论中国农村留守儿童形成根源与解决机制

当代中国农村留守儿童问题是我国社会转型时期的一个独特社会问题,这是由我国大量农村剩余劳动力不断涌向城市直接引起的.由于种种原因,农村留守儿童普遍无法接受与其他城市儿童一样良好的教育,于此各种社会问题随之而生并引起了社会的广泛关注.阐述了留守儿童的社会现状和形成根源,并提出了解决留守儿童问题的解决机制.     

Study of seasonal settlements of five scallop species in the western Mediterranean

Scallop settlements were examined over a 16 month period using collectors placed in the sea at 28 day intervals near Torre la Sal Beach, Castellón, Eastern Spain in the western Mediterranean. Collectors were set over depths of 20 m and five species, Flexopecten flexuosus, Chlamys varia, Aequipecten opercularis, Pecten jacobaeus and Palliolum incomparabile, were collected. The most frequent species, F. flexuosus, had peak settlements in August 1992 and in July 1993. C. varia had peaks in April and July 1992 and in June 1993. Both of these species settled throughout the year, except in winter. A. opercularis settled predominantly from February to June and in October, but with a few settlements during the rest of the year. Pecten jacobaeus had a small peak in February and another peak from April to July. P. incomparabile spat were rare. Settlements were studied in relation to chlorophyll a, suspended particulate and sea temperature data in the area of study.     

Nitrogen dynamics in the settlement ponds of a small-scale recirculating shrimp farm (<Emphasis Type="Italic">Penaeus monodon</Emphasis>) in rural Thailand

Rural shrimp farmers in Thailand are being encouraged to adopt practices that will reduce the quantity and improve the quality of their effluent. A simple and cheap option for small-scale shrimp farmers is to use settlement ponds to store and remediate discharge water before being re-used. We undertook a detailed study of the settlement ponds in a small-scale commercial black tiger shrimp farm typical of rural Thailand. We found that over a 9-week period, following the harvest of one of the two farm production ponds, total nitrogen (TN) concentrations in the water column were reduced by 30%, with the greatest removal (56%) occurring during the fifth week. There was a 10% increase in dissolved organic nitrogen (DON) concentrations during the trial. Sediments were a source of total ammonia nitrogen (TAN), and the re-mineralisation rate was the highest in the first two settlement ponds. Coconut fronds added to two of the four settlement ponds to increase the surface area available for microbial activity were found to provide a site for microbial re-mineralisation of TAN, the photosynthetic uptake of TAN and oxidised nitrogen (NO _x ) and nitrification. The water column was a net assimilator of TAN through autotrophic uptake. This study has shown that settlement ponds are capable of reducing water column N concentrations; however, sediment must be managed to reduce re-mineralisation during successive cropping cycles. In addition, coconut fronds were shown to improve N removal, although they should be periodically removed to maintain efficiency.