Nutrient dynamics across the Israeli coastal shelf: An unusual oligotrophic coastal system

The nutrient dynamics and biogeochemical properties of the Pelagic Eastern Mediterranean (EMS), an inland sea with many of the characteristics of a mid-ocean gyre, has been well studied. By contrast there are few studies designed to understand these processes on the coastal shelves of the EMS. In this study the nutrient dynamics were determined using ultra-sensitive analytical procedures together with Primary Productivity (PP) and Chlorophyll across the Israeli Coastal shelf (ICS). This includes 12 monthly cruises over a seasonal cycle at an outer shelf station and several transects from the coast to 100 m depth. Sample preservation tests were carried out for nutrients at the ultra-low concentrations found on the ICS, which found that filtered fresh samples were optimal while confirming that using unfiltered frozen samples gave results for Dissolved Inorganic Phosphorus (DIP) that were too high and NO₃⁻ and NO₂⁻ (N&N) were too low. The extra DIP released from frozen unfiltered samples may be due to DIP stored in the periplasm of cyano- and other bacteria (Kamennaya et al., 2020). Nutrient data for the ICS showed moderate concentrations of N&N (∼300–500 nM) throughout the water column in winter, which decreased to less than 50 nM in mid-late summer. DIP was depleted (<10 nM) throughout the year. Net PP at the outer shelf station (120 m) was 30 gC m² y⁻¹. The seasonality and concentration of nutrient concentrations and Net PP as well as the observed seasonal signal of Chlorophyll from remote sensing were similar to that found in the pelagic EMS. The ICS is ultra-oligotrophic and is very different from the eutrophied coastal shelves found in many locations globally because of the unusual anti-estuarine circulation.