The impact of NO inf3sup- loading on the freshwater macrophyte Littorella uniflora: N utilization strategy in a slow-growing species from oligotrophic habitats

Effects of ammonium pulse on the growth of three submerged macrophytes

Ammonium pulse attributed to runoff of urban surface and agriculture following heavy rain is common in inland aquatic systems and can cause profoundly effects on the growth of macrophytes, especially when combined with low light. In this study, three patterns of NH₄-N pulse (differing in magnitude and frequency) were applied to examine their effects on the growth of three submersed macrophytes, namely, Myriophyllum spicatum, Potamogeton maackianus, and Vallisneria natans, in terms of biomass, height, branch/ramet number, root length, leaf number, and total branch length under high and low light. Results showed that NH₄-N pulse caused negative effects on the biomass of the submerged macrphytes even on the 13th day after releasing NH₄-N pulse. The negative effects on M. spicatum were significantly greater than that on V. natans and P. maackianus. The effects of NH₄-N pulse on specific species depended on the ammonium loading patterns. The negative effects of NH₄-N pulse on P. maackianus were the strongest at high loading with low frequency, and on V. natans at moderate loading with moderate frequency. For M. spicatum, no significant differences were found among the three NH₄-N pulse patterns. Low light availability did not significantly aggregate the negative effects of NH₄-N pulse on the growth of the submersed macrophytes. Our study contributes to revealing the roles of NH₄-N pulse on the growth of aquatic plants and its species specific effects on the dynamics of submerged macrophytes in lakes.

Massive uprooting of Littorella uniflora (L.) Asch. during a storm event and its relation to sediment and plant characteristics

During spring storms massive uprooting of Littorella uniflora occurred in a shallow Dutch softwater lake. The aim of this study was to test whether changes in plant morphology and sediment characteristics could explain the observed phenomenon. Uprooting was expected to occur in plants having a high shoot biomass and low root to shoot ratio (R:S), growing on sediments with a high organic matter content. Normally, uprooting of the relative buoyant L. uniflora is prevented by an extensive root system, expressed as a high R:S. This was studied by sampling floating and still rooted L. uniflora plants, as well as sediment and sediment pore water, along a gradient of increasing sediment organic matter content. Increasing organic matter content was related to increasing L. uniflora shoot biomass and consequently decreasing R:S. Furthermore, the results indicated that uprooting indeed occurred in plants growing on very organic sediments and was related to a low R:S. The increased shoot biomass on more organic sediments could be related to increased sediment pore water total inorganic carbon (TIC; mainly CO2 ) availability. Additionally, increased phosphorus availability could also have played a role. The disappearance of L. uniflora might lead to higher nutrient availability in the sediments. It is suggested that this could eventually promote the expansion of faster-growing macrophytes.

Crassulacean acid metabolism in the context of other carbon-concentrating mechanisms in freshwater plants: a review

Inorganic carbon can be in short supply in freshwater relative to that needed by freshwater plants for photosynthesis because of a large external transport limitation coupled with frequent depleted concentrations of CO(2) and elevated concentrations of O(2). Freshwater plants have evolved a host of avoidance, exploitation and amelioration strategies to cope with the low and variable supply of inorganic carbon in water. Avoidance strategies rely on the spatial variation in CO(2) concentrations within and among lakes. Exploitation strategies involve anatomical and morphological features that take advantage of sources of CO(2) outside of the water column such as the atmosphere or sediment. Amelioration strategies involve carbon-concentrating mechanisms based on uptake of bicarbonate, which is widespread, C(4)-fixation, which is infrequent, and crassulacean acid metabolism (CAM), which is of intermediate frequency. CAM enables aquatic plants to take up inorganic carbon in the night. Furthermore, daytime inorganic carbon uptake is generally not inhibited and therefore CAM is considered to be a carbon-conserving mechanism. CAM in aquatic plants is a plastic mechanism regulated by environmental variables and is generally downregulated when inorganic carbon does not limit photosynthesis. CAM is regulated in the long term (acclimation during growth), but is also affected by environmental conditions in the short term (response on a daily basis). In aquatic plants, CAM appears to be an ecologically important mechanism for increasing inorganic carbon uptake, because the in situ contribution from CAM to the C-budget generally is high (18-55%).

Aquatic macrophytes as bioindicators of carbon dioxide in groundwater fed rivers

Aquatic plants have been used as hydrological tracers in groundwater fed river systems. In nature, patterns in plant distribution have been attributed to ammonium (NH(4)) toxicity and phosphate (PO(4)) limitation, while some laboratory studies have focused on the role of the partial pressure of CO(2) (pCO(2)). The aims of this study were (i) to test whether plant distribution was more related to pCO(2) than NH(4) and PO(4) in nature, (ii) to develop and test the predictive power of new plant indices for pCO(2), NH(4) and PO(4), and (iii) to test the potential causality of the relationships using species eco-physiological traits. These tests were carried out with field data from the Rhine, Rhône and Danube river basins. Species composition was best related to the effect of pCO(2). The pCO(2) plant index was well calibrated (r(2)=0.73) and had the best predictive power (r(2)=0.47) of the three indices tested on independent datasets. The plant-pCO(2) relationship was supported by a biological mechanism: the ability of strictly submerged species of aquatic vascular plants to use HCO(3) under low pCO(2). This was not the whole story: the effects of pCO(2), NH(4) and PO(4) on plant distribution were partially confounded and interacted all together with temperature. However, neither NH(4) toxicity nor P limitation could be asserted using species eco-physiological traits. Moreover, the predictive power of the NH(4) and PO(4) plant indices was not as strong as pCO(2), at r(2)=0.24 and r(2)=0.27, respectively. Other potentially confounding variables such as spatial structure, biotic and physical factors were unlikely to confound the findings of this study.

Distribution of aquatic macrophytes in contrasting river systems: a critique of compositional-based assessment of water quality

A brief summary of the historical developments relating to plant distribution and aquatic macrophyte-nutrient indices provided a means of assessing the general context and validity of previous assumptions. This has particular current relevance because of the prominent use of bioindicators for defining nutrient enrichment. A survey of 161 sites distributed across two broadly contrasting groups of rivers (circum-neutral versus alkaline) recorded 110 species of aquatic macrophytes and these have been statistically analyzed to (i) rank and separate the individual effects of local environmental conditions and spatial isolation on species distribution in the two contrasting groups of sites; (ii) calculate a macrophyte index based on plant cover and species indicator values (Mean Trophic Rank, MTR); and finally (iii) investigate the implications for biomonitoring. Chemical, physical and hydrological site attributes together with spatial isolation, each explained a significant and at least partially independent influence over plant species distribution. It was extremely difficult, however, to separate the single effects of different site attributes on plant distribution. While some plant species are more restricted to certain environmental conditions, many appeared indifferent to the range of those being tested. The role played by nutrients (nitrogen (N) and phosphorus (P)) were either mostly indistinguishable from other site attributes (e.g., nitrate from conductivity) or subordinate (e.g., soluble reactive phosphorus, ammonium). It is therefore very unlikely that macrophyte species composition could provide a reliable bioindicator of the surrounding nutrient (N, P) status. The calculation of the plant index illustrated this unreliability by showing that strong correlations existed with many environmental variables, not just inorganic N and P.