Phytoplankton productivity during mixing season in the Gulf of Aqaba

Abstract

The Gulf of Aqaba is an enclosed, desert-surrounded oligotrophic sea with high water transparency and almost no terrestrial influx. The gulf receives its waters from the Red Sea through the Straits of Tiran, a narrow and shallow sill. Unlike in the northern Red Sea, in the Gulf of Aqaba a deep vertical winter mixing occurs with far reaching consequences for phytoplankton productivity. From our data we extracted the following pattern: Nitrate usually is depleted within the mixed layer, and consequently the overall phytoplankton productivity is nutrient limited in the Gulf of Aqaba even during the mixing season. Not every year the critical depth is passed, and daily phytoplankton production becomes light limited. If light limitation occurs, nitrate shows up in the mixed layer as a result of fast regeneration due to high water temperatures in general. If the weather conditions are such that the mixing is interrupted or stops in this situation, high production events are triggered. These can last maximum about 5 days. The consequences of these findings for production estimates in the Gulf of Aqaba will be discussed.