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-year period, from June 1996 through May 1998 that examined the seasonal variations in the nutrient (NO₃/^-, PO₄/^3-, and Si(OH)₄) and in the chlorophyll a (Chl a) concentrations at two permanent stations across the SE Mediterranean continental shelf and slope (120 and 400 m water depth off the Israeli coast). Seasonally-dependent Chl a concentrations ranged between 0.003 and 0.415 mg m^-3. Following autum and winter a distinct biomass phytoplankton peak was produced (∼ 30 mg Chl a m^-2 in the upper 120 m) and a subsequent moderate spring peak was observed. In the decomposition zone (120-400 m), an opposite pattern was observed with decreasing amounts of NO₃/^- and PO₄/^3- during winter. The integrated Si(OH)₄ pattern in the euphotic zone indicates that diatoms grow during spring and summer and that their frustules are rapidly settled during the breakdown of stratification. At depths between 150 and 400 m, the geometric mean NO₃/^-/PO₄/^3- ratio (∼ 24) was significantly higher than in the upper 150 m (∼ 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 reconcile the relatively low (< 16) NO₃/^-/PO₄/^3- ratios and apparent N dependence in the euphotic zone with the high (> 20) ratios of exogenous upwelling and atmospheric inputs. The determination of the possible presence of additional nutrient phases and preferential uptake of nutrients by the phytoplankton is an important next step toward the understanding of nutrient limitation in such ultra-oligotrophic systems.