Short-term variability in primary productivity during a wind-driven diatom bloom in the Gulf of Eilat (Aqaba)

Abstract

In the northern Gulf of Eilat (Aqaba), sharp increases in the biomass of diatoms and rates of primary production occurred in April 2008. Within 24 h, diatom abundance rose from 8 x 10(3) to 228 x 10(3) cells l(-1), and photosynthetic rates concomitantly doubled from 15 to 35 mu g C l(-1) d(-1). Water transparency declined, as indicated by the vertical diffusion attenuation coefficient K-d for photosynthetically active radiation (PAR), which increased from 0.076 to 0.090 m(-1) and decreased the euphotic depth from 60 to 45 m. During this time, a significant increase in silica deposition by the diatoms was also detected. We attribute the mentioned changes in environmental characteristics to wind-generated surface currents. Strong winds (up to 10 m s(-1)) during the measurements enriched the surface layers with unusually high nutrient concentrations within <1 d. Hence, primary production rates were observed at a relatively eutrophic nearshore station (MP) and pelagic station (A(1), 10 km towards the center of the Gulf). They were compared with rates measured on the subsequent day. Values measured were twice as high as those at the pelagic station during the previous calm day. Routine monitoring programs with monthly or semi-weekly sampling are thus likely to miss brief but significant injections of nutrients, leading to the underestimation of seasonal and annual primary production. Our results demonstrate the impacts of transient events on the function and annual yield of aquatic ecosystems.