Frantic Spawners in a Lazy River: When Sea Lampreys Match Hydrology as a Factor of Sediment Mobility and Morphological Changes

ABSTRACT

A wide range of aquatic or semi‐aquatic animal species can greatly modify their environments by acting as river ‘engineers’. While there has been much focus on certain species (e.g., crayfish, salmon), other species have rarely been studied, such as sea lampreys. Because sea lampreys spawn outside of high‐water periods, they are a good species to investigate to assess the contribution of hydraulic vs. biotic factors to morpho‐sedimentary processes. In addition, unlike salmon, lamprey can reproduce in low‐ to medium‐energy rivers. We thus analysed the influence of sea lampreys as an engineer species and quantified their relative contribution to physical morpho‐sedimentary processes in a low‐energy river. We hypothesised that because these systems have low sediment inputs and hydraulic capacity, sea lampreys contribute greatly to morphogenic processes. We studied the mobility of riverbed sediments by using passive integrated transmitters and surveying riverbed topography with centimetric horizontal and vertical accuracy in spawning grounds at two sites. During spawning, lampreys locally modified the topography by generating multi‐decimetre forms, moving sediments by a mean of more than 1 m and helping to diversify the riverbed's particle‐size distribution. In turn, their nests greatly influenced the frequency and distance of sediment movement during winter floods. The results indicated that lamprey activity has at least as much influence as the action of moderate winter floods (< Q ₂ ) does, and that lamprey activity is more frequent and regular than floods and can reshape the riverbed greatly (> Q ₂ ). Thus, conserving lamprey populations is a challenge for maintaining or restoring morpho‐sedimentary and biotic processes and restoring the physico‐chemical processes of aquatic environments, particularly in low‐energy rivers.