Physical and chemical properties of seawater during 2013–2015 in the 400 m water column in the northern Gulf of Aqaba, Red Sea

This research investigated physical (temperature, salinity, and density) and chemical (dissolved oxygen, ammonium, nitrate, nitrite, phosphate, and silicate) properties of offshore seawater in the Red Sea northern Gulf of Aqaba; Jordanian Site were measured during 2013–2015 to assess the temporal and seasonal variation of the upper 400 m of the water column. The study also investigated seasonal variations, assessing the relationships of temperature with physical and chemical parameters. The average value of temperature for all data was 23.03 ± 1.58 °C. Temperature followed an expected seasonal cycle during 2013–2015, with well-mixed conditions in the upper 400 m of the water column during spring (Feb–Apr) and stratification during summer (Jul–Aug). There were no significant differences among years for temperature, but highly significant differences among months and depths. The average value of salinity (psu) for all data was 40.60 ± 0.10 with significant positive or negative differences among years, months, and depths. In general, dissolved oxygen, ammonium, nitrate, nitrite, and phosphate data showed positive or negative significant differences among months and depths with no significant annual variations. Silicate only showed significant differences among depths. Correlation tests between temperature and other parameters in the upper 25 m of the water column revealed significant inverse-relationships between temperature and all other parameters (other than salinity) that were attributed to the dominant thermal controls on seawater density, to the thermodynamic controls on oxygen solubility and to seasonal increases in light irradiance that allowed nutrient consumption by primary producers. In the intermediate water column (100–150 m), similar correlations were found as in the 0–25 m data, except for silicate. In the deeper waters (300–400 m), only salinity, density, and phosphate showed significant correlations with temperature, and indicated that the seasonal effects of primary production at depth were minimal. In general, the values of all parameters during the years 2013–2015 in the upper 400 m were comparable with previous studies (e.g., 1998–2003). In conclusion, this research manifested the strong correlation of temperature with some chemical parameters and presumed seasonal controls on primary production. Given the general lack of interannual variation, water quality in the northern Gulf of Aqaba appears relatively stable