RESPONSES OF A PRAIRIE WETLAND TO PRESS AND PULSE ADDITIONS OF INORGANIC NITROGEN AND PHOSPHORUS - INVERTEBRATE COMMUNITY STRUCTURE AND INTERACTIONS

Bottom-up experimental manipulation of a wetland food web via press or pulse nutrient additions in Delta Marsh, Canada, demonstrated a diffe rential response among primary producers and associated invertebrate g razers. Microinvertebrate grazers, dominated by Ceriodaphnia dubia, ef fectively depressed phytoplankton biomass in control, press, and pulse enclosures in the absence of fish predators. Similarly, microinverteb rate grazers increased in density in response to increased availabilit y of periphyton on acrylic rods in both press and pulse nutrient treat ments. Subsequently, proliferation of inedible filamentous green algae , in part as a consequence of size-selective herbivory, especially wit h the press nutrrient additions, led to a marked decline in grazer den sity. Macrophyte-associated invertebrates, mainly chydorid cladocerans , increased in abundance in parallel with macrophyte biomass changes a nd reduced epiphyton biomass through most of the season. Metaphyton sh ading (primarily in the press treatment) led to macrophyte decline and eventual decomposition, and substantial reduction in phytophilous inv ertebrate density. These grazer-algal interactions support the hypothe sis of top-down control by grazers on algae (phytoplankton and epiphyt on) as a regulatory mechanism in macrophyte-dominated aquatic ecosyste ms. The transition from epiphyton-dominated to metaphyton-dominated we tland is shown to be facilitated by herbivory, nutrient addition, and continuous availability of nutrients.