Scarcity of medium-sized phytoplankton in the northern Red Sea explained by strong bottom-up and weak top-down control

This study tested whether the extreme scarcity of larger nanophytoplankton and micro-phytoplankton in the Gulf of Aqaba and in the open northern Red S ea is caused by nutrient limitation or by selective removal by grazers. Sam ples of near surface phytoplankton were incubated on board under a fully fa ctorial combination of release from grazing pressure and release from nutri ent stress. Release from grazing pressure by different size classes was obt ained by sieving through 100, 20, and 10 mu m size mesh screens. Release fr om nutrient stress was obtained by enrichment of Si alone and a full enrich ment by N, P, Si and trace elements. Growth rates of most phytoplankton tax a showed a strong, positive response to the full nutrient enrichment and a weaker, but significant response to grazer exclusion. Several diatom taxa s howed a weak positive response to Si enrichment. Thus, bottom-up control of medium-sized algae appears to be more important than top-down control.