Aquatic ecosystems are increasingly stressed not only by increased nutrient loads (eutrophication) but also by changing forms and proportions of nutrients. Nutrient enrichment, composition and stoichiometry interact with aquatic food web dynamics in complex ways. Both algal species composition and emergent properties within species change with changing nutrient composition, in turn affecting food webs at all levels. Consumers further regulate - and may even accelerate - discrepancies in nutrient stoichiometry by various feedbacks, release,.and recycling pathways. Stoichiometric regulation of aquatic ecosystem structure also occurs at the sediment interface via altered biogeochemical processes and benthic food webs when nutrient composition changes. Thus, multiple feedbacks serve to alter food web structure when nutrient loads are altered. Such feedbacks may also lead to conditions conducive to invasive species and altered stable states as illustrated for the San Francisco Bay Delta and the Rhine River.