Studies on the transfer of lead in an intertidal food-chain

Abstract

The levels of lead contamination in the intertidal sediments and flora at Lindisfarne, NE England, were studied from September 1990 to September 1991. The daily throughput of lead for Wigeon Anas Penelope was estimated, and liver lead burdens were determined. Around the upper shore, the mean density of spent shot in the top 10 cm of sediment was 53.1 pellets/m(^2), the product of many seasons' wildfowling activity. The density encountered was higher than any previously reported from wildfowling sites in Britain. An experiment was established to examine settlement and degradation rates of lead shot under natural intertidal conditions. Settlement rates were slow, except in loose sand. Degradation was also slow. Only 2% weight loss occurred in thirteen months. The levels of chemically available lead in the sediments were determined by acid extraction. The grain size composition was the principal factor explaining variation in lead concentration between sites. The highest level recorded was 16.2 ppm. Lindisfarne is therefore considered to be a site of low to moderate lead contamination. The lead concentrations in Zostera spp. were higher during Autumn 1990 than during Summer 1991. This may be due to seasonal effects or yearly differences in growth rates. During Autumn 1990, when they formed the main food of Wigeon, Zostera leaves contained around 20 ppm lead. Enteromorpha and Salicornia contained around 14 and 8 ppm lead respectively. The lead content of Wigeon faeces was around 15 ppm. It was estimated that the difference between the quantity of lead ingested and excreted in the faeces for a Wigeon feeding on Zostera leaves was 1.06 mg per day. Only 8% of a sample of Wigeon shot at Lindisfarne had a liver lead concentration above 5 ppm dry weight. Such low liver lead concentrations, generally considered as normal background levels, are inconsistent with the estimated lead retention rate.