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Importance of flood regime to invertebrate habitat in an unregulated river–floodplain ecosystem

Unlike most large rivers of the northern hemisphere, several medium-sized rivers in the southeastern USA Coastal Plain remain unregulated. These smaller rivers possess 2 habitat types (snags and floodplain) that were historically important for invertebrate assemblages in many rivers and are strongly dependent on flood regime. I reviewed and compared 2 models of habitat inundation (snags and floodplain) that were developed for the Ogeechee River (Georgia, USA) to understand the ecological significance of these habitats. These models showed that snag habitat surfaces varied from only ∼20 to 50% of channel bottom surfaces, but floodplain inundation varied from 0 to 37 times the width of the channel at 100% inundation. Long-term analysis of inundation patterns from a 58-y record of discharge demonstrated that substantial flooding occurred almost annually for 1 to 2 mo/y. Habitat-specific invertebrate biomass was highest on snags (mostly aquatic insects), followed by the main channel (dominated by Corbicula), and then the floodplain (oligochaetes, crustaceans, aquatic insects). After correction for total amount of habitat surface area, invertebrate biomass contributions were highest in the floodplain > main channel > snag. However, arthropods and oligochaetes, the most likely prey of higher trophic levels, were clearly dominant on snags and in the floodplain. In many rivers around the world, invertebrate productivity from snags and floodplains is likely to have been significantly diminished because of snag removal, channelization, and floodplain drainage for >2 centuries. Understanding the interaction between flood regime and invertebrate habitat in unregulated rivers like the Ogeechee River can serve as a benchmark in restoration efforts.