The lethal and sublethal effects of the aquatic macrophyte Myriophyllum spicatum on Baltic littoral planktivores

Macrophyte architecture can structure predator-prey interactions, but it is the chemicals within the plant that may actually be lethal. We conducted aquarium experiments to study the effects of common aquatic macrophytes (Myriophyllum spicatum, Myriophyllum sibiricum, and Chara tomentosa) and a predator (perch, Perca fluviatilis) on the survival, habitat choice, swimming, and feeding activities of Baltic littoral planktivores, mysids Neomysis integer and Praunus flexuosus, and three-spined stickleback (Gasterosteus aculeatus) larvae. Chemicals excreted by M. spicatum in a dense patch caused high mortality (73% to 89%) in both mysid species but not in sticklebacks, whereas M. sibiricum and C. tomentosa had no lethal effects. In lower stem densities stickleback larvae and N. integer avoided M. spicatum even in the presence of predator signals, and M. spicatum lowered the swimming and feeding activities of stickleback larvae. Only P. flexuosus did not avoid M. spicatum vegetation. Areas occupied by M. spicatum seem to be highly unsuitable habitats for littoral mysids and three-spined stickleback larvae. Because M. spicatum is a dominant macrophyte in the study area and eutrophication further increases its abundance, it may strongly influence the occurrence and distribution of mysids and fish larvae in the littoral ecosystems of the Baltic Sea.