Effects of macrophytes on phytoplankton: nutrient uptake versus allelopathy

remainsdifficulttoiso-late as a mechanism (Gross et al. 2007).One potentially confounding factor is competitionbetween macrophytes and phytoplankton for nutrients. Forrooted macrophytes, sediments may be the principal site foruptake of phosphorus and nitrogen as well as iron, manga-nese, micronutrients, and trace metals (Barko et al. 1991).Availability of these same elements for phytoplanktongrowth within macrophyte stands may be low due to uptakeby epiphyton or incr eased nitrification (Korner 1999) anddenitrification (Weisner et al. 1994). Nonrooted macro-phyte species cannot use the sediment nutrient pool, and dis-solved products important for macrophyte growth are takenup principally from the water column (Barko et al. 1988).Though plants may not be a nutrient (especially phosphorus)sink over the long-term (weeks to months; Lombardo &Cooke 2003), net foliar uptake in the short term (days), atwhich allelopathy experiments are often performed, may bequite high (Pelton et al. 1998, Lombardo & Cooke 2003).The nutrient constraint on phytoplankton due to uptake bymacrophytes is likely more significant in the lower end of thenutrient regime (Jeppesen et al. 1999).Coexistence experiments testing allelopathic effects ofmacrophytes on phytoplankton are therefore often performedat very high nutrient concentrations, although sensitivity ofphytoplankton to allelochemicals is expected to be higherunder additional stress such as nutrient limitation (Reigosaet al. 1999, Hilt et al. 2006). Inderjit & Del Moral(1997) wondered whether separating resource competitionfrom allelopathy is at all realistic.Here we analyze the nutrient dynamics observed in labora-tory-scale coexistence experiments that targeted allelopathiceffects of