Seasonal patterns of ocean biogeochemistry at the U.S. JGOFS Bermuda Atlantic time-series study site

Seasonal patterns in hydrography, oxygen, nutrients, particulate carbon and nitrogen and pigments were measured on monthly cruises at the Bermuda Atlantic Time-series Study site, 80 km southeast of Bermuda. Between October 1988 and September 1990, the annual cycle was defined by the creation of 160–230 m-deep mixed layers in February of each year and a transition to strong thermal stratification in summer and fall. The 230 m mixed layer in February 1989 resulted in mixed-layer nitrate concentrations of 0.5–1.0 μmole kg−1, carbon fixation rates over 800 mg C m−2 day−1, and a phytoplankton bloom with chlorophyll concentrations over 0.4 mg m−3. Chlorophyll a, particulate organic matter, inorganic nutrients and primary production had returned to prebloom levels the following month with the exception of a chlorophyll maximum layer at 100 m. Particle fluxes at 150 m in February 1989 reached 56 mg C m−2 day−1 and 11 mg N m−2 day−1 (0.77 mmole N m−2 day−1). Estimates of new production during the bloom period calculated from changes in oxygen and nitrate profiles ranged from 100 to 240 mmoles N m−2, significantly higher than the sediment trap fluxes and approaching the measured total production rates. In spring of 1990, mixed layer depths did not exceed 160 m, nitrate was rarely detectable in the upper euphotic zone, chlorophyll a concentrations were similar to 1989, and particulate organic matter concentrations were lower. The period of elevated biomass lasted for 3 months in 1990, and phytoplankton pigment composition varied between cruises. The average rates of primary production and particle flux were higher in 1990 than those measured in the spring of 1989, despite the differences in mixed layer depth. Throughout both years, NO3 : PO4 ratios in the upper thermocline exceeded Redfield ratios. The maintenance of this pattern requires a net uptake of PO4 between 150 and 250 m, a depth range usually associated with net remineralization. The exact mechanism that maintains elevated PO4 uptake and its implication for the nutrient supply to the euphotic zone remain unknown.

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