International Journal of Oceans and Oceanography

The interaction between phytoplankton and zooplankton was studied weekly in Boughaz El-Maadia, a mixing area connecting Lake Edku to Abu-Qir Bay on the Egyptian Mediterranean coast. Zooplankton grazing has been postulated to play at times an effective role in regulating the phytoplankton cycle and causes losses in its standing crop. In spite of large weekly variations in the abundance and community structure, phytoplankton and zooplankton usually showed an obvious inverse relationship during different periods. However, dense accumulation of phytoplankton seems to be due to the existence of unfavorable phytoplankton species to be grazed by the existing zooplankters. The grazing pressure appeared to be affected by the environmental conditions, like temperature, as well as the species composition of both phytoplankton and zooplankton communities. The multiple regression analysis indicated that some zooplankton species had no food selectivity, such as the rotifers Brachionus plicatilis, Br. urceolaris, the cladoceran Moina micrura, and cirripede larvae, while others showed clear selectivity to certain phytoplankton species, such as the rotifers Br. calyciflorus, Synchaeta pectinata, the copepod nauplii and adult copepod species Oithona nana, Paracalanus parvus, Euterpina acutifrons, Acanthocyclops americanus and Halicyclops magniceps. Rotifers were more effective grazers than copepods. However, both groups had strong correlation with the dominant phytoplankton groups (diatoms, blue green and green algae). The abundance of phytoplankton as a whole, and that of different groups were affected by dissolved oxygen, nitrite, ammonium, phosphate and temperature, while zooplankton appeared to be influenced by phytoplankton biomass, salinity and temperature.