In shallow lakes and ponds, macrophytes are believed to play a pivotal role in creating the alternative stable states through their effects on phytoplankton abundance. However, it remains uncertain how macrophytes affect on phytoplankton. Several studies suggested that other than competitive interactions for light and nutrients, macrophytes have adverse effects on phytoplankton though the allelopathy. However, the magnitudes of allelopathic effects on phytoplankton assemblages are rarely examined at community level. Moreover, few studies have examined empirically if phytoplankton abundance related negatively with macrophyte abundance.
To clarify the impacts of allelochemicals on natural phytoplankton assemblages, we performed incubation experiments at Cornell University Research Ponds Facility (CURPF). We cultured in-situ algal assemblages from the ponds with and without exudates of Chara vulgaris and/or Elodea nuttallii, which are reported to have allopathic effects on some of phytoplankton species. In addition, we quantified abundances of macrophytes and phytoplankton in summer for total 35 ponds in CURPF to examine if there are negative relationship among these abundances. In this talk, we present these results and discuss allelopathic effects of macrophytes on phytoplankton assemblages and if macrophytes have indeed adverse effects on phytoplankton abundance.