More than half of the species of British prosobranchs living intertidally have planktonic larvae. Settlement involves a positive response to certain factors which results in a speedy loss of structures fulfilling larval needs and the introduction of others associated with locomotion and feeding on the sea-bed (Fretter, 1972). Unless this response relates to the needs of the species, settlement could lead to mass mortality of larvae settling on unsuitable ground.
In February/March, 1993, a range of water-quality variables were measured in nine freshwater lakes at different locations in Region XI, Chile . Seven of the lakes, situated between the Rio Ibanez (46°08'S) and Lago Lapparent (46 ° 14'S), are characterised by low concentrations of minerals and nutrients, similar to the oligotrophic lakes of the Araucanian district of Chile (39 °S to 42 °S) .Sulphate concentrations were disproportionately high at each of these sites and it is proposed that this results from the deposition of volcanic ash following the eruption of Volcan Hudson in August, 1991 . The concentration of sulphate found in a ninth lake well to the north of the ash deposition zone was 35 fold lower than those typically measured within the main study area.Lago Lapparent is a large `blue' lake receiving glacial sediments, that forms the southern boundary of the area studied . Concentrations of minerals and nutrients found at this site were generally even lower than those found in the other seven lakes within the area . Organochlorine pesticides were not detected at any of the sites after solid phase extraction of 250 ml water samples .
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