Flux of lead in submerged plants and its relevance to a freshwater system

Abstract

The fluxes of lead in submerged freshwater angiosperms, mosses and benthic algae were investigated by laboratory and field observations. Laboratory experiments showed that the initial uptake of lead by angiosperm shoots and Navicula sp. was rapid (up to 0.07 mg Pb g −1 dry weight min −1 from a solution containing 1.0 mg Pb l −1), and over-whelmingly passive. Mosses were especially efficient at sorbing lead from solutions containing less than 1.0 mg Pb l −1 . Rates of uptake increased with rising external concentrations, but decreased with time. Anatomical and morphological characteristics of plant tissues affected metal flux and an epiphytic covering of Navicula (i.e., an Aufwuchs community) sorbed metal and reduced uptake into tissues by a third. Up to 90% of the lead sorbed in the first hour by shoots of Elodea canadensis Michx from a solution containing 1.0 mg Pb l −1 was released within 14 days of transfer into a lead-free medium; 10% was irreversibly bound. Moss monitors placed in Ullswater, a lake with sediments containing up to 26 mg Pb g −1 dry weight, confirmed that the metal is brought into circulation during turbulence, but not in periods of calm. Shoots of Elodea collected from the lake contained up to 0.3 mg g −1 dry weight non-exchangeable lead. Young shoots tended to contain less than older shoots (0.12 compared with 0.3 mg Pb g −1), and leaf more than stem tissues. The fluxes of lead from the environment through Aufwuchs and submerged plants are discussed.