Abstract

  Seawater samples were collected weekly over fourteen months at 25 m intervals between the surface and 200 m from the offshore waters of the Gulf of Aqaba, Red Sea. Water temperature and dissolved oxygen concentration and percent saturation were measured in situ. Within two hours of sampling, samples were analysed for chlorophyll a, ammonia, nitrate, nitrite, phosphate and silicate. The temperature field depicted a well defined seasonal pattern of winter mixing from December to April and summer stratification from May to November. All other analysed parameters were intimately related to this pattern. Dissolved oxygen showed a maximum and homogeneous distribution in winter and a minimum in summer. Chlorophyll a showed a seasonal pattern close to that of dissolved oxygen, but with a distinct summer peak between 50 and 75 m. Ammonia was absent from the entire water column during summer and relatively abundant and homogeneously distributed in winter. Nitrite had a seasonal pattern similar to that of chlorophyll a and exhibited a summer subsurface maximum just below that of chlorophyll a. Nitrate, phosphate and silicate had similar seasonal patterns characterised by high concentrations in deeper water during summer overlaid by vanishingly low concentrations of nitrate and phosphate and relatively low in the case of silicate. In winter the three nutrients exhibited relatively high concentrations homogeneously distributed in the entire water column. These findings are analysed and discussed with reference to previous records from the Gulf of Aqaba and other oligotrophic water bodies.

Keywords: oligotrophic, primary productivity, mixing-stratification, thermocline