Effects on river macroinvertebrate communities of tsunami propagation after the 2011 Great East Japan Earthquake

Abstract

Summary The 2011 Great East Japan Earthquake generated extreme tsunami waves that entered river mouths, causing strong and long‐distance propagation along the water corridors. This study reports the first empirical survey of this rare and extreme ecological impact of tsunami propagation on river communities. We sampled macroinvertebrate communities at 15 river sites on the Sendai Plain in Northeast Japan along a gradient of tsunami disturbance intensity [i.e. altitude: range = 1–29 m above sea level (a.s.l.)] two and 16 months after the earthquake. In comparison with data collected before the tsunami, we found evidence of significant reductions in taxon richness (−54% on average) and total abundance (−91%) after the tsunami in inundated river reaches (up to 25 km from the river mouth). There were large spatial variations in local impacts, with altitudes of 7–9 m a.s.l. separating heavily damaged inundated communities from intact non‐inundated communities. The degraded communities exhibited variable recovery rates extending beyond 16 months after the tsunami. Mobility trait analysis revealed a relatively rapid recovery of swimmers (e.g. Baetis), reflecting their high mobility and active drift behaviours in the recolonisation phase. We observed the rare phenomenon of ‘upstream drift’ of marine worms (Phyllodocidae) to freshwater rivers in three of the 10 inundated sites. These survived in fresh water for up to 14 months, suggesting that some marine‐derived immigrants can persist in fresh water for extended periods.