Inundated floodplain gravels in a stream with an unstable bed: Temporary shelter or true invertebrate refugium?

Abstract

At near‐peak flow of a large flood in a New Zealand stream, we took 10 Surber samples in calm, shallow areas of an inundated flood plain. The next day, when discharge was still high enough to cause bed movement in the main channel but the flood plain had dried out again, we collected 10 sediment samples in the dry flood plain. Invertebrate taxon richness in during‐flood samples was 83% of that found in the main channel after a period of stable flow, and total invertebrate densities were 10% of densities during stable conditions. Taxon richness, total densities, and densities of the four taxa most common in the benthos of the river were significantly lower in the dry flood plain than in during‐flood samples, implying that many invertebrates left these patches during drying. Nevertheless, mean total invertebrate densities in the dry flood plain were still 37% of during‐flood values. We conclude that inundated floodplain gravels provided temporary shelter for lotic invertebrates during the flood, but several animals were trapped as the flood plain dried, and those that managed to return to the baseflow channel were still exposed to bed‐moving shear stresses. We doubt that floodplain gravels can act as true invertebrate refugia in our study river, primarily because of the unstable nature of its sediments.

Keywords:

• stream ecology
• disturbance
• invertebrates
• refugia
• floodplain gravels
• streambed stability

Notes

Corresponding author.