Abstract:
A comparison of monthly biogeochemical measurements made from 1993 to
1995, combined with hydrography and satellite altimetry, was used to observe the
impacts of nine eddy events on primary productivity and particle flux in the Sargasso
Sea. Measurements of primary production, thorium-234 flux, nitrate+nitrite, and
photosynthetic pigments made at the US JGOFS Bermuda Atlantic Time-series Study
(BATS) site were used. During the three years of this study, four out of six high thorium-
234 flux events over 1000 dpm/m2/d occurred during the passage of an eddy. Primary
production nearly as high as the spring bloom maximum was observed in two modewater
eddies (May 1993 and July 1995). The 1994 spring bloom at BATS was
suppressed by the passage of an anticyclone. Distinct phytoplankton community shifts
were observed in mode-water eddies, which had an increased percentage diatoms and
dinoflagelletes, and in cyclones, which had an increased percentage cyanobacteria
(excluding Prochlorococcus). The difference in the observations of mode-water eddies
and cyclones may result from the age of the eddy, which was very important to the
biological response. In general, eddies that were one to two months old elicited a large
biological response; eddies that were three months old may show a biological response
and were accompanied by high thorium flux measurements; eddies that were four months
old or older did not show a biological response or high thorium flux. Our conceptual
model depicting the importance of temporal changes during eddy upwelling and decay fit
the observations well in all 7 upwelling eddies. Additional information is needed to
determine the importance of deeper mixed layers and winter mixing to the magnitude of
the eddy impacts. Also, sampling generally captured only the beginning, end, and lor
edge of an eddy due to the monthly to semi-monthly frequency of the measurements
made at BATS. Lagrangian studies, higher resolution time-series, and/or more spatial
coverage is needed to provide additional information for improved C and N budgets in
the Sargasso Sea and to complete our understanding of the temporal changes that occur in
an eddy.
