洱海苦草(Vallisneria natans)水深分布和叶片C、N、P化学计量学对不同水深的响应

湖泊水深是影响沉水植物生长、繁殖与分布的重要环境因子,水深增加改变了水下光照、风浪和底泥特性等,因而可能导致沉水植物的生理生化指标发生相应变化.本研究通过对云南洱海沉水植物苦草(Vallisneria natans)随水深分布的情况进行调查,并分析了定植于不同水深的苦草叶片碳(C)、氮(N)和磷(P)含量及其比率,以阐明水深变化对苦草叶片生态化学计量学的影响.结果表明洱海苦草定植的水深范围为0.5~5.6 m,在1.5~2.4 m处达到最大频度,在2.5~3.4 m处达到最大相对生物量,这表明苦草在洱海中的最适生长深度在1.5~3.4 m范围内;苦草叶片C、N和P含量平均值分别为356.10、26.13和3.54 mg/g,C:N、C:P和N:P的平均值分别为14.38、113.46和7.85;苦草叶片C含量、C:N和C:P均随水深增加而降低,N和P含量则随水深增加而升高,N:P在0.5~1.4 m较高,其余水深梯度之间则没有显著差异.总体上,苦草叶片C、N、P含量及其化学计量特征显著地受到了湖泊水深的影响.另外,本研究还发现水深的增加使得苦草叶片N、P含量发生聚敛,这导致其N、P之间的耦合性变弱.

Distribution and leaf C, N, P stoichiometry of Vallisneria natans in response to various water depths in a large mesotrophic lake, Lake Erhai, China

Water depth is an important environmental factor which affects growth, reproduction and distribution of submersed macrophytes. Increased water depth leads to changes of underwater light, wave and sediment properties, and may consequently affects physiological-biochemical characteristic of submersed macrophytes. To explore carbon (C), nitrogen (N) and phosphorus (P) stoichiometry of submersed macrophytes in response to various water depths and evaluate significance of these responses to fitness of submersed macrophytes, we investigated depth distribution of Vallisneria natans, a common submersed rosette macrophyte with strap-like leaves on a stunted basal stem, in a plateau deep-water lake, namely Lake Erhai, and measured leaf C, N, P concentrations of this species colonizing in different water depths. The results suggested that the colonization depths of V. natans were between 0.5 and 5.6 m. The depth range of optimum growth for V. natans was 1.5-3.4 m, where the plant reached its highest frequency of occurrence and relative biomass. The mean values of leaf C, N and P concentrations for V. natans were 356.10, 26.13 and 3.54 mg/g, respectively, and the ratios of C:N, C:P and N:P were 14.38, 113.46 and 7.85, respectively. N:P ratios were relatively stable, except for a higher mean value between 0.5 and 1.4 m than the other water depths. Leaf C concentration, C:N and C:P ratios decreased, while leaf N and P concentrations increased with increased water depth. These results indicated that leaf C, N, P stoichiometry of V. natans was significantly affected by water depth. In addition, we also found that increased water depth lead to convergent leaf nutrients in V. natans, which resulted in a weaker coupling of N and P concentrations in deep water than that in shallow water.