THE IMPACT OF INVASIVE SUBMERGED WEED SPECIES ON SEED BANKS IN LAKE-SEDIMENTS

Seed banks within submerged sediments were sampled from 21 New Zealand lakes with varying degrees of invasion by vegetatively reproducing, a dventive hydrocharitacean species, to examine the influence of vegetat ion type on seed bank characteristics. Seed number and seed species ri chness were significantly lower at sites where the submerged vegetatio n was dominated by the adventive weeds Elodea canadensis Michaux, Eger ia densa Planchon or Hydrilla verticillata (Linn. f.) Royle, compared with a predominantly native vegetation. The greater the degree of excl usion of native seed producing plants by adventive weed beds, the grea ter the apparent impact upon seed abundance, In contrast, seed numbers beneath the adventive weed beds did not differ significantly from sit es in lakes where a large scale decline in submerged vegetation had ta ken place subsequent to hydrocharitacean invasion. The observed impact of weed invasion on lake sediment seed banks is attributed to reduced in situ seed production over time and may be accentuated by increased sediment accumulation beneath tall growing, high biomass weed beds. A s seed density influences the speed and extent of germination and plan t establishment, our results suggest a large reduction in the re-veget ation potential of invaded lakes. Seed bank decline may also provide a n additional 'ecological feedback mechanism' contributing to the maint enance of stable, turbid de-vegetated lakes.