Spatial and temporal variability in the phytoplankton community of the Western Arabian Gulf and its regulation by physicochemical factors and zooplankton

Phytoplankton abundance can be affected both by physicochemical factors such as nutrients, temperature or light availability (bottom-up control), and by the abundance of predators (top-down control). Phytoplankton research in the Arabian Gulf has so far focusedon bottom-up control of the phytoplankton community, this study was the first to include top-down control as well. High abundances of the three diatom genera Chaetoceros, Guinardia and Bacteriastrum were observed locally during summer. Other important phytoplankton genera observed in this study included the cyanobacteria Trichodesmium and Nostoc, the diatom Leptocylindrus, and the dinoflagellate Prorocentrum. Whereas Trichodesmium was characteristic for the main body of the Arabian Gulf, from Khafji to Dhahran, Nostoc, Leptocylindrus and Prorocentrum characterized the phytoplankton community in the southern waters, the Gulf of Salwa and Half Moon Bay. This is a high salinity environment that is separated from the main body of the Arabian Gulf by a shallow sill. Here, we observed a seasonal change in the phytoplankton community, with a dominance of Nostoc and Leptocylindrus during summer, followed by a dominance of Prorocentrum during winter. Chaetoceros and Bacteriastrum showed significant correlation with the abundance of copepod Oithona brevicornis and the ghost shrimp Belzebub hanseni. Nostoc, Leptocylindrus and Prorocentrum all showed significant correlation with salinity and the concentration of total suspended solids.