The spring bloom in the northwestern Sargasso Sea: spatial extent and relationship with winter mixing

The interannual variability of the spatial extent of the spring bloom in the central Sargasso Sea was quantified by remote-sensing approaches. Proxy measurements employing satellite-derived sea-surface temperature and chlorophyll a concentrations were used to estimate the amount of new production derived from inorganic nitrate supplied by convective winter mixing. Nitrate supply and new productivity were estimated from temperature–nitrate and chlorophyll a–euphotic-zone depth relationships derived from in situ measurements made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site near Bermuda (31°40′ N, 64°10′ W). Proxy estimates of springtime new production at the BATS latitude fall within the range of previous estimates from geochemical approaches, numerical modeling, and analysis of hydrographic measurements. However, estimates of new production made at BATS are poor predictors of the regional-scale new production due to large interannual differences in the spatial gradients of the controlling factors. Moreover, the BATS latitude is in a region of local maxima of meridional gradients in sea-surface temperature and thereby new production. The BATS site lies in a boundary region between the dynamic, spatially variable and productive northern Sargasso Sea and the spring-bloom-free region of the southern Sargasso. Hence, interpretations of spring bloom-forced regional scale biogeochemical processes from point observations made at BATS should be made with caution.