Present and long-term changes of phytoplankton communities in hypertrophic Mediterranean lagoon, Lake Manzala, Egypt.

摘要

This work was carried out to throw light on the present status of phytoplankton and the long-term changes resulted from heavy nutrients loading during the period 1979-2007. Based on cell density, present phytoplankton populations were dominated by chlorophytes followed by cyanoprokaryotes, whereas bacillariophytes ranked the third predominant positions. On cell biovolume base, Bacillariophytes were the most dominant class. Bacillariophytes were dominated by centric forms, Cyclotella spp and small pinnate forms, Nitzschia spp, whereas chlorophytes and cyanoprokaryotes were dominated by pico-coccoid taxa. Long-term changes of chlorophyll a showed a duplication from 34 micro g/l in 1979 to 78.2 micro g/l in 1987/88, followed by a climax of 988.3 micro g/l, in 2007. Regressing the pooling data of chlorophyll a with TP and TIN indicated that chlorophyll a was significantly (P<0.001) correlated with TP, R2=0.45, rather than TIN, R2=0.14. Regarding phytoplankton community composition, Bacillariophyceae was the main component of phytoplankton during 1979, followed by a graded waning till 2007 when they harbored their least percentage abundance of 17.8%. Chlorophyceae and cyanoprokaryotes were of a significant importance in 1979. During the eutrophication phase, cyanoprokaryotes became the dominant algal group till year 2000, whereas chlorophytes still unchanged till 1987/88 followed by a slight increase during 2000, then they became the main component of the phytoplankton abundance during 2007. In the early years, the filamentous and large diatoms dominated. Later, however, small cells appeared in 1987/88 and recently they become more dominant. Cyanoprokaryotes during the commencement of the eutrophication were represented by large filamentous Spirulina sp and heterocystous Anabaena sp, Chroococcus and Microcystis appeared thereafter and became dominant during 2000. Among chlorophytes, Tetraspora, Scenedesmus and Pediastrum contributed significantly in late 1970s. After the peak of eutrophication, Pediastrum and Tetraspora declined whereas Scenedesmus and other pico-chlorococcales increased, specifically Dictyosphaerium spp till 2007.