Effects of Prolonged Flooding on Nektonic Invertebrates in Small Diked Marshes

Abstract

This study examined the response by nektonic invertebrates to prolonged flooding of a series of experimental marshes in the Delta Marsh in south-central Manitoba. Prior to flooding, nektonic invertebrate densities, biomass, and numbers of taxa were at their highest levels during midsummer in open-water sites with dense submersed vegetation; during the same period, nekton levels within the experimental marshes were minimum in stands of emergent vegetation. These differences appear to be related to available habitat structure and dissolved oxygen concentrations. Flooding to 1 m above normal eliminated the submersed vegetation and existing emergent vegetation. This resulted in a single large input of litter at the time of flooding with little or no subsequent input during flooding. The herbivore–detritivore functional group reached high levels early in the first year of flooding and remained high until late in the second year of flooding. The predator–parasite group did not exhibit a similar increase following flooding. Changes in the amount of plant litter available as both food and habitat affected these changes in the nekton following flooding. Wind-induced water currents also probably had an effect on nekton distributions during the flooding period.