Effect of freshwater flow on the succession and abundance of phytoplankton in Rosetta Estuary, Egypt

Patterns in the spatial and temporal composition, dominance and abundance of the phytoplankton community in Rosetta Estuary were studied during one year at seven different sampling stations. A total of 152 taxa was recorded, of which Bacillariophyta (67 taxa) contributed the highest portion of the total number of taxa, followed by Chlorophyta (41 taxa), Cyanophyta (18 taxa) and Dinophyta (16 taxa). The main phytoplankton associations were characterized by green algae and diatoms, in distinctive patterns of dominance. Fresh and marine water fluxes are responsible for phytoplankton heterogeneity. There is a gradual reduction in phytoplankton abundance towards the mouth of the estuary. The inner stations showed two bloom periods; the first in spring and the other in winter, both associated with low water salinities resulting from discharge of fresh Nile water into the estuary. The phytoplankton dynamics of the outer stations were characterized by highly spatial variable system.

Species diversity ranged from 0.3 to 3.37, with the highest index in May. Phytoplankton standing crop was observed to have a negative effect on species diversity.

Scenedesmus, Selenastrum, Actinastrum, Crucigenia, Cyclotella, Nitzschia and Skeletonema were possibly the most prevalent phytoplankton genera in the estuary at different periods.

The similarity analysis demonstrated two assemblages, the first one appeared in the outer stations with similarity of 60–72%, and the inner stations with similarity 72–80%. The inner stations had pronouncedly high annual average counts and were characterized by more chlorophytes occurrence, reached >63% at stations 5 and 6, and the less productive outer stations, by the appearance of brackish water species of dinophytes (32%, at station 3).