Jonas Schoelynck

Silica uptake in aquatic and wetland macrophytes: a strategic choice between silica, lignin and cellulose?

*Although silica (Si) is not an essential element for plant growth in the classical sense, evidence points towards its functionality for a better resistance against (a)biotic stress. Recently, it was shown that wetland vegetation has a considerable impact on silica biogeochemistry. However, detailed information on Si uptake in aquatic macrophytes is lacking. *We investigated the biogenic silica (BSi), cellulose and lignin content of 16 aquatic/wetland species along the Biebrza river (Poland) in June 2006 and 2007. The BSi data were correlated with cellulose and lignin concentrations. *Our results show that macrophytes contain significant amounts of BSi: between 2 and 28 mg BSi g(-1). This is in the same order of magnitude as wetland species (especially grasses). Significant antagonistic correlations were found between lignin, cellulose and BSi content. Interestingly, observed patterns were opposite for wetland macrophytes and true aquatic macrophytes. *We conclude that macrophytes have an overlooked but potentially vast storage capacity for Si. Study of their role as temporal silica sinks along the land-ocean continuum is needed. This will further understanding of the role of ecosystems on land ocean transport of this essential nutrient.

Effects of Wind Waves versus Ship Waves on Tidal Marsh Plants: A Flume Study on Different Life Stages of Scirpus maritimus

Recent research indicates that many ecosystems, including intertidal marshes, follow the alternative stable states theory. This theory implies that thresholds of environmental factors can mark a limit between two opposing stable ecosystem states, e.g. vegetated marshes and bare mudflats. While elevation relative to mean sea level is considered as the overall threshold condition for colonization of mudflats by vegetation, little is known about the individual driving mechanisms, in particular the impact of waves, and more specifically of wave period. We studied the impact of different wave regimes on plants in a full scale flume experiment. Seedlings and adult shoots of the pioneer Scirpus maritimus were subjected to two wave periods at two water levels. Drag forces acting on, and sediment scouring occurring around the plants were quantified, as these are the two main mechanisms determining plant establishment and survival. Depending on life stage, two distinct survival strategies emerge: seedlings present a stress avoidance strategy by being extremely flexible, thus limiting the drag forces and thereby the risk of breaking. Adult shoots present a stress tolerance strategy by having stiffer stems, which gives them a higher resistance to breaking. These strategies work well under natural, short period wind wave conditions. For long period waves, however, caused e.g. by ships, these survival strategies have a high chance to fail as the flexibility of seedlings and stiffness of adults lead to plant tissue failure and extreme drag forces respectively. This results in both cases in strongly bent plant stems, potentially limiting their survival.

Hydrodynamically mediated macrophyte silica dynamics

In most aquatic ecosystems, hydrodynamic conditions are a key abiotic factor determining species distributions and abundance of aquatic plants. Resisting stress and keeping an upright position often relies on investment in tissue reinforcement, which is costly to produce. Silica could provide a more economical alternative. Two laboratory experiments were conducted to measure the response of two submerged species, Egeria densa Planch. and Limnophila heterophylla (Roxb.) Benth., to dissolved silicic acid availability and exposure to hydrodynamic stress. The results were verified with a third species in a field study (Nuphar lutea (L.) Smith). Biogenic silica (BSi) concentration in both stems and leaves increases with increasing dissolved silica availability but also with the presence of hydrodynamic stress. We suggest that the inclusion of extra silica enables the plant to alternatively invest its energy in the production of lignin and cellulose. Although we found no significant effects of hydrodynamic stress on cellulose or lignin concentrations either in the laboratory or in the field, BSi was negatively correlated with cellulose concentration and positively correlated with lignin concentration in samples collected in the field study. This implies that the plant might perform with equal energy efficiency in both standing and running water environments. This could provide submerged species with a tool to respond to abiotic factors, to adapt to new ecological conditions and hence potentially colonise new environments.

Ecosystem Engineering by Plants on Wave-Exposed Intertidal Flats Is Governed by Relationships between Effect and Response Traits.

In hydrodynamically stressful environments, some species—known as ecosystem engineers—are able to modify the environment for their own benefit. Little is known however, about the interaction between functional plant traits and ecosystem engineering. We studied the responses of Scirpus tabernaemontani and Scirpus maritimus to wave impact in full-scale flume experiments. Stem density and biomass were used to predict the ecosystem engineering effect of wave attenuation. Also the drag force on plants, their bending angle after wave impact and the stem biomechanical properties were quantified as both responses of stress experienced and effects on ecosystem engineering. We analyzed lignin, cellulose, and silica contents as traits likely effecting stress resistance (avoidance, tolerance). Stem density and biomass were strong predictors for wave attenuation, S. maritimus showing a higher effect than S. tabernaemontani. The drag force and drag force per wet frontal area both differed significantly between the species at shallow water depths (20 cm). At greater depths (35 cm), drag forces and bending angles were significantly higher for S. maritimus than for S. tabernaemontani. However, they do not differ in drag force per wet frontal area due to the larger plant surface of S. maritimus. Stem resistance to breaking and stem flexibility were significantly higher in S. tabernaemontani, having a higher cellulose concentration and a larger cross-section in its basal stem parts. S. maritimus had clearly more lignin and silica contents in the basal stem parts than S. tabernaemontani. We concluded that the effect of biomass seems more relevant for the engineering effect of emergent macrophytes with leaves than species morphology: S. tabernaemontani has avoiding traits with minor effects on wave attenuation; S. maritimus has tolerating traits with larger effects. This implies that ecosystem engineering effects are directly linked with traits affecting species stress resistance and responding to stress experienced.

Tussocks : biogenic silica hot-spots in a riparian wetland

Understanding the factors determining the size and extent of Si cycling in wetlands is important, as more and more research shows they interact strongly within riverine Si fluxes. One key factor is the size of the ecosystem Si reservoir, which strongly depends on the occurrence of organisms specialized in biological Si processing. Our study was aimed to test whether tussocks, a common growth form of sedges, can efficiently retain biogenic silica. As such, they take advantage of efficient recycling of a private Si stock, providing them with a competitive advantage. We showed that tussock development caused a patch-like distribution of biogenic silica (BSi) in wetlands. While in a managed wetland (where tussocks are absent) BSi was uniformly distributed over surface layers, tussock development in an unmanaged wetland strongly interfered with BSi distribution. A mosaic of BSi richer inter-tussock soils and BSi poorer soils under tussocks developed, resulting from the active uplift of Si into the tussock from soil below the tussocks. Tussocks affected the role of wetlands as silica hot-spots and biogenic Si sinks near rivers. This implies that future studies should focus on quantifying the effect of tussock development, and human management, on system scale BSi storage.

Grazers: biocatalysts of terrestrial silica cycling

Silica is well known for its role as inducible defence mechanism countering herbivore attack, mainly through precipitation of opaline, biogenic silica (BSi) bodies (phytoliths) in plant epidermal tissues. Even though grazing strongly interacts with other element cycles, its impact on terrestrial silica cycling has never been thoroughly considered. Here, BSi content of ingested grass, hay and faeces of large herbivores was quantified by performing multiple chemical extraction procedures for BSi, allowing the assessment of chemical reactivity. Dissolution experiments with grass and faeces were carried out to measure direct availability of BSi for dissolution. Average BSi and readily soluble silica numbers were higher in faeces as compared with grass or hay, and differences between herbivores could be related to distinct digestive strategies. Reactivity and dissolvability of BSi increases after digestion, mainly due to degradation of organic matrices, resulting in higher silica turnover rates and mobilization potential from terrestrial to aquatic ecosystems in non-grazed versus grazed pasture systems (2 versus 20 kg Si ha−1 y−1). Our results suggest a crucial yet currently unexplored role of herbivores in determining silica export from land to ocean, where its availability is linked to eutrophication events and carbon sequestration through C–Si diatom interactions.

Silicon in aquatic vegetation

Summary Silicon (Si) use by plants has not always received the research attention of other elements. Yet today, the importance of Si for plant functioning is slowly becoming better understood. Si is a crucial element for many terrestrial plant species (especially grasses), yet a recent surge of research has shown that some species of aquatic plants contain significant amounts of Si too. We argue that degree of Si accumulation is a functional trait in aquatic vegetation, with plants adapting to environmental conditions. Aquatic vegetation can show apparent plasticity regarding Si uptake, adaptive to water and wind dynamics, light interception, herbivory and nutrient stress. Beyond a plant physiological viewpoint, high Si uptake results in high BSi in plant litter, which can impact on aquatic decomposition processes. Si content in aquatic vegetation shows intriguing relations with other strength components such as cellulose and lignin. Si content has also been linked to fungal and microbial community, litter stoichiometry and invertebrate shredders: all factors that potentially influence organic turnover in aquatic sediments. Uptake of Si by aquatic vegetation is thus not only an important transient sink for Si in the global biogeochemical Si cycle, it can also affect carbon turnover in aquatic ecosystems. Experimental and field studies should be conducted to elucidate controls on aquatic plant Si uptake, especially focusing on interactive effects of multiple biotic and abiotic factors. This review provides an overview of the state-of-the-art knowledge on silicon in aquatic vegetation.

Resistance and reconfiguration of natural flexible submerged vegetation in hydrodynamic river modelling

In-stream submerged macrophytes have a complex morphology and several species are not rigid, but are flexible and reconfigure along with the major flow direction to avoid potential damage at high stream velocities. However, in numerical hydrodynamic models, they are often simplified to rigid sticks. In this study hydraulic resistance of vegetation is represented by an adapted bottom friction coefficient and is calculated using an existing two layer formulation for which the input parameters were adjusted to account for (i) the temporary reconfiguration based on an empirical relationship between deflected vegetation height and upstream depth-averaged velocity, and (ii) the complex morphology of natural, flexible, submerged macrophytes. The main advantage of this approach is that it removes the need for calibration of the vegetation resistance coefficient. The calculated hydraulic roughness is an input of the hydrodynamic model Telemac 2D, this model simulates depth-averaged stream velocities in and around individual vegetation patches. Firstly, the model was successfully validated against observed data of a laboratory flume experiment with three macrophyte species at three discharges. Secondly, the effect of reconfiguration was tested by modelling an in situ field flume experiment with, and without, the inclusion of macrophyte reconfiguration. The inclusion of reconfiguration decreased the calculated hydraulic roughness which resulted in smaller spatial variations of simulated stream velocities, as compared to the model scenario without macrophyte reconfiguration. We discuss that including macrophyte reconfiguration in numerical models input, can have significant and extensive effects on the model results of hydrodynamic variables and associated ecological and geomorphological parameters.

Silicon Affects Nutrient Content and Ratios of Wetland Plants

PurposeThe main objective of this study is to enhance our knowledge about the interplay between the plant Si content and the plant nutrient content as well as the nutrient stoichiometry for a broad range of submerged and emergent wetland plants.MethodsWe investigated the carbon, nitrogen, phosphorus and silicon content of 16 species (10 submerged and 6 emergent) along a transect within the Biebrza river (Poland) at 10 locations for the years 2006 and 2007. In addition, the relationships between silicon and carbon, nitrogen and phosphorus content as well as the calculated nutrient ratios (C/N, C/P and N/P) were analyzed.ResultsWe found evidence for a significant relation of Si to major nutrients carbon and nitrogen (negative correlation) and phosphorus (positive correlation) within wetland plants. Our results show that especially submerged plants are able to substitute carbon compounds in a significant share by silicon compounds. Nitrogen however is partially substituted by Si for emergent plants. This change in plant nutrient content in turn alters the nutrient stoichiometry of C : N : P.ConclusionOur data is the first showing a possible important effect of plant Si content on plant nutrition and nutrient cycling in wetlands. Plant Si has to be considered in plant ecology, physiology and biogeochemistry of wetland plants, because of its effects on the plant nutrient state. Furthermore, Si may also possibly affect the carbon and nutrient turnover via altered plant nutrient content and subsequently via litter decomposition.

Mapping of Submerged Aquatic Vegetation in Rivers From Very High Resolution Image Data, Using Object Based Image Analysis Combined with Expert Knowledge

The use of remote sensing for monitoring of submerged aquatic vegetation (SAV) in fluvial environments has been limited by the spatial and spectral resolution of available image data. The absorption of light in water also complicates the use of common image analysis methods. This paper presents the results of a study that uses very high-resolution image data, collected with a Near Infrared sensitive DSLR camera, to map the distribution of SAV species for three sites along the Desselse Nete, a lowland river in Flanders, Belgium. Plant species, including Ranunculus peltatus, Callitriche obtusangula, Potamogeton natans L., Sparganium emersum R. and Potamogeton crispus L., were classified from the data using object-based image analysis and expert knowledge. A classification rule set based on a combination of both spectral and structural image variation (e.g. texture and shape) was developed for images from two sites. A comparison of the classifications with manually delineated ground truth maps resulted for both sites in 61% overall accuracy. Application of the rule set to a third validation image resulted in 53% overall accuracy. These consistent results not only show promise for species-level mapping in such biodiverse environments but also prompt a discussion on assessment of classification accuracy.