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This study tested whether the extreme scarcity of larger nanophytoplankton and microphytoplankton in the Gulf of Aqaba and in the open northern Red Sea is caused by nutrient limitation or by selective removal by grazers. Samples of near surface phytoplankton were incubated on board under a fully factorial combination of release from grazing pressure and release from nutrient stress. Release from grazing pressure by different size classes was obtained by sieving through 100, 20, and 10 µm size mesh screens. Release from nutrient stress was obtained by enrichment of Si alone and a full enrichment by N, P, Si and trace elements. Growth rates of most phytoplankton taxa showed a strong, positive response to the full nutrient enrichment and a weaker, but significant response to grazer exclusion. Several diatom taxa showed a weak positive response to Si enrichment. Thus, bottom-up control of medium-sized algae appears to be more important than top-down control.

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