Primary production in the northern Red Sea

Rates of uptake of carbon and nitrogen (ammonium, nitrate and urea) by phytoplankton, along with concentrations of nutrients and chlorophyll a, in the Saudi Arabian waters of the northern Red Sea (23 °N-28 °N) were measured in autumn, 2012. Concentrations of nitrate, nitrite and phosphate within the euphotic zone were in trace amounts while those of silicon were in excess of 0.5μmolL^-1. Concentrations of chlorophyll (Chl a) were very low within the euphotic zone (0.01-0.6μgL^-1 at discrete depths and 1.53-21.5mgm^-2 as column-integrated values). A deep chlorophyll maximum and a nitrite maximum were present between 60 and 80m at almost all of the stations occupied. Rates of carbon uptake at discrete depths ranged from 0.02 to 3μgCL^-1h^-1. Chl-normalized carbon uptake rates related with ambient light in a Michaelis-Menten kinetic pattern. About 80% of the carbon uptake was attributable to the <20μm fraction. Ammonium and urea were the nitrogen compounds taken up in preference by phytoplankton and accounted for close to 90% of the total N uptake. Considered together, these results indicate that the waters of the northern Red Sea are oligotrophic and that the primary production is strongly N-controlled. Analyses of the data and interpretation of the results led to the following speculations: (1) the perceived north-south gradient in Chl a (and possibly in primary production) in the Red Sea is maintained by circulation of Chl- and nutrient-rich waters through a series of gyres, (2) there is a greater role for heterotrophy and microbial loop in the trophic dynamics, and (3) in situ nitrification in the euphotic zone is an important source of N for phytoplankton and consequently export of carbon to deep sea could be lesser than that indicated by f-ratios.