The effect of cold and warm core eddies on the distribution and stoichiometry of dissolved nutrients in the northeastern Mediterranean

1998-10-01
Yılmaz, Ayşen
TUĞRUL, SÜLEYMAN
The nutrient distribution and phytoplankton production in the Levantine Sea of the eastern Mediterranean are principally determined by the duration and the intensity of deep winter mixing in the quasi-permanent anticyclonic and cyclonic eddies. In the seasons of stratification, a nutrient-poor aphotic layer is formed between the euphotic zone and the nutricline; interestingly, it consistently extends down to depths of about 29.0-29.05 isopycnal surfaces, but nearly vanishes in the core of the cyclonic Rhodes Gyre (RG) due to the upwelling of the Levantine deep water (LDW) up to the base of the euphotic zone. Accordingly, the nutricline is much sharper and shallower in the cyclonic RG; nevertheless, it is consistently established between the density surfaces of 29.00-29.05 and 29.15 throughout the basin. In the severe winters of 1992 and 1993, the upper 1000 m of the cyclonic Rhodes Gyre was occupied by the LDW with its associated chemical properties and abnormally high nutrient concentrations (NO3 = 3.8-4.7 mu M; PO4 = 0.14-0.16 mu M and Si = 7.3-7.8 mu M) were observed in the euphotic zone. However, the surface nutrient concentrations of the anticyclonic regions were raised merely from the summer-autumn values of < 0.02 and nearly 0.2 mu M to about 0.03 and 0.8 mu M for phosphate and nitrate, respectively. The molar ratios of nitrate to phosphate in the water column range between 5 and 20 in the euphotic zone but exhibit well-defined peak values (as large as 40-120) at the top of the nutricline (corresponding to nearly the depths of the 29.05 isopycnal surfaces) for most of the year. Such prominent maxima are the result of the apparent shift between the onsets of the nitracline and phosphacline due to as yet undefined factors. Below the nutricline the N/P ratios decrease regularly and reach an almost constant deep value (= 28) over the basin. The mean ratio, derived from linear regression of the pooled phosphate and nitrate data from March 1991 to March 1994 is about 23.6, substantially higher than deep ocean values.
Citation Formats
A. Yılmaz and S. TUĞRUL, “The effect of cold and warm core eddies on the distribution and stoichiometry of dissolved nutrients in the northeastern Mediterranean,” pp. 253–268, 1998, Accessed: 00, 2020. [Online]. Available: https://hdl.handle.net/11511/30246.