Shading constrains the growth of invasive submerged macrophytes in streams

Abstract

Submerged macrophytes are an important component of stream ecosystems but invasion by non-native species can threaten the natural structure and function of these systems. Restoration of riparian shading may help to regulate the growth of macrophytes, particularly invasive species, which often proliferate in open streams. In a large, flowing outdoor mesocosm we studied the establishment and colonization success of three non-native macrophyte species (Egeria densa, Elodea canadensis and Ceratophyllum demersum) and one native charophyte (Nitella spp. aff. cristata) that commonly occur in stream systems. Plant shoots were grown at five shade levels (35%, 63%, 79%, 90% and 95%) for eight weeks with regular monitoring of growth, morphology and biomass accrual. All species were able to establish at all shading levels. At high shade (≥ 90%) we found a reduction in lateral spread and branching and an increase in main stem length for the non-native species, while Nitella did not show any response for these traits. Biomass accumulation was delayed at high shade for all species. At the end of the experiment, the highest biomass accrual was observed at the lowest shade level (35%) for E. densa and E. canadensis, at 63% shade for C. demersum and at 79% shade for Nitella. Our results confirm, and provide additional examples, that both native and non-native submerged macrophytes can establish and colonize streams that are highly shaded but that the growth of non-native species, in particular, can be limited. Our study also highlights that there are differences in shade tolerance among non-native species.