Ahmed¹, Saiyed I., A. Ali², S. Amjad², N. Bano², N. Khan², M. Nisa², S.H.N. Rizvi² and M. Saleem²

¹Department of Marine Science and Fisheries, Sultan Qaboos University, P.O. Box 34, Al- Khod 123, Muscat, Sultanate of Oman, Tel: 968-515-191 and ²National Institute of Oceanography, ST-47, Block 1, Clifton, Karachi 75600, Pakistan.

Summary and conclusions from the NASEER (North Arabian Sea Environmental and Ecosystem Research) cruises, conducted between Karachi, Pakistan and Muscat, Oman to examine impacts of monsoonal-driven physical forcing effects on the dynamics of biological and chemical parameters

The annual cycle of monsoonally driven changes in Chl_a, size-fractionated primary productivity (PP), zooplankton abundance and composition together with changes in other associated physical and chemical parameters were measured during several NASEER cruises as the Pakistan national contribution to the International JGOFS program. The cruises were between Karachi and Muscat on a 1,500 nautical mile track covering 64 ocean observation stations and 6 time series stations in the North Arabian Sea.

The SW monsoon in August 1992 with the associated strong winds and nutrient upwelling produced shallow mixed layer depths (20 – 50m) and the highest rates of PP (1640 mg C/m²/day) at Station 45 off the Omani shelf near Muscat. During the SW monsoon small picoplankton species (<1.0 micrometer size fraction) clearly dominated the larger phytoplankton (>1.0 micrometer size fraction) at Station 45 where the highest surface concentrations of Chl_a were observed. During the NE monsoon in January 1992, the mixed layer deepened to 80 – 100 m due to convective cooling and mixing resulting in the entrainment of higher nutrient concentrations. A subsurface Chl_a maximum at about 50m depth at Station 45 was also observed. PP at Station 45 and overall during the NE monsoon in January, 1992 was lower than the values observed during the SW monsoon. However, consistent with the US JGOFS observations, overall PP rates were higher in the North Arabian Sea than those observed in many other oceanic regions. Similarly, the zooplankton biomass (105ml/100m³) collected during the turbulent SW monsoon was on average 17% higher than that recorded during the calmer NE monsoon (89ml/100m³). Thus, the primary forces of wind driven mixing and upwelling during the summer period in concert with special physical features such as eddies and filaments together with winter induced convective cooling and mixing provide the necessary settings for the expression of high productivity so characteristic of the North Arabian Sea.