Succession of heterotrophic and mixotrophic dinoflagellates as well as autotrophic microplankton in the harbour of Alexandria, Egypt

In the Eastern Harbour of Alexandria, three trophic categories of microplankton, heterotrophic dinoflagellates (HTD), mixotrophic dinoflagellates (MTD) and obligately autotrophic microplankton (OA) were studied in terms of species richness, numerical standing crop, biovolume and carbon biomass during the summer of 1999. Although represented by a small number of species, MTD proved to be the most important trophic category, contributing more than half the standing crop and the carbon biomass. Almost all blooms were dominated by the MTD species Prorocentrum minimum and Scrippsiella trochoidea. The HTD and the OA were richer in species but lower in biomass. Mixotrophic and autotrophic cells < 30 mum contributed 98% of the respective biomass, while 90% of the heterotrophic biomass was contributed by cells > 50 mum.