Andreas Hussner

Growth response and root system development of the invasive Ludwigia grandiflora and Ludwigia peploides to nutrient availability and water level.

Ludwigia grandifl ora and Ludwigia peploides, both native to South America, are aquatic plants, which favourably grow in stagnant or slow fl owing waters. Due to their rapid growth both species are considered two of the most important aquatic weeds worldwide. The species have invaded several countries in SW Europe and often cause problems, e.g. in France, The Netherlands or Belgium. In the present study, growth and root system develop- ment of L. grandifl ora and L. peploides was investigated under three different hydro regimes and three different substrates with different nutrient contents. Both species showed their strong abilities to grow on drained soils by an increase of the relative amount of root biomass and changes in the root system development. Relative growth rates (RGR) of L. grandifl ora and L. peploides did not differ between the species, but were signifi cantly differ- ent amongst the treatments for each species, i.e., RGR were signifi cantly infl uenced by nutrients (both p < 0.001) and water level (L. grandifl ora n.s., L. peploides p < 0.001). L. grandifl ora and L. peploides showed a signifi cant increase in the relative amount of roots with decreasing water level (both p < 0.001) and nutrients (both p < 0.001), when the relative amount of shoots decreased signifi cantly (both: water level p < 0.001 and nutrients p < 0.001). In addition, both species responded to water level and nutrients with signifi cant changes in the depth distribution of their root system. In conclusion, L. grandifl ora and L. peploides showed a high tolerance to different water levels, which may be important for potential habitat distribution in the future.

From first reports to successful control: a plea for improved management of alien aquatic plant species in Germany.

Alien aquatic plant species can strongly affect all types of freshwater ecosystems. Their number has more than doubled between 1980 and 2009 in Germany, and currently 27 are known and their number is still increasing. Eleven have been classified as invasive, but only four are managed yet, mainly by weed cutting. Most of the alien aquatic plant species were probably introduced as aquarium and pond waste. Despite this fact, 18 of the 27 known alien species are traded as ornamentals for aquaria or garden ponds in German shops. Alien species can most successfully be controlled when their management starts as soon as possible after their introduction. In Germany, the delay between first records and start of management actions seems too long for successful control. The public awareness of alien aquatic plants and problems they can cause in Germany is still limited despite a number of recent projects. At present, Black lists are developed that help nature conservationists, stakeholders and politicians to select those alien species for which prevention measures should be implemented. These, however, are not legally binding and laws regulating trade in Black listed plant species are strongly needed to reduce their impact on the environment and economy.

Hygraula nitens, the only native aquatic caterpillar in New Zealand, prefers feeding on an alien submerged plant

Hygraula nitens is a New Zealand native moth with aquatic larvae that feed on submerged aquatic plants. The larvae have been mainly observed using native Potamogeton and Myriophyllum species as a food source, although some studies reported larvae feeding on the alien macrophytes Hydrilla verticillata, Lagarosiphonmajor and Ceratophyllum demersum. Experimental mesocosm studies showed larvae had a major effect on H. verticillata, C. demersum, L. major, Elodea canadensis and Egeria densa. In both no choice and choice experiments H. nitens larvae showed a clear preference for and the highest consumption of C. demersum, while the native macrophyte Myriophyllumtriphyllum ranked fourth out of five alien and two native plant species, indicating a preference of the larvae for alien macrophytes. Additional choice experiments using C. demersum, sampled from different waters in NZ, illustrated that there was a clear difference in H. nitens preference for plants based on their source. However although C. demersum had the lowest leaf dry matter content (LDMC) compared with the other macrophytes, neither the LDMC nor leaf carbon, nitrogen, phosphorus or total phenolic contents alone could explain the preferences of H. nitens, and we conclude that food choice is based on a combination of these and/or additional factors.

Sensitive response of sediment-grown Myriophyllum spicatum L. to arsenic pollution under different CO2 availability

Aquatic plants may face resource constraints or anthropogenic pollution, and effects might be heightened under multiple stress conditions. We investigated if arsenate effects on Myriophyllum spicatum L. would be stronger under CO2 limitation and low phosphorus availability. In a factorial design, we exposed sediment-grown plants to either CO2 (high carbon or HC) or bicarbonate (low carbon or LC) and four levels of arsenate. We observed strong effects of arsenate exposure on growth, biomass allocation (leaf, stem and root mass fractions), pigments and phenolic compounds. CO2 availability strongly affected the content in phenolic compounds and a few other response variables, yet overall effects were less pronounced than expected. Strong interactive effects of CO2 availability and arsenic concentration were only observed for carotenoids, the carotenoid/chlorophyll ratio and phenolic compounds in leaves. Only the carbon content declined with increasing arsenic concentration, otherwise leaf elemental content and stoichiometry were not affected by arsenic or CO2 availability, suggesting that plants strived to maintain leaf functions. The observed effects on biomass allocation and plant quality, specifically dry matter content and phenolic compound content of M. spicatum not only show direct changes in plant performance but suggest also indirect effects on ecological interactions such as competition or herbivory.