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Seasonal variability of size-classes of phytoplankton biomass in a sub-tropical embayment, Muscat, Sea of Oman
Al-Hashmi K.; Claereboudt M.; Piontkovski S.; Al Azri A.; Amin S.M.N.

Abstract
The contribution of three different cell size classes of picoplankton: 0.74-2 μm, nanoplankton:2–20 μm and microplankton, >20 μm of the phytoplankton population and their relationship to environmental conditions were studied over two annual cycles at one station in Bandar Khyran Bay, Sea of Oman, from May 2006 to August 2008. Nanoplankton was the most important class contributing 54.4% to total Chl a (range 6-82%). Its seasonal highest concentrations was during the cold periods when temperature ranged from 28-29 °C in fall and near 24 C in winter when the supply of nutrients was sufficient to sustain their growth. Picoplankton had the second level of the contribution, comprising (23.5%, range 4-74 %) of the total Chl a. and their concentration was generally constant (0.04-.06 μg l–1) throughout the study period. The drop of picoplankton population coincided with an increase in the microplankton and nanoplankton populations indicating a high grazing pressure exerted on the picoplankton population. Microplankton size-class occupied the third level of the contribution comprising (22.2%, range 3-65 %). Their general concentration was below 0.1 μg l–1 and only dominant when temperatures were lowest and nitrate, nitrite, silicate and phosphate concentrations were the highest. The temporal variability observed was associated with changes in the nanaoplankton indicating that in some cases, it is the small fraction of phytoplankton that drives changes in abundances and productivity.
Keywords: Phytoplankton, Chlorophyll a, Size-class, Picoplankton, Nanoplankton, Upwelling, Sea of Oman

Seasonal and Interannual Changes of Indian Oil Sardine, Sardinella longiceps Landings in the Governorate of Muscat (the Sea of Oman)

SERGEY A. PIONTKOVSKI, HAMED S. AL-OUFI, and SAUD AL-JUFAILI

ABSTRACT
Monthly data on Muscat’s landings of the Indian oil sardine, Sardinella longiceps, along with 23 environmental parameters ( sea surface temperature, temperature of the mixed layer, wind speed, kinetic energy of mesoscale eddies, concentration of nitrates, dissolved oxygen, chlorophyll-a, abundance of phytoplankton, zooplankton, and several others) were analyzed for the period 1994–2011. Seasonal changes were associated with the time of t h e winter (Northeast) monsoon, with maximal landings in February. The multiple regression analysis of the statistically signifi cant variables selected through the Principal Component Analysis has implied that 51% of seasonal variability in sardine landings might be approximated by the seasonal variations of the zonal component of wind speed and chlorophyll-a concentration in the coastal and open-sea regions. In terms of interannual changes, sardine landings exhibited a declining trend from 2001 to 2011 (the time covered by the most complete data set). Rising sea temperature, thermal stratifi cation of the water column, and the trophic pressure imposed on sardine populations by large pelagic predators (talang queenfi sh, Scomberoides commersonnianus; kingfi sh, Scomberomorus commerson; longtail tuna, Thunnus tonggol; and some others) might be the factors mediating this trend.

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