Key-words:Lake Poyang, tributaries, chlorophyll a, light, water retention timeVariations in phytoplankton chlorophyll a were studied in Lake Poyang, and these variations were compared with those observed in its tributaries. Samples were collected from January 2009 to January 2013 during the dry, mid-dry and wet seasons. The study showed that chlorophyll a concentrations were significantly higher in the wet season than in the dry and mid-dry seasons (p < 0.0001) in the lake; a comparative analysis of Lake Poyang and its tributaries showed significant differences in chlorophyll a between the two ecosystems during the wet season but not during the dry and mid-dry seasons. During the wet season, the underwater light conditions were significantly elevated and water retention time increased substantially (25.5 d). In combination, these factors contributed to a significantly higher chlorophyll a concentration during the wet season than that during the dry and mid-dry seasons in the lake; the effect of nutrients on the phytoplankton was not obvious. The large variations in water retention time during the three seasons indicated that Lake Poyang has the combined characteristics of a river and a lake. The short retention times during the dry and mid-dry seasons produced no difference in chlorophyll a between the lake and its tributaries, whereas the long retention time during the wet season facilitated phytoplankton growth in the lake. ces facteurs a contribué à une augmentation significative de la concentration en chlorophylle a pendant la saison des pluies ; l'effet des nutriments sur le phytoplancton n'était pas évident. Les grandes variations dans le temps de rétention de l'eau au cours des trois saisons impliquent que le lac Poyang combine les caractéristiques d'une rivière et d'un lac. Les temps de rétention courts pendant les saisons sèche et mi-sèche n'entraînent aucune différence en chlorophylle a entre le lac et ses affluents, tandis que le temps de rétention long pendant la saison des pluies facilite la croissance du phytoplancton dans le lac.

Key-words:Yangtze River, floodplain lake, macrozoobenthos, community structure, sand mining Lake Poyang is the largest freshwater lake in China and contains unique and diverse biota within the Yangtze floodplain ecosystem. However, knowledge of its macrozoobenthic assemblages remains inadequate. To characterize the current community structure of these assemblages and to portray their decadal changes, quarterly investigations were conducted at 15 sites from February to November 2012. A total of 42 taxa were recorded, and Corbicula fluminea, Limnoperna fortunei, Gammaridae sp., Nephtys polybranchia, Polypedilum scalaenum and Branchiura sowerbyi were found to dominate the community in terms of abundance. The bivalves Corbicula fluminea, Lamprotula rochechouarti, Arconaia lanceolata and Lamprotula caveata dominated the community in biomass due to their large body size. The mean abundance of the total macrozoobenthos varied from 48 to 920 ind·m −2 , the mean biomass ranged from 28 to 428 g·m −2 . The substrate type affected strongly the abundance, biomass, and diversity of the macrozoobenthos, with muddy sand substrates showing the highest values. Compared with historical data, remarkable changes were observed in the abundance of macrozoobenthos and the identity of the dominant species. The mean total abundance decreased from 724 ind·m −2 in 1992 to 228 ind·m −2 in 2012. The dominant species have shifted dramatically. Large unionids were dominant before 1998, whereas pollution-tolerant species (e.g., Branchiura sowerbyi) increased in dominance after 2008. Our findings should have implications for the conservation of the benthic biodiversity of this large Yangtzeconnected lake. RÉSUMÉLa structure des communautés et les changements décennaux dans les assemblages du macrozoobenthos du lac Poyang, le plus grand lac d'eau douce en Chine Mots-clés :fleuve Yangtze, Le lac Poyang est le plus grand lac d'eau douce en Chine et contient des biotes uniques et diversifiés de l'écosystème de la plaine inondable du Yangtzé. Cependant, la connaissance de ses assemblages macrozoobenthiques reste insuffisante. Pour caractériser la structure de la communauté actuelle de ces assemblages et présenter leurs changements décennaux, des inventaires