Ecology of aquatic Ranunculus communities under the Mediterranean climate

The Iberian Peninsula encompasses more than 80% of the species richness of European aquatic ranunculi. The floristic diversity of the phytocoenosis characterised by aquatic Ranunculus and the main physical–chemical factors of the water were studied in 43 localities of the central Iberian Peninsula. Four aquatic Ranunculus communities are found in most of the aquatic environments. These are species-poor and have an uneven distribution: three species of Batrachium are heterophyllous and their communities are distributed in different aquatic ecosystems on silicated substrates; one species is homophyllous and its community occurs in various aquatic ecosystems with carbonated waters. In the Mediterranean climate, Ranunculus species are present in different habitats, as shown by the results of all the statistical analyses. Ranunculus trichophyllus communities occur in base-rich waters with a high buffering capacity (2273.44 ± 794.57 mg CaCO₃ L⁻¹) and a high concentration of cations (Ca²⁺, 121 ± 33.12 mg L⁻¹; Mg²⁺, 71.64 ± 82.77 mg L⁻¹), nitrates (2.89 ± 4.80 mg L⁻¹), ammonium (2.19 ± 1.36 mg L⁻¹) and sulphates (216.25 ± 218.54 mg L⁻¹). Ranunculus penicillatus communities are found in flowing waters with a high concentration of phosphates (0.48 ± 0.6 mg L⁻¹) and intermediate buffering capacity (683.66 ± 446.76 mg CaCO₃ L⁻¹). Both Ranunculus pseudofluitans and Ranunculus peltatus communities grow in waters with low buffering capacity (R. pseudofluitans, 385.91 ± 209.2 mg CaCO₃ L⁻¹; R. peltatus, 263.3 ± 180.36 mg CaCO₃ L⁻¹), and a low concentration of cations (R. pseudofluitans: Ca²⁺, 12.57 ± 9.42 mg L⁻¹; Mg²⁺, 3.42 ± 1.67 mg L⁻¹; R. peltatus: Ca²⁺, 15 ± 18.26 mg L⁻¹; Mg²⁺, 6.26 ± 8.89 mg L⁻¹) and nutrients (R. pseudofluitans: nitrates, 0.23 ± 0.2 mg L⁻¹; phosphates, 0.09 ± 0.1 mg L⁻¹; R. peltatus: nitrates, 0.19 ± 0.21 mg L⁻¹; phosphates, 0.09 ± 0.12 mg L⁻¹); the first in flowing waters, the latter in still waters.     

Effects of retrogressive permafrost thaw slumping on sediment chemistry and submerged macrophytes in Arctic tundra lakes

1. Global warming is predicted to cause changes in permafrost cover and stability in the Arctic. Zones of high ion concentration in regions of ice‐rich permafrost are a reservoir of chemicals that can potentially be transferred to fresh waters during thawing. Consequently, input of enriched runoff from the thaw and sediment and vegetation from the landscape could alter lakes by affecting their geochemistry and biological production. 2. Three undisturbed lakes and five lakes disturbed by retrogressive permafrost thaw slumps were sampled during late summer of 2006 to assess the potential effects of thermokarst shoreline slumping on water and sediment chemistry, the underwater light regime, and benthic macrophyte biomass and community structure. 3. Undisturbed lakes had sediments rich in organic material and selected micronutrients, while disturbed lakes had sediments richer in calcium, magnesium and strontium, greater transparency of the water column, and a well‐developed submerged macrophyte community. 4. It is postulated that enriched runoff chemistry may alter nutrient availability at the sediment–water interface and also the degradation of organic material, thus affecting lake transparency and submerged macrophytes. The results suggest that retrogressive permafrost slumping can significantly affect food webs in arctic tundra lakes through an increase in macrophyte biomass and development of a more complex benthic habitat.     

How a bottom‐dweller beats the canopy: inhibition of an aquatic weed (Potamogeton pectinatus) by macroalgae (Chara spp.)

1. Bottom‐dwelling charophytes have been observed to replace canopy‐forming pondweeds within a few years in de‐eutrophied shallow lakes. Competition for bicarbonate (HCO₃^?) may explain this shift in vegetation dominance but inhibition of pondweeds by Chara spp. through direct competition has not been shown experimentally. 2. We tested whether charophytes inhibited growth of fennel pondweed (Potamogeton pectinatus) in the absence of belowground competition by growing plants in pots in mesocosms following a replacement series experimental design. To further understand the role of bicarbonate, we studied main and interactive effects of Chara, light and bicarbonate on P. pectinatus growth in a laboratory study. 3. Early in the mesocosm experiment, high charophyte densities had a negative effect on P. pectinatus cover at a time when bicarbonate levels were low. However, bicarbonate levels eventually converged to low levels in all treatments. At final harvest, both species exhibited lower biomasses at higher densities of conspecific pots, indicating that ultimately intraspecific competition was limiting. In a laboratory study, Chara inhibited P. pectinatus the most under a combination of high light and high bicarbonate concentrations, suggesting that Chara may negatively affect P. pectinatus by acting as a general nutrient sink. 4. Our results suggest that Chara growth can reduce bicarbonate levels, delaying but not preventing a P. pectinatus growth pulse. Given the recorded inhibition under ample bicarbonate supply, Chara’s ability to act as nutrient sink may contribute to the decline of P. pectinatus under Chara recovery in shallow lakes.     

Photosynthetic and growth responses of Egeria densa to photosynthetic active radiation

Egeria densa, a submerged aquatic macrophyte native to South America, has successfully invaded many reservoirs in Brazil and elsewhere. Ecophysiological responses of E. densa to light availability were assessed in microcosm experiments. Under low light conditions, we found that apical shoots expanded more rapidly than those under higher light exposure, allowing the plant to reach the higher light conditions of the surface. E. densa showed low k_m (15.6-34.8 μmol m⁻² s⁻¹ PAR) and light compensation point values (7.5-16.2 μmol m⁻² s⁻¹ PAR), indicating that it is able to effectively exploit the low radiation levels available at high depths and turbid waters. This may represent a competitive advantage over other submerged species, and it helps to explain the successful spread of E. densa in Brazilian reservoirs.     

Invasion strategies in clonal aquatic plants: are phenotypic differences caused by phenotypic plasticity or local adaptation?

Background and Aims

The successful spread of invasive plants in new environments is often linked to multiple introductions and a diverse gene pool that facilitates local adaptation to variable environmental conditions. For clonal plants, however, phenotypic plasticity may be equally important. Here the primary adaptive strategy in three non-native, clonally reproducing macrophytes (Egeria densa, Elodea canadensis and Lagarosiphon major) in New Zealand freshwaters were examined and an attempt was made to link observed differences in plant morphology to local variation in habitat conditions.

Methods

Field populations with a large phenotypic variety were sampled in a range of lakes and streams with different chemical and physical properties. The phenotypic plasticity of the species before and after cultivation was studied in a common garden growth experiment, and the genetic diversity of these same populations was also quantified.

Key Results

For all three species, greater variation in plant characteristics was found before they were grown in standardized conditions. Moreover, field populations displayed remarkably little genetic variation and there was little interaction between habitat conditions and plant morphological characteristics.

Conclusions

The results indicate that at the current stage of spread into New Zealand, the primary adaptive strategy of these three invasive macrophytes is phenotypic plasticity. However, while limited, the possibility that genetic diversity between populations may facilitate ecotypic differentiation in the future cannot be excluded. These results thus indicate that invasive clonal aquatic plants adapt to new introduced areas by phenotypic plasticity. Inorganic carbon, nitrogen and phosphorous were important in controlling plant size of E. canadensis and L. major, but no other relationships between plant characteristics and habitat conditions were apparent. This implies that within-species differences in plant size can be explained by local nutrient conditions. All together this strongly suggests that invasive clonal aquatic plants adapt to a wide range of habitats in introduced areas by phenotypic plasticity rather than local adaptation.     

Functional traits composition predict macrophytes community productivity along a water depth gradient in a freshwater lake

Functional trait composition of plant communities has been proposed as a helpful key for understanding the mechanisms of biodiversity effects on ecosystem functioning. In this study, we applied a step‐wise modeling procedure to test the relative effects of taxonomic diversity, functional identity, and functional diversity on macrophytes community productivity along water depth gradient. We sampled 42 plots and 1513 individual plants and measured 16 functional traits and abundance of 17 macrophyte species. Results showed that there was a significant decrease in taxonomic diversity, functional identity (i.e., stem dry mass content, leaf [C] and leaf [N]), and functional diversity (i.e., floating leaf, mean Julian flowering date and rooting depth) with increasing water depth. For the multiple‐trait functional diversity (FD) indices, functional richness decreased, while functional divergence increased with water depth gradient. Macrophyte community productivity was strongly determined by functional trait composition within community, but not significantly affected by taxonomic diversity. Community‐weighted means (CWM) showed a two times higher explanatory power relative to FD indices in determining variations in community productivity. For nine of sixteen traits, CWM and FD showed significant correlations with community productivity, although the strength and direction of those relations depended on selected trait. Furthermore, functional composition in a community affected productivity through either additive or opposite effects of CWM and FD, depending on the particular traits being considered. Our results suggested both mechanisms of mass ratio and niche complementarity can operate simultaneously on variations in community productivity, and considering both CWM and FD would lead to a more profound understanding of traits–productivity relationships.     

太湖环棱螺(Bellamya sp.)及其与沉水植物的相互作用

选用太湖常见的环棱螺及沉水植物,研究了不同状态下环棱螺营养盐的释放特征,探讨了环棱螺对水体营养盐、透明度和浮游藻类的影响,及其与沉水植物的相互作用.结果表明:在一定温度范围内,环棱螺营养盐的释放速率随温度升高而增加,且进食状态下释放速率高于饥饿状态;环棱螺能在短时间内提高水体透明度,但其营养盐释放又引起局部水体溶解态氮磷含量的增加;适宜的条件下,水体中藻类的再生能力超过环棱螺对其的抑制力;水体营养盐含量增加,促进与环棱螺共存的伊乐藻和轮叶黑藻的生长.在太湖的不同湖区,草型湖区螺类的生物量远高于藻型湖区,这表明沉水植物可能是影响螺类分布的重要生态因子之一.     

The possibility of submerged macrophyte recovery from a propagule bank in the eutrophic Lake Mikołajskie (North Poland)

Changes in submerged macrophyte communities in the eutrophic Lake Mikołajskie have been studied for the last 40 years. As the recent commissioning of a number of sewage treatment plants within the complex of the Mazurian Great Lakes has led to an improvement in water transparency, it was expected that species composition would start to return to the state present before intensive eutrophication. The role of seed and oospore banks in the reconstruction of submerged macrophytes is analysed on the basis of laboratory experiment. Cores of sediments (9 cm long) were collected from the littoral of Lake Mikołajskie. The cores were divided in 3 cm layers and were incubated under laboratory conditions. Five species of submerged macrophytes were germinated from the propagule banks. The richest in viable propagules was the deepest layer and poorest in active propagules was the shallowest layer of sediment. The recolonization of the littoral zone of Lake Mikołajskie by species, which occurred at a earlier period in the lake’s history is possible because viable propagules have persisted in deeper sediments.     

Effects of two common macrophytes on methane dynamics in freshwater sediments

The methane cycle in constructed wetlands without plants and withPhragmites australis (reed) and Scirpus lacustris (bulrush) wasinvestigated. Variations in CH₄production largely determined variations in CH₄ emission among the systems, rather than variations inCH₄ storage and oxidation. Twofoldlower CH₄ production rates in theScirpus system (5.6–13 mmol m^-2 d^-1) relative to the control (16.7–17.6 mmolm^-2 d^-1) were accompanied by a lower contribution ofmethanogenesis to organic carbon metabolism (20% for Scirpus vs.80% for control). Sedimentary iron(II) reservoirs were smallerin the Scirpus than control sediment (300 vs. 485 mmol.m^-2) and a shuttle role for iron asan intermediate between root O₂release and carbon oxidation, attenuating the availability of substrate formethanogens, is suggested. Differences in CH₄ production among the Phragmites and Scirpus systemswere controlled by the interspecific variation in sediment oxidationcapacities of both plant species. Comparatively, in the Phragmites sediment,dissolved iron reservoirs were larger (340 mmol.m^-2) and methanogenesis was a more importantpathway (80%). Methane transport was mainly plant mediated inthe Phragmites and Scirpus systems, but ebullition dominated in thenon-vegetated control systems as well as in the vegetated systems when plantbiomass was low.     

The ecology of periphytic rotifers

The ecology of rotifer assemblages in the periphyton has received little attention relative to that of pelagic rotifers. This paper reviews the ecology of periphytic rotifers, with particular emphasis on the role of macrophytes in the structuring of rotifer assemblages spatially and temporally, and compares these aspects with the dynamics of better known pelagic rotifer communities. Littoral rotifer periphyton communities are typically diverse in lakes, and have composition dissimilar to that of the open water. In rivers, diversity and composition in the pelagic and littoral appear to be similar. Rotifers show preference for macrophyte species they associate with, probably through differences in physical structure or complexity, food concentration or composition, chemical factors, macrophyte age, and differences in the degree of protection from predation provided by macrophytes. These mechanisms are in general not well investigated in rotifers. Factors affecting the seasonal dynamics of periphytic communities appear to be similar to pelagic communities, with seasonal dynamics of substrates and disturbance by flooding or drying also being important.