Abstract
Dominance of harmful pennate diatom Pseudo-nitzschia delicatissima was observed during a regular environmental monitoring study carried out in the coastal waters of Jeddah, Saudi Arabia. Neither any discoloration of water nor any fish kill was reported during the period of this specific phenomena. This diatom species dominated the area with a higher percentage (97.5%) of the total phytoplankton community encountered. Apart from these, the occurrence of harmful algal bloom causing heterotrophic dinoflagellates was also noticed in a few numbers. Phytoplankton biomass in terms of chlorophyll a obtained was 4.1 mg m(-3) and the silicate values (1.92 mu mol l(-1)) were comparatively higher than the other inorganic nutrients. Zooplankton abundance was relatively low (705 ind. m(-3)) and were dominated mainly by copepods (79.9%) followed by mollusc and decapod larvae.