New chemical, bio-optical and physical observations of upper ocean response to the passage of a mesoscale eddy off Bermuda

A mesoscale eddy advected across the Bermuda Testbed Mooring site over a 30 -day period centered on July 14, 1995. Temperature and current measurements along with biogeochemical measurements were used to characterize the biolo gical response of the upper ocean associated with the introduction of nitra te into the euphotic layer due to the doming of isotherms associated with t he eddy. Complementary shipboard data showed an anomalous water mass, which extended from a depth of similar to 50 to 1000 m, manifesting as a cold su rface expression and warm anomaly at depth. Although mesoscale eddies are f requently observed in the Sargasso Sea, the present observations are partic ularly unique because of the high-temporal-resolution measurements of the n ew instrumentation deployed on the mooring. Analyzers that measure nitrate plus nitrite were placed at depths of 80 and 200 m and bio-optical sensors were located at depths of 20, 35, 45, 71, and 86 m. Peak nitrate values of nearly 3.0 mu M at 80 m and chlorophyll nitrate values of 1.4 mg m(-3) at 7 1 m were observed, as well as a 25- to 30-meter shoaling of the 1% light le vel depth. A Doppler shift from the inertial period (22.8 hours) to 25.2 ho urs was observed in several time series records due to the movement of the eddy across the mooring. Inertial pumping brought cold, nutrient-rich water s farther into the euphotic zone than would occur solely by isothermal lift ing. Silicic acid was depleted to undetectable levels owing to the growth o f diatoms within the eddy. The chlorophyll a values associated with the edd y appear to be the largest recorded during the 8 years of the ongoing U.S. JGOFS Bermuda Atlantic Time Series Study (BATS) program.