| Popis: |
The effect of nutrient addition, macrophyte removal and fathead minnow addition on the invertebrate community and habitat quality of Delta Marsh was assessed using 'in situ' enclosures in the Blind Channel. Factors important in determining the stable state of the marsh were chosen as treatments (nutrient addition, submersed macrophyte removal, and fathead minnow addition). The clear water stable state, characterized by low turbidity, low phytoplankton biomass and abundant submersed macrophytes, is most likely when nutrient loading is low, macrophytes are abundant, and top-down control from planktivorous fish is low. The turbid water state, characterized by high turbidity, high phyloplankton biomass and few submersed macrophytes, is most likely when nutrient loading is high, submersed macrophyte biomass is sparse, and top-down control is high. Inorganic nutrient addition (N and P) was found to cause phytoplankton blooms, and thus turbid conditions when submersed macrophyle biomass was relatively low. However, nutrient addition did not cause phytoplankton blooms or turbid conditions when submersed macrophytes were abundant. Addition of fathead minnows resulted in decreased densities of microinvertebrates, and thus a greater biomass of phytoplankton, due to decreased grazing pressure via the trophic cascade. Submersed macrophytes did not provide a refuge for zooplankton from predation by planktivorous young of the year fathead minnows. |
