Bermuda nitrate concentrations

In the South Atlantic, with similarly low surface nitrate concentrations, DON is consistently <5 pmol kg, or 2 pmol kg lower than the North Atlantic. In fact, the western Sargasso Sea (near Bermuda) also has relatively low surface DON values (Hansell and Carlson, 2001), similar to values found in the South Adantic. The relatively elevated DON concentrations in the low latitude North Adantic are suggestive of unique inputs there (such as atmospheric deposition, N2 fixation, or upweUed nitrate with horizontal transport) compared to the South and western North Atlantic. 

Figure 33-5 Comparison of observed data near Bermuda with the Fasham et al. (1990) model output for (A) nitrate concentration, (B) phytoplankton N biomass, (C) bacterial N biomass, and (D) integrated bacterial N biomass. Reproduced with permission from Fasham eta/. (1990).

Figure 33-7 Basin-wide contour plots of the Fasham et al. (1993) model solution of (A) modeled average phytoplankton concentration (mmol N m ) at 5 m during April, (B) modeled average nitrate concentration (mmol N m ) at 5 m during April, (C) and (D) as for Figures (a) and (b) but for August. Concentrations greater than 1 mmol N m are shaded. The positions of Bermuda Station S and Ocean Weather Station India are indicated by the letters S and I, respectively. Reproduced with permission from Fasham etal, 1993.

Depth profiles of cell densities in the photic zone generally show E. huxleyi to live within the mixed layer. Cortes et al. (2001) studied the seasonal depth distribution of coccohthophorid species off Hawaii. Sampling showed that the main production occurred in the middle photic zone (50-100 m), which lay within the mixed layer for most of the year. While the depth of maximum E. huxleyi density varied during the annual cycle, it generally lay between the shallowest sampling level (10 m) and 100 m. Depth profiles off Bermuda (Haidar and Thierstein, 2001) found that maximum densities of E. huxleyi were nearly always shallower than 100 m, and more commonly within the upper 50 m. The highest cell densities for E. huxleyi recorded were at 1 m depth in March, after the seasonal advection of nitrate into the mixed layer. Seven years of water-column particulate data off Bermuda confirm that alkenone concentrations in the surface mixed layer are 2-4 times higher than in the deep fluorescence maximum at 75-110 m (Conte et al., 2001). 

---