Contrasting Effects of Increasing Invasive Crayfish Densities on Competing Submerged Macrophytes in Shallow Lakes

1. Non-native crayfish are among the most widespread aquatic invasive species worldwide. High densities of crayfish in lakes can potentially cause complete loss of macrophytes through herbivory, physical destruction, shading by suspension of sediments and nutrient release facilitating phytoplankton, thus altering ecosystem state and functions. However, lower crayfish densities may affect different macrophyte species in different ways, thus altering competition between macrophyte species with different growth forms. We hypothesize that high crayfish densities can completely destroy submerged macrophyte communities and shift systems into more turbid conditions whereas low crayfish densities can promote low-growing macrophytes that compete with canopy-forming species.
2. To test these hypotheses, we applied three different densities (2, 4, 8 crayfish m⁻²) of the invasive crayfish Procambarus clarkii and a control (no crayfish) to 36 mesocosms of monocultures and mixed stands of low-growing and canopy-forming submerged macrophytes for 60 days. We measured concentrations of nutrients, suspended solids, phytoplankton and periphyton and above- and belowground biomass of low-growing Vallisneria natans and canopy-forming Hydrilla verticillata.
3. In addition, we conducted a meta-analysis of crayfish impacts on macrophytes to compare effect sizes between different crayfish densities. Using data from published studies, we derived the relationship between crayfish density and crayfish-induced reductions in macrophyte abundance.
4. Our meta-analysis revealed a significant decline of macrophytes with increasing crayfish densities. In line with existing studies, high crayfish densities caused a strong loss of both low-growing and canopy-forming macrophytes and increased nutrient concentrations, phytoplankton biomass and suspended sediment, thus favouring an ecosystem shift to more turbid conditions. Low and medium crayfish densities reduced macrophyte biomass similarly for both species in monocultures. However, in mixed cultures, low crayfish densities (2 crayfish m⁻²) reduced the above- and belowground biomass of the canopy-forming H. verticillata more than that of low-growing V. natans, supporting our hypothesis. This support of low-growing macrophytes occurred despite a temporal facilitation of shading by increased periphyton biomass.
5. Our results show that the impact of crayfish invasions on aquatic ecosystems is strongly dependent on their density. High densities facilitate the loss of macrophytes and increase the likelihood of ecosystem collapse into a turbid state. However, low densities can provide a competitive advantage to low-growing macrophytes that would otherwise be outcompeted by canopy-forming species. Although complete eradication is often impossible, management of invasive crayfish can target densities that have potentially positive effects on invaded ecosystems.