The impact of invasive submerged weed species on seed banks in lake sediments

Abstract

Seed banks within submerged sediments were sampled from 21 New Zealand lakes with varying degrees of invasion by vegetatively reproducing, adventive hydrocharitacean species, to examine the influence of vegetation type on seed bank characteristics. Seed number and seed species richness were significantly lower at sites where the submerged vegetation was dominated by the adventive weeds Elodea canadensis Michaux, Egeria densa Planchon or Hydrilla verticillata (Linn. f.) Royle, compared with a predominantly native vegetation. The greater the degree of exclusion of native seed producing plants by adventive weed beds, the greater the apparent impact upon seed abundance. In contrast, seed numbers beneath the adventive weed beds did not differ significantly from sites in lakes where a large scale decline in submerged vegetation had taken 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. As seed density influences the speed and extent of germination and plant establishment, our results suggest a large reduction in the re-vegetation potential of invaded lakes. Seed bank decline may also provide an additional ‘ecological feedback mechanism’ contributing to the maintenance of stable, turbid de-vegetated lakes.