Sphagnum bleaching: Bicarbonate 'toxicity' and tolerance for seven Sphagnum species.

Growth and functioning of Sphagnum mosses are closely linked to water level and chemistry. Sphagnum mosses occur in wet, generally acidic conditions, and when buffered, alkaline water is known to negatively impact Sphagnum. The effects of time, dose and species-specific responses of buffered, alkaline water on Sphagnum are largely unknown. We investigated the effects of bicarbonate and calcium on the survival, growth and physiological functioning of seven Sphagnum species occurring in contrasting environments, from raised bogs to (rich) fens. Mosses were submerged in different concentrations of bicarbonate and calcium solutions for 10 weeks under climate-controlled circumstances. After 2 weeks, all species exposed to the high bicarbonate treatment (2.0 mM) showed severe potassium leakage and swift discoloration. In contrast, species showed differential responses to the intermediate bicarbonate treatment (0.8 mM), some with a later onset of potassium leakage. S. squarrosum, S. teres & S. contortum generally persisted the longest, with all species dying after 6 to 10 weeks. Calcium alone, in contrast, negatively affected S. squarrosum, S. teres & S. contortum, causing discoloration and potassium leakage. Our study shows enrichment with bicarbonate, but not calcium, is detrimental for most Sphagnum species tested. A mechanistic model was developed that is consistent with dose and duration dependence and the species specificity. Future conservation and restoration measures for Sphagnum-dominated habitats and Sphagnum farming (cultivation, production and harvest of Sphagnum moss biomass) should limit flooding with bicarbonate-rich waters while investigating new management options, like acidifying surface waters to lower bicarbonate levels.

Radial oxygen loss by the cushion plant Eriocaulon schimperi prevents methane emissions from an East-African mountain mire.

Groundwater-fed fens are known sources of methane (CH4 ) emissions to the atmosphere, and these are known to be mediated by the vegetation. In a fen located in the Bale Mountains, Ethiopia, we assessed the effects of a cushion plant (Eriocaulon schimperi) and a sedge (Carex monostachya) on rhizosphere biogeochemistry. Methane and CO2 concentrations and pH were measured in pore-water at different depths in the profile. Redox potentials and NaCl-extractable element concentrations were analysed in soil samples from sites dominated by either E. schimperii or C. monostachya. Nutrient and element concentration were analysed in plant tissues. At Carex-dominated sites, CH4 concentrations increased from 70 µmol·l-1 at a depth of 10 cm to 130 µmol·l-1 at a depth of 100 cm. CH4 concentrations at Eriocaulon-dominated sites were almost zero (<1 µmol·l-1 ) to a depth of 100 cm. Simultaneously, soil redox potentials and CO2 concentrations were higher at Eriocaulon-dominated sites, indicating a low potential for CH4 production and a high potential for CH4 oxidation. Eriocaulon schimperi displayed a root investment strategy to cope with the harsh environment, similar to the cushion plant Astelia pumila in Patagonian bogs. This strategy is characterised by high root/shoot ratios, high root porosity and density under high redox conditions. Both cushion plant species create an aerobic rhizosphere through radial oxygen loss from deep roots, which strongly reduce CH4 fluxes to the atmosphere.

Differential responses of two wetland graminoids to high ammonium at different pH values.

Enhanced soil ammonium (NH4+) concentrations in wetlands often lead to graminoid dominance, but species composition is highly variable. Although NH4+ is readily taken up as a nutrient, several wetland species are known to be sensitive to high NH4+ concentrations or even suffer toxicity, particularly at low soil pH. More knowledge about differential graminoid responses to high NH4+ availability in relation to soil pH can help to better understand vegetation changes. The responses of two wetland graminoids, Juncus acutiflorus and Carex disticha, to high (2 mmol·l(-1) ) versus control (20 µmol·l(-1) ) NH4+ concentrations were tested in a controlled hydroponic set up, at two pH values (4 and 6). A high NH4+ concentration did not change total biomass for these species at either pH, but increased C allocation to shoots and increased P uptake, leading to K and Ca limitation, depending on pH treatment. More than 50% of N taken up by C. disticha was invested in N-rich amino acids with decreasing C:N ratio, but only 10% for J. acutiflorus. Although both species appeared to be well adapted to high NH4+ loadings in the short term, C. disticha showed higher classic detoxifying responses that are early warning indicators for decreased tolerance in the long term. In general, the efficient aboveground biomass allocation, P uptake and N detoxification explain the competitive strength of wetland graminoids at the expense of overall biodiversity at high NH4+ loading. In addition, differential responses to enhanced NH4+ affect interspecific competition among graminoids and lead to a shift in vegetation composition.

Early warning indicators for river nutrient and sediment loads in tropical seagrass beds: a benchmark from a near-pristine archipelago in Indonesia.

In remote, tropical areas human influences increase, potentially threatening pristine seagrass systems. We aim (i) to provide a bench-mark for a near-pristine seagrass system in an archipelago in East Kalimantan, by quantifying a large spectrum of abiotic and biotic properties in seagrass meadows and (ii) to identify early warning indicators for river sediment and nutrient loading, by comparing the seagrass meadow properties over a gradient with varying river influence. Abiotic properties of water column, pore water and sediment were less suitable indicators for increased sediment and nutrient loading than seagrass properties. Seagrass meadows strongly responded to higher sediment and nutrient loads and proximity to the coast by decreasing seagrass cover, standing stock, number of seagrass species, changing species composition and shifts in tissue contents. Our study confirms that nutrient loads are more important than water nutrient concentrations. We identify seagrass system variables that are suitable indicators for sediment and nutrient loading, also in rapid survey scenarios with once-only measurements.

Limited toxicity of NH(x) pulses on an early and late successional tropical seagrass species: interactions with pH and light level.

Seagrasses have declined at a global scale due to light reduction and toxicity events, caused by eutrophication and increased sediment loading. Although several studies have tested effects of light reduction and toxicants on seagrasses, there is at present no information available on their interacting effects. In a full-factorial 5-day laboratory experiment, we studied short-term interactive effects of light conditions, pH and reduced nitrogen (NH(x)) in the water layer, mimicking pulses of river discharge, on the tropical early successional species Halodule uninervis and the late successional species Thalassia hemprichii. In contrast to recent results reported for the temperate species Zostera marina, increased NH(x) supply did not affect leaf mortality or photochemical efficiency in H. uninervis and in 7 out of 8 treatments for T. hemprichii. However, both tropical species demonstrated striking differences in nitrogen accumulation, free amino acid composition and free NH3 accumulation. The increase in tissue nitrogen content was two times higher for H. uninervis than for T. hemprichii. Nitrogen stored as free amino acids (especially asparagine) only increased in H. uninervis. High pH only affected T. hemprichii, but only when not shaded, by doubling its free NH3 concentrations, concomitantly decreasing its photosynthetic efficiency. Our results indicate that the early successional H. uninervis has higher tolerance to high NH(x) loads as compared to the late successional T. hemprichii. H. uninervis was better able to avoid toxic internal NH(x) levels by further assimilating glutamine into asparagine in contrast to T. hemprichii. Moreover, both tropical species seem to cope much better with high NH(x) than the temperate Z. marina. The implications for the distribution and succession of seagrass species under high nutrient loads are discussed.