Interannual thermohaline (1979–2014) and nutrient (2002–2014) dynamics in the Levantine surface and intermediate water masses, SE Mediterranean Sea

Abstract

In this study a >30years dataset of 1382 CTD casts in the Levantine Basin (LB) was analyzed to examine the thermohaline trends of the Surface (~0–50m) and Intermediate (~150–350m) Water masses (LSW, LIW). In addition, a 13years (2002–2014) dataset of 3 deep water stations (>1000m) in the eastern Levantine Basin (Haifa Section cruises) that were visited 2–3 times annually was used to explore the relations between the physical and nutrient properties in the LIW. Over the past 30years the LSW and LIW masses displayed positive long-term trends in salinity of +0.008±0.006 and +0.005±0.003year−1, respectively, and temperature of +0.12±0.07 and +0.03±0.02°Cyear−1, respectively. Decadal variations in salinity and temperature were superimposed on all long-term trends. Throughout the period 2002–2014 nutrient levels in the LIW core and corresponding integrated values of chlorophyll a also varied in nearly opposite phase with temperature and salinity. Furthermore, these variations occurred with a similar decadal periodicity, but with shifted phase with those observed in the Southern Adriatic and North Ionian Seas in the same water mass. The latter were considered to be caused by decadal reversals in the North Ionian Gyre, i.e. Bimodal Oscillation System (BiOS). These results indicate that the thermohaline flux variations attributed to the BiOS mechanism have a significant impact in magnitude on the available nutrients and the dynamics of the eastern basin primary productivity. These results should be taken into consideration in assessing the relative contribution of external nutrient loads in comparison to those attributed to variations in thermohaline fluxes and in the assessment of long-term and interannual primary productivity (chlorophyll a and nutrients) trends in the LB.