Responses of vascular (Egeria densa) and non-vascular (Chara globularis) submerged plants and oospores to contrasting sediment types

In this study, we investigated the growth responses of a native, non-vascular charophyte (Chara globularis var. globularis Thuill.) and an invasive vascular angiosperm (Egeria densa Planch.) on three contrasting New Zealand lake sediment types (fine silt from beneath charophyte beds, sand from an exposed shoreline and low density organic-rich substrate from beneath Ceratophyllum demersum L. beds) under low and high light conditions (6 and 15% ambient). In a shallow outdoor trough, the growth responses of transplanted C. globularis and E. densa plants were examined over a 12-month period and the germination response of C. globularis oospores over a 5-month period. Under low light, growth of both species, but particularly C. globularis, was poor and no significant differences were observed between sediment types. Under high light, growth of C. globularis plants was strongly inhibited (by 96 and 49%) on the sand and C. demersum sediments relative to the charophyte sediment (320 g DM m−2 y−1). However, germination and short-term (<20 weeks) survival of C. globularis oospores was comparable between sediment types (8-12%, no significant differences). Since growth of E. densa was not similarly affected by C. demersum sediment (955, 1051 and 270 g DM m−2 y−1 on C. demersum, charophyte and sand sediments), this could indicate higher tolerance of unfavourable conditions associated with organic-rich sediments (inhibitory substances and/or low density substrate) by vascular submerged plants relative to non-vascular species.