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To evaluate the relative importance of photosynthetic versus morphological adaptations of submersed macrophytes to low light intensity in lakes, rapid light curves RLCs , morphological parameters, relative growth rate RGR , clonal reproduction and abundance of two submersed macrophytes Potamogeton maackianus and Vallisneria natans were examined under 2. Accordingly, the RGR, plant height and abundance of P.
At the 2. The results revealed that P. Underwater light availability is one of the most important environmental factors affecting the growth, morphology, species composition and distribution of submersed macrophytes in lakes 1. For many submersed macrophyte species growing on different light regimes required physiological adaptations and patterns of it might be species-specific 2. In lakes, low light stress on submersed macrophytes could be induced by many biotic and abiotic factors such as resuspension of soft surface sediment, increases in periphyton and phytoplankton, and shading of neighboring plants 3 , 4.
Eutrophication decreases the water transparency and combined with water level fluctuations, it not only affects the vertical distribution of underwater light intensity but also the amount of light reaching the sediment shifting the relative importance of nutrients versus light availability in affecting growth, distribution and species interaction of submersed macrophytes 5 , 6.
Based on the morphological characteristics of submersed macrophytes, Chambers 7 classified them into four types of growth forms: bottom dweller, rosette, erect, and canopy former. Each form prefers different habitats relative to nutrient status and light regime, indicating species-specific adaptive strategies to the various environments.
Several studies have examined the physiological and morphological responses of submersed macrophytes to various light regimes 6 , 8 , 9 , Few studies, however, have linked the two low light adaptive strategies to the distribution of submersed macrophytes along various light gradients and evaluated their relative importance.
Rapid light curve RLC estimates the relative electronic transport rate rETR as a function of photosynthetic active radiation PAR by using modulated chlorophyll fluorescence technology, which provides information on the temporal acclimated state of photosynthesis. Morphological parameters, relative growth rate RGR and clonal reproduction provide information on the ecological adaptation of submersed macrophytes to various light regimes, because the survival, fulfillment of life history and population expansion of the plants require extra photosynthetic carbohydrate supply than those using for leaf maintenance alone In lakes, two common submersed macrophytes, Potamogeton maackianus A.
Hara rosette grow well in waters with medium turbidity.
They can both be dominant in such an environment despite their different growth forms 13 , indicating that their low light adaptive strategies differ significantly. In this study, P. Specifically, we hypothesized that 1 P. In the experiment, P. At the beginning of the experiment, three healthy seedlings of V.
As to P. Four replicate treatments were carried out for each of the four light regimes Fig. During the experiment, chlorophyll a Chl-a , pH, temperature T , and PAR was determined in the overlying water every two weeks according to the methods described by Clesceri et al.
The mean and range in the parentheses were 0.
The mean and range in the parentheses of pH and T were 8. The pH value of lake water nearby our experimental site was 9. The mean and range in the parentheses of PAR in the water surface in the I1 to I4 light regimes were Ramet numbers of V. The shoot length of P.
At the end of the experiment, all plants were harvested, gently washed and dried with tissue paper and then weighted. The RLC of V. RLC parameters were calculated by a fitting formula of White and Critchley Field investigations of the abundance of P.
Lakes Taihu, Erhai, and Donghu the experimental site are located in the eastern, middle and south-western parts of China, respectively and thus belong to subtropical climate zone, and all three lakes have undergone eutrophication contributing to the decline of submersed vegetation during the past three decades In the field investigations, submersed macrophytes were collected by an underwater reaping hook covering a bottom surface area of 0.
In Lake Erhai, the sampling sites were set at intervals of 0. A total of samples were collected from 87 transects uniformly distributed around the lake shore. At transects with a steep lake bottom, submersed macrophytes were sampled at fewer sites. In the Xukou Bay of Lake Taihu, the sampling sites were set at intervals of m distance along transects starting from the shore to the deepest depths where the plants occurred. A total of samples were collected from 5 transects uniformly distributed around the lakeshore.
We measured the PAR in each sampling site at several water depth 0, 0. I 1 and I 2 is PAR at water depth d 1 and d 2 , respectively Then the K in the sediment Ks was obtained by fitting K in each water depth 0, 0. We excluded the values bang in the middle of two treatments.
The mean and range in the parentheses of actual water depth ranges of two species in the RI1 to RI4 in Lake Erhai were 4. One-way analysis of variance ANOVA was conducted to determine the statistical significance for each variable between the treatments. Non-normal data were Sqrt-transformed to obtain normality. The E k values of V.
The E k values of P. The ETRm values of V. The average ETRm values of P. The I1, I2, I3 and I4 stand for 2. The decreasing light regimes influenced the growth of V. The RGR of V. The RGR of P. The leaf length of V. The average leaf lengths of V.
Characterising the underwater light field 6. During the field experiment, water samples were collected at three water depths on each observation day to measure the concentrations of total nitrogen TN and total phosphorus TP. Because sunlight is the ultimate source of energy used by organisms on the earth's surface, a basic distinction lies between those who use its energy directly— autotrophs —and those who receive it indirectly by consuming other organisms— heterotrophs. In the experiments, the leaf lengths of V. This may contribute to the more rapid decline in P.
The average shoot length of P. The branch numbers of P. Plant height means average maximum leaf length of each individual for V. Branch number were the number of shoots except the highest main branch of plant in a cup. The A indicates plant height of two species and B indicates branch number of P. The ramet number increased substantially with increasing light intensity during the experimental period for V.
At the end of the experiment, V. The A and B indicate ramet number of V. The biomass of V. The average biomass density of V. The average biomass density of P. For V. For P. The field investigations revealed that P. The changes in abundances of P. The photosynthetically saturating light intensity promote shoot elongation and increased branch numbers of this canopy-forming submersed macrophyte. These results are in agreement with our first and second hypotheses.
Both species tended to exhibit increased E k and ETRm with increasing light intensity, which is consistent with the results of several other studies 11 , 21 , However, the lower E k and ETRm of V. The higher shoot length and high branch number in the relatively high light environment induced the canopy formation of P.
However, the continuous spectrum of increasing E k relative to the light intensities of V. This may contribute to the more rapid decline in P. In the experiments, the leaf lengths of V. In the I1 light regime, the growth of P. Similar to the leaf or shoot length responses observed in the experiments, in the field investigations the biomass of V.