The seasonal dynamics of nutrient and chlorophyll a concentrations on the SE Mediterranean shelf-slope

We report the results of a sequence of twelve research cruises over a two-y ear period, from June 1996 through May 1998 that examined the seasonal vari ations in the nutrient (NO3- PO43-, and Si(OH)(4)) and in the chlorophyll a (Chl a) concentrations at two permanent stations across the SE Mediterrane an continental shelf and slope (120 and 400 m water depth off the Israeli c oast). Seasonally-dependent Chl a concentrations ranged between 0.003 and 0 .415 mg m(-3). Following autumn and winter a distinct biomass phytoplankton peak was produced (similar to 30 mg Chl a m(-2) in the upper 120 m) and a subsequent moderate spring peak was observed. In the decomposition zone (12 0-400 m), an opposite pattern was observed with decreasing amounts of NO3- and PO43- during winter. The integrated Si(OH)(4) pattern in the euphotic z one indicates that diatoms grow during spring and summer and that their fru stules are rapidly settled during the breakdown of stratification. At depth s between 150 and 400 m, the geometric mean NO3-/PO43- ratio (similar to 24 ) was significantly higher than in the upper 150 m (similar to 7), where it varied seasonally with low ratios (3, 5) during autumn and winter compared to summer and spring (9, 11). At the present time, it is not possible to r econcile the relatively low (< 16) NO3-/PO43- ratios and apparent N depende nce in the euphotic zone with the high (> 20) ratios of exogenous upwelling and atmospheric inputs. The determination of the possible presence of addi tional nutrient phases and preferential uptake of nutrients by the phytopla nkton is an important next step toward the understanding of nutrient limita tion in such ultra-oligotrophic systems. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.