Variable importance for sustaining macrophyte presence via random forests: data imputation and model settings.

Data sets plagued with missing data and performance-affecting model parameters represent recurrent issues within the field of data mining. Via random forests, the influence of data reduction, outlier and correlated variable removal and missing data imputation technique on the performance of habitat suitability models for three macrophytes (Lemna minor, Spirodela polyrhiza and Nuphar lutea) was assessed. Higher performances (Cohen's kappa values around 0.2-0.3) were obtained for a high degree of data reduction, without outlier or correlated variable removal and with imputation of the median value. Moreover, the influence of model parameter settings on the performance of random forest trained on this data set was investigated along a range of individual trees (ntree), while the number of variables to be considered (mtry), was fixed at two. Altering the number of individual trees did not have a uniform effect on model performance, but clearly changed the required computation time. Combining both criteria provided an ntree value of 100, with the overall effect of ntree on performance being relatively limited. Temperature, pH and conductivity remained as variables and showed to affect the likelihood of L. minor, S. polyrhiza and N. lutea being present. Generally, high likelihood values were obtained when temperature is high (>20 °C), conductivity is intermediately low (50-200 mS m-1) or pH is intermediate (6.9-8), thereby also highlighting that a multivariate management approach for supporting macrophyte presence remains recommended. Yet, as our conclusions are only based on a single freshwater data set, they should be further tested for other data sets.

Genetic diversity and population structure of Nuphar submersa (Nymphaeaceae), a critically endangered aquatic plant endemic to Japan, and implications for its conservation.

Nuphar submersa (Nymphaeaceae) is a critically endangered freshwater macrophyte indigenous to central Japan, with only four small extant populations represented across its entire range. We investigated the genotypic and genetic diversity as well as the genetic structure of all extant individuals of N. submersa based on analysis of 15 microsatellite loci. Among 278 individual ramets, 52 multilocus genotypes were detected: 30 genotypes in Nikko City (NIK), 18 in Nasukarasuyama City (NAS), 3 in Mooka City (MOK), and 1 in Sakura City (SAK). The average number of alleles per locus ranged from 1.20 to 1.93, whereas the observed and expected heterozygosities ranged from 0.11 to 0.33 and from 0.10 to 0.24, respectively. With the exception of SAK, all populations contained multiple clones, but our results indicated low levels of within-population genetic diversity. The populations NIK and NAS comprised few large or middle-sized genets and many small genets. The populations NIK and NAS were suggested to comprise large old, old fragmented, and/or young small genets resulting from seedling establishment. All four populations were differentiated, and gene flow between the populations was restricted (average level of gene flow (Nm) = 0.122, G' ST  = 0.639). Of the total genetic diversity, 67.20 and 9.13% were attributable to inter- and intra-population diversity, respectively. STRUCTURE analysis revealed two or three well-differentiated groups of populations. Cluster I comprised one population (NIK) and cluster II comprised the remaining populations at K = 2. The populations NIK, NAS, and the remaining populations were assigned to clusters I, II, and III, respectively, at K = 3. For conservation practices, we recommend that each cluster be regarded as a different management unit. We further suggest that artificial gene flow among MOK and SAK populations is an appropriate option, whereas NIK should not be reinforced with genotypes from the remaining populations.

[Structural and functional aspects of heterophylly in Nuphar lutea (L.) Smith: ultrastructure and photosynthesis].

Ultrastructure and functional characteristics of photosynthetic apparatus of the floating and submersed leaves of heterophyllous aquatic plant Nuphar lutea (L.) Sibth. et Sm. were examined. Differences in the mesophyll cell chloroplast ultrastructure, pigment content and chlorophyll fluorescence parameters of floating and submersed leaves as well as of submersed leaves at the different water depth were detected. The efficiency of PS II was established to reduce significantly under the chlorophyll fluorescence induction with light intensity higher than 60 micromol m(-2)s(-1). The obtained results may be considered as plant adaptation to reduced light intensity and changed light spectrum in the water.

Ecological modulation of environmental stress: interactions between ultraviolet radiation, epibiotic snail embryos, plants and herbivores.

1. The distribution of egg masses of the freshwater snails Lymnaea stagnalis and Planorbarius corneus on the undersides of water lily leaves (e.g. Nuphar lutea) is related to the prevalence of the leaf-mining beetle Galerucella nymphaeae. 2. When given the choice, Planorbarius significantly avoids leaves that were infested by the mining beetle. Conversely, Lymnaea did not discriminate against mined leaves. 3. Intact Nuphar leaves block over 95% of incident ultraviolet radiation. Yet, ultraviolet transmission reaches almost 100% under beetle mining scars. These are several times wider than snail embryos. 4. When exposed to natural sunlight, Lymnaea embryos proved to be resistant to ambient ultraviolet, while Planorbarius embryos were rapidly killed. Thus, one selective advantage of Planorbarius discrimination against mined leaves when depositing its eggs could be the avoidance of ultraviolet radiation passing through mining scars. 5. Other mining-related modifications of the leaves, reduced area, decreased longevity, altered aufwuchs (i.e. biofilm and epibionts) are discussed but seem less relevant for the oviposition preference of Planorbarius. 6. The discriminatory behaviour of this snail species was triggered by water-borne cues emitted by the damaged leaf, not by the eggs or larvae of the beetle. 7. This study illustrates how environmental stress on a given species, ultraviolet radiation in this case, can be ecologically buffered (shading by Nuphar) or enhanced (reduction of Nuphar shading through beetle mining) by associated species. It highlights how the impact of a given stress depends on the identity of the target species as well as on the identity and role of other species in the community.

Alterations in Rubisco activity and in stomatal behavior induce a daily rhythm in photosynthesis of aerial leaves in the amphibious-plant Nuphar lutea.

Nuphar lutea is an amphibious plant with submerged and aerial foliage, which raises the question how do both leaf types perform photosynthetically in two different environments. We found that the aerial leaves function like terrestrial sun-leaves in that their photosynthetic capability was high and saturated under high irradiance (ca. 1,500 mumol photons m(-2) s(-1)). We show that stomatal opening and Rubisco activity in these leaves co-limited photosynthesis at saturating irradiance fluctuating in a daily rhythm. In the morning, sunlight stimulated stomatal opening, Rubisco synthesis, and the neutralization of a night-accumulated Rubisco inhibitor. Consequently, the light-saturated quantum efficiency and rate of photosynthesis increased 10-fold by midday. During the afternoon, gradual closure of the stomata and a decrease in Rubisco content reduced the light-saturated photosynthetic rate. However, at limited irradiance, stomatal behavior and Rubisco content had only a marginal effect on the photosynthetic rate, which did not change during the day. In contrast to the aerial leaves, the photosynthesis rate of the submerged leaves, adapted to a shaded environment, was saturated under lower irradiance. The light-saturated quantum efficiency of these leaves was much lower and did not change during the day. Due to their low photosynthetic affinity for CO(2) (35 muM) and inability to utilize other inorganic carbon species, their photosynthetic rate at air-equilibrated water was CO(2)-limited. These results reveal differences in the photosynthetic performance of the two types of Nuphar leaves and unravel how photosynthetic daily rhythm in the aerial leaves is controlled.