans）和冠层型穗花狐尾藻（Myriophyllum spicatum）是我国长江中下游浅水湖泊中常见的沉水植物种类，二者在形态特征上具有较大的差异.在自然水体中，水深变化对这两种植物的生长以及竞争格局的影响还有待研究.本文设计了3个水深梯度（水深0.5、1.5、2.5 m），探讨混栽条件下苦草和穗花狐尾藻生长和竞争格局对水深变化的响应.结果显示在实验系统内，中水深（1.5 m）处理组对两种植物的生长均最有利，表现为两种植物的相对生长率和生物量均最高.低水深（0.5 m）处理组苦草的生物量和相对生长率均显著低于高水深（2.5 m）处理组；穗花狐尾藻则相反，高水深对其生长的抑制作用更大.2种沉水植物在高水深胁迫时均表现出地上部分（叶长或茎长）增加，地下部分（根长）减少的形态响应特征.此外，随着水深由高到低，苦草与穗花狐尾藻生物量之比逐渐减小，表明苦草在两种植物中的竞争优势逐渐降低.研究表明湖泊水深变化不仅能够影响沉水植物的丰度，同时还可能会影响沉水植物的群落结构，而在我国浅水湖泊的生态修复实践中，在通过水位调控恢复沉水植物时，调控范围应考虑目标植物（如苦草）的光合特征.

Effects of water depth on growth of submerged macrophytes Vallisneria natans and Myriophyllum spicatum

Water depth is one of the primary factors that affected submerged macrophyte growth. The rosette plant (Vallisneria natans) and canopy plant (Myriophyllum spicatum) are two common submerged macrophyte species in lakes of the middle and lower reaches of Yangtze River, China. The two species differ greatly in morphological characteristics. How the fluctuation of water depth could affect their growth and their competitive patterns remains to be studied. In this study, three water depth levels (0.5 m, 1.5 m and 2.5 m) were established to explore the responses of growth and competitive patterns of the two plant species to water depth under the condition of mixed planting. Our results showed that both macrophyte species favored in moderate water depth (1.5 m) environment, with highest values of plant biomass and relative growth rate (RGR) in this water depth. The biomass and RGR of V. natans in low water depth treatment (0.5 m) were significantly lower than that in high water depth treatment (2.5 m). However, the opposite pattern was observed in M. spicatum, high water depth treatment has a greater repression effect on its growth. The two species showed the similar responses in morphology, with the indictors of the aboveground part (leaf length or shoot length) increased and that of the underground parts (root length) decreased under the stress of high water depth. Moreover, the biomass ratios of V. natans to M. spicatum are gradually reduced with the water depth, indicated that the competitive advantage of V. natans were increased with water depth. Our study indicates that water depth fluctuations in lakes may affect not only the abundance of submerged macrophytes, but also their community structure. In restoration of shallow lakes, our study suggests that the photosynthetic characteristics of the key species (such as V. natans) should be fully considered when adjusting water levels to rebuild submerged macrophytes.