Seasonal changes in the abundance of the herbivorous moth larvae Acentria ephemere/la on submersed macrophytes in Lake Constance were studied between 1998 and 2000. Acentria were found feeding on many pondweeds (Potamogeton lucens, P. pectinatus, P. perfoliatus), Ceratophyllum demersum and Myriophyllum spicatum, but not on Chara spp., Najas marina ssp. intermedia or Elodea nuttallii. During the vegetated periods of 1998 and 2000 the abundance of larvae on P. perfoliatus and M. spicatum increased exponentially from approx. 10 ind.lm 2 to a maximum of 10,000 ind.lm 2 or 20-165 ind.lg dm. The much lower abundance in 1999 may be a result of the massive flooding of the lake in spring. Our data suggest that at least part of the Acentria population in Lake Constance may have two generations per year. Larvae use predominantly the upper shoot sections during the active feeding period and move to the lower stern sections for winter diapause. Herbivory caused substantial damage especially to the apical meristems of P. perfoliatus and M. spicatum. This study indicates that aquatic shredder-herbivores may have astronger impact on submersed macrophytes than previously considered.

Epiphytes cause a strong light attenuation for submersed macrophytes. Various abiotic and biotic factors determine epiphyte density, among them nutrients, water clarity and possibly allelopathy. We compared epiphyte biomass on the major occurring macrophytes, among them allelopathically active species in four shallow eutrophic lakes in Upper Swabia (Southern Germany). We determined epiphyte chlorophyll a, organic carbon, nitrogen and ashfree dry mass. Mean epiphyte chlorophyll a per macrophyte ranged between 100 and 400 µg g −1 dm in summer and increased up to 1700 µg g −1 dm in autumn. Few differences between species and lakes were observed. No apparent relation was found between epiphyte density and physical or chemical variables of the lakes. Epiphytes had a molar stoichiometry of organic carbon to nitrogen (C:N) slightly below 7:1, a ratio considered to be optimal also for benthic algae. Lakes with higher concentrations of humic compounds exhibited a significant higher ratio of organic carbon to epiphytic chlorophyll a, indicating a higher proportion of heterotrophs in the epiphyte layer. In one lake, one phenotype of Elodea nuttallii exhibited a significant higher C:chl a ratio than other macrophytes. Allelopathy as a factor responsible for this difference is discussed.