Past, present and future patterns in the nutrient chemistry of the eastern mediterranean

Abstract

This review summarises the distribution of dissolved nutrients (nitrate, phosphate and silicate) over the period of modern measurements (~30 years) and aspects of the biogeochemical processes which control their distribution in the Eastern Mediterranean Sea (EMS). The levels of nitrate, phosphate and silicic acid (~6 µ mol N kg -1, 0.25 µ mol P kg -1 and 11 µ mol Si kg -1 in deep water) in the EMS are much lower than all other parts of the ocean due to the unusual anti-estuarine circulation which exports nutrient replete intermediate water at the Straits of Sicily and to the relatively young age of the deep water (τ ≈ 120 years). Dissolved oxygen decrease and nutrients increase eastwards, in the direction of the deep- water circulation with nitrate increasing by ~0.5 µ mol kg -1 from the Western Ionian to the Eastern Levantine, phosphate by 0.05 µ mol kg -1 and silicic acid by ~4.5 µ mol kg -1 representing the amount (rate) of organic matter and biogenic silica (BSi) breakdown over the residence time of Deep Water in the basin. The East Mediterranean Transient interrupted this simple pattern causing lower nutrients in the deep water and upwelled nutrients into the upper layers. The high nitrate/phosphate found in the deep water (25-28:1) is due to a combination of high Nitrate:Phosphate waters advected from the surface of the Adriatic during deep water formation during the P-limited winter bloom and P recycling more efficiently than N from the descending particulate organic matter (POM). High N:P ratios in POM and dissolved organic matter (DOM) show that the entire EMS is P starved. There are seasonal changes in nutrient limitation with conventional P limitation during the winter phytoplankton bloom which becomes N&P co-limitation in early summer and can, under some circumstances, become even N limited in mid-summer in the nutrient starved upper waters. N 2 fixation rates are constantly low in the EMS compared to somewhat higher levels in the western MS. Nutrient budgets have been used to explain that the reason for the unusual N:P ratio in the basin is high N:P ratio in the external inputs combined with low denitrification rates caused by the ultra-oligotrophic status of the basin. However the external nutrient (both riverine and atmospheric) inputs have increased dramatically between 1950 and 2000. Estimates of the pristine flux of nutrients by riverine sources to the EMS are 2.0 × 10 9 moles N/y and 0.095 × 10 9 moles P/y while the atmospheric flux has increased by 85 % for NOx and 65 % for NH3 between 1910 and the end of the twentieth century. It is suggested that climate change may result in dramatic threshold changes in trophic status of the EMS if surface circulation rates decrease as has been predicted.