Nutrient budget for the Eastern Mediterranean: Implications for phosphorus limitation

Abstract

The eastern Mediterranean has a high nitrate to phosphate (N : P) ratio (~28 : 1) in the deep water and a highly unusual P limitation of the primary productivity. We present a detailed nutrient budget of inputs to the basin, which shows that there is a high N: P ratio (>16 : 1) in all the input sources, particularly from the atmospheric source, where the N: P ratio was 117 : 1. The high N: P ratio is retained within the system because there is no significant denitrification in either the sediments or intermediate water. This is because of the extreme oligotrophic nature of the system, which is caused by the unusual anti-estuarine flow at the Straits of Sicily. Support for this conclusion is provided by the observation that the only area of the eastern Mediterranean where the N: P ratio in deeper water is ~16 : 1 is the northern Adriatic, which is also the only area with significant denitrification. The N budget (total input to basin vs. net output at the straits of Sicily) balances closely. This N balance suggests that N fixation is an insignificant process in this P-limited system. The unusually light 15N values in the deep water nitrate and particulate organic nitrogen can be explained by processes other than nitrogen fixation. These processes include a lack of significant denitrification in the basin and by particulate organic matter exported from surface waters during the Plimited winter plankton bloom.