Nutrient dynamics and new production in a warm-core eddy from the Eastern Mediterranean Sea

Abstract

The nutrient dynamics of a warm-core eddy from the Eastern Mediterranean was studied through a complete annual cycle. Winter mixing of the upper 450 m of the eddy core resulted in phytoplankton biomass build-up (70 mg Chl a m −2) of stock well above that present in the boundary (44 mg m −2). During this biomass build-up phosphate was reduced to undetectable levels while excess nitrate (0.6 μM) remained which, taken with other evidence, showed that this area of the Eastern Mediterranean is phosphorus limited. After stratification in late March, nitrate and phosphate concentrations were at undetectable levels in the upper 120 m, a deep chlorophyll maximum had developed (90–120 m), and a gradual systematic accumulation of nutrients occurred in the zone from 120 to 450 m. The permanent nutricline lay between 450 and 650 m. The net supply of nutrients to the euphotic zone (0–120 m), which is the annual new production, was 300 mmoles N m −2 y −1 and 14.9 mmoles P m −2 y −1. Most of this new production (∼90%) was supplied by deep winter mixing and occurred over a limited period of time (days to a few weeks). Previous estimates for new production in the Eastern Mediterranean by Dugdale and Wilkerson (1988, Oceanologica Acta, 9, 170–184), based on bottle incubations, were low because the major input of nutrients by deep winter mixing was not included in the calculation. Most of the nutrients supplied to the euphotic zone in the eddy (60–70%) were derived from the decomposition of the previous year's productivity.