Nutrient gradients in the Red Sea : The coastal area off Jeddah and the central Red Sea

The present study deals with two different environments of the Red Sea: The polluted coastal waters off the Jeddah Metropolitan Area (JMA), Saudi Arabia, and the strictly oligotrophic waters of the central Red Sea. Daily loads of total nitrogen and phosphorus from JMA amount to 6,564 kg N and 2,241 kg P, respectively. Steep gradients prevail nearshore, ranging from hypertrophic city lagoons to oligotrophic open water offshore. N:P ratios, as well as the comparison of nutrient levels with half-saturation constants, suggest that nitrogen is the limiting factor restricting primary production. A numerical model based on the Delft3D Modelling System was developed and validated using field data. Scenario runs to study the impact of changes in nutrient loads showed that modification of nitrogen loads led to a more pronounced effect on coastal phytoplankton biomass than changes in phosphorus loads. An 80% reduction in nitrogen discharges led to a 57% decrease in both nutrient and chlorophyll-a concentrations in a confined area in the inner Jeddah bight.

Along a transect in the central Red Sea a common feature were the oligotrophic inorganic nitrogen and phosphorus concentrations in the upper mixed layer with a N:P ratio well below the Redfield value. Nitrate and phosphate surface concentrations were in most cases below the detection limit but increased substantially below the upper mixed layer. Chlorophyll-a concentrations were very low in the upper mixed not exceeding 0.1 ug L-1. There was a typical deep chlorophyll maximum which reached up to 0.5 ug L-1. Surface photomineralization of dissolved organic nitrogen averaged 1.7 umol N m-2 d-1. When compared to other inorganic nitrogen sources, photomineralization is a process bringing only moderate amounts of bioavailable nitrogen to the N-limited environment prevailing in the surface waters of the central Red Sea.


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