Falling into pieces: In situ fragmentation rates of submerged aquatic plants and the influence of discharge in lowland streams

Abstract

The formation and dispersal of viable plant fragments are considered as key determinants for the vegetative spread potential and competitive strength of submerged aquatic macrophytes. Although it is known that the disturbance by flow facilitates fragment dispersal in streams, detailed information on in situ fragmentation rates and the influence of discharge are still lacking. We determined the fragmentation rates (i.e. number of fragments per biovolume of a given species) of the four widespread aquatic plant species Myriophyllum spicatum, Potamogeton crispus, Elodea canadensis and Elodea nuttallii in defined sections of small to medium-sized German streams with different flow regimes. We further measured chlorophyll a fluorescence (Fv/Fm: maximum quantum yield of photosystem II) to denote the viability of plant fragments collected in the field. We documented stream- and species-specific differences in fragmentation rate, ranging from 74 ± 20 (P. crispus) up to 1260 ± 376 fragments m−3 specific biovolume d-1 (E. canadensis; ± SE). Fragmentation rates generally increased in streams characterized by rather high discharge conditions but were negligibly associated with minor discharge fluctuations at a given stream section. This effect was significant for M. spicatum and E. nuttallii, but not for P. crispus and E. canadensis. Overall, a high portion of fragments was viable, as indicated by Fv/Fm values > 0.58 in 95% of all fragments. Our results demonstrate that fragmentation rates of submerged aquatic plants are to a certain extent controlled by the discharge conditions of a stream and highlight the strong vegetative spread potential of the four species studied.