Hydraulic effects of vessel-induced waves on early life stages of lake macrophytes

Abstract Background The impact of vessel-induced waves on macrophyte communities in lakes remains controversial, due to a lack of comprehensive assessments which also consider mechanistic effects on ecological processes during early life stages. This study investigates both the direct and indirect effects of such waves on the early life stages of macrophytes in a case study of Lake Wörthersee, Austria. The study focuses on Characeae species and Najas intermedia which have both experienced significant declines in Lake Wörthersee. Results The linear wave theory was applied to model typical small vessel-induced waves, characterized by wave heights of 0.05–0.30 m and periods of 1 and 3 s. Relevant characteristics for testing remobilization of oospores and seeds like geometric dimensions and density were experimentally determined by field data. Sediment samples from ten locations across six beach sites at depths ranging from 0.8 to 2.2 m were collected and analyzed for oospore presence and sediment texture. Results indicate that maximum wave scenarios can affect oospores and seeds directly by motion activation at water depths down to 3.75 m. Moderate wave scenarios, which are assumed to occur more frequently, can mobilize those particles between 0.75 and 2.25 m water depth. This corresponds to our field data, where 95% of oospores were found in samples from water depths ≥ 2 m. The mobilization disrupts germination processes and impedes the recovery of macrophyte populations. Additionally, in the study lake and other fine-sediment dominated lakes, a significant fraction of sediments can be initiated to motion by small vessel-induced waves, which may indirectly affect germination by altering habitat conditions like turbidity and nutrients. However, grain size distribution showed minimal variation with depth at the same sampling sites, indicating limited sediment redistribution at the selected sampling sites by small vessel-induced waves. No significant correlation was found between oospore abundance and sediment grain size. Conclusions These findings suggest that the impacts of increased traffic of small vessels in lakes may be a key factor contributing to the decline of macrophyte species in shallow waters by motion triggering of oospores and seeds, with broader implications for the recreational management.